Symposium EM11 : Wide-Bandgap Materials for Energy Efficiency—Power Electronics and Solid-State Lighting

As the demand for more energy efficient electronics increases, GaN has emerged as an attractive candidate material for high-voltage power management applications. The most promising device structure at the moment is that of a metal-insulator-semiconductor high-electron mobility transistor (MIS-HEMT) in which a gate oxide is placed between the gate metal and the AlGaN/GaN heterostructure of a HEMT. This is an attractive device architecture because of its high current, high breakdown voltage and low gate leakage current, all desirable attributes for power transistors. A concern with this new device technology is reliability and instability under prolonged high-field and high-temperature conditions. In particular, issues associated with the gate oxide have not been studied in the better established GaN HEMTs and bring in new concerns. Understanding sources of instability and reliability associated with the gate oxide is the goal of the present research.

The physical mechanisms responsible for bias stress instability (BTI) in GaN MIS-HEMTs are poorly understood. This is because of their complex gate stack structure with multiple interfaces and many trapping sites. In order to isolate the role of the gate oxide and its interface in BTI, we are studying a simpler GaN MOSFET structure in which the gate oxide is placed directly on top of the GaN channel. Even in this case, GaN substrate trapping complicates the interpretation of the results. Our research reveals the importance of electron trapping and detrapping inside the oxide as well as trap state generation at or near the oxide/semiconductor interface. Separately, we are studying time-dependent dielectric breakdown (TDDB) in GaN MIS-HEMTs, a catastrophic condition that arises after prolonged high-voltage gate bias stress. We have developed an experimental methodology to characterize TDDB through time-dependent current-voltage and capacitance-voltage measurements. Our techniques isolate different roles of threshold voltage shift, oxide trap formation and trapping, interface state generation, stress-induced leakage current (SILC), and eventual oxide breakdown. Out of this research, a classical signature for TDDB in GaN MIS-HEMTs emerges with evidence of progressive breakdown that supports the percolation model of defects.

These studies should be instrumental in understanding the complex instability and reliability issues of GaN MIS-HEMTs for power electronics applications.

This paper reviews the most relevant dielectric-related trapping mechanisms in GaN-based transistors with MIS-type and p-GaN type gate. Metal-insulator-semiconductor (MIS) devices with partially-recessed gate have been submitted to pulsed and constant voltage stress, with the aim of evaluating the impact of charge trapping processes on the dynamic properties of the devices and on the negative-bias threshold instabilities (NBTI) induced by negative gate bias. Three different dielectrics were considered for this investigation: SiN deposited by rapid thermal chemical vapour deposition (RTCVD), SiN deposited by plasma enhanced atomic layer deposition (PE-ALD), and Al₂O₃ deposited by atomic layer deposition (ALD). In addition, we investigate the failure processes of GaN-based HEMTs with p-type gate submitted to long-term stress tests. The results obtained within this paper are critically compared to previous literature reports, to provide a more complete view of the state-of-the-art.

GaN has been employed for high temperature, high power, high voltage and high frequency devices due to its wide band gap, excellent electron mobility and high breakdown electric field[1]. But achieving a dielectric with low interfacial defect density on GaN, high permittivity and low leakage current still remains challenging. Previously, we have shown that a defect-free interface can be achieved by growing lattice matched epitaxial lanthanum oxide (La₂O₃) on GaAs (111) using atomic layer deposition (ALD)[2]. However, there is no binary metal oxide has a suitable lattice constant, band gap, and conduction band offset for passivating the GaN(0001) surface and serving as an effective dielectric.
In this work, we demonstrate for the first time that an epitaxial Mg_xCa_1-xO film can be deposited on GaN by ALD. By adjusting the ratio between the Mg and Ca concentrations in the film, a lattice matched Mg_xCa_1-xO/GaN(0001) interface can be achieved with low interfacial defect density. High resolution XRD has shown that the lattice parameter of this ternary oxide obeys Vegard’s Law. Cross-sectional TEM shows an atomically sharp interface, confirming the high quality of the epitaxy. The valence band offset between Mg_xCa_1-xO and GaN was measured to be ~1 eV and thus the conduction band offset is ~3eV assuming the oxide band gap is ~7.4eV. Therefore both the band gap and band offset of Mg_xCa_1-xO are suitable as gate dielectric for GaN device applications. High temperature capacitance-voltage characterization shows that the film with composition Mg_0.25Ca_0.75O has the lowest density of interfacial defects. With this optimal oxide composition, a Mg_0.25Ca_0.75O/AlGaN/GaN MOS-HEMT device was fabricated. An ultra-high on/off ratio of 10¹¹ and a near ideal subthreshold swing of 62 mV/dec were achieved with this device. Thus we believe epitaxial Mg_xCa_1-xO films on GaN can be applied for the future high-power and high-frequency applications.


[1] S. Dimitrijev, J. Han, H. A. Moghadam, and A. Aminbeidokhti, “Power-switching applications beyond silicon: Status and future prospects of SiC and GaN devices,” MRS Bull., vol. 40, no. 05, pp. 399–405, 2015.
[2] X. Wang, L. Dong, J. Zhang, Y. Liu, P. D. Ye, and R. G. Gordon, “Heteroepitaxy of La2O3 and La2−xYxO3 on GaAs (111)A by Atomic Layer Deposition: Achieving Low Interface Trap Density,” Nano Lett., no. 111, 2013.

Understanding degradation of AlGaN/GaN high electron mobility transistors (HEMTs) is critical for enabling optimal performance and reliability of these devices. Mechanical strain is believed to be a critical degradation mechanism based on finite element simulations and experimental observations of cracks and pitting in the devices. However, there is a lack of experimental techniques capable of measuring mechanical strains in AlGaN/GaN HEMTs. In this work, we present atomic force microscope (AFM) measurements of thermomechanical and inverse-piezoelectric (IPE) deformation in periodically biased AlGaN/GaN HEMTs. Periodic applied bias induces periodic Joule heating and an associated periodic thermomechanical surface deformation, which the AFM cantilever tracks with sub-picometer precision. Periodic applied bias also induces measurable periodic IPE deformation. This technique is able to measure deformation in regions of the device with sub-micron features, including near the metallic gate, which is inaccessible by Raman microscopy. We investigate devices with a range of operating conditions: drain-source voltage, V_DS, of 0 to 50 V, gate-source voltage, V_GS, of -10 to 1 V, drain-source power of 0 to 6 W/mm, and frequency of 30 – 400 kHz. To study thermomechanical behavior, we open the channel and apply periodic V_DS. As V_DS increases, deformation decreases, especially near the gate and on the source side of the channel. We present an electro-thermo-mechanical finite element model which agrees well with and aids interpretation of the measurements. The model reveals that as V_DS increases, the hotspot moves away from the gate and toward the drain, leading to decreased gate temperature rise and decreased thermomechanical deformation, especially near the gate and source. The model also reveals that tensile thermal stress develops in the AlGaN layer near the drain-side edge of the gate due to a large mismatch of thermal expansion coefficients between the metallic gate and the AlGaN layer. This is important because the total stress is a combination of IPE, intrinsic, and thermal stresses. IPE and intrinsic stress in the AlGaN layer are both believed to be tensile. Therefore, tensile thermal stress will increase the total stress and may accelerate device degradation. This indicates that the gate temperature (and associated thermal strain mismatch with the AlGaN layer) is more important for thermal stress than the maximum device temperature. As the hotspot moves away from the gate for increasing V_DS, the gate temperature decreases, leading to 55% greater tensile thermal stress for V_DS = 10 V than for V_DS = 48 V for the same device power. To study IPE deformation, we set V_DS = 0 V to prevent heating and apply periodic V_GS. The measurements indicate that IPE deformation above the gate is 1.5 pm/V above pinch-off voltage and < 0.5 pm/V below pinch-off voltage. We conclude with a discussion of the implications of our experimental results on device degradation.

Recently, GaN-based transistors have been expected to be used in the low voltage applications such as DC/DC converters. For the switching devices, normally-off operation is strongly required from the viewpoint of safety. In order to obtain the normally-off characteristic, on-resistance (R_on) has been sacrificed and normally-off GaN HFETs show higher R_on. Thus, decreasing R_on, especially decreasing contact resistance (R_c) is a critical issue for these devices.
We have proposed a normally-off GaN HFET with p-type NiO gate for reduction of R_on by decreasing device dimensions [1]. The p-type NiO gate is successfully fabricated and normally-off operation is also realized with a threshold voltage (V_th) of 0.8 V. Although R_on is reduced by downscaling with introducing NiO gate, R_c becomes the main part of R_on in the limitation of downscaling. Then, we introduce a novel fabrication technique for source and drain electrodes to reduce R_c. In this technique, a heavily doped n⁺⁺-GaN layer regrown by MOCVD is introduced using Ge as a dopant [2]. A carrier concentration of up to 1 x 10²⁰ cm^-3 is achieved. Using this layer for source and drain electrodes, the total R_c is markedly reduced to 0.25 Ωmm. From these novel techniques, the fabricated GaN HFET with the 350 nm NiO gate exhibits a low R_on of 0.95 Ωmm, a maximum drain current of 1.1 A/mm, and a peak transconductance of 490 mS/mm, maintaining normally-off operation. These excellent results will contribute to the marked increase of conversion efficiency in switching devices.

[1] A. Suzuki, Y. Yamada, H. Tanaka, H. Ueno, N. Otsuka, Y. Anda, T. Ueda, T. Tanaka, and D.Ueda, Proc. Workshop on Compound Semiconductor Device and ICs, 2013, p. 77.

[2] A. Suzuki, S. Choe, Y. Yamada, S. Nagai, M. Hiraiwa, N. Otsuka, and D. Ueda, IEDM Tech. Dig., 2014, p. 275.

With the emergence of novel high power applications such as the automotive market, the development of a new generation of power devices operating well above 1 kV with high efficiencies is needed. Alternatives to existing Silicon (Si) technology have to be found since Si power devices are thermally limited and show high specific on-resistance at those operating voltages. GaN’s wide band-gap semiconductor properties and the compatibility with silicon technology lead to high expectations in low-cost power electronics with breakthrough performance, especially for high voltage DC-DC converters. However, this technology still suffers from the limitation of the silicon substrate since the breakdown occurs when the electric field reaches the silicon for large gate to drain spacing. In order to overcome this limitation, we have developed a process in which the Si substrate is locally removed near the high electric field region. This allowed us to achieve state-of-the-art 3-terminal lateral breakdown voltage GaN-on-Si transistors above 3000 V while delivering low specific on-resistance. These devices still shows lateral breakdown voltages well-above 2 kV at 600 K.
In order to make this approach viable for high power applications, high thermal dissipation has to be ensured and the substrate needs to be grounded. An integrated thermal management based on thick PVD AlN is being implemented into the trenches, which is expected to maintain the outstanding breakdown voltage properties. In this presentation, first results corresponding to the development of this technology will be depicted.

Even though the majority of development efforts for high power and high frequency applications have focused on
AlGaN/GaN High Electron Mobility Transistors, InxAl1-xN shows promise as a candidate material for the gate
barrier and polarization charge-inducing layer because of its wide bandgap, high spontaneous polarization charge,
and lattice-matching to GaN [1]. The advances in epitaxial growth of HEMTs on silicon substrates have enabled
both improved economic efficiencies and technical functionalities. However, the growth of InAlN by MOCVD is
challenging as the optimal conditions for AlN and InN growth are substantially different, and potential incorporation
of Ga into the film compromises the sharp interfaces required in HEMT structures encompassing AlInN. It has been
reported that both 2DEG density and mobility degrade as a result of the parasitic formation of a GaN layer at the
InAlN/AlN interface [2].
This paper reports on InAlN films and InAlN/GaN HEMT structures epitaxially grown on 150 mm <111> Si, using
Veeco’s Propel� single wafer MOCVD system. The TurbodiscÒ, vertical rotating disk reactor encompasses the
core reaction and high velocity laminar flow characteristics of its predecessor batch systems, thus enabling stable,
repeatable operation, long PM cycles, and direct, model-based process scale up. Moreover, it incorporates advanced
features for alkyl/hydride flow distribution and temperature uniformity, which translate into excellent uniformity in
epitaxial layer thickness, alloy composition, and doping profile across the wafer. [3]
Material quality was studied by growing InAlN films, 100 nm thick, with indium content of 17% on Si substrates
using a simplified stress compliance AlN/GaN buffer structure. Smooth surfaces with root mean square (rms)
roughness of 0.68 nm were observed in a 5x5 μm2 AFM scan. X-ray Diffraction analysis shows well defined layer
peaks and fringes, indicating good structural quality and abrupt layer interfaces. Thickness uniformity of InAlN is
0.87% for a 7-point XRD measurement across the 150 mm wafer. SIMS analysis confirms the uniform In depth
profile and the presence of abrupt layer interfaces. Negligible Ga (< 100 ppm, atomic) incorporation was detected in
the InAlN bulk film. Film sheet resistance of 230�/�, charge of 2.1×1013/cm2, and mobility of 1270 cm2/V.s were
measured on a prototypical InAlN/GaN HEMT structure comprising a 10 nm-thick, 17% In, InAlN barrier.
References
[1] J. Kuzmik, IEEE Electron Device Lett. Vol. 22, No. 11, pp. 510 (2001);
[2] Lin Zhou, et al, Phys. Status Solidi C 7, No. 10, 2436-2439 (2010);
[3] Jie Su, et al, Phys. Status Solidi A 213, No. 4, 856–860 (2016).

Today, 40% of the energy in the United States is consumed as electric energy. Power electronics, which are used to efficiently convert, control, and process the flow of electric power, play a significant and growing role in the delivery of this electricity. It has been estimated that as much as 80% of electricity could pass through power electronics between generation and consumption by 2030 [1]. Therefore, advances in power electronics promise enormous energy efficiency gains. A key element of any power electronic system is the semiconductor power switching device which determines the frequencies and power levels at which a power electronic system may operate. A Significant portion of the power losses in power electronic converters is dissipated in their power semiconductor devices. Silicon (Si) has been the semiconductor material of choice for power devices for quite some time, due to cost, ease of processing, and the vast amount of information available about its material properties. Si devices are, however, reaching their operational limits in blocking voltage capability, operation temperature, and switching frequency due to the intrinsic material properties of Si. Wide bandgap (WBG) power semiconductors, such as gallium nitride (GaN), silicon carbide (SiC), and diamond, are an attractive emerging alternative to Si in many applications. Power converters based on WBG devices can achieve both higher efficiency and higher gravimetric and volumetric power conversion densities. However, high cost and challenging fabrication of practical devices remain important barriers to the widespread adoption of WBG devices. In 2014, ARPA-E launched a program entitled SWITCHES (Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High-Efficiency Systems) to catalyze the development of WBG devices using new fabrication innovations and/or new device architectures. This paper gives an overview of the technical progress to date in the SWITCHES program. The performance of various high voltage and high current rectifiers and transistors in the GaN, SiC, and diamond material systems is discussed. Material and processing challenges and reliability concerns for wide-bandgap power devices are also described. A glimpse into the future trends in device development and commercialization is offered.

It is undeniable that wide badgap materials will be the face of next generation electronics offering solutions never thought of before. The roadmap beyond Si in power electronics, introduced by Silicon carbide and Gallium nitride, is now expanding further with wider bandgap materials like Gallium Oxide [1], Aluminum (Gallium) Nitride [2] and Diamond [3] technologies gaining maturity. Higher blocking electric field definitely sets up wider bandgap semiconductors for operations between tens of kilowatts to megawatts rage. Medium power GaN HEMTs supporting applications up to 10KW have shown the pathway to commercialization. While GaN based diodes have already established the capability to block multiple KVs[4], transistors still fall short of delivering blocking voltages as predicted by GaN’s materials limit and reliable normally off single chip solutions. Although reducing the overall cost of the chip continue to impede market penetration, one can surely rely on the examples set by lateral GaN devices, to extend the roadmap using vertical GaN and ultimately diamond based devices.
Our recent success achieved with vertical GaN devices, viz. CAVETs[5], demonstrating over 500V blocking capability (complying with the drift layer design) suggests that with improvement in bulk GaN substrates these vertical devices will be competitive with SiC devices. Our predictive models [6] based on experimentally calibrated simulation tools, clearly show the manifestation of higher electron mobility [7] in bulk GaN in lower power losses in switches compared to its SiC counterparts. For example a 1.2KV GaN vertical JFET incurs significantly less switching loss compared to its SiC equivalent since the gate charge is reduced by 50%.
A very important feature of the GaN based power devices is realized when these devices are run ‘hotter’, thereby simplifying or eliminating cumbersome cooling techniques at the system level. Our recent study on contact metallurgy [8] creates a very promising landscape where the contacts are experimentally proven to sustain temperatures over 400^oC.
Finally, the promise of the ultimate power electronics lies in diamond. The outstanding material properties in diamonds are best utilized in devices designed to support over 1KV, at the very least. Some of our recent results present an encouraging future of diamond power electronics demonstrating bipolar action and high blocking voltages suitable for KV operations.
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For fabricating n-channel power-switching MOSFETs as well as high breakdown voltage junction diodes based on GaN, the site-controlled fabrication of p-type GaN of controlled hole concentrations is indispensable. For this purpose, Mg ion-implantation with subsequent annealing is an attractive process, because Mg concentration ([Mg]) and its depth profile can be controlled by the multiple-energy implantation method. However, there have been few reported results on p-type conductivity of Mg-implanted GaN, and the relation between the point defects generated by the implantation and photoluminescence (PL) spectra is scarcely known [1]. In this presentation, the defect species detected using the positron annihilation method [1] and PL spectra are correlated for ion-implanted and epitaxial Mg-doped GaN.

Mg⁺-ions were implanted into 4-μm-thick unintentionally doped (UID) GaN epilayers, creating box profiles of [Mg] at 1×10¹⁷, 1×10¹⁸, and 1×10¹⁹ cm^-3. For comparison, 1-µm-thick Mg-doped epilayers of similar [Mg] being 5×10¹⁷, 2×10¹⁸, and 4×10¹⁹ cm^-3 were grown on the 4-µm-thick UID GaN by MOVPE. For all samples, c-plane freestanding GaN substrates grown by HVPE was used as a substrate, in order to get rid of extrinsic effects originating from threading dislocations. All the samples were annealed at 1300 °C for 5 minutes with N₂ gas atmosphere at 1 atm. The static and temporal PL measurements were carried out between 10 and 300 K.

A broad ultraviolet luminescence (UVL) band with the peak at 3.28 eV was observed in all ion-implanted and epitaxial Mg-doped GaN at 10 K. As it originates from the radiative transition of electrons in the conduction band or shallow donors to Mg acceptors, the result implies the presence of Mg acceptors on Ga sites. The result is consistent with the fact that PL spectra at 10 K of low [Mg] samples exhibited a peak originating from the recombination of excitons bound to a neutral acceptor.

It should be noted that Mg-implanted GaN commonly exhibited a broad green luminescence (GL) band at 2.35 eV, which was almost unseen in epitaxial Mg-doped GaN. Moreover, overall PL intensity at 10 K of Mg-implanted GaN was more than an order of magnitude lower than that of epitaxial Mg-doped GaN, indicating higher concentration of nonradiative recombination centers (NRCs) in the Mg-implanted samples. As Uedono et al. have shown [1] that high temperature annealing increased the size of vacancy complexes composed of Ga vacancies (V_Ga) and N vacancies (V_N) and Chichibu et al. [2] have clarified the origin of NRCs in GaN as V_Ga-complexes like V_Ga(V_N)_n, additional introduction of V_N by Mg-implantation is likely. Accordingly, GL band is most likely originating from V_N. We will discuss the PL dynamics at the meeting.
This work has been supported in part by NEDO-SIP and MEXT programs (Research Alliance and Grant-in Aids of Scientific Research), Japan.
[1] Uedono et al., PSS B 252, 2794 (2015). [2] Chichibu et al., APL 86, 021914 (2005).

Due to the high-speed switching and high-voltage blocking capabilities, GaN power transistors can enable high-efficiency, compact and low-cost power converters. This presentation provides an overview of the development of GaN power switching device technology at HRL, for both lateral and vertical device structures. For GaN-on-Si lateral transistors, we have tackled key challenges in normally-off gate structure and dynamic on-resistance, resulting in KW-level power switching with ~ns switching time. For GaN-on-GaN vertical didoes and transistors, we addressed issues in carbon impurity, Schottky junction design, trench gate structure, and P-body contact, leading to 600V class Schottky barrier diodes and normally-off transistors.

GaN HEMTs for power switching are being actively developed by major power electronics companies due to their superior combination of low on-resistance and high off-state voltage capability. However the on-state resistance after switching from off-state (dynamic R_ON) is often found to be dramatically higher than the static resistance resulting in unacceptable switching losses. This problem now results largely from transient charge trapping in the semi-insulating carbon doped layer situated below the 2DEG. Here we will show that good performance not only requires good control of doping but also, rather surprisingly, control of the vertical leakage from the 2DEG to the GaN:C layer.
Recent calculations have placed the carbon acceptor level in the lower half of the GaN bandgap making the material slightly p-type. We will demonstrate using simulation and experiment the consequences that follow for device operation, since the GaN:C layer is isolated from the 2DEG by a reverse biased P-N junction under normal operational bias. This floating buffer can easily charge resulting in the dynamic R_ON dispersion, so suppression requires a leakage path to prevent the undesirable charging.
Using an innovative silicon substrate ramp technique we show that leakage between layers and charge storage in the key parts of the GaN epitaxial stack does indeed exist in epitaxy displaying low dynamic R_ON dispersion. It will be demonstrated that good performance of devices requires a blocking barrier below the 2DEG and a vertical leakage path allowing positive charging of the GaN:C, presumably along active dislocations.

This work was funded by the UK EPSRC PowerGaN project.

β-Ga₂O₃ is a promising candidate for future high-power device applications because of its extremely large band gap of about 4.5 eV and associated high breakdown electric field. Recently, we demonstrated a record breakdown voltage of over 750 V for Ga₂O₃ MOSFETs with a channel layer grown by molecular beam epitaxy (MBE) [1]. There have been several reports on homoepitaxial growth of Ga₂O₃ thin films on Ga₂O₃ (100) and (010) substrates. However, Ga₂O₃ MBE growth on Ga₂O₃ (001) substrates has received little investigation. In this work, we performed systematic studies on ozone MBE growth of Ga₂O₃ thin films on Ga₂O₃ (001) substrates.
Ga₂O₃ thin films were grown on β-Ga₂O₃ (001) substrates by MBE using Ga metal source and ozone gas. We investigated their structural features as a function of growth temperature by using reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM).
The growth rate of a Ga₂O₃ thin film on a Ga₂O₃ (001) substrate strongly depended on the growth temperature. At 600°C, the growth rate was as high as about 0.6 μm/h, which was almost the same as that of Ga₂O₃ thin films grown on Ga₂O₃ (010) substrates. However, the growth rate of Ga₂O₃ (001) monotonically decreased with increasing growth temperature and reached a very small value of less than 10 nm/h at 750°C. Note that the growth rate of Ga₂O₃ (010) thin films grown under similar conditions is almost constant at 0.5-0.6 μm/h in the wide temperature range of 550-750°C [2]. The surface AFM image of a Ga₂O₃ (001) thin film grown at 600°C revealed a wire-like-shaped morphology running in the [010] direction with a periodicity of about 20 nm, which was consistent with the [010] azimuthal RHEED patterns that had implied the formation of micro-facets. The average surface roughnesses (R_a) of Ga₂O₃ (001) films grown at 600°C and 650°C were 1~2 nm and three to four times larger than those of MBE-grown Ga₂O₃ (010) surfaces. In contrast, straight and ordered steps were observed on a Ga₂O₃ (001) surface grown at 700°C, and the absence of two-dimensional islands on the surface grown at 750°C suggested the transition of growth mode from two-dimensional nucleation to step flow at around 700°C. The meandering step-edges running in the [010] direction observed on Ga₂O₃ (001) surfaces grown above 700°C indicated that Ga adatoms were incorporated only at the (010) step-edges. These results suggested that the substrate off-cut angle (i.e. step density) and direction could be important parameters for the high-speed growth of atomically-flat Ga₂O₃ thin films on Ga₂O₃ (001) substrates.
This work was partially supported by Council for Science, Technology, and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), "Next-generation power electronics" (funding agency: NEDO).

[1] M. H. Wong et al., IEEE Electron Device Lett. 37, 212 (2016), [2] K. Sasaki et al., J. Cryst. Growth 392, 30 (2014).

Gallium oxides are promising materials. There are 5 known polymorphs[1], the α-, β-, γ-, δ-, ε-Ga₂O₃. Especially the thermodynamically stable phase β-Ga₂O₃ has attracted interest as a material for high power devices[2]. The other metastable polymorphs are moving into focus as well. Oshima et al. have grown ε-Ga₂O₃ thin films by MOCVD[3] and Orita et al. used PLD to grow ε-Ga₂O₃. Basic material properties of ε-Ga₂O₃, such as the size of the band gap are comparable to those of β-Ga₂O₃, but it is also expected to exhibit a high spontaneous polarization, possibly allowing the realization of two dimensional electron gases with high sheet carrier densities[4] when used in heterostructures. α-Ga₂O₃ was grown homoepitaxially on c-plane sapphire by mist CVD[5]. Schewski et al. observed pseudomorphic films on c-plane sapphire with a thickness of 3 monolayers independent from growth technique[6].
In our work we concentrate on the metastable phases ε-Ga₂O₃ and α-Ga₂O₃. We present the growth of ε-Ga₂O₃ and of thicker α-Ga₂O₃ layers by plasma assisted molecular beam epitaxy (PAMBE). ε-Ga₂O₃ thin films were grown in a tin-assisted process on c-plane sapphire. Usually, in Ga-rich conditions the growth of gallium oxide is attenuated by an etching of the growing film due to Ga₂O formation, which re-evaporates at growth temperature[7]. With the addition of a small tin flux this etching is shown to be suppressed, hence allowing the formation of ε-Ga₂O₃ in metal-rich conditions. α-Ga₂O₃ thin films with thickness of up to 200 nm are grown pseudomorphically on r-plane sapphire. However, the film thickness is still limited as nucleation of β-Ga₂O₃ can occur on the c-plane facets of the growing layers. The influence of different growth parameters on the phase formation and properties of the thin films is analyzed with HRXRD, AFM, and optical reflection measurements.


[1] Roy et al. J. Am. Chem. Soc. (1952)

[2] M. Higashiwaki et al. Phys. Status Solidi A 211 (2014)

[3] Y. Oshima et al. J. Appl. Phys. 118 (2015)

[4] M. B. Maccioni and V. Fiorentini Appl. Phys. Express 9 (2016)

[5] D. Shinohara and S. Fujita Jap. J. Appl. Phys. 9 (2008)

[6] Schewski et al. Appl. Phys Express 8 (2015)

[7] P. Vogt and O. Bierwagen Appl. Phys. Lett. 108 (2016)

Gallium oxide (Ga₂O₃) is a wide-bandgap semiconductor with attractive properties being exploited for the development of a range of electronic and electro-optic applications. In this work, we report on the growth of epitaxial Ga₂O₃ thin films on various substrates using metal organic chemical vapor deposition (MOCVD) method. Triethylgallium (TEGa) and trimethylgallium (TMGa) were used as precursors for gallium to grow Ga₂O₃ thin films at chamber pressures of 40 Torr, 65 Torr, 90 Torr and 120 Torr using 1800 sccm of pure oxygen. As the chamber pressure increases, the growth rate of the films was observed to decrease. Moreover, the films grown from the TEGa source were generally thinner than those grown from the TMGa source, despite identical growth conditions. The crystal structure and surface quality of the films were assessed by XRD, Raman spectroscopy, and atomic force microscopy (AFM). The film grown using TEGa at 65 Torr yielded a single phase epitaxial (-201) oriented β-Ga₂O₃ material with a surface roughness of ~ 4.0 nm. However, the films grown using the TMGa source were polycrystalline. Although, the reaction path between TEGa or TMGa and O₂ to form Ga₂O₃ by MOCVD is unknown, the fact that the TEGa pyrolyzes at lower temperature than the TMGa indicates greater susceptibility of the TEGa source to gas phase parasitic reactions than the TMGa source. This suggests that the TEGa source is highly depleted before it reaches the substrates surface, leading to a slow growth rate and, hence, a thinner epitaxial film. The UV-visible transmission spectra for films grown from both Ga sources had a band gap of~4.9 eV. Cathodoluminescence (CL) spectroscopy measurements presented a broad blue emission band, regardless of the Ga source used for the film growth, suggesting the presence of donor-acceptor-pair (DAP) processes. Ga₂O₃ thin film growth with water vapor as an oxidant was also studied and will be discussed.

Vertical n-Ga₂O₃ power devices require insulating or p-type materials for current blocking layers (CBLs), guard rings, or inversion-mode channels. Mg-ion (Mg⁺⁺) implanted Ga₂O₃ was investigated in this work as a CBL in light of semi-insulating Ga₂O₃ obtained by Mg compensation doping of n-type bulk crystals. Systematic thermal anneals and electrical measurements presented evidence of implant activation and illustrated a pathway for forming Mg⁺⁺-implanted CBLs in Ga₂O₃ devices.

Two-terminal electrical test structures comprising n-Ga₂O₃/Ga₂O₃:Mg(CBL)/n-Ga₂O₃ were fabricated on 7-μm-thick Si-doped (n~10¹⁶ cm^-3) β-Ga₂O₃ (001) epilayers grown by halide vapor phase epitaxy on Sn-doped (n~3×10¹⁸ cm^-3) substrates. Their design resembled typical transistor structures with a thick n^- drift layer. Mg⁺⁺ implantation was performed at 560 keV and a dose of 4×10¹³ cm^-2 with a peak concentration of 1×10¹⁸ cm^-3 at ~600 nm below the surface. Capless thermal anneals were carried out at 800°C, 900°C, and 1000°C for 30 min in N₂ to attempt Mg⁺⁺ activation and implantation damage recovery. A 100-nm-thick n⁺ top contact layer was formed by Si ion implantation, followed by activation annealing at 800°C for 30 min in N₂. Patterned-top and blanket-bottom Ti/Au ohmic electrodes were subsequently deposited. Current-voltage measurements were performed to assess vertical conduction through the Mg⁺⁺-implanted CBL by grounding the top electrode and applying positive substrate bias.

Ga₂O₃ as-implanted with Mg⁺⁺ was highly resistive owing to extensive lattice damage. Annealing at increasing temperatures was expected to result in progressive damage recovery and hence reduced current blocking by the implanted region. Leakage through the CBL annealed at 800°C remained low (<1 mA/cm² at 200 V) as only limited damage reversal had taken place. A higher annealing temperature of 900°C led to significantly increased conduction through the CBL (1 mA/cm² at 60–90 V) consistent with improved crystal quality. However, the effect of damage recovery saturated beyond 900°C and only slight degradation in current blocking capability (1 mA/cm² at 40–80 V) was observed with 1000°C annealing. The similar blocking characteristics between structures annealed at 900°C and 1000°C suggested that the barrier was no longer dominated by lattice defects; instead, a distinct mechanism that could be unambiguously ascribed to Mg⁺⁺ activation as compensating acceptors in the host material had given rise to a new barrier in the current path. These results established Mg⁺⁺-implantation with an activation temperature beyond 900°C as a feasible CBL technology in vertical Ga₂O₃ transistors, for which the implantation and annealing conditions could be further optimized to realize effective electron barriers.

This work was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO).

Ga₂O₃ has recently generated significant interest and high quality growth (both thin-film and bulk) has been achieved with several techniques. Its distinctive feature compared to other transparent semiconducting oxides is the high transparency all the way to UV thanks to a wide 4.9 eV band gap. Hence this material has potential applications in future UV devices and high power electronics. n-type doping is achieved with Sn and Si, and highly resistive material can be produced by doping with Fe and Mg. p-type doping is yet to be achieved. Ga vacancies have been shown to act as efficient compensating centers in n-type material [1]. In order use Ga₂O₃ as a semiconductor in electronics, detailed understanding and control of defects and doping are required.

In this work, we analyze the formation mechanisms of Ga vacancies with positron annihilation spectroscopy [2] in Ga₂O₃ thin films grown by metal-organic chemical vapor deposition [3]. To this end, we studied samples grown on different substrates (Al₂O₃, Ga₂O₃), with different doping impurities (Si, Sn) and using different precursors for Ga (TMGa, TEGa) and O (H₂O, O₂). In addition, post-growth thermal annealings were performed to manipulate the defect balance in the films. We show that the choice of substrate, precursor and n-type dopant all have a dramatic effect on the efficiency of Ga vacancy formation and hence on the electrical properties of thin-film Ga₂O₃.

[1] E. Korhonen et al., Appl. Phys. Lett. 106, 242103 (2015).
[2] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).
[3] G. Wagner et al., phys. status solidi (a) 211, 27 (2014).

Solar-blind photodetectors have great potentials for numerous applications including flame sensors, radiation detectors, and analysis in chemical, environmental, and biological fields. Various material can be used for solar-blind photodetectors such as In₂Ge₂O₇, InAlN, AlGaN, GaN, MgZnO diamond and so on. However, these materials have still several problems to be solved such as difficulty of film growth with high quality, chemical instability, difficulty of handling the substrate due to intrinsic properties, high cost and so on. Therefore, the development of novel materials and designs of devices with 5S (high stability, high speed, high signal-to-noise, high sensitivity, high selectivity) is necessary in deep-UV detectors research field. β-Ga₂O₃ is a promising candidate for deep-UV photodetectors due to its desirable electrical and optical properties such as ~4.9 eV of direct band gap, and excellent chemical and thermal stabilities. Because it also has intrinsic solar-blindness, blinding the light with wavelength over 280 nm, β-Ga₂O₃ based solar-blind photodetectors need not the additive filters. The solar-blind photodetectors were fabricated using two-dimensional β-Ga₂O₃ micro-flakes, which are mechanically exfoliated from bulk sample. The exfoliated flakes were transferred to as-prepared SiO₂/p-Si/back gate metal (Ti/Au) structure and then the Cr/Au contact electrodes were formed via photolithography and electron-beam evaporators technique. The crystal structure and quality of the exfoliated flakes were confirmed by using transmission electron microscope, selected area electron diffraction pattern and micro-Raman spectroscopy. The photoresponse properties of device were investigated using the semiconductor parameter analyzer and the UV lamp with wavelengths of 254 nm and 365 nm. The devices have back-gate field-effect transistor structure. Therefore, dark currents were effectively reduced and the photoresponse properties were enhanced by applying the negative gate voltage. In this study, we demonstrated the solar-blind photodetectors using exfoliated β-Ga₂O₃ micro-flakes for the first time and investigated the output characteristics and photoresponse properties. The fabricated devices exhibited the highest responsivity among the reported literatures. We believe that these excellent results resulted from the following two reasons; the decrease of dark current and the high surface-to-volume ratio of 2-dimensional β-Ga₂O₃ flakes. The details of the results will be presented at the conference.

Atomic-layer-deposition (ALD) Al₂O₃ films are the most promising solution to gate insulation and surface passivation for non-Si, non-SiC semiconductor devices because of their high thermal stability, relatively high dielectric constant (~ 9), and large bandgap (~ 7 eV) together with high conformity, uniformity, and reproducibility due to the self-limiting process of ALD.[1] The reliability of the Al₂O₃ films as gate insulators needs to be assessed based on the right understanding of current conduction process in the films. To meet this requirement, we recently proposed a model, called space-charge-controlled field emission (SCC-FE) model, and successfully reproduced quantitatively experimental current-voltage (I–V) characteristics of Al₂O₃ films.[2] The SCC-FE analysis revealed that the negative- and positive-bias leakage currents of Al₂O₃ films are enhanced by a sheet of virtual dipoles near the gate and a sheet of positive charge near the substrate, respectively.
To reduce the leakage currents, this study investigated the effect of temperature, a key condition of ALD, on the current conduction in Al₂O₃ films, formed on Si substrates using H₂O oxidant, and obtained the following results. First, from a practical viewpoint, I–V characteristics of insulators need to be compared at the same equivalent oxide field (EOF). In terms of the EOF, the negative-bias leakage current of Al₂O₃ films is approximately independent of ALD temperature above 150°C, because of the compensation of two competing effects of increasing electron affinity and increasing permittivity of Al₂O₃. On the other hand, the positive-bias leakage current increases with increasing ALD temperature above 210°C, due to the increasing sheet of charge near the substrate. Intriguingly, we observed an oscillatory change of leakage current with increasing ALD temperature from 125°C to 210°C, and attributed it to an oscillatory change of dipoles located at the Al₂O₃/underlying chemical SiO₂ interface. Hence, 125°C- or 175°C-grown Al₂O₃ films have the minimal leakage current under positive bias. Considering, however, that these films cause the so-called blisters problem when heated above 400°C,[3] a 450°C ALD process is presently the most promising technology for growth of high-reliability Al₂O₃ film, because of the absence of blistering. It is open to further investigations to develop technologies for reducing charges and dipoles near and at the Al₂O₃/underlying insulator interface, thus suppressing leakage current under positive bias.

[1] A. Hiraiwa, et al., J. Appl. Phys. 117, 215304 (2015); doi: 10.1063/1.4921824
[2] A. Hiraiwa, et al., J. Appl. Phys. 119, 064505 (2016); doi: 10.1063/1.4941547
[2] O. Beldarrain, et al., J. Vac. Sci. Technol. A 31, 01A128 (2013); doi: 10.1116/1.4768170

An alloy system of corundum-structured oxide semiconductors constituted with α-Al₂O₃, α-Ga₂O₃, and α-In₂O₃ offers novel opportunity for wide-bandgap heterostructure oxide semiconductor devices. Marked evolution of α-Ga₂O₃[1], recently, has launched Schottky barrier diodes with record-low on-resistance and high-breakdown voltage[2]. One of other promising materials is α-In₂O₃, whose bandgap is as high as 3.7 eV and electron mobility seems to be higher compared to that of α-Ga₂O₃. We have reported the preliminary operation of α-In₂O₃ MOSFETs[3], and in this presentation we show the recent advancement of crystal growth, conductivity control, and device performance.
We have used the mist CVD technology, which is a safe and cost-effective technology suitable for oxide materials, for the crystal growth on c-plane substrates. As a buffer layer in order to relax the lattice mismatch between α-In₂O₃ and sapphire (∼15%), we used α-Fe₂O₃, α-Ga₂O₃, or α-(Al,Ga)₂O₃. The w-scan x-ray diffraction FWHM of α-In₂O₃ was as small as 110 arcsec. UID α-In₂O₃ showed n-type conductivity with electron concentration and Hall mobility at room temperature of, for example, 3.1×10¹⁸ cm^-3 and 143 cm²/Vs, respectively. More doping of Zn or Mg systematically reduced the electron concentration, suggesting that they acted as compensating acceptors. This means that p-type conductivity might be expected by reducing unintentionally-doped donors.
MOSFETs were fabricated using amouphous Al₂O₃ as a gate insulating layer and Au as gate and source electrodes. The device showed saturated drain current characteristics and clear pinch-off with subthreshold swing of 1.83 V/dec, field-effect mobility of 187 cm²/Vs, and effective mobility of 240 cm²/Vs. The relatively high mobility values are attractive for future evolution.
[1] D. Shinohara and S. Fujita, Jpn. J. Appl. Phys. 47, 7311 (2008)..
[2] M. Oda et al., Appl. Phys. Express 9, 021101 (2016).
[3] K. Kaneko et al., Appl. Phys. Express 8, 095503 (2015).

Transparent conducting oxides (TCOs) and transparent semiconducting oxides (TSOs) have received extensive interests and demands for the application of current optoelectronic devices. Alkaline earth stannates have recently attracted much attention as an excellent TCOs and TSOs. In particular, La-doped BaSnO₃ (LBSO) has excellent room-temperature (RT) carrier (electron) mobility (μ_e ~ 320 cm²V^-1s^-1 at n = 8.0 × 10¹⁹ cm^-3 in a single crystal) and higher electrical conductivity than previously-reported TCOs. Despite the great potential of LBSO for use in Indium-free TCOs and TOSs, epitaxial LBSO films were reported to have much lower μ_e than single crystals. This difference has been attributed to the high density of extended defects in the films, i.e., threading dislocations, which are considered as scattering centers for carriers.
In this presentation, we show significant increase in the room-temperature electron mobility of LBSO by post-annealing under N₂ at higher temperature than the growth temperature. Regardless of pre-annealing growth temperature, simple annealing under N₂ consistently doubled the RT μ_e of LBSO films on STO substrates and could achieve the maximum electron mobility of 78 cm²V^-1s^-1 at n = 3.0 × 10²⁰ cm^-3, which is comparable to the best mobility of PLD-grown LBSO films on STO substrates. The enhancement of room-temperature mobility by post-annealing is attributed to the annihilation of extended defects, along with compressive strain induced by the difference in the thermal expansion coefficients of substrates and LBSO films. Our finding suggests that the thermal strain induced by high-T annealing can be exploited to reduce the density of extended defects in LBSO films to boost their room-temperature mobility.

Gallium oxide (Ga₂O₃) has attracted great interest because of its potential for the next-generation of power electronics for its ultra wide band gap of 4.8 eV, a higher breakdown field of 8 MV/cm and potential low-cost mass production of native substrates. In this paper, we study the integration of GaN film on Ga₂O₃ by MOCVD epitaxy. We used (0 1 0) β-Ga₂O₃ substrates compared to those with other orientation like (-2 0 1), (1 0 0), (1 0 1), etc, where c-plane GaN epitaxy has been performed.
A closed coupled showerhead reactor was used for GaN epitaxy on 10 x 15 mm² Ga₂O₃ substrates. It was found that the Ga₂O₃ substrate started to decompose below 600 °C under H₂ ambient. By introducing ~5 slm NH₃, the decomposition stopped and the substrate surface stayed stable until the temperature reached ~880 °C. By using a thin low-temperature (LT) AlGaN nucleation layer (NL), the top 1.36 μm GaN layer was grown at ~1000 °C. Selected area diffraction patterns taken by TEM confirmed the GaN film was grown along the [1 1 -2 0] direction. In Ga₂O₃’s (0 1 0) plane, the GaN’s c-axis was rotated 8.7° away from Ga₂O₃’s [1 0 2] direction towards the [2 0 7] direction. Ω-2θ scan by X-ray diffraction measurement showed the crystal peaks of (1 1 -2 0) GaN and (0 2 0) Ga₂O₃ with the Ω separation of ~1.5°. All these results indicate that single-crystal a-plane GaN film was grown on the (0 1 0) Ga₂O₃ substrate. To our knowledge, this is the first time that non-polar GaN epitaxy on Ga₂O₃ has been reported. Such non-polar film may have broad application in both electronic and optoelectronic devices.
Further growth optimization indicated the GaN layer preferred low V/III molar ratio and low growth pressure. Compared to using thick NL, thin AlGaN NL helped remove pits in the surface and reduced the root mean square roughness to 0.98 nm for 1 x 1 μm² scan and 3.15 nm for 10 x 10 μm² scan respectively. This result is comparable to those achieved from GaN homoepitaxy on a-plane GaN bulk substrates.
Oxygen was found to be one of the major impurities in the GaN film, with ~10²⁰ cm^-3 in the LT NL and stabilized at ~10¹⁹ cm^-3 in the top GaN. Coating the backside of substrates by sputtered AlN helped reduce the oxygen concentration by 5X in the LT NL, but did not improve the top GaN layer grown at high temperature. A comparative study showed the growth window of c-plane GaN on (-2 0 1) Ga₂O₃ substrates was much wider. By optimizing the growth conditions, the oxygen concentration was reduced and stabilized at ~4x10¹⁷ cm^-3 in the 3 μm c-plane GaN grown. Additional effort is undergoing to further reduce the oxygen concentration in the GaN film grown on (0 1 0) Ga₂O₃ substrates.

The key to the efficient transmission and conversion of low-carbon electrical energy is the improvement of power electronic devices. Diamond is considered to be the ultimate wide bandgap semiconductor material for applications in high power electronics due to its exceptional thermal and electronic properties. Two recent developments - the emergence of commercially available electronic grade single crystals and a scientific breakthrough in creating a MOS channel in diamond technology, have now opened new opportunities for the fabrication and commercialisation of diamond power transistors.
These will result in substantial improvements in the performance of power electronic systems by offering higher blocking voltages, improved efficiency and reliability, as well as reduced thermal requirements thus opening the door to more efficient green electronic systems.
The current research carried out mainly in Japan and Europe will be presented, with the various device architectures explored, including MOSFET, MESFET, JFET and rectifiers. Results obtained in the framework of the first European research collaboration on diamond devices, aiming at fabricating the first HVDC diamond based converter will also be presented.

AlGaN/GaN-on-diamond microwave devices have demonstrated at least three times higher power density than devices grown on SiC substrates. We demonstrate the benefit of optimized seeding of the diamond growth on achieving defect free GaN-on-diamond wafers, high mechanical stability and homogenous thermal properties for use in ultra-high power microwave electronic devices. These material structures benefit from the high electron mobility of the AlGaN/GaN device layer and the high thermal conductivity of the diamond, however, the rather large coefficient of thermal lattice expansion (CTE) between the diamond and the GaN pose challenges.

In situ focused ion beam cross-sectioning was used to study GaN-on-diamond wafers, fabricated at Element-Six, seeded with different size nano/microsize particles to gain insight on the microstructure-properties relationship. Voids can form at the GaN-diamond interface if an inappropriate seeding scheme is used; physical mechanisms for the occurrence of these defects will be discussed. Using an optimized seeding approach, we demonstrate that GaN-on-Diamond wafers with no macroscopic defects can be fabricated. For the investigation of the mechanical strength of the GaN-diamond interface, a novel micro-mechanical testing approach was employed. Micro-size pillars were fabricated containing GaN, diamond and the interface between the two layers; a mechanical load was then applied onto the GaN layer to ‘pull’ it away from the diamond layer to test the strength of the interface. We find that stress in excess of 3 GPa is required to break the ‘bond’ between GaN and the diamond. This has demonstrated high interface mechanical stability, which is essential for real-life deployment of this novel material structure in device applications. To correlate the local microstructure with the thermal properties of the GaN-on-Diamond wafers, mapping of the thermal properties using a transient reflectance technique was applied. The results showed a high homogeneity of the thermal properties for defect free wafers and this provides an excellent basis for achieving high performance ultra-high power electronic devices in a manufacturing environment. The most recent advances and challenges in these areas will be presented and discussed.

This work is in part supported by DARPA under Contract No: FA8650-15-C-7517 monitored by Dr. Avram Bar Cohen, supported by Dr. John Blevins, Dr. Joseph Maurer and Dr. Abirami Sivananthan.

We fabricated hydrogen-terminated (C-H) diamond MOSFETs using the two-dimensional hole gas (2DHG) induced by coating the C-H diamond surface with Al₂O₃ insulator by high temperature atomic layer deposition (ALD) method. We have reported high breakdown voltage (>1600 V) characteristics [1] and wide temperature (10 K-673 K) operations [2, 3]. Generally, the transport properties of C-H diamond MOSFETs shows normally-on because 2DHG is induced even when applying no bias. Normally-off characteristics are required for power devices for safety operations. The normally-off type Diamond FETs have reported by using partially oxidized diamond MISFET [4], HfO₂-gated diamond FETs [5] and JFETs with narrow channel widths [6]. In this paper, we fabricated C-H Diamond MOSFETs aim at gate threshold voltage (V_th) by partial oxidation (partial C-O) and nitrogen-doped to C-H channel. V_th can be controlled by changing the level of valence band maximum. As a result, normally-off operation, high breakdown voltage and high current density characteristics were obtained.
The fabrication process was as follows. First, undoped CVD layer was deposited on Ib (001) diamond substrate and Ti/Au source/drain were deposited. Second, the diamond surface was C-H by remote plasma. Third, the channel region was partial C-O channel or nitrogen-doped. To form partial C-O channel, UV irradiation in oxygen atmosphere. And the dose of nitrogen doping was varied to 10¹⁸–10¹⁹ cm^-3. The sheet resistance of partial C-O and N-doped surface was 10⁵–10⁶ Ωsq, which is one or two orders of magnitude higher than that of typical C-H diamond. Then, Al₂O₃ passivation layer was deposited to cover partial C-O channel by ALD. Finally, Al gate was deposited.
V_th of C-H diamond MOSFETs with partial C-O channel was -3.0 V (@RT) which is high enough for power device application. V_th control and normally-off operation were achieved. The maximum drain current density of -20 mA/mm (V_DS = -50 V) was obtained, which is compatible to C-H diamond MOSFETs with no partial C-O channel. Here, the sizes of the device were L_SG = 4 μm, L_G = 15 μm and L_GD = 21 μm, respectively.The breakdown voltage of a partial C-O channel C-H diamond MOSFET was obtained 1790 V (L_GD = 21 μm, @RT). In addition, the highest breakdown voltage was obtained to 2021 V (L_GD = 24 μm, @RT) with V_th = -3.5 V. The breakdown voltage of >2 kV is the highest for diamond FETs ever reported. Threshold shift characteristics of C-H diamond MOSFETs with N doping will be exhibited on site.

[1] H. Kawarada et al., IEEE IEDM 14933800, pp.279 -282 (2014) and ISPSD pp483-486 (2016).
[2] A. Hiraiwa, H. Kawarada, et al., J. Appl. Phys. 112 (2012) 124504.
[3] H. Kawarada et al., Appl. Phys. Lett. 105 (2014) 013510.
[4] H. Umezawa, H. Kawarada, et al., J. Appl. Phys. Vol. 44, No. 11, pp.7789 -7794 (2005).
[5] J. W. Liu, Y. Koide, et al., Appl. Phys. Lett. 103 (2013) 092905.
[6] T. Suwa, M. Hatano, et al., IEEE Electron Device Lett. Vol. 37, No. 2, pp. 209 -211 (2016).

Gallium Nitiride (GaN) and diamond are two important materials in next generation power electronics and LEDs. Specifically, GaN-based transistors have high breakdown voltages and high carrier densities resulting in low resistance and high efficiency. However, as the gate size of GaN-based transistors decreases to achieve higher operating frequency, heat dissipation worsens due to boundary scattering and growth defects. It is important to limit the temperature rise in these devices because an increase in temperature will reduce electron mobility and degrade the lifetime of the device. Currently, the thermal properties of size-constrained GaN is not well characterized. In this work, we measure the thickness dependent thermal conductivity of a 15-1000nm thick GaN film heteroepitaxially grown on 4H-SiC using a unique pump-probe thermal imaging technique. Additionally, diamond grown on GaN has been proposed as a near-junction heat sink for GaN based devices. However, the thermal conductivity of these thin diamond films is difficult to measure. In this work, we also measure the anisotropic thickness dependent thermal conductivity of 1-100μm thick diamond films grown on silicon.

Diamond as a semiconductor material for electronics has potential due to its material properties including high thermal conductivity, high electric field breakdown strength, and high carrier mobilities. In this paper we will report on the diamond based power electronics work at the MSU/Fraunhofer Center for Coatings and Diamond Technologies (CCD). We will present our work to improve the quality of bulk and epitaxial mono-crystalline diamond material and its use in making vertical diamond diodes for power electronics. The desired diode characteristics in this project includes a reverse bias breakdown voltage exceeding 1000 V and a forward current exceeding 10 A. Work will be described that improves the quality of the bulk substrates by reducing the line defect (dislocation) density. Boron doped epitaxial layers are then grown on the cut substrates with conditions and processes to minimize the generation of new dislocation defects. Diode architectures being studied include a Schottky vertical diode, a Schottky quasi-vertical diode and these same structures with field plates of Al₂O₃. To make the diamond diodes, a heavily-doped p-type layer and a lightly-doped p-type layer are deposited in microwave plasma-assisted CVD reactors using boron as the dopant. Efforts are made during the lightly boron doped deposition to minimize the unwanted nitrogen and other impurity incorporation.

Diodes have been fabricated with both small Schottky contact areas of 150 micrometer diameter and larger Schottky contact areas of 2 mm². Various types of Schottky contacts have been used including gold, platinum and molybdenum. Diodes with the smaller contacts have been fabricated with breakdown voltages of over 1000 V and forward current flow densities of 500 A/cm². Diodes with the larger contacts have been fabricated with current flows up to 18 A and a current density of 900 A/cm². Diode characteristics are measured in the temperature range from 300-600 K and comparisons are made to device simulations using the MEDICI and Sentaurus TCAD semiconductor device simulators. In particular, simulation studies to better understand the reverse bias breakdown voltage and the forward on resistance will be discussed.

The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000455.

Based on Baliga’s FOM, GaN, AlN, and AlGaN-based Schottky diodes are expected to be superior to SiC by a factor of 6 to 200. Advances in native substrates have led to the possibility of vertical devices with low dislocation densities, thus approaching the materials’ ultimate performance. After effectively removing dislocations, new material challenges become apparent: epitaxy morphology on native substrates, point defect control, and surface manipulation. These influence the development of these switches as they determine: thickness and composition, carrier concentrations and mobility at the contact and drift layers, and Schottky barrier and passivation/isolation. Surface morphology was found to strongly depend on the substrate miscut with characteristic differences between AlN and GaN, as determined by surface kinetics. These differences become significant for AlGaN, determining composition and lateral uniformity. Point defect control leads to the achievement of the necessary high carrier concentrations in the back contacts while allowing for controllable low carrier concentrations in the drift layers; a carrier concentration of 2x10¹⁶ cm^-3 with a mobility of 1100 cm²/Vs was shown for GaN on sapphire. Besides all these achievements, it is recognized that the semiconductor surface plays a significant role in determining the performance of the devices. Therefore, understanding the state of the surface after processing and fabrication steps is necessary to develop methods to achieve the best performance on such devices. The need for proper passivation leads to the evaluation of a variety of dielectrics based on different deposition methods. Surface sensitivity techniques such as XPS allow for the evaluation of the surface electrical properties, examining proper passivation, processing steps and possible leakage mechanisms. Based on this evaluation, it was found that LPCVD silicon nitride offers a good alternative as a passivation layer. Removal of band bending in AlGaN (x < 0.6) and reduction in band bending for x > 0.6 strongly indicate no or greatly reduced surface/interface states. Results on the performance and further limitations of Schottky diodes based on these materials achievements will be discussed.

AlGaN is an attractive material for optoelectronic emitters and high power electronic (HPE) devices because its energy gap (E_G) is large, it emits in the UV, E_G can be tuned to a specific emission frequency, has a large breakdown field, the donor depth of the dopant, Si, is relatively small out to about 80% Al, and it forms a dense 2-dimensional electron gas (2DEG) when combined with GaN in a hetero-structure. Since obtaining the largest possible electron concentration in the 2DEG is frequently required for HPE devices because it minimizes the on-resistance (R_ON), most of the HPE device structures are grown with an (0001) orientation with the top layer containing more Al. This produces a tensile stress that cannot be relieved by the first order basal plane or second order prismatic dislocations slipping in from the surface because the plane strain caused by the lattice mismatch does not create a shear stress on their slip planes. As a result, the structure becomes bent if there is no plastic deformation, or plastic deformation is created by the formation of pyramidal or partial dislocations or by basal plan dislocations being formed spontaneously. By analyzing nano-indentations into, and micro-compressing pillars made from, high quality GaN material and calculating their associated stress fields, we provide insight into how the mismatch stress will be relieved in the (0001) grown AlGaN. This is done by determining the load necessary for the nano-indenter to create a large strain in a high quality GaN crystal with no increase in the load, which is often described as a ‘pop-in’ event; determining a similar load for (0001) oriented micro-pillars etched into the crystal, and also measuring the angles the slip traces on different exposed planes make with the vertical using an AFM; determining the the burgers vector and slip planes of the dislocations formed by both processes using a TEM; and analyzing the shear stresses in selected basal, prismatic, and pyramidal slip systems and comparing them with the experimental results.

AlN thin films has become a promising candidate for high-temperature power electronics, piezoelectric sensors, ultraviolet light emitting diodes (LEDs) and other optoelectronic devices, due to its unique physical properties, such as high thermal conductivity, high thermal stabilities, high surface velocity of acoustic wave(SAW) and ultra wide band gap.
In this study, the temperature-dependent optical and electrical properties of AlN/sapphire thin films with different thickness of epitaxial layers were investigated. The samples were prepared by metal-organic chemical vapor deposition (MOCVD) technique. Temperature-dependant (from 300K to 825K) ellipsometry parameters(psi and delta) of the three samples in the wavelength ( 195nm-1650 nm) at three incident angles of 50^o, 55^o and 60^o were measured by using a dual rotating-compensator Mueller matrix ellipsometer. Based on the best fit optical models and fitting algorithms by using Eometrics software, surface roughness, every layer thickness, the dependence of optical constants ( n & k ) and absorption coefficient on temperature were derived, and the fitted results are in good agreement with measured ones. It was observed that, when the temperature was raised up to 650K, the band tail of absorption edge for the samples all became more obviously and had a larger red shift than before. Comparing the fitted results of these three samples, the sample with 404nm-thick epitaxial layer had much better absorption properties and higher band gap energy than other two samples especially at high temperature. The sample with 117nm -thick epitaxial layer had the highest surface roughness and the worst absorption properties. So we argue that the thickness of epitaxial layer probably play a crucial role in the physical performance and the crystalline quality of AlN thin films at high temperature. To confirm this inference and elucidate the fundamental mechanism of the physical phenomena, other measurements such as PL, HRXRD, and high temperature Hall-effect equipment have also been employed to investigate the effect of epitaxial layer thickness on the electrical and optical performance.

For ultrahigh-voltage (UHV: > 10 kV) applications, SiC bipolar devices are of academic and technological interest. UHV and low-loss power devices are key components for future smart grids and high-voltage power supplies [1,2]. Since a typical voltage of power distribution is 6.6-7.2 kV, 13–15 kV power devices are required for constructing single-phase converters. However, several issues in the material and device physics must be addressed before UHV SiC devices exhibit a full potential and good reliability. This paper describes control of carrier lifetime and analyses of a breakdown phenomenon in SiC, aiming at development of UHV SiC bipolar devices.
The acceptor level of carbon vacancy (Vc) has been identified to be the primary carrier-lifetime killer in SiC [3]. The authors succeeded in reduction of Vc density from 1E13 to below 3E10 cm^-3 by high-temperature oxidation. The nearly Vc-free region can be extended to a 150–200 μm deep region by increasing the oxidation temperature or oxidation period. On the other hand, the Vc defects can be intentionally generated by either low-energy electron irradiation (kick-out of carbon atoms by particles) or high-temperature thermal treatment (approaching the equilibrium defect density). By these techniques, the Vc density can be changed in the wide range from 1E11 to 1E16 cm^-3. Although the typical carrier lifetime of as-grown SiC epitaxial layers is 1 us, the carrier lifetime can be controlled from about 10 ns to over 50 μs by combination of the elimination and intentional generation of Vc defects.
Toward quantitative analyses of breakdown phenomena in high-voltage devices, the authors have started a basic study on the impact ionization coefficients in SiC [4]. Various types of dislocation-free pn structures having different doping and thicknesses were fabricated, and the photo-multiplication characteristics were measured by using a filtered short-wavelength light (250–270 nm). Separation of electron-initiated and hole-initiated avalanche phenomena was made by employing p⁺/p/n⁺ and n⁺/n/p⁺ diode structures. From the electric-field dependence of multiplication factors experimentally obtained, the impact ionization coefficients were accurately determined in the electric-field range from 1.0 to 3.3 MV/cm. Furthermore, the ionization coefficients were extracted in the wide temperature range from 150 to 673 K. Device simulation by using the new set of impact ionization coefficients has enabled prediction of accurate breakdown voltage (including its temperature dependence) for any kinds of SiC devices.
Impacts of carrier-lifetime control and accurate determination of the impact ionization coefficients on 10–20 kV PiN diodes are described at the symposium.
[1] J. Wang et al., IEEE Indust. Electronics Magazine, June, 2009, p. 16.
[2] T. Kimoto, Jpn. J. Appl. Phys., 54, 040103 (2015).
[3] K. Danno et al., Appl. Phys. Lett., 90, 202109 (2007).
[4] H. Niwa et al., IEEE Trans. Electron Devices, 62, 3326 (2015).

SiC-MOSFETs, which are key device for high efficient electric power conversion, have still had serious problems (e. g. low channel electron mobility and threshold voltage variation) whose origin is thought to be insufficient quality of SiO₂/SiC interface. For further improvement of interface quality, it is important to clarify the origin of poor interface quality. There are several techniques for characterizing MOS interface properties. Deep level transient spectroscopy (DLTS) [1] is one of powerful techniques capable of macroscopic quantitative evaluation of trap density at/near MOS interface. For not only macroscopic but also microscopic evaluation, scanning DLTS, which measure trap distribution by stimulating trap using electron beam, was proposed. On the other hand, DLTS using SPM [2] has also been proposed and its feasibility has been shown. However, in our best knowledge, in spite of SPM’s advantage of high spatial resolution 2-dimensional (2D) imaging ability, SPM based DLTS 2D imaging has not been reported. In this paper, super-higher-order scanning nonlinear dielectric microscopy (SHO-SNDM) [3] based local DLTS and its 2D imaging are proposed. This method is demonstrated with oxidized SiC wafer. We measured three n-type silicon face (4°-off) 4H-SiC wafer samples on which 45-nm-thick thermally grown silicon dioxide film was formed. Two of them were followed by post oxidation annealing (POA) in nitric oxide ambient with different annealing conditions: (a) 10 min in 1250°C and (b) 60 min in 1150°C. We label the samples without POA, with POA in condition (a) and with POA in condition (b) as #1, #2 and #3, respectively. By analyzing the acquired images, time-constant t and magnitude c(t) of local transient capacitance response were obtained at each pixel. At first, all local DLTS spectra were added and averaged in each images. Clear peaks were observed between 0.4ms and 1ms, which was roughly consistent with the time-constant of O2 trap at RT, i.e. one of the previously reported traps detected by macroscopic constant capacitance DLTS [5]. In addition, highest peak value was obtained from #1 (without POA) and lowest peak value was obtained from #3 (POA at 1250°C for 10min), which is consistent with macroscopically obtained result. Furthermore, we obtained local DLTS images and found they had dark and bright areas, which can be translated as trap distribution. Thus, we conclude that this technique can contribute to understanding of physical properties of SiO₂/SiC interface.
[1] D. V. Lang: J. Appl. Phys. 45 (1974) 3023.
[2] A. L. Tóth et al.: Mat. Sci. Semi. Proc. 4 (2001) 89.
[3] N. Chinone et al.: J. Appl. Phys. 116 (2014) 084509.
[4] Gang Liu et al.: Appl. Phys. Rev. 2 (2015) 021307.
[5] A. F. Basil et al.: J. Appl. Phys. 109 (2011) 064514.

Silicon Carbide (SiC) is one of excellent semiconductor materials for power semiconductor application because of its large bandgap. SiC metal-oxide-semiconductor (MOS) field-effect-transistors (FETs) still suffer from severe degradation of channel electron mobility due to imperfect SiO₂/SiC interface [1]. Usually, SiO₂/SiC interface quality has been characterized by fabricating MOS capacitor on the sample, which needs additional processes following the oxidation and annealing process. Because, in research & development (R&D) of devices, many sample fabrications and characterizations are required, simple characterization technique with shorter characterization time is effective to shorten the R&D cycle.
Previously, it has been suggested that scanning nonlinear dielectric microscopy (SNDM) [2] gives images which may correlate with the interface state density of SiO₂/SiC interface [3]. SNDM measurement does not require additional process after oxidation and annealing process. Therefore, SNDM is a candidate of easy and sophisticated method for interface quality characterization.
In this paper, oxidized (45-nm thick SiO₂) four silicon-face (Si-face) and five carbon-face (C-face) wafers heat treated under various post-oxidation-annealing conditions are measured by SNDM and method for evaluating SiO₂/SiC interface quality using SNDM is proposed.
2μm × 2μm areas on these nine samples were measured by SNDM in air at room temperature. For interface quality evaluation, we calculated a standard deviation (STD) of signal level in each SNDM image and then this STD value was divided by the mean value of SNDM signal in each image. We call this value “normalized STD”. We examined the relationship between normalized STD and macroscopically measured interface trap densities (Dit) evaluated by conventional High-Low method using MOS capacitor. It was found that normalized STD had very good correlation with Dit, i.e. it was lineally proportional to Dit. In addition, the data were reproducible even with different tip. This means that normalized STD is a universal parameter showing the SiO₂/SiC interface quality both for Si and C-face. These results demonstrate that Dit in oxidized Si-face and C-face can be estimated from SNDM image without fabricating MOS capacitor. Thus, this technique measuring normalized STD by SNDM enables us to quickly examine the effect of variation of process parameters in MOS fabrication and to effectively reduce the time needed for R&D cycle.
[1] Gang Liu et al.: Appl. Phys. Rev. 2 (2015) 021307.
[2] Y. Cho et al.: Rev. Sci. Instrum. 67 (1996) 2297.
[3] N. Chinone et al.: Proceedings of ICSCRM2015(in print).

The interface between 4H-SiC and SiO₂ in metal oxide semiconductor field effect transistors (MOSFET) contains many electrically active defects, which adversely affect the performance of SiC microelectronic devices by lowering the electron mobility. The devices' electrical properties are improved by a number of treatments, the most prevalent of which is a nitric oxide (NO) post-anneal. Additionally, devices fabricated on different crystallographic faces of SiC show markedly different performance. Furthermore, newer passivation schemes such as boron and phosphorous anneals cause significant performance gains, with relatively poor understanding of the underlying mechanisms. Our previous work on NO annealed devices has shown an inverse relationship between anneal time and the width of the transition layer (w_TL) at this interface, which is correlated with improved channel mobility, increased N interfacial density, and decreased charged interface trap density. More recent work analyzing w_TL at interfaces of varying orientation has revealed much narrower interfaces that do not appear to decrease in thickness when subject to an NO post-oxidation anneal, contradicting the expected trend.

To further explore the characteristics of these interfaces, high resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), and spatially resolved electron energy-loss spectroscopy (EELS) have been used. We have investigated SiC/SiO₂ interfaces fabricated on the Si-face, with and without miscut, as well as on the a-face. Transition layer information was obtained using EELS at the Si-L_2,3 C-K, and O-K edges. Hyperspectral unmixing of the EELS data using machine learning techniques has revealed previously obscured bonding states at the interfaces, helping to explain the origins of mobility enhancement caused by NO anneals. Various interfacial states are presented and compared, and the effect of both substrate orientation and NO post-oxidation annealing is explored.

Boron and phosphorous annealed devices were analyzed with similar techniques, revealing significantly different mechanisms of incorporation for the passivating species compared to NO. P was found to preferentially cluster throughout the oxide, potentially explaining the origins of polarization instability observed in such devices. In contrast, B distributes more uniformly, but with a pile-up at the interface accompanied by an adjacent depletion region, which is expected to have significant effects on device performance.

Our results explore the differences in the chemical and electronic structure of the 4H-SiC/SiO₂ interface for different processing methods, and demonstrate the importance of controlling the quality of the interface in SiC power electronics. Our methods provide a framework for analyzing devices processed under a range of various conditions.

*Supported by ARL under Grants No. W911NF-11-2-0044 and W911NF-07-2-0046, and NSF GRFP Grant No. DGE 1322106

Electronically detected magnetic resonance (EDMR) is a technique, which allows for the observation and characterisation of defects within semiconductors. Offering selectivity and sensitivity advantages over traditional electron paramagnetic resonance (EPR) techniques. Even with these advantages interpretation has long proved challenging, with many potential candidate defects seeming to qualitatively explain an observed signal. In this approach defect spectra are identified by comparing EDMR measurement to extensive ab initio calculations. This allows a defect identification based upon atomic composition, symmetry, and hyperfine (HF) structure. This approach was initially successfully applied to the identification of bulk defects has now been extended to defects at the SiC / SiO₂ interface.

A universally agreed upon description of the SiC / SiO₂ interface is a challenge with models varying from the abrupt to the highly disordered, thankfully as a starting point the EDMR was able to guide the way. From the anisotropy of the g-tensor it is clear that the defect(s) of interest is on the SiC side of the interface in a crystalline environment. This has allowed the simulations using an abrupt interface based upon the model of Devynck and Pasquarello, allowing the defects studied to be considered in a crystalline environment, at and just below the interface. After an exhaustive survey of the potential defects, both suggested by previous literature and imagined, we were able see an excellent agreement between the EDMR and the simulation from the family of defects based upon carbon dangling bond(s), with the carbon dangling bond(s) aligned along the surface. This is in good agreement with the work of Bardeleben et al where a defect of similar type was observed in an oxidised porous sample (PbC - Centre).

Silicon Carbide (SiC) based devices have been an emergent device technology for the last 20 years. Huge leaps forward have been made during this time, taking devices from the lab bench to commercially available diodes, MOSFET, and integrated power modules. For the full potential of SiC to be realized, a number of challenges remain to be overcome. Key amongst these is the high defect concentration inherent in SiC devices, to tackle this, defect identification and passivation is critical.

Combining the single defect selectivity of electrically detected magnetic resonance measurement (EDMR) with theoretical modelling gives the potential to unambiguously identify performance limiting defects within fully processed devices. Comparing the EPR parameters, defect symmetry, charge state, multiplicity, and elemental composition allows defect identification with a high degree of confidence. Through this combination of techniques a systematic approach has been formulated to facilitate defect identification. Once a defect is identified the interaction of the defect with various atomic and molecular species are considered. This allows a systematic approach to passivation, moving away from the current bottom-up approach.

This approach was initially used to study defects within N-implanted pn-junctions, allowing the N_CV_Si to be identified as the main source of N-dopant deactivation and poor mobility. The work of Kimoto et al. and Miyake et al. showed an improvement in device characteristics after a high temperature C-anneal. This was suggested to be due to the ‘healing’ of V_C present within the lattice. With the N_CV_Si identified a competing mechanism has been identified resulting in the conversion of the N_CV_Si defect into the N_CC_Si which shows excellent agreement with the available experiment data.

Silicon carbide power devices switching greater than 100kHz enable new benchmarks in power converter performance. These converters will enable power systems where all the sources and loads are connected by converters. These new systems are multifunctional and highly-integrated. However, their realization requires research in areas such as: advanced power electronic control across many converters, concepts for distributed storage, and active filtering across many converters.

A primary concern of SiC field-effect structures for power electronics is the reliability of the gate oxide under operating conditions, such as the high internal temperatures attained during power switching operations. Applications of a potential bias on the gate electrode of a metal-oxide-SiC device regulates the carrier concentration in the transistor channel. Stable device operation requires that the electric field across the dielectric remain constant in time, i.e. during the entire life-cycle of the device, as well as space, i.e. from source to drain. Imperfections at the SiO₂/SiC interface result in localized points for electrical breakdown and also scattering centers that reduce the carrier mobility within the FET channel. Although circuit designers prefer large area gates for optimum device performance, oxide reliability issues usually force a compromise between gate size and device lifetime. We report on large area (1000µm diameter) n-MOS capacitors with extremely low gate leakage current densities (<10 nA/cm²), measured during accelerated age testing at high temperature (600C). Additionally, the thermal generation of equilibrium minority carriers (holes) during inversion biasing in these structures, provides a measure of the structural and electrical quality of the SiO₂/SiC interface itself.

n-MOS capacitors were fabricated on 6H-SiC and 4H-SiC substrates. The gate oxide (~ 39 nm) was grown via dry oxidation at 1150C, followed by a 900C Ar anneal and a 2 hour 1175C NO anneal. The gate metal is 100 nm of Pt sputtered at 350C. Each sample has fifty two 100 - 1000 µm diameter gates. The noise of our electrical characterization system at 630C is ±2pA for the current-voltage data and ±2pF for capacitance-voltage data.

Gate Leakage Current 330C < T < 630C: Following thermal stress, oxide leakage measurements were made on independently processed chips, with 500 & 1000 µm gates. Chip 1 was held at 530C for 5hours; the current density (J) of all devices is below 2pA/cm². Chips 2 and 3 were measured at 330C < T < 630C. Below 450C, all 10 devices exhibit J< 5nA/cm², irrespective of size or location on the 1cm² substrate. On chip 2, two 1mm capacitors have J<4nA/cm² even at 630C and 50% of the gates are <10nA/cm² at all temperatures. This data along with the fact that the flat band voltage for all the gates is close to theory demonstrates the stability of the oxide with respect to time-dependent dielectric breakdown.

High frequency inversion capacitance at 300C < T < 630C: 1 MHz C-V characteristics of a 1000 µm capacitor were obtained by sweeping from accumulation (Vg=10V) to Vg=-8V at 0.4 V/s without using optical illumination to generate minority carrier, i.e. in the dark. For T>450C as the gate bias voltage drops below 0V, holes thermally generated from the n-type substrate create an inversion layer under the gate. The temperature dependence of the measured inversion capacitance is in agreement with the delta depletion approximation.

Ultrafast laser irradiation offers a unique approach to modifying material properties at the surface. One of the most intriguing phenomenon observed in ultrafast laser-matter interaction studies is the formation of high spatial frequency laser induced periodic surface structures (HSFL). In this work, the characteristics and possible mechanisms of HSFL formation in SiC as well as its effect on electrical conductivity is discussed.
Recent studies have suggested a connection between the surface defect density and HSFL formation in SiC evidenced by the laser spot size dependence on HSFL formation threshold. In our study, the formation of HSFL is observed to also be highly dependent of temperature. Preliminary result indicates that the threshold of HSFL formation increases when irradiated at 800°C compared to room temperature. This suggests stronger interactions between the irradiating laser and the electronic states induced by crystallographic defects and impurities. This is reasonable considering infrared laser irradiations would be absorbed mainly through multi-photon processes due to the relatively large band gap of SiC. Additionally, at the same irradiation fluence, significantly less debris is observed at 800°C. This debris, at room temperature, was studied with XPS and Auger and shown to be silicon oxides. Hence, temperature is observed to have a significant effect on the ablated materials chemistry.
The formation threshold of HSFL decreases as the number of pulses is increased. We speculate defect accumulation within the crystal leads to a higher degree of laser absorption with each laser pulse. The absorbed laser energy is then proportional to the magnitude of local ablation leading to the HSFL morphology. Laser interactions with semiconductors have indicated significant bond softening occurs during ultrafast laser induced band gap collapse and are strongly linked to the formation of point defects in crystals. The mechanism of point defect formation in semiconductors will be thoroughly discussed in the context of SiC.
Finally, large area irradiations of 4H-SiC substrates show significant electrical conductivity increase when HSFLs are present. The experiments are conducted in air (1 atm), vacuum and high purity N₂ (0.8 atm) environments. Notably, laser irradiations in N₂ environment yield consistently, several orders of magnitude, higher electrical conductivity compared to air and vacuum environments. We suggest a higher degree of nitrogen incorporation to the SiC surface chemistry during and after strong transient non-equilibrium conditions induced by laser irradiation. We will discuss the role that vacancy concentration and bonds softening play in Nitrogen incorporation. The use of N₂ ambient environment may also prevents oxide formation during irradiation and subsequent native oxide formation outside the irradiation chamber.

The advent of high quality SiC wafers has stimulated the use of this material in applications ranging from high power, high frequency and/or high temperature MOSFET-like devices to applications in bio-technology. Such SiC biomedical applications make use of its good bio-compatibility and high chemical inertness. In such bio applications, understanding and controlling the surface termination is critical, since can influence the biocompatibility. For example it has been found that when a polymeric surface is treated to become N-enriched, it promotes cell adhesion. Due to the important role of N in the electrical passivation of SiC and foreseeing its applications in bio-compatible devices, the characterization of nitrogen up-take at the SiC surface was investigated in this work.
Different SiC nitridation methods were performed: N₂ plasma exposure, NO or NH₃ thermal nitridations. The atomic N quantification, its chemical environment, wettability, and surface morphology were obtained after HF etching to remove the formed film. Different amounts of N and oxygen (O) that remained after etching were quantified at the SiC wafer surface. N incorporation was mainly in the silicon nitride-like form, with smaller proportion of silicon oxynitride. The more N incorporated, the less O was detected, and the increase in the N/O ratio resulted in a significant increase in the hydrophobicity on the SiC surfaces. Qualitatively this surface wettability effect is consistent with the behavior of N as a passivating agent of the SiC/N/SiO₂ MOSFET devices. Variable control of SiC surface energies, from primarily hydrophilic to hydrophobic, with a simple and controllable atomic additive that is also effective in device performance, can provide new opportunities for SiC biomedical applications.

Cubic silicon carbide (3C-SiC) is a promising new material for switching devices in the medium voltage range because of its superior channel mobility compared to hexagonal silicon carbide (4H and 6H-SiC). Due to the lack of 3C-SiC bulk material as substrates for epitaxy, 3C-SiC is typically grown on either silicon or hexagonal SiC. Growth on silicon substrates reduces cost, but also leads to a large number of defects in the grown layer. These defects are caused by the mismatch in the lattice constant, thermal expansion coefficienct, and the basic lattice structure (polar vs. unipolar). This leads to defects including anti-phase boundaries, dislocations, stacking faults, and twins.

We will present the heteroepitaxial growth of 3C-SiC on hexagonal silicon carbide substrates and homoepitaxially on 3C-SiC templates using our hot filament chemical vapor deposition (HF-CVD) technique. These substrates promise superior layer quality compared to silicon substrates due to their much better match in lattice parameters and thermal expansion. While conventional CVD has been extensively used to grow 3C-SiC, there are only a limited number of reports of HF-CVD growth of 3C-SiC and even fewer reporting the FWHM of the double rocking curve. Still, the HF-CVD method holds the promise of improved growth because of the hot filaments that precondition the gases, cracking silane and propane and creating monoatomic hydrogen. This additional degree of freedom enables further optimization of the growth process and growth at lower temperatures.

3C-SiC epi layers ranging in thickness from 0.6 to 15 µm were grown in a high-vacuum HF-CVD system on both on-axis 6H wafers and 3C-on-4H templates purchased from Ascatron (Sweden). Silane (25-50 sccm) and propane, diluted with 5 slm hydrogen, flow through an array of 1800°C filaments before reaching the substrate. Nominal growth parameters are a substrate temperature and pressure of 1450°C and 3 Torr respectively. Growth is initiated after a hot-hydrogen etch and a carbon-precursor preflow. Typical growth rate is 1-4 µm/hour. Multiple runs were performed with varying carbon to silicon ratio. Crack-free layers were obtained with surface topology varying from rough to smooth. The domains are clearly oriented. Micro-Raman confirmed the presence of crystalline 3C-SiC and XRD omega scans revealed a FWHM as low as 149 arcsec. High quality material, comparable to values in the literature as determined by XRD, was obtained for a C:Si ratio in the range around 0.45 to 0.50.

We will present the latest results using high resolution scanning electron microscopy and atomic force microscopy for surface imaging, X-Ray rocking measurements to assess the crystalline quality, and micro-Raman to evaluate doping and strain. A comparison to growth on low-cost silicon substrates will be included.

This talk presents the current research activities of the BU Computational Electronics Group aimed at developing multi-scale modelling approaches to study semiconductor materials and devices. The first part of this talk will discuss the application of this methodology to understand the breakdown properties of wide band gap semiconductors, in particular GaN, AlGaN and AlInN. Starting from the DFT-based electronic structure calculation and computing the carrier-phonon interaction without including fitting parameters, it will be shown that it is possible to predict macroscopic device performance of GaN p-i-n structures. Subsequently, we will discuss the breakdown characteristics of AlGaN and AlInN alloys with regard to their potential use in power devices. Finally we will show hoe the modelling approach can be applied to the study of complex power devices, such has GaN-MOS power switches.

The work at Boston University has been supported in part by the U.S. Army Research Laboratory through the Collaborative Research Alliance (CRA) for MultiScale multidisciplinary Modeling of Electronic materials (MSME), and NSF.

For decades, density functional theory (DFT) has systematically underestimated one-electron band gaps in insulators and semiconductors. Unfortunately, this problem has been conflated with other difficulties surrounding the calculation of defect levels across these gaps, especially in semiconductors. It is often claimed that a good DFT band gap is required to obtain accurate estimates of defect levels and that hybrid screened exchange (HSE) functionals, which admix a fraction of non-local exchange - a fraction that can be semi-empirically adjusted to fit the experimental band gap, provide a path to the accurate calculation of defect levels. However, Schultz and Edwards have shown that defect levels calculated from total energies have no relation to the one-electron band gap — that for a variety of exchange correlation potentials that yield widely varying one-electron band gaps, the defect levels are to a surprising degree invariant [1]. Moreover, Schultz and coworkers have shown that, within the Local Moment Counter Charge (LMCC) approximation [2], DFT can do extremely well estimating experimental band gaps in a variety of materials including Si, GaAs, and CsI using the full spectrum of calculated defect levels to define the gap when these defect levels span the experimental gap [1,3,4]. We have recently applied the LMCC method to cubic SiC. The experimental band gap (2.39 eV) compares well with the defect band gap (2.4 eV) obtained from the LMCC. We will compare these results to results for hexagonal SiC, and also present results comparing defect levels in cubic and hexagonal GaN.

[1] P. A. Schultz and A. H. Edwards, Nulear Instruments & Methods in Physis Researh, Section
B: Beam Interactions with Materials and Atoms 327 , 2 (2014).
[2] P. A. Schultz, Phys. Rev. Lett. 84, 1492 (2000)
[2] P. A. Schultz, Phys. Rev. Lett. 96, 246401 (2006).
[3 R. Van Ginhoven and P. A. Schultz, J. Phys. Condens. Matter 25, 495504 (2013).

GaN is a highly promising material for solid state lighting and high power electronics. Light emission and charge transport in GaN are regulated by the dynamics of excited carriers at femtosecond to nanosecond timescales. The microscopic carrier scattering processes occurring in this ultrafast regime are challenging to access experimentally and not completely understood, yet they are crucial to understand GaN based light emitting devices. In particular, the rate at which carriers with excess energy with respect to the band edges (so-called hot carriers, HCs) lose energy and relax to the band edges remain the subject of debate. This talk will discuss first principles calculations of HC dynamics in GaN and address the rate and microscopic mechanisms of HC energy loss. Employing density functional theory plus ab initio electron-phonon (e-ph) calculations, we compute the e-ph scattering relaxation times (RTs) for electrons and holes with energies within 5 eV of the band edges, and resolve the contribution of the different phonon modes to the RTs. We find a significant difference in the dynamics of hot electrons and holes in GaN, which we attribute to the different density of states and orbital character of the valence band and conduction band states. We compute a RT of approximately 10 fs for electrons at the conduction band minimum, in excellent agreement with the experimental lifetime of 16 fs. While we find a dominant contribution to the RTs by the long-range polar interaction of the longitudinal optical (LO)-mode at the band edges, we also find dominant contributions by the non-polar phonon modes at higher energies into bands, a result that challenges the accepted notion that the relaxation mechanism for HCs in GaN is LO phonon emission. Our work is the first ab initio calculation of e-ph RTs in a polar material; it introduces novel grid sampling and e-ph matrix element interpolation techniques, which will be discussed briefly.

Due to its unique optical and electrical properties, wurtzite gallium nitride (GaN) is an important compound semiconductor in a variety of electronic and optoelectronic device technologies. In many of these devices, high electric fields and current densities often lead to elevated temperatures and mechanical stresses, both of which may compromise device performance and reliability. Owing to its high spatial resolution, micro-Raman spectroscopy is one of the most useful techniques in measuring local temperature rise, stress, and electric field in GaN-based high electron mobility transistors (HEMTs). In this work, we discuss the complex dependence of the optical phonon frequencies of wurtzite GaN on a finite electric field, which is critical in properly decoupling the contributions of strain, stress, temperature rise, and electric field to changes in the Raman spectrum of devices under bias.
In the presence of a finite electric field, wurtzite crystals deform by changes in the lattice constants a and c (associated with macroscopic strain) and in the internal structural r parameter (describing the Ga-N bond length along the c-axis). These changes in atomic coordinates affect the interatomic force constants (IFCs) and shift the phonon frequencies. The electric field, however, is different from an imposed mechanical stress in that the electric field changes the internal structural parameters of the unit cell and phonon frequencies even at zero macroscopic strain (fixed values of the lattice constants). To quantify these frequency shifts, it is important to include separate contributions of the strain and electric field in the linear potential deformation theory formalism, which shows that there must be a contribution of the field to the frequency shift apart from the IPE-induced strain.
To support this theory, we have calculated the contribution of the electric field apart from the IPE-induced strain with density functional theory (DFT) and measured the zone center optical phonon frequency shifts in wurtzite GaN HEMTs biased in the pinched OFF state using micro-Raman spectroscopy. Our DFT calculations show that the E₂ high and A₁ (LO) modes shift by -1.38 and 2.16 cm^-1/(MV/cm), respectively, with the electric field along the c-axis apart from the IPE-induced strain. These coefficients are an order of magnitude larger than those associated with the IPE-induced strain and properly account for the observed Raman peak shifts in GaN HEMTs with increasing drain bias in the pinched OFF state. This result helps to clearly explain the reason the pinched OFF state removes the effect of the electric field and IPE-induced stress on micro-Raman temperature measurements in the ON state. In a broader context, a quantitative understanding of this mechanism is also required for accurate measurements of temperature, stress, strain, and electric field via micro-Raman spectroscopy in all wurtzite and zincblende crystals.

We are simulating how misfit dislocation arrays evolve during growth and annealing of lattice-mismatched III-nitride heteroepitaxial thin films. III-nitrides typically have very high densities of dislocations due to a lack of inexpensive native substrates. While devices in III-nitride materials typically have a higher tolerance to dislocations than other semiconductors, there is still a strong correlation between the defect density and the device performance and lifetime. In addition to the defects which propagate from the substrate, lattice mismatch can lead to nucleation and growth of misfit dislocations to relieve the layer strain.

Our goal is to quantify the kinetic and energetic parameters that govern the evolution of the dislocation array, and to integrate that knowledge into a predictive simulator. In particular, we are initially exploring misfit dislocations which enable relaxation of layer strain for a-plane AlGaN/GaN films. Here the shear stress is resolved onto both the prismatic and less mobile pyramidal planes, providing the opportunity to simultaneously observe kinetic relaxation processes that are expected to proceed at different rates in the same sample.

The simulator has been adapted from our previous work on Ge_xSi_(1-x)/Si films, recognizing the substantially different dislocation densities, energetics, configurations, and kinetics between these materials. For Ge_xSi_(1-x)/Si, we were able to show that refinement of dislocation nucleation models using only around two dozen experimental data points allowed for quantitative prediction on (110) interface films. We are extending such refinement methods to the III-nitrides through growth and in-situ TEM annealing of films based on critical thickness and excess stress calculations.

In addition to the experimental refinement of the dislocation simulator, we have also performed first principles calculations to explore the effect of dislocation core structures on defect levels in the bandgap of GaN. Using density functional theory, we have identified the presence of a midgap state for the a-type edge dislocation, and predict that it can be passivated by the migration of oxygen interstitials to the dislocation core. Using the results from hybrid theory DFT for the position of the deep state for the a-type edge dislocation, initial Shockley-Read-Hall calculations predict that the deep state is unlikely to lead to significant recombination currents.

In summary, our overall goal is to combine experiment, simulation, and refinement to produce a robust simulator of misfit dislocation arrays and – through first principles calculations – their (opto)electronic properties in III-nitride heteroepitaxial thin films.

We would like to acknowledge the support of the NSF through grant DMR-1309535.

Time-dependent cathodoluminescence (CL) and specimen current (SC) in GaN have been monitored to evaluate trapping behaviour and evolution of charge storage. Examination of CL and SC suggests that the near band edge (NBE) emission in GaN is reduced primarily by the activation of traps upon irradiation, and Gallium vacancies are prime candidates (V_Ga). At the steady state, measurement of the stored charge by empiric-analytical methods, suggests that all available traps within the interaction volume have been filled, and that additional charge is being stored interstitially, necessarily beyond the interaction volume.
The proposed model was developed to explain dynamics behind optoelectronics devices, such as lasers and LEDs, that operate at high current density, and their degradation by monitoring the in situ evolution of electrical and optical signals. Using momentary current density values analogous to those in operational devices, NBE CL was found to decrease systematically with experimental parameters. SC dynamics were found to be more erratic, and both were difficult to correlate. This poor correlation led to the conclusion that direct space charge effects (by way of affecting generation or recombination efficiency of electron-hope pairs) are not the main actors at early irradiation stages. It is still uncertain if V_Ga beyond the nominal interaction volume can be activated through diffusion currents or whether these traps can be restored to its original V_Ga-3H complex. However, elucidation of these phenomena could offer solutions to aging laser diodes, besides the obvious, growth of GaN systems with fewer V_Ga^. Once established, the space charge region is responsible for the steady state CL emission and prior to build up, it is responsible for the generation of diffusion currents. Since the non-recombination effects resulting from diffusion currents that develop early on are analogous to those leading to device failure upon aging, this study is fundamental towards a holistic insight into optical properties in GaN.

Epitaxial metal/semiconductor superlattices with latticed-matched coherent interfaces are attractive for a range of applications including high-temperature thermoelectric devices, plasmonic and hyperbolic metamaterials and next-generation refractory electronics and plasmonics. We have recently developed the first epitaxial metal/semiconductor multilayers and superlattices based on (Zr,Hf)N/ScN and TiN/(Al,Sc)N systems. In this presentation, we show structural and electronic defects that the novel nitride metamaterials exhibit at ambient and elevated temperatures, and demonstrate the effects of such defects on their electronic properties.

High-resolution X-ray difraction (XRD) along with high-resolution transmission electron microscopy (HRTEM) analysis of the (Zr,Hf)N/ScN multilayers grown on [001]MgO substrates reveal that the multilayers are grooved at regular intervals along the threading dislocations that mark the boundaries of the columnar microstructure. The threading dislocations are believed to originate from misfit dislocations near the interface between MgO and the nitride superlattice due to a 7% lattice mismatch. Annealing of the multilayers at high-temperatures (1000°C) showed that the metallic atoms (Zr and Hf) diffuse along the boundaries forming dislocation pipes directly observed in the lattice-resolved HRSTEM for the first time. The TiN/(Al,Sc)N superlattices are free from these structural defects due to the close lattice-matching between the constituent nitrides and the MgO substrate. Significant interdiffusion, however, has been observed during prolonged annealing at elevated temperature (1000°C) in these superlattices.

In terms of the electrical properties, we have found that unintentional oxygen impurities degenerately dope the semiconducting ScN and (Al,Sc)N layers with high carrier concentrations (~10²⁰ cm^-3). Such high carrier densities deplete semiconducting layers in epitaxial metal/semiconductor interfaces resulting in the dominance of field emissions (or electron-tunelling) over the intended thermionic emissions. We have successfully overcome this challenge, reducing the carrier concentrations in ScN by alloying it with Mg_xN_y and eventually turning ScN into a p-type semiconductor. High thermoelectric power–factors of the p-type ScN and strategies to overcome the effect of such defects on electrical properties will be addressed in this presentation.

Semiconductor superlattices with layer thicknesses down to a few nanometers are the crucial component in developing technologies such as quantum cascade lasers. Proper thermal management in these devices requires understanding the thermal resistance of the “bulk” crystal of each layer as well as the resistance due to phonon scattering at the interfaces between two layers. Here we present for the first time measurements of the thermal conductivity of the III-V semiconductor InAlAs using the 3ω method¹. With this measurement and that of bulk InGaAs in hand, we proceed to investigate interfacial thermal resistances (ITRs) in InGaAs/InAlAs superlattices. Quantifying how different growth and material properties such as alloy composition and strain contribute to the ITR between superlattice layers will provide the foundation for a robust theoretical description of phonon behavior at these semiconductor interfaces. This experiment is an important step toward removing the current disparity theoretical and experimental values for ITRs and will facilitate high power applications of quantum cascade lasers. This work was supported by the U.S. Department of Energy.
[1] D. G. Cahill and R. O. Pohl, Phys. Rev. B, vol. 35, no. 8, pp. 1–7, 1987.

The performance of commercial GaN-based LEDs in the visible spectral range has continued to improve significantly in recent years. Still, the efficiency droop towards high currents remains a major loss mechanism at typical operation conditions. We present the various experimental studies that led us to the conclusion that nnp- (or eeh-) Auger recombination is the dominant physical root cause for the efficiency droop phenomenon [1,2,3]. This understanding has contributed to our most recent improvements in LED quantum efficiency for the blue spectral range. We discuss these recent developments quantitatively for different operation conditions. Finally, the role of the efficiency droop for the current status of the green gap problem is analyzed.

[1] M. Binder, A. Nirschl, R. Zeisel, T. Hager, H.-J. Lugauer, M. Sabathil, D. Bougeard,
J. Wagner, and B. Galler: Appl. Phys. Lett. 103 071108 (2013).
[2] B. Galler, H. Lugauer, M. Binder, R. Hollweck, Y. Folwill, A. Nirschl, A. Gomez-
Iglesias, B. Hahn, J. Wagner, and M. Sabathil: Appl. Phys. Express 6, 112101 (2013).
[3] A. Nirschl, M. Binder, M. Schmid, I. Pietzonka, H. Lugauer, R. Zeisel, M. Sabathil, D.
Bougeard, and B. Galler, Optics Express 24 3, 2971 (2016)

Modern commercial InGaN/GaN blue LEDs continue to suffer from efficiency droop, a reduction in efficiency with increasing drive current. External quantum efficiency (EQE) typically peaks at low drive currents (< 10 A cm^-2) and drops monotonically at higher current densities, falling to <85% of the peak EQE at a drive current of 100 A cm^-2. Mitigating droop-related losses will yield tremendous gains in both luminous efficacy (lumens/W) and cost (lumens/$). Such improvements are critical for continued large-scale market penetration of LED technologies, particularly in high-power and high flux per unit area applications.
However, device structures that reduce droop typically require higher indium content and are accompanied by a corresponding degradation in material quality which negates the droop improvement via enhanced Shockley-Read-Hall (SRH) recombination. In this work, we use advanced characterization techniques to identify and classify structural defects in InGaN/GaN quantum well (QW) heterostructures that share features with low-droop designs. Using aberration-corrected scanning transmission electron microscopy (C_s-STEM), we find the presence of structural defects in high indium content architectures that correlate with SRH recombination measured in device operation. We apply geometrical phase analysis to lattice-resolved C_s-TEM micrographs to investigate the role of non-uniform strain fields in defect formation in the QW region. Further, we couple C_s-STEM, electron energy loss spectroscopy (EELS), and deep level optical spectroscopy (DLOS) to relate atomic-scale variations in microstructure with defect spectroscopic signatures in order to identify electronically or optically active structural defects within the QW region. Finally, we use the insights gained through advanced characterization to inform epitaxial growth strategies and develop InGaN/GaN QW heterostructures exhibiting low droop while maintaining high material quality.

In state-of-the-art InGaN based visible light emitting diodes the peak quantum efficiency occurs at low current density. Under high injection, these devices suffer from a strong decrease in efficacy often referred to as the efficiency droop. One of the most widely accepted mechanisms for this efficiency droop is Auger recombination, a three-body nonradiative process where, e.g., an electron recombines with a hole, while transferring the released energy to a third carrier without emitting a photon [1]. This process is expected to be proportional to the third power of the carrier density. As a consequence, a proper measurement of the efficiency droop requires an accurate knowledge about the actual carrier density. However, through electrical injection measurements, the exact quantum well (QW) carrier density (n) is difficult to obtain. To achieve a more accurate estimate of n, we have recently demonstrated that a comprehensive spectral modeling of the photoluminescence (PL) of a single QW under high injection can provide such information [2].
In this work, InGaN/GaN and GaN/AlGaN QWs grown by metal organic vapor phase epitaxy are studied by means of time-resolved photoluminescence spectroscopy [3]. In a first set of experiments, it is shown that the investigated polar InGaN/GaN QWs under non-resonant high optical excitation show clear signatures of an Auger loss mechanism and thus behave very differently from their less disordered GaN/AlGaN counterpart. In order to remove potential influences of the built-in polarization field and exemplify the dominant role of localization, identical experiments have been conducted with two m-plane InGaN/GaN QWs, featuring a similar In-composition and geometry but a different degree of indium alloy disorder. We can thus clearly demonstrate that carrier localization due to alloy fluctuations in the QW strongly enhances the Auger recombination process in InGaN/GaN QWs. This effect may be further enhanced by the presence of polarization fields. We suggest that the relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can consistently explain the enhancement of the efficiency droop in InGaN/GaN QWs.

[1] J. Iveland et al., Physical Review Letters, Vol. 110, 177406, 2013.
[2] G. Rossbach et al., Physical Review B, Vol. 90, 201308(R), 2014.
[3] M. Shahmohammadi et al., Physical Review B (submitted).

InGaN-based heterostructures have attracted considerable interest for a variety of different applications. For example, the key building blocks of modern light-emitting devices operating in the blue spectral region are c-plane InGaN/GaN quantum wells (QWs). It is remarkable that these devices are so successful, given the extremely high defect densities in InGaN/GaN systems. This defect insensitivity has been mainly attributed to carrier localization effects introduced by alloy fluctuations, preventing carriers to reach non-radiative recombination centers. It is only recently that localization effects in InGaN QWs have been addressed in theoretical studies. But most of these studies focus mainly on ground state properties. These are important to understand low temperature experimental data; however, in order to understand outcomes of experimental studies conducted at ambient temperature and/or transport properties, many excited states must be considered.

Here, we present a detailed analysis of the electronic structure of c-plane In_xGa_1-xN/GaN QWs with indium contents of x = 0.1, 0.15 and 0.25. The study is carried out by means of an atomistic tight-binding model, including local alloy, strain and built-in field variations arising from random alloy fluctuations as well as well width fluctuations [1]. We conclude that for as little as 10% indium in the QW, the valence band structure is strongly affected by localization effects. Also our data indicate that well width fluctuations could lead to electron wave function localization effects in addition to localization effects introduced by random alloy fluctuations. We find that not only hole ground states but also excited hole states show strong localization features. Based on our data we estimate that even at x=0.1, an energy range of order 100 meV into the valence band should be dominated by strongly localized states. This energy range increases with increasing indium content. Experimental data, such as the “S-shape” dependence of the PL peak position with temperature gives clear experimental evidence of the presence of this broad range of (excited) localized states.

By studying the (modulus) wave function overlaps between the first 40 hole or electron states, we gained initial insights into the probability of transferring carriers from one site/state to another. This analysis revealed different regimes ranging from strongly “localised states” up to “delocalised states”. Especially, the observed strong hole wave function localization features should impact the vertical hole transport along the c-axis in c-plane InGaN-based multi-QW LEDs.

We find here also that built-in field, random alloy and well width fluctuations lead to the situation of independently localized electron and hole wave functions, consistent with the models required to explain time resolved photoluminescence measurements of c-plane InGaN QWs [2].

[1] D. Tanner et al., submitted (arXiv:1606.03616)
[2] A. Morel et al., PRB 68, 045331 (2002)

Three-dimensional core-shell GaN-based microrods with embedded InGaN multi-quantum-well structures (MQW) on non-polar sidewalls are promising candidates for a novel LED architecture based on GaN material free of extended defects. The large active area on the 3D structures in relation to the surface area of the substrate is a further significant advantage of the rod structures. In order to optimize the internal quantum efficiency (IQE) of the 3D LED structures, a detailed knowledge of the radiative and non-radiative recombination channels and their rates is required.

We compare the spectrally and temporally resolved luminescence of InGaN/GaN microrod structures (3D LEDs) with the ones of planar InGaN/GaN LEDs under different excitation conditions. All LED structures were grown by MOVPE. The 3D LEDs were fabricated by continuous selective area growth (SAG) through a SiO_x mask on a GaN/sapphire substrate. We used time-integrated photoluminescence (PL) measurements to analyse the concentration and homogeneity of the indium in the QWs, and we study the influence of the excitation intensity on the GaN and InGaN luminescence. The luminescence dynamics of the InGaN MQWs were investigated by time-resolved experiments using a femtosecond laser system (repetition rate 1 kHz, pulse length ~ 100 fs) and a streak camera (time resolution < 10 ps) in order to characterise the fundamental optical relaxation and recombination processes and compare them for the 2D and 3D LED structures.

Due to the quantum confined Stark effect (QCSE), we find a biexponential decay characteristic in the planar LEDs with τ_fast^2D ~ 100 ps – 700 ps and τ_slow^2D ~ 3 ns – 6 ns, depending on the excitation conditions. The 3D LEDs with the non-polar m-plane InGaN quantum wells as light-emitters, on the other hand, show a monoexponential decay of the InGaN PL with τ^3D ~ 200 ps – 600 ps in a good approximation, so that τ^3D ~ τ_fast^2D. For increased excitation intensities in the range of 0.2 MW – 1 MW pulse power, we find a change of the intensity ratio I_PL^GaN/I_PL^InGaN: while for low excitation intensities the InGaN luminescence dominates, the GaN luminescence is the strongest at higher laser powers.

We will further discuss the influence of the laser photon energy on the recombination dynamics of the InGaN luminescence for the two different cases E_photon > E_gap^GaN and E_gap^InGaN < E_photon < E_gap^GaN. In order to investigate the direct impact of the structural difference between layer and 3D structures, a layer LED will be deep etched by ICP-RIE and wet chemical etching to rod like structures. The recombination dynamics of the layer LED can be directly compared to the 3D rods with the same (axial) MQW. Our results will help to develop strategies for further increasing the internal quantum efficiency of the 3D LED structures.

III-nitride semiconductors are well-suited for solid-state lightening, such as LEDs and LDs. Conventional GaN-based optoelectronic devices suffer from rather high p-type contact resistance (~10^-4 ohmcm), leading to high threshold voltage for high current optoelectronic devices. Generally, Mg is used as p-type dopant of GaN. Mg-doped GaN (GaN:Mg) has an intrinsically low ionized-acceptor concentration (10¹⁷-10¹⁸ cm^-3) due to the large ionization energy E_a of Mg (~0.18 eV) and high compensation by donor-like defects such as nitrogen vacancy. These low effective-acceptor concentrations (N_A-N_D) prevent p-type contact resistance from decreasing through field-emission (FE) tunneling. Recently, the formation of compensation defects in GaN:Mg was hindered thanks to the low growth temperature by MBE using ammonia gas. This allows achieving high acceptor concentrations over 10¹⁹ cm^-3. Here we report on the electrical properties of heavily Mg-doped GaN and carry out systematic investigation of p-type contact resistance.

N_A-N_D in GaN:Mg layers as a function of the Mg flux was investigated. N_A -N_D linearly increases with Mg flux. We could control the lower limit of N_A-N_D to 3x10¹⁷ cm^-3. N_A -N_D agrees well with [Mg] measured by SIMS until a critical value of N_A -N_D = 7x10¹⁹ cm^-3. By further increasing the Mg flux, N_A -N_D dramatically decreases. The relationship between hole mobility and N_A -N_D below the critical concentration was determined using Hall-effect measurements. The hole mobility decreases with increasing N_A-N_D, as expected from ionized-impurity scattering. For GaN:Mg layer with N_A-N_D = 2x10¹⁹ cm^-3, the room-temperature resistivity, hole mobility, and hole concentration are 0.2 Wcm, 13.5 cm²/Vs, and 2x10¹⁸ cm^-3, respectively. A hole concentration of 2x10¹⁹ cm^-3 is measured for GaN:Mg layers with the highest N_A-N_D value (=7x10¹⁹ cm^-3). This increase of the ionization efficiency for high [Mg] is due to reduction of E_a to less than 0.1 eV due to Coulomb interaction with ionized acceptors and screening of the Coulomb potential by mobile charges.

The sheet resistance (R_s) of GaN:Mg layers was determined using transmission line measurements. In the high [Mg] range (2 to 7x10¹⁹ cm^-3), R_s slightly decreases with increasing [Mg]. The p-type GaN layer with [Mg] = 7x10¹⁹ cm^-3 exhibits the lowest R_s (6x10³ ohm/sq). R_s values are calculated from the fit of the Hall-effect data. The calculated values qualitatively agree with the experimental data. When further increasing [Mg] to more than 7x10¹⁹ cm^-3, R_s increases too. The relationship between [Mg] and contact resistance r_c was examined. In the high [Mg] region, r_c dramatically reduces with increasing [Mg]. At [Mg] = 5x10¹⁹ cm^-3, r_c was minimum at 2x10^-5 ohmcm². According to the FE model, depletion widths at a metal/p-GaN interface decrease with increasing N_A -N_D, resulting in high carrier-tunneling probability through the potential barrier. We consider that FE current is dominant in the high [Mg] range.

We demonstrate the substantial impact of misfit strain on the indium incorporation in semi- and nonpolar multi quantum well (MQW) structures. By inserting a partially relaxed AlInN buffer layer, a modified growth template is offered for the subsequent MQW growth. This leads to an increased incorporation efficiency up to In contents of 40% in the quantum wells (QWs) without accumulation of additional strain.

GaInN QWs emitting in the long-wavelength regime of the visible spectrum require high In concentrations. Due to the large lattice mismatch to GaN, such highly strained layers are subjected to the strain-induced generation of defects, acting as nonradiative recombination centers [1]. For semipolar (11-22) structures, deliberate relaxation of AlInN has been demonstrated [2]. Further, such AlInN buffer layers have been shown to affect the strain state of m-plane MQW structures [3].

A series of non- and semipolar samples was grown by low pressure metalorganic vapor phase epitaxy. As substrates, pseudo-bulk m-plane GaN and (11-22) GaN on patterned sapphire has been used. The active zone comprises five-fold GaInN/GaN MQWs grown on top of AlInN buffer layers with nominal thicknesses up to 400 nm. The AlInN is nominally lattice-matched to GaN along the c-direction or its projection, respectively. For both crystal orientations, reference structures without the AlInN buffer were grown using identical MQW growth parameters. Structural analysis is carried out via high-resolution X-ray diffraction (HRXRD). Optical investigations are performed by temperature and power dependent photoluminescence (PL) measurements.

Evaluation of reciprocal space maps from HRXRD measurements suggests that the MQWs are grown coherently on top of the partially relaxed AlInN buffers. For both crystal orientations, the metamorphic structures show a significant increase of the In incorporation up to concentrations of 40% in the QWs. However, the strain energy density in the QWs is reduced as compared to the reference samples. Grown in m-orientation, this effect persists upon increasing the QW growth temperature. Further, the main emission peak of the PL spectrum shifts towards longer wavelengths, reflecting the reduced band gap energy. For the (11-22) structures, the internal quantum efficiency is measured to be 30% at an emission wavelength of 575 nm.

These observations clearly demonstrate that the strain state plays a crucial role in the In incorporation efficiency in GaInN QWs. Insertion of a partially relaxed AlInN buffer reduces the lattice mismatch of the QWs, enabling more In to be incorporated without accumulation of additional strain. Thus, metamorphic non- and semipolar structures are a very promising approach for realization of efficient green light emitters in order to close the “green gap”.


[1] T. Langer et al. Appl. Phys. Lett. 103, 022108 (2013).
[2] E.R. Buss et al. Appl. Phys. Lett. 105, 122109 (2014).
[3] P. Horenburg et al., Appl. Phys. Lett. 108, 102105 (2016).

Photonic devices operating via exciton-polaritons may offer a fast and energy-efficient alternative to the modern electronics. However, there are important physical problems with implementation of this idea on the base of semiconductor heterostructures. The limitations come from (i) relatively small Coulomb interaction energy between the electrons and holes forming the excitons and (ii) a weak coupling between the photons and excitons. The former limitation can be released by using GaN and related wide-bandgap materials with much larger exciton binding energy compared to traditional III-V materials. As a result, stability of excitonic states can be provided at room temperature, especially in quantum wells. Competition of radiative decay with non-radiative damping of the excitons becomes the major problem of this material system. While non-radiative damping is technologically dependent, the radiative decay rate can be improved in excitonic Bragg structures, where electro-magnetic coupling of individual quasi-2D excitons leads to formation of a superradiant optical mode.
In this paper we demonstrate and investigate optical Bragg lattices formed by quasi-two-dimensional excitons in periodic systems of InGaN quantum wells separated by GaN barriers. We reached an enhancement factor larger than 2 at room temperature for a periodic system of 60 quantum wells. We show that this factor can be doubled by using a complex periodic supercell. The resonant optical response is shown to be tunable by applying an external electric field. The optical transmission and reflection spectra were modelled using transfer matrix approximation. By fitting the experiment we evaluate the radiative and non-radiative broadening parameters in our structures. The latter appears to be (40+-5) meV and mostly reflects inhomogeneous broadening of the excitonic states in InGaN quantum wells. The value of the radiative broadening is (0.20+-0.02) meV. That means that coupling of excitons with light is about 10 times more efficient in InGaN/GaN system compared to traditional III-Vs.

One of the factors limiting the performances of GaN light-emitting diodes (LED) is light extraction. Many strategies have been proposed and demonstrated, but there is still plenty of room for light extraction efficiences to improve. One of the most effective ways would be for the LED to be completely free-standing so that the light extraction surfaces would be maximized; this would of course be impossible as the device should be mounted in a package. In a previous demonstration the LEDs were mounted in a vertical orientation for maximal exposure of light emission surfaces [1]; wire-bonding of such devices proves to be challenging, as was heat-sinking due to the contact area.

In the present demonstration LED stripes which are partially free-standing are demonstrated. The free-standing portions are formed by selective laser-liftoff through a shadow mask, so that a part of the LED stripe remains attached to the sapphire substrate which is mounted onto a package. In this way, wire-bonding of the device from the planar attached region remains convenient, while heat conduction only the GaN stripe remains possibe up to a certain length due to its high thermal conductivity. The optical characteristics and performances of the devices will be reported in the presentation. The design considerations and limitations will be discussed.

[1] L. Zhu, Z.T. Ma, P.T. Lai and H.W. Choi, “Vertically-mounted InGaN-on-Sapphire Light-emitting Diodes”, IEEE Transactions on Electron Devices 58, 490 (2011).

Germanium war recently introduced as a highly favorable n-type dopant in hexagonal GaN (h-GaN). In regard to the standard Si donor, the superiority of Ge doped h-GaN is demonstrated by very high free carrier concentrations above 10²⁰cm^-3 with smooth surfaces and reduced tensile strain. However, due to symmetry reasons h-GaN, if grown in c-plane, exhibits on strong spontaneous and piezoelectric polarization fields at interfaces and surfaces, which limit the recombination efficiency in e.g. double hetero-structures or quantum wells. To overcome these harmful effects non-polar or semi-polar h-GaN may be grown or as an additional alternative way the metastable cubic phase of GaN (c-GaN) may be used, where these fields are absent. Therefore, for device applications it will be very attractive to investigate the behavior of Ge as an alternative n-type dopant also in cubic group III-nitrides.
In this contribution we report on recent doping experiments of cubic GaN epilayers by Ge and investigate in detail the optical characteristics by photoluminescence measurements. Plasma-assisted molecular beam epitaxy was used to deposit Ge-doped cubic GaN layers with nominal thicknesses of 600 nm on 3C-SiC(001)/Si(001) substrates. The Ge doping level could be varied by around six orders of magnitude by changing the Ge effusion cell temperature. A maximum donor and free carrier concentration of 3.7×10²⁰ cm^-3 was incorporated into the GaN layers as verified by secondary ion mass spectrometry (SIMS) and Hall-effect measurements. This free carrier concentration was about one order of magnitude higher than that reached by Si doping under similar growth conditions. Low temperature (13 K) photo-luminescence showed a clear shift of the donor-acceptor emission to higher energies with increasing Ge-doping. Above a Ge concentration of about 2x10¹⁸cm^-3 the near band edge lines merge to one broad band due to band gap renormalization and conduction band filling effects. From temperature dependent measurements of the observed donor-acceptor transition a donor-energy of about 30 meV could be estimated for Ge. In addition the integral luminescence intensity and the SIMS data nicely follow the Ge vapor pressure curve. All these results demonstrate that Ge is well suited for n-type doping of cubic group III-nitrides.

In general, light emission with a high degree of linear polarization (DLP) is observed for InGaN/GaN quantum wells of nonpolar or high-inclination semipolar orientation, both in electroluminescence and photoluminescence. The origin are the band structure and transition matrix elements of the anisotropically strained quantum well, in particular the p character of the uppermost valence bands. However, a quantitative comparison of DLP measurements with k.p band structure simulations shows, that the measured DLP at low temperature (below ca. 70 K) is lower than expected, while higher at room temperature. We show that this behavior is due to different localization length of electrons and holes, such that inhomogeneous broadening is probed differently [C. Mounir et al, Phys. Rev. B 93, 235314 (2016)]. This leads to redistribution of carriers between valence subbands and consequently to a modified DLP.
When taking into account the different effective inhomogeneous broadening of the individual valence bands, we were able to explain a wide range of published measurements of DLP as well as our own measurements of m-plane and (20-2-1) quantum wells, including m-plane facets of core-shell microrods. At low temperature the contribution of an exponential Urbach tail to inhomogeneous broadening is the cause for the decreased DLP. Overall, a quantitative agreement between k.p simulation and experiments of the DLP was achieved over a wide range of excitation density and temperature, supporting our interpretation of the role of inhomogeneous broadening on polarized light emission.

InGaN nanowires have been investigated for the use of optoelectronic devices, especially LEDs and lasers.[1-4] One challenge in the growth of infrared InGaN disc-in-nanowire lasers is to incorporate large amount of In. It has been shown that as the amount of In increases, branching and faceting occurs.[5-7] This work focuses on examining the shape evolution and compositional distribution of InN/InGaN nanowire heterostructures, which has direct impact on the performance of these devices. The nanowires are grown on (001) Si substrates with 260nm thick of n-GaN, followed by a 150-nm cladding layer graded in composition from n-GaN to n-In_0.4Ga_0.6N. The active region is composed of 3nm InN layers separated by 12 nm In_0.4Ga_0.6N barriers. The structure is capped by a 150-nm p-GaN layer. Scanning electron microscopy of the structures shows that increasing In composition changes the growth mode, and thus the crystal shape of the nanowires heterostructures. The active regions take on a polyhedral shape and appear at the top of the nanowires instead of at the midpoint, suggesting that the p-GaN does not grow on the c-plane of the nanowire. Indeed, scanning transmission electron micrographs reveal that the InN regions are enveloped by a quasi-conformal GaN shell. Energy-dispersive X-ray spectroscopy shows that the polyhedral shapes are enriched in Ga on their upper half, while the bottom half is enriched in In and encased in a Ga-rich shell. Line profiles along the growth direction suggest that there is significant In segregation and composition modulation into the barrier layers. Occasionally, cracks develop near the InN-In_0.4Ga_0.6N interfaces, indicating the relief of significant amounts of lattice mismatch strain. Other complex shapes were also observed, including branches and voids, depending on the amount of In in the active region. Despite the complex morphology of the nanowires, strong luminescence was frequently observed. Low temperature photoluminescence measurement shows a primary peak at 1.6 μm and a secondary peak at 1.3 μm.

Reference:
[1] Nguyen, Hieu Pham Trung, et al. Nano letters 11.5 (2011): 1919-1924.
[2] Nguyen, Hieu Pham Trung, et al. Nanotechnology 23.19 (2012): 194012.
[3]Qian, Fang, et al. Nature materials 7.9 (2008): 701-706.
[4] Jahangir, Shafat, et al. Quantum Electronics, IEEE Journal of 50.7 (2014): 530-537.
[5] Yan, Lifan, et al. Nano letters 15.3 (2015): 1535-1539.
[6] Kehagias, Th, et al. Nanotechnology 24.43 (2013): 435702.
[7] Zhang, Xin, et al. Nanotechnology 27.19 (2016): 195704.

Since the discovery of graphene, numerous studies have been conducted to explore its unique electrical properties. Another interesting property that has not yet been fully exploited is its weak atomic bonding to other materials. Because of this characteristic, graphene can be a template for the growth and transfer of single-crystalline films if the films can be epitaxially grown on graphene. Layer transfer of single-crystalline semiconductors is attractive for cost reduction as well as for enabling many novel applications including flexible electronics and hybrid device integration. A number of methods to achieve layer transfer have been reported including mechanical lift-off and chemical lift-off. However, it is challenging to release high modulus semiconductors such as III-nitrides and silicon carbides while maintaining their original quality.Graphene forms weak van der Waals bonding to most materials. Thus, graphene can be an ideal template for growth and transfer of single-crystalline semiconductor films if such films can be epitaxially grown. In this study, we employed epitaxial graphene formed on a single-crystalline SiC substrate as a template. The epitaxial graphene on SiC has a single orientation over the entire substrate. More importantly, the vicinal macro steps of the SiC surface can provide periodic nucleation sites for the thin film. We obtained single-crystalline, 3 Å root mean square (RMS) roughness GaN films with low defectivity (as low as 4 × 10⁸/cm²) via GaN epitaxy on epitaxial graphene. The entire single-crystalline GaN films were then released from the graphene and transferred on an arbitrary substrate. Further reuse of the graphene/SiC substrate allowed growth and transfer cycles of GaN films for multiple times. InGaN/GaN epitaxial LED stacks were grown on a recycled graphene/SiC substrate (reused three times), and blue light emission was observed from the released LED stacks. Here we, for the first time, demonstrate direct growth of high-quality single-crystalline GaN films on graphene. The GaN film was released and transferred onto arbitrary substrates. The post-released graphene/SiC substrate was reused for multiple growth and transfer cycles of GaN films. By using this technique, fully-functional flexible blue light emitting diodes (LEDs) were demonstrated. Our results pave a promising path for wide application of III-nitride semiconductors by allowing their device integration with conventional semiconductors.

Although there are several reports on physical properties of boron nitride nanosheets (BNNS), still BNNS for graphene electronics and for optical devices need to be address further. In this presentation, we will provide a detail description in the synthesis of wafer scale boron nitride nanosheets (BNNS) and their applications in graphene electronic and optical devices. We used ammonia borane (NH₃BH₃) precursors and applied tube furnace chemical vapor deposition (CVD) technique at low pressure conditions for the growth of wafer-scale high-quality BNNS of size ≈ 50×50 µm² on conducting (copper) and non-conducting (SiO₂) media. The samples prepared on Cu substrates were mechanically transferred to the TEM grid and to the other substrates of interest for the analysis of crystal structure and optical properties of the material. Electron microscopy technique was used to analyze surface morphology, number of layered and crystal structure of as synthesized BNNS. SEM shows large assembly of overlapped BNNS. Corresponding HRTEM analysis of the collected piece of a sample have shown bilayer BNNS with slightly interpretable hexagonal lattice of B₃-N₃ hexagons similar to carbon atoms in graphene. Raman spectroscopy confirmed the hexagonal structure of the nanosheets. For optical analysis of the samples, we conducted cathodoluminescence spectroscopy and analyzed deep UV band in BNNS that give us information about optical properties of the material. We also produced the heterostructure of bilayer graphene (G) with fewlayer BNNS (G/BNNS/SiO₂) and measured the graphene sheet resistance. We figured out that the sheet resistance of graphene goes an order of the magnitude down in case of its contact with BNNS as compared to SiO₂. The overall study will show the results that will help to analyze physical properties of wafer scale BNNS for graphene electronics and for optical devices.

Zinc sulphide (ZnS) is a wide bandgap II-VI compound semiconductor having a range of applications in optoelectronic devices, especially in photovoltaic cells, high-efficiency blue light emitting devices etc. Most applications of ZnS are, however, related to its optical properties rather than the electrical properties due to the high value of resistivity (~10⁷ Ω-cm). In this study, ZnS has been alloyed with varying amount of Cu via RF sputtering method at 350 °C and studied the electrical, optical and structural properties. Hall-effect measurement confirms that the film with 40% Cu exhibit highest conductivity of ~ 182 S-cm^-1 at room temperature. XRD, TEM-SAED and Raman analysis confirm the presence of sphalerite phase of Cu alloyed ZnS with the presence Cu₂S in minor quantity. The material is primarily oriented along (111) and (200) planes. XPS analysis reveals the presence of Cu in the monovalent state which gives rise to p-type conductivity in the film. The reflection-corrected transmittance of the most conducting film shows > 70% in visible range. The obtained results indicate that the RF sputtered p-type Cu alloyed ZnS would be a promising candidate as a carrier selective contact in solar cells. Also, the enhanced conductivity along with high transmittance of Cu alloyed ZnS is expected to be useful in the fabrication of transparent diodes, transparent transistors and light-emitting diodes.

Transparent conductors (TCs) are unique class of materials that exhibit both high optical transparencies in the visible region with high electrical conductivity. Till now, much of the development in TCs have been focused predominantly on n-type transparent conducting oxides (TCOs). In this report, we are emphasizing the development of Zinc Sulfide (ZnS) based p-type transparent conductor by Atomic Layer Deposition (ALD). Due to the interest towards the uniform, pinhole-free and conformal deposition capabilites with unique thickness control, Atomic Layer Deposition (ALD) process has been emerged as a promising deposition technique in the last decade.

Cu:ZnS thin films are deposited in a custom built laminar flow type ALD reactor on Si(111) and amorphous glass substrates at 150°C. Diethylzinc (DEZ) and Cu(II)hexafluoropentanedionate (Cu(hfac)₂) are used as Zn and Cu metal sources respectively while Hydrogen sulfide (H₂S) is used as a sulfur source at 150°C. A single layer of Cu_xS cycle (Cu(hfac)₂-H₂S) is inserted after a certain number of ZnS cycles (DEZ-H₂S) to achieve a particular atomic concentration of Cu that is doping into ZnS films that constituted one “super-cycle” of Cu:ZnS.

Growth mechanism and surface chemistry are studied by in-situ Quartz crystal microbalance (QCM) and in-situ Fourier transform infra-red spectroscopy (FTIR) at 150°C. These gives the most valuable information about complete surface coverage and to avoid any possibility of Cu metal formation.

Electrical characterization of films using Hall measurement under van-der-pauw configuration revealed p-type conductivity of as deposited films. Film resistivity as a function of the Cu concentration as expected is observed with a minima at ~1 × 10^-3 Ω-cm and a carrier concentration of the order of 10²¹ cm^-3. Additionally, transparency of > 80% in the visible range is confirmed from optical spectroscopy. The direct band gap is found to be constant at 3.65eV but with a small drop (~0.1eV) after heavy Cu doping.

We have fabricated a new semiconducting material, ZnInON (ZION), which is a pseudo-binary alloy of wurtzite ZnO and wurtzite InN [1-3]. This ZION has a tunable band gap over the entire visible spectrum and a high optical absorption coefficient of 10⁵ cm^-1, making ZION a promising material for solar cells and light emitting devices. Here we report strong blue and green photoluminescence of ZION at RT.
ZION films were deposited by RF magnetron sputtering on single crystalline ZnO templates on sapphire substrates [4]. O_2, N₂ and Ar gases were used and the total pressure was 0.3 Pa. The gas flow rate of O₂, N₂ and Ar were 3.0, 33.3, and 6.6 sccm, respectively. ZnO and In targets with a purity of 99.99% were used. The substrate temperature was RT. The film thickness was 100 nm. The chemical composition ratio of ZION films was (ZnO)_0.92(InN)_0.08. The crystal structure of ZION films was examined by X-ray diffraction (XRD). Photoluminescence of ZION films was excited by the 325 nm line of a cw He-Cd laser (6 mW).
ZION films epitaxially grow in a two dimensional mode at RT. The film shows a step-terrace structure with the step height of 0.27 nm, which corresponds to the height of a single-atomic-layer step and the half length of the c-lattice parameter of ZION. ZION film has the same a-lattice parameter of 0.325 nm as ZnO and a longer c-lattice parameter of 0.536 nm, indicating the coherent growth of ZION films on ZnO templates. This ZION film shows strong blue (440 nm) and green (550 nm) photoluminescence (PL) at RT. The PL spectrum depends on the excited position of the film, suggesting inhomogeneous chemical composition. The PL intensity tends to increase with decreasing the temperature from RT. Origin of PL will be discussed at the meeting.
This work was partially supported by Grant-in-Aid for JSPS Fellows 26･5011, Iketani Science and Technology Foundation and JSPS KAKENHI Grant Number 15H05431.
[1] N. Itagaki, et al., “Metal oxynitride semiconductor containing zinc”, U.S. Patent No. 8274078 (2008).
[2] N. Itagaki, et. al., Mater. Res. Express 1, 036405 (2014).
[3] K. Matsushima, et. al., Thin Solid Films 587, 106 (2015).
[4] K. Kuwahara, et. al., Thin Solid Films 520, 4674 (2012).

The zinc oxide (ZnO) has widely utilized for wide-bandgap (approximately 3.2~3.3 eV), high electron mobility, and well-known fabrication methods for nanoparticles and bulk films. Due to its unique properties, numerous attempts have been progressed for introducing ZnO on light-emitting diodes. Especially, its energy level and electron mobility were suitable for quantum-dot based light-emitting-diodes (QLEDs). In order to commercialize the QLEDs, realization of efficient charge carrier injection and exciton recombination in quantum-dot (QDs) active layers was one of the most important issues for high performance of QLEDs. In detail, the thickness, crystallinity, size, surface morphology of electron transport layer (i.e., ZnO) were critical factors which affected charge injection properties in QLEDs. To optimize the ZnO layer for the balance of charge injection, using interfacial dipole-induced polymers has been suggested for their ability of shifting energy level of the ZnO surfaces. However, it was difficult to control the specific energy level for matching the various types of QDs, and to apply all-solution-process system due to their weak interaction between QDs and inserted molecules.
Herein, we introduced selective ligand exchange methods for controlling charge injection using spherical amine-functionalized molecules, such as poly(amidoamine) dendrimer (PAD), for modifying energy level of ZnO surface. The energy levels of work function, conduction band minimum (CBM), and valance band maximum (VBM) were reduced by PAD generation number. The higher generation number of PAD, the amine functionalities and size of molecules were increased, which induced energy level shift toward lower energy direction. Moreover, the insulating oleic acid (OA) which caused increase of resistance of devices were selectively replaced by PAD molecules and followed washing step. For these reasons, the amine-functionalized PAD which used charge injection controlling ligands (CICLs) could impede the electron injection properties from ZnO to QDs. Based on our experimental results, we fabricated QLEDs with PAD ligands for improving external quantum efficiency (EQE, [%]), current efficiency (CE, [cd/A]), and power efficiency (PE, [lm/W]) through selective ligand exchange process. Especially, the selective ligand exchange process enhanced EQE, CE, and PE of quasi-type II 16 nm red QDs over 2.94, 2.92 and 2.99 times, respectively. Moreover, this modified ZnO could be applied to various type of QDs, such as type I red, green, and blue QDs which had different band energy levels.
As a result of the finely designed QLEDs structure with the CICLs, the charge balance with radiative recombination was considerably enhanced, realizing QLEDs with high efficiency (i.e., EQE = 11.4 %) and brightness through systematically shifted energy levels, easily removed insulating OA ligands, and strongly binding among ZnO, CICLs, and QDs that were essential to realize all-solution-processed QLEDs.

Quantum dot-based light-emitting diodes (QLEDs) have received considerable attention owing to their extraordinary properties including tunable emission wavelengths covering the whole visible spectrum, narrow full-width at half-maximum (FWHM), simple colloidal synthetic processes and solution processability. Recent years have witnessed significant achievements in both the design and optimization of both QDs and QLEDs, demonstrating its comparable performance with mature organic LED technology. However, to date, much of such progress has been made by using the standard sandwich-like architecture incorporating poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole injection material. Although it shows superior transparency, smooth surface over large areas, good conductivity and moderate work function, PEDOT:PSS has some limitations. The acidic and hygroscopic nature as well as the thermal instability will influence the device performance and result in fast degradation. Furthermore, due to its semi-metallic nature, PEDOT:PSS cannot be qualified as a good electron blocking layer. These shortcomings might hinder the possible commercialization of QLEDs in the future.
Herein, we reported QLEDs that utilize solution-processed copper thiocyanate (CuSCN) film as the hole injection layer for the replacement of PEDOT:PSS. With its high transparency across the whole visible spectrum and intrinsic p-type conductivity, CuSCN, which is available at low cost and can be directly solution-processed or doctor-bladed by dissolving in suitable solvents, has been demonstrated as a qualified hole transport layer or HIL in thin film transistors, perovskite solar cells, organic/QD photovoltaic cells and organic LEDs. Despite of the above-mentioned merits, there is a unique advantage of using CuSCN as the HIL in QLED. Compared with PEDOT:PSS, CuSCN has a higher hole mobility and a deeper valance band maximum (VBM), therefore hole injection in QLED is more efficient and consequently, and better device performance can be further expected because of the improved charge balance. As a result, CuSCN-based QLED demonstrated lower turn-on voltage (decreased by 0.6 V) and comparable or superior brightness, external quantum efficiency and current efficiency with PEDOT:PSS-based devices. Our results clearly suggests that this inorganic material can be further applied to the realization of all-inorganic QLEDs, which is of great importance for its commercialization in solid-state lighting and display applications.

In recent years bandgap (E_g) - modification of ZnO thin films (3.3 eV) by alloying with a wider bandgap semiconductor MgO (7.8 eV) has been investigated extensively for optical and optoelectronic applications. Specifically, thin films of Zn_1-xMg_xO doped with AI, Ga or In have been targeted for use in photovoltaic devices. We report a facile method to produce good quality Zn_1-xMg_xO thin films via slurry-painting of ZnO-MgO-ZnCl₂ paste in a proportion of 85:10:5 (w/w) prepared in propylene glycol on a ceramic base followed by air-bake at 400 ^oC – serving as target in radio frequency (RF) sputtering. Thin films of 500 nm in thickness were obtained from 3-inch sputter targets at RF power of 180 W, 6 mTorr Ar pressure, with a deposition rate of 3 nm per min. The slurry was modified through the addition of 5 % (w/w) of Al₂O₃ or InCl₃, which resulted in thin films with improved electrical conductivity, optical transparency and thermal stability. In this work we present a procedure to prepare targets for radio frequency (RF) sputtering from different slurry mixtures: (i) ZnO and ZnCl₂; (ii) ZnO, MgO and ZnCl₂; (iii) ZnO, MgO, Al₂O₃, and ZnCl₂; (vi) ZnO, MgO and InCl₃; and (v) ZnO, MgO, ZnCl₂ and InCl₃. The slurry mixture was added on a partially used-up ZnO commercial target layer-by-layer with repeated air-sintering at 400 ^oC. Thin films of ZnO, Zn_1-xMg_xO, Zn_1-xMg_xO:In and Zn_1-xMg_xO:Al were deposited on corning glass substrates by RF sputtering from these targets. The average optical transmittance of all these films in the visible region was 80%. The highest E_g of 3.8 eV was for Zn_1-xMg_xO:Al films. The Zn_1-xMg_xO thin films doped with In or Al showed electrical conductivity of nearly 10² (Ω cm)^-1. The carrier density and mobility were measured by Hall Effect. The carrier density and Hall mobility in all the films are in the interval: 10¹⁸ to 10¹⁹ cm^-3 and 2 to 12 cm²/Vs respectively. X-ray diffraction patterns on the films show that preferential orientation exists for different crystallographic planes, depending on the slurry mix used. Peaks relating to MgO do not appear in the XRD, which reveals introduction of Mg into the wurtzite structure of ZnO. Lattice stress associated with the texture coefficients exists in many cases. We present how the stress may be released through post-deposition annealing of the films in air or nitrogen at 350 ^oC, but which also changes their electrical and optical properties. The films doped with Al are relatively more stable to thermal processing. The use of these transparent films, as-prepared and those subjected to heat treatment, in thin film solar cells of antimony sulfide-selenide absorbers produced by thermal evaporation is reported. Characteristics and stability of these solar cells are discussed.

Gallium oxide, a wide bandgap (WBG) material, shows promise for high voltage and power applications when compared to silicon carbide (SiC) and gallium nitride (GaN) using Baliga’s Figure of Merit (BFOM). The monoclinic form of gallium oxide, β-Ga₂O₃, has a bandgap of 4.9 eV, high breakdown field of 8 MV/cm, and optical transparency in the visible range. In this work, pulsed laser deposition (PLD) has been used for β-Ga₂O₃ growth on c-plane sapphire. Characterization of the resulting thin films using x-ray diffraction (XRD) shows that the optimum growth temperature and chamber pressure are 887°C and 0.1 mT, respectively. Films of 200 nm thickness offer a high transmittance of visible light (>93%) and have a sharp 250 nm cutoff wavelength. Metal-semiconductor-metal (MSM) photodetectors are fabricated by depositing interdigitated nickel/gold (50 nm / 100 nm) electrodes on undoped β-Ga2O3 thin films and characterized. With a dark current of less than 14 nA, these devices show a strong photoconductive response in the ultraviolet region for electrode spacing of 5 μm. Responsivity data as a function of wavelength will be presented.

The monoclinic gallium oxide, β-Ga₂O₃, is the preferred polymorph phase for most Ga₂O₃-based semiconductor devices because of its thermal and chemical stability as well as its high bandgap and breakdown electric field. These material properties determine the advantage of β-Ga₂O₃ over silicon carbide and gallium nitride for high power and voltage applications based on Baliga’s figure of merit (BFOM), which describes resistive losses in semiconductor materials.

In this work, we report on growth optimization of undoped and Sn-doped β-Ga₂O₃ on c-plane sapphire by using pulsed laser deposition for growth of single crystalline epitaxial layers. A KrF excimer laser (λ = 248 nm) was used as an ablation source and stoichiometric Ga₂O₃ target (99.99% purity) and targets with variable Sn mole fractions (up to 10%) were employed for the deposition of undoped and doped materials, respectively. The growth conditions were varied in wide ranges in order to optimize the layer morphology and crystalline quality. The layers were studied by different characterization methods (AFM, SEM, X-ray diffraction, SIMS, transmission, THz spectroscopy). Under optimized growth conditions (deposition temperature of 887°C and pressure of 0.1 mT) the epitaxial grown layers were transparent and monocrystalline with smooth surface morphology, RMS of 1.9 nm, as revealed by AFM imaging. The Ga₂O₃ epitaxial layers show a sharp optical absorption cutoff in the UV region (~250 nm) in correspondence with the expected bandgap, which shifts noticeably with increasing Sn doping. Details on morphology, XRD, bandgap, conductivity and Sn-doping data as a function of growth parameters will be presented.

Gallium nitride (GaN)-based high electron mobility transistor (HEMT) has been widely used in high power electronics. Due to safety-consideration, enhancement-mode (E-mode) operation is preferred for power device application. In power switching applications, low ON-state gate leakage current and large on-state operation swing are required to lower the power consumption (i. e. high leakage increases more heat) and to prevent the power switches from electromagnet interfere. Recently, owing to the simple process, E-mode device has been fabricated using recessed-gate with MIS (metal-insulator-gate) structure. However, the recessed-gate is usually performed with dry etch technique, which generates large number of lattice damages due to plasma bombardment at the surface. Thus, it results in large gate leakage current and high density of interface traps.
In this work, a gate-recessed E-mode GaN HEMT with ex-situ N₂ plasma treatment before gate dielectric deposition is demonstrated. The ex-situ N₂ plasma treatment was performed using PECVD machine before 12 nm Al₂O₃ gate dielectric deposition by ALD system. Compared to other wet chemical treatments (KOH, NH₄OH, HCl), ex-situ N₂ plasma treatment could effectively suppress the ON-state gate leakage current (lower than 1 order) and increase the forward gate voltage swing (from +6 V to +8 V) defined at gate leakage current of 1 mA/mm at V_DS= 0 V. However, the increase of OFF-state gate leakage current was observed in the device with ex-situ N₂ plasma treatment due to the contamination and oxidation at recessed-surface during the wafer transferred process into the ALD chamber. To protect the sensitive GaN surface from contamination, a thin SiN_x layer was capped on the recessed-surface as an interfacial layer between gate-recessed surface and Al₂O_3. Thus, much lower forward and reverse gate leakage currents were obtained. In this study, the detailed effect of ex-situ N₂ plasma treatment and SiN_x interfacial layer will be discussed. Besides, hysteresis effect and material analysis will also be conducted.

Zinc oxide (ZnO) has attracted lots of attention since it has outstanding characteristics, such as direct and wide band gap, large exciton binding energy, transparency in the visible range, synthesizability with various geometrical shapes, low temperature, and low-cost producibility, and non-toxicity. Accordingly, it has been used in a wide variety of applications and one of the promising applications of ZnO is in optoelectronic devices. For these applications, p-n homojunction is always preferred to achieve better device performance. However, due to its intrinsic defects, as-grown ZnO shows n-type property, which results in difficulty to achieve stable p-type conductivity in it. Thus, it is challenging as well as promising for researchers to produce stable and reproducible p-type ZnO.
In this report, we propose a facile hydrothermal method to synthesize p-type ZnO nanorods (NRs) using autoclave at a temperature of 120 °C. To achieve p-type conductivity in ZnO, we have used antimony (Sb) as an acceptor dopant. Antimony acetate was used as the precursor of Sb and the doping concentration was controlled by varying the precursor concentration. The X-ray photoelectron spectra indicated the possibility of formation of acceptor states, which was subsequently confirmed by Hall measurements. The morphology of Sb-doped ZnO NRs with different Sb concentrations was characterized by scanning electron microscope. The photoluminescence spectra revealed a strong near band edge emission at 380 nm. The p-n homojunction light-emitting diode (LED) was fabricated by growing n-type ZnO NRs on polymer-molded Sb-doped ZnO NRs with a conventional solution method (95 °C). The I-V characteristics of the diode showed a typical p-n junction diode property with a threshold voltage of 2.5 V and a rectification ratio of 1.38x10² at 4 V. The electroluminescence spectra revealed an eminent peak around 400, 612 and 742 nm corresponding to UV, orange, and red emission respectively, which were attributed to defects related transitions.

Keywords: Solution process, Sb doping, p-type ZnO, homojunction, Light-emitting diode

Many studies based on metal oxide semiconductors have been performed for the next generation optoelectronic devices and materials development to replace the existing gallium nitride-based devices due to their outstanding properties; high crystallinity, environmental stability, solution processibility. Because of a need for control of perpendicular current flow caused by high integration density, individual vertical structure is more advantageous than multi-layer thin film structure. In this sense, vertically grown epitaxy structure has a high application capability for the next generation optoelectronic devices. However, epitaxial growth of single crystal requires expensive vacuum equipments or high temperature processes. Besides, in the case of heteroepitaxial growth, it is difficult to control the lattice orientation and lattice distance between the two different materials within a certain value.
In this report, the heteroepitaxial growth of materials was induced by a low temperature hydrothermal method. Cobalt oxide (Co₃O₄) p-type nano material which acts as a seed layer of zinc oxide (ZnO) was used in order to match lattice orientation and lattice distance with n-type ZnO. After synthesizing single crystal cobalt hydroxide (Co(OH)₂) nanoplates through the hydrothermal method, Co(OH)₂ nanoplates were converted into Co₃O₄ exhibiting p-type characteristics by a rapid thermal anneling treatment. Then, ZnO nanorods (NRs) were epitaxially grown on Co₃O₄ nanoplates by using the hydrothermal synthesis.
Surface and cross-section of nanomaterials were characterized using a scanning electron microscope. Crystallographic characteristics of vertically oriented ZnO NRs and Co₃O₄ were analyzed by using an X-ray diffraction technique. The optical and electrical properties of heteroepitaxial p-Co₃O₄/n-ZnO junction were investigated by using photoluminescence and current-voltage measurement.

Keywords : Cobalt oxide nanoplate, Zinc oxide nanorod, Heteroepitaxial p-n junction, Hydrothermal methods, Light-emitting diode

Bulk crystals of SiC and AlN have been grown by high-temperature (>2000 °C) sublimation technique. The Si and Al vapors sublimed from the source powders are highly reactive and gradually corrode most of conventional crucible materials during the growth process. These corrosive effects potentially deteriorate quality of bulk crystals, and increase material and processing costs. Graphite is an affordable material commonly used in bulk SiC sublimation growth processes. However, there still are issues in terms of production cost and crystal quality of SiC wafers due to limit of graphite material properties. We have demonstrated the fabrication of very thick (~100 µmt) TaC protective coatings on graphite crucibles (Sintered Tantalum carbide Coatings on graphite: SinTaC) using a novel low-cost process, which have been successfully applied as crucibles for AlN sublimation growth. Here we demonstrate SiC sublimation growth with SinTaC crucibles (by a simple and quick procedure of replacing conventional graphite crucibles with the novel TaC-coated graphite crucibles), which resulted in enhanced long-duration growth rate and larger crystal size as well as long-term durability of the crucibles thanks to the TaC protective layers.
Both of the SinTaC crucibles and naked-graphite crucibles were employed for SiC sublimation growth processes. The dimensions of the SinTaC and naked-graphite crucibles were strictly identical. The SinTaC layers were formed as follows. A TaC slurry, which is consists of TaC powder, solvent mixture, and etc., was applied on the crucible with a spray gun. The TaC powder compact films on the graphite crucibles were sintered in a reduced-pressure Ar atmosphere at a temperature over 2000 °C. The sublimation growth processes of SiC with the SinTaC and graphite setups were carried out at the temperatures of 2200 °C for a seed and 2330 °C for a source powder in a mixed gas atmosphere of Ar and N₂ for 24 hours.
From the appearance of after-growth SinTaC crucibles and weight-loss analysis, there was no prominent damage on TaC layers and underlying graphite substrates during the growth. Thus, it is confirmed that SinTaC crucibles can endure SiC sublimation growth conditions. Comparing the time-dependent growth heights with SinTaC and graphite crucibles, the growth height with SinTaC crucibles overrides that with graphite crucibles, and the difference in growth height increases with increasing growth duration. The growth height values after 24 h with SinTaC crucibles was higher by a factor of ~1.2 than that with graphite crucibles, and after 53.1 h the SinTaC growth height is estimated to be higher by a factor of ~1.5 (extrapolated value) than the graphite growth height. The comparative analysis also reveals that the growth with the TaC-coated crucible results in lower leakage of source gas and higher material yield for single crystal growth. Their causes are attributed to gas-tight and low-emissivity properties of the thick TaC layers.

Nowadays, white LEDs are the first choice in solid state lighting thanks to their high efficiency, low cost, compactness and robustness. The optimization of luminescent nanomaterials with tailored emission properties in the visible when illuminated with UV light is crucial for the development of white LEDs with better performances. Metal oxide nanopowders are suitable candidates. On the one hand, their emission properties often meet the requirements needed for white phosphors: high luminescence quantum yield and structure-tailored emission properties. On the other hand, their reduced size, typically less than 100 nm, allows for their incorporation in suitable transparent matrices preserving light emission features without strong detrimental scattering losses.
Here we present new results on UV-excited light emission properties of several oxide nanopowders prepared by microwave-assisted non-aqueous sol-gel. In a recent work, three main components in the emission of Eu doped γ-Ga₂O₃ nanoparticles (NPs) were identified: 1) intrinsic emission from oxide NPs, 2) intrinsic emission of Eu ions and 3) extrinsic emission originated by the presence of an organic capping layer resulting from the peculiar synthesis route. Proper thermal treatments, able to remove the organic capping without altering the oxide nanophase, could control the optical emission. In this work, we investigate the photoluminescence behavior of Ti, Zr, Hf, Ta, W, Zn and Ga oxides in order to recognize some possible common optical feature of extrinsic origin as well as to study the intrinsic emission in view of application in UV-pumped white-light emitting devices. This sample set comprises both strong luminescent oxides and “PL-silent” materials used as reference, evaluated before and after organic capping removal, operated by annealing at 420°C for 1 h. Samples were characterized by means of X-ray diffraction (XRD) patterns, absorption/reflection spectra in the UV-Vis-IR range, Raman spectroscopy, steady state and time-resolved photoluminescence emission and excitation spectra. XRD patterns confirmed a NP size of less than 10 nm even after annealing. Raman spectra showed that the mild thermal treatment is able to burn the organic residuals without altering the inorganic core. Finally, emission spectra provided us basic information on the overall spectroscopic features of the systems as regard intrinsic and extrinsic contributions.
In conclusion, untreated oxide nanopowders show emissions centered at 320-450 nm ascribable to the organic capping layer. Emission spectra after thermal treatment show instead features originated mainly from defect species and exciton emission, typical of the oxide matrices. The opportunity of tuning both intrinsic and extrinsic properties of metal oxide nanoparticles will be discussed.

Hexagonal boron nitride (h-BN) exhibits a wide indirect bandgap of 5.955 eV, as recently demonstrated by phonon-assisted emission and absorption measurements [1]. h-BN has attracted an enormous interest as an alternative dielectric for electronic devices based on van der Waals heterostructures resulting from stacking grapheme and transition metal dichalcogenide layers, because its surface is atomically flat and it is free of trapped charges. In addition, the thermal conductivity in the basal plane of the h-BN crystal is two orders of magnitude higher than that for the conventional SiO₂ dielectric, and can be beneficial for the thermal management of these novel devices.

Despite the current interest in h-BN, phonon dynamics in this material has not been thoroughly investigated. The analysis of the temperature dependence of Raman spectra can provide valuable information about the phonon decay processes in crystals. We have carried out a systematic study of both Raman active modes over a temperature range from 80 to 600 K. Whereas the low-energy mode (E_2g^low) corresponds to a gliding motion of the rigid hexagonal layers and is thus governed by weak van del Waals interactions, the high-energy mode (E_2g^high) probes the strong in-plane interatomic interactions. The high structural anisotropy of h-BN is reflected in its thermal expansion coefficient, which is negative in the basal plane and positive along the c direction. This has a strong influence on the E_2g^low and E_2g^high Raman frequencies.

The frequencies and linewidths of the E_2g modes are analyzed on the basis of Cowley’s second-order perturbation theory. Density functional theory calculations of the phonon dispersion are carried out to identify the main phonon decay channels. On account of the lack of efficient decay channels, the E_2g^low mode is extremely narrow and exhibits weak anharmonic interactions (negligible broadening), its frequency downshift being mainly a consequence of thermal expansion. In contrast, the E_2g^high mode displays a substantial broadening that can be accounted for by a dominant 4-phonon decay process. Decay processes, however, are not sufficient to reverse the strong frequency upshift caused by the lattice thermal contraction and thus explain the observed E_2g^high downshift. Similarly to the case of the E_2g mode of graphite [2], the contribution of the first-order 4-phonon scattering term is found to be dominant. This is related to the low-lying modes of the layered structure of h-BN and is confirmed by ab-initio estimations of the fourth derivative of the total energy with respect to the phonon coordinates. A good agreement is found by fitting the anharmonic model to the experimental data using the anharmonic coupling potentials as free parameters.

References
[1] G. Cassabois, P. Valvin, and B. Gil, Nat. Photonics 10, 262 (2016).
[2] P. Giura, N. Bonini, G. Creff, J. B. Brubach, P. Roy, and M. Lazzeri, Phys. Rev B. 86, 121404(R) (2012).

Gallium oxide (Ga₂O₃) is a wide-band-gap semiconductor which has attracted much attention over the past few years because its physical properties make it a promising candidate for applications in a wide range of fields. Ga₂O₃ exhibits five different polymorphs: α, β, γ, δ and ε, of which the monoclinic β-Ga₂O₃ is the thermodynamically stable phase at ambient conditions. This compound has been widely studied as an ultraviolet transparent conducting oxide, deep ultraviolet solar-blind detector, basic material for high-temperature oxygen sensors, and for spintronic applications. Recently,Ga₂O₃ has also been investigated for its potential in applications in power electronic devices and a high breakdown voltage has been demonstrated [1]. The high temperature corundum-like phase α-Ga₂O₃ is metastable at ambient conditions and experimental studies of its optical properties are rather scarce.

Mist-CVD is a cost-effective technique that can provide large-area, high quality α-Ga₂O₃ layers which are susceptible to be used as buffer layers for the growth of epitaxial hexagonal GaN layers, since they have the rhombohedric crystalline structure of sapphire and an excellent lattice match to wurtzite GaN. The availability of high-quality, mist CVD-grown α-Ga₂O₃ films has triggered a renewed interest in this material and its fundamental optical properties.

Absorption measurements in the near-UV region at RT performed on mist-CVD films grown on c-face sapphire substrates exhibit two structures which can be fitted by assuming excitonic effects with large Gaussian broadening in both cases. By applying the Elliott-Toyozawa model, a band gap of 5.62 eV is obtained with an exciton binding energy of 110 meV. A second optical transition at higher energy is observed at 6.3 eV. Ellipsometry measurements performed in the same energy range show two structures in the imaginary part of the dielectric function at similar energies. The band gap value found is higher than those experimentally reported previously after applying the Urbach rule to absorption measurements [2, 3].

Temperature dependent measurements of the absorption coefficient yield a gap band value of 5.67 eV at 6K. Both, temperature dependence of the band gap energy and of the exciton width can be well accounted for through a Bose-Einstein model in the 6-300 K temperature range involving scattering by a phonon mode.

Interference fringes observed in reflectivity measurements performed in a wide-range energy (230-1700 nm) have allowed us to determine the refraction index dispersion, and hence, the electronic dielectric constant at zero frequency ε_e (0)= 3.96 is determined. This value is in very good agreement with the one obtained by means of the ellipsometry measurements.

References
[1] M. Higashiwaki et al., Appl. Phys Lett. 103, 123511 (2013)
[2] G. Sinha et al., J. Cryst. Growth 276, 204 (2005)
[3] D. Shinoara and S. Fujita, Jpn. J. Apl. Phys. 47, 7311 (2008)

To realize single-package white down-converted light-emitting diodes (DC-LEDs), we synthesized green Zn-Ag-In-S (ZAIS) and red Zn-Cu-In-S (ZCIS) alloy-shell quantum dots (QDs) using a hot-injection method. To improve the photoluminescence quantum yield (PLQY), the synthesized green ZAIS and red ZCIS alloy-shell QDs were repeatedly injected with shell precursors. The obtained green ZAIS and red ZCIS alloy-shell QDs showed high PLQY values of 0.72 and 0.85 with peak wavelengths of 506 nm and 585 nm, respectively. Additionally full-width-at-half-maximum (FWHM) values of 84 nm and 103 nm, respectively, were realized. We also fabricated the synthesized green ZAIS and red ZCIS alloy-shell QDs in a cup-type blue-emitting InGaN LED to realize single-package white DC-LEDs, and we characterized the color performance by means of the luminous efficacy (LE), the color rendering index (CRI), the CRI for strong red (R₉), and the external quantum efficiency (EQE) at a correlated color temperature (CCT) in the range of 2,700 K to 10,000 K. The realized single-package white DC-LEDs showed a luminous efficacy (LE) of 37 lm/W and a high color qualities (CRI = 96, R₉ = 92, EQE = 0.14) at a CCT of 5,000 K.

Optoelectronic devices based on organometallic halide perovskites have recently emerged as a promising inexpensive optoelectronic technology. Hybrid organic-inorganic CH₃NH₃PbI₃ is a novel material that has attracted great concern as active layers in solar cells with efficiencies higher than 20%. As for light-emitting diodes based on CH₃NH₃PbI₃, color-pure electroluminescence can be tuned from blue to infrared region by tuning the halide composition. Recently, quantum dots based on organometallic halide perovskites with various colors have already been synthesized by many methods. High photoluminescence quantum yield above 80% has been observed. In this work, we report a method to fabricate highly luminescent CH₃NH₃Pb(Br_xI_1-x)₃ colloidal quantum dots in a short time. The color is tunable in 500 -700 nm range by adjusting halide composition. Besides, by increasing the solution temperature while preparing the colloidal quantum dots, the size of the quantum dots can be increased and the red-shift of the color can be observed. Furthermore, we blend CH₃NH₃Pb(Br_xI_1-x)₃ colloidal quantum dots with polymer hosts to realize quantum dot light-emitting diodes. The electroluminescence spectra can be controlled by blending ether various amounts or different color of colloidal quantum dots into the polymer hosts and blending. Upon properly choose the color of the CH₃NH₃Pb(Br_xI_1-x)₃ colloidal quantum dots and the color of polymer hosts, white light emission can be achieved.

Recently, we have reported a new fabrication method of buffer layers utilizing RF magnetron sputtering, “nitrogen mediated crystallization (NMC)”, which enables us to control the density of crystal grains through the introduced N atoms that hinder the crystal growth [1, 2]. By utilizing such buffer layers, atomically flat ZnO films were fabricated even on 18% lattice mismatched sapphire substrates. During NMC process, N atoms play a key role in modifing the crystal growth of ZnO. Here, we report the absolute N atom density ([N]_plasma) in Ar/N₂ sputtering plasma and discuss effects of N atoms on heteroepitaxial growth of ZnO on lattice mismatched substrates.
We measured [N]_plasma in Ar/N₂ sputtering plasma by means of vacuum ultraviolet absorption spectroscopy (VUVAS). NMC-ZnO buffer layers were deposited by RF magnetron sputtering at 700 ^oC on c-plane sapphire substrates. The RF power supplied to the cathode was 100W. The flow rate of N₂ and Ar were 0.5–10 and 24–15 sccm, respectively, and the total gas pressure was 0.35 Pa. ZnO films were deposited on the NMC–ZnO buffer layers by RF magnetron sputtering at 700 ^oC. Ar/O₂ mixtures were used for sputtering gases, and the flow rate of O₂ and Ar were 5.0 and 45 sccm, respectively. The total gas pressure was 0.70 Pa and the RF power supplied to the cathodes was 60 W. Post-deposition anneal on the ZnO films were performed at 1000 ^oC for 3 hours in air.
By utilizing ZnO films fabricated via NMC as buffer layers, single crystalline ZnO films were fabricated on sapphire substrates. The crystal quality of the single crystalline ZnO depends on [N]_plasma. For [N]_plasma ≥ 3.3×10¹⁰ cm^-3, the pit and dislocation density of ZnO films were high of 6.0×10⁷–3.6×10⁸ cm^-2 and 5.4×10⁸–7.7×10⁸ cm^-2, respectively. While, at [N]_plasma = 2.2×10¹⁰ cm^-3, a ZnO film with almost pit-free surface and low dislocation density of 3.6×10⁸ cm^-2 was fabricated, attributed to the large surface reaction probability and small incorporation ratio of N atoms into ZnO films. These results indicate that control of [N]_plasma is importance to obtaine high crystal quality of ZnO.

[1] N. Itagaki, et al., Appl. Phys. Express 4, (2011) 011101.
[2] K. Kuwahara, et al., Thin Solid Films 520, (2012) 4674.

Organometallic lead perovskite-based solar cells have demonstrated high power conversion efficiencies of about 22%. Since the photoluminescence quantum efficiency up to 70% has been reported, perovskite materials are believed to be promising candidate for LEDs and lasers. Indeed, perovskite-based light-emitting diodes have also demonstrated a high external quantum efficiency of about 8.5%. However, most of the light-emitting diodes emit on one color with narrow full width at half maximum. In this work, we demonstrate that the electroluminescence spectra of a light-emitting diode based on perovskite-conjugated polymer bilayer can be tuned by adjusting the recombination region. The CH₃NH₃PbI₃ perovskite material was prepared on the ITO/PEDOT:PSS substrate as the first emissive layer. A mixture of 1,3-Bis[2-(4-tert -butylphenyl)-1,3,4-oxadiazo-5-yl]benzene, and poly(9-vinylcarbazole) were deposited on the perovskite layer as the second emissive layer. Under forward bias, the electroluminescence spectra of this light-emitting diode changes with bias and the white electroluminescence is observed. This is the first time that white light can be generated from such a bilayer structure.

Single crystal barium tungstate (BaWO₄) has attracted significant attention due to its important optoelectronic properties. For example, it has applications in all solid-state lasers emitting in specific spectral regions and, owing to its blue luminescence, BaWO₄ with a scheelite structure is important in electrooptics. Since crystal properties depend on microstructure, much research has been pursued to control BaWO₄ crystal morphology, leading to growth of nanowires, spheres, cylinders, whiskers, penniform nanostructures, and flower-like structures. However, few reports have investigated their formation mechanism. While some concluded certain topologically complex morphologies resulted from oriented attachment (OA) of primary particles, this conclusion was based on post-growth analysis with no systematic evaluation of morphological evolution either in time or with changes in growth conditions. Here we detail the mechanisms of BaWO₄ formation by using SEM) to examine morphologies over a wide range of conditions and time-points and in situ AFM to investigate the generation and propagation of growth sources on BaWO₄ crystal surfaces. Our results show that the morphology exhibits a systematic dependence on solute and ethanol concentrations. At low ethanol and moderate solute levels, well faceted dipyramidal crystals form. As supersaturation is increased by raising either solute or alcohol levels, crystals develop a well-known “shuttle” morphology with a regular pattern of undulations along the fast growth direction and a four-fold “knot” structure at the mid-plane. While earlier reports attribute these features to OA, we show they are simply remnants of dendritic branchingAs supersaturation is decreased through a reduction of solute concentration, crystals exhibit a flower-like morphology. However, by filtering the starting solutions, the majority of these structures are eliminated in favor of simple dipyramidal crystals, showing the “flowers” are a result of heterogeneous nucleation events that dominate at low supersaturation. Even at low solute levels, increasing ethanol concentrations, and hence supersaturation, drives growth back towards formation of the shuttle morphology. In situ AFM experiments show the advance of BaWO₄ faces over the range of conditions investigated occurs through island birth and spread, leading to rough faces. The addition of ethanol has two effects. The first is overgrowth by macrosteps, whose speed increases with ethanol level. The second is the emergence and rapid growth of hillocks perpendicular to the dipyramid axis, which evolve into the knot structure found at the mid-plane. These results demonstrate that complex BaWO₄ morphologies occur through purely classical growth mechanisms influenced by ethanol as a modifier of island birth and step propagation and highlight the need for caution in deciphering growth mechanisms through interpretation of ex situ images collected long after growth is complete.

Wide bandgap semiconductor oxides present real applicability on devices as transparent electrodes (TCO), displays, optical waveguides, gas sensors. In particular, ZnO presents high optical transparency in visible range as well as large exciton binding energy (60 meV) which provides an effective use in optoelectronic devices. Inherently to its production processes, the loss of stoichiometry between II-VI elements plays a fundamental role in optical and electrical properties due to presence of oxygen/zinc vacancies as well as presence of interstitial atoms. These defects create localized states in the bandgap which act as donor/acceptor levels. In addition, these defects can act as traps to localize carriers and influence negatively the electrical conduction and optical emission.
Here we present an investigation on ZnO thin films grown by spray-pyrolysis method and employ photoluminescence spectroscopy as a tool to characterize the trapping process. Our samples are produced by pulverization of a solution consisting of zinc acetate dihydrate in distilled water. The solution with molarity M=10^-2 allow us the growth of films on glass substrates at the temperature range 220-300^oC in atmosphere. The structural, morphological and optical properties of ZnO thin films were studied by X-rays diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements as function of temperature. The optical spectra reveal two bands: one associated to emissions in the near-band-edge absorption range (NBE) and the green-band related to zinc and oxygen vacancies. As the temperature increases, a systematic increase on optical emission is observed for temperatures up to 250 K and 200K respectively. Above this critical temperature, the PL intensity decays as conventional Arrhenius behaviour observed in semiconductors emissions. These effects are attributed to traps which behave as potential fluctuation responsible to localize the photogenerated carriers. In low temperature regime, the effect of localization is strong as the carrier has no thermal energy to overcome these fluctuations. As the the temperature increases, the carriers avoid the fluctuations with depth around k_BT. In a critical temperature T_C, the influence of traps has a blockage and TC indicates the higher depth of potential fluctuations. An thermal annealing of ZnO film up to 500^oC increased the crystallite size from 12.6 nm to 23.8 nm which indicates an enhancement of crystal quality. As a consequence the influence of traps in defect-related optical band is completely removed as we observe in the Arrhenius plot of intensity as a function of 1/k_BT. The higher depth of potential fluctuations as well as their influence in NBE emission are significantly reduced.

High-brightness AlGaInP-based light-emitting diodes (LEDs) have received considerable attention in the area of solid-state lighting. Considering the internal quantum efficiency (IQE) of near 100%, to achieve a high light output power of the AlGaInP-based LEDs, the best approach is to enhance the light extraction efficiency (LEE). A thick conductive p-GaP window layer was proposed as a current spreading layer to enhance LEE. However, it is time consuming and not cost effective. Furthermore, the large difference in refractive index between p-GaP window layer (n~3.4) and surrounding epoxy layer (n~1.5) will result in a small critical angle for the light to escape from the LEDs. Therefore, transparent conductive layer (TCL) has been introduced to enhance the current spreading and light extracting effects.
Recently, ZnO-based TCLs are intensively investigated as alternatives of ITO, owing their advantages of nontoxic nature, high thermal stability, and abundance on our planet. Nevertheless, there are very rare related works on ZnO-based TCLs in AlGaInP-based LEDs. The challenge is to make an excellent direct ohmic contact between ZnO-based TCL and p-GaP window layer. In our previous work, MOCVD growing AZO-TCLs have demonstrated an excellent ohmic contact on GaN-based LEDs. However, by far, it has not been explored for AlGaInP-based LEDs application. Since this AZO-TCL was grown at high temperature of 550 ^oC using O₂ as oxidizer, if it is used directly, it is very possible that the AZO reacts with GaP window layer and degrades the ohmic contact.
In this study, an improved-AZO-TCL grown by MOCVD at 400 ^oC using H₂O as oxidizer was employed in AlGaInP-based LEDs with a carbon-doped p⁺-GaP contact layer (p⁺-GaP:C). The AZO-TCL shows high transparency (98%), high refractive indexes (2.1), and low resistivity (5 × 10^-4Ωcm). A direct ohmic contact was formed between the AZO-TCL and carbon-doped p⁺-GaP contact layer with a specific contact resistance of 2.2 × 10^-4 Ω cm². It is mainly attributed to the formation of the heavily unintentional hydrogen doping clear AZO interfacial layer with concentration of 10²¹/cm³, which was confirmed by SIMS profiles of H and O, ECV profiling, and HRTEM measurements. A low dynamic resistance of 4.2 ohm and a low forward voltage of 1.99 V are obtained under an injection current of 20 mA, which is lower than that of ITO-TCL LED (V_f ~2.07V and dynamic resistance of 8.5 Ω @20 mA), and are almost identical with the conventional LED without TCL. Furthermore, the light output power and wall plug efficiency were increased than conventional LED without TCL by 31% and 30%, LED with ITO-TCL by 12% and 16%, respectively. The effective current spreading ability and the low series resistance of the AZO-TCL provide the promise for high-performance AlGaInP-based LED applications. This research could pave the way for the production of the high brightness AlGaInP-based LEDs.

Transparent n-type oxide semiconductor β-Ga₂O₃ has a bandgap (E_g) of ~4.9 eV, which is even wider than SiC and GaN. Modifying crystallinity and orientation as epitaxial β-Ga₂O₃ thin films, and controlling conduction property by dopant such as Si or Sn does encourage its application to deep-UV optoelectronic and power devices. The epitaxial β-Ga₂O₃ thin film decreases thermal carriers, avoid breakdown, and enables high-efficient and low-loss device operation at high temperature or at high voltage. Epitaxial β-Ga₂O₃ thin films have been reported grown by some techniques such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and also pulsed laser deposition (PLD) at relatively high temperature of >400°C. On the other hand, it is of importance to develop a thin film process that takes place at low temperature, in which suppressed interdiffusion and thermal roughening would lead to stoichiometry as well as sharp interfaces in building a thin film device. In this study, epitaxial and ultra-flat β-Ga₂O₃ thin films were obtained without heating the substrate by PLD of precursor amorphous films at room temperature and subsequent excimer laser annealing (ELA). The effect of buffer layer on initial oriented growth of β-Ga₂O₃ crystals and on properties of epitaxial thin films was also investigated. The precursor amorphous Ga₂O₃ thin film was prepared on an atomically stepped α-Al₂O₃ (0001) substrate with NiO (111) buffer layer by pulsed laser deposition at room temperature using KrF excimer laser (λ=248 nm, d=20 ns, 1.5 J/cm²), and targets of β-Ga₂O₃ ceramics and β-Ga₂O₃:Sn single crystal. The amorphous thin films were subsequently solid-phase crystallized by ELA irradiating non-focused KrF excimer laser (250 mJ/cm²) in air at room temperature. The β-Ga₂O₃ (-201) thin film epitaxially crystallized on α-Al₂O₃ (0001) substrate with six-fold in-plane symmetry in the case NiO (111) buffer layer was introduced. The optical bandgap of ~4.9 eV was obtained for the epitaxial β-Ga₂O₃ (-201) thin film formed at room temperature comparable to which deposited at high temperature. The epitaxial thin film revealed ultra-flat surface with roughness of ~0.2 nm reflecting that of the the substrate. The β-Ga₂O₃ was found to crystallize from the interface instead of the surface and along step edges of substrate, verified by reducing irradiated laser pulse number and removing residual amorphous part with phosphoric acid. The effect of NiO (111) buffer layer on easing lattice mismatch between β-Ga₂O₃ thin film and the substrate and initial crystallization, and on the electronic property would be presented.
[1] D. Shiojiri et al., J. Crystal Growth 424 (2015) 38.

Indium Tin Oxide (ITO), benefiting from the high electron density of 10²⁰~10²¹ cm^-3, is widely used as transparent conductive layer (TCL) for visible-light optoelectronic semiconductor devices. Unfortunately, the large amount of free electrons results in plasma oscillation so that the transmittance of ITO begins to decrease at near-infrared wavelength. Generally, the higher the electron density is, the earlier the transmittance begins to decrease. Therefore, there is a trade-off between the conductivity and the near-infrared transparency, which prevents the ITO from being used as the near-infrared transparent conductive layer.
In this work, Tin-doped Indium Oxide (ITO) is grown on sapphire substrates by Metal-Organic Chemical Vapor Deposition (MOCVD). The Tin doping level is carefully controlled to result in an electron density of about ~1×10²⁰ cm^-3. The growth condition is optimized in order to increase the carrier mobility of the medium-doped ITO thin films. The ITO grown directly on the sapphire substrates is polycrystalline in nature. However, by use of a proper buffer layer, it is found that the ITO becomes c-axis aligned, which obeys the In₂O₃:Sn<111>//Al₂O₃<0001> epitaxial relationship. Therefore, the hall mobility of the thin film could be increased to 50~100 cm²V^-1s^-1, which results in acceptable conductivity without losing the near-infrared transparency. The sheet resistance of the optimized 90-nm-thick ITO film is 60 Ω and the transmittance at 380 nm, 780 nm and 3000 nm is 94%, 97% and >80% respectively.
The electrical and optical poreperties of the MOCVD-grown medium-doped ITO is compared with some recent reported transparent conductive materials like carbon nanotube network, siver nanowire network, topological insulator nanosheet network, graphene and so on. It is found that the high mobility ITO films are with superior visible-to-near-infrared optical and electrical properties and could be used as near-infrared transparent conductive layer.

The key challenge in designing high performance electronic devices based on diamond is to identify shallow acceptor and donor dopants and to create an effective doping strategy enabling their implementation into the diamond lattice. Amongst variety of potential n-type elements phosphorus was successfully doped in diamond and extensively investigated by means of both theoretical and experimental methods. Since phosphorus-doped diamond is fabricated using chemical vapor deposition, hydrogen impurity may simultaneously enter diamond too and form complexes with phosphorus-vacancy defects. However, relatively small amount of information about these complexes have been reported so far. To fill the gap we carried out a systematic study of PV:H centers in diamond using spin-polarized, hybrid density functional theory approach. The revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) was applied for the total energy calculation. For each defect the equilibrium geometry, formation energy as a function of charge state, nett spin and defect charge transition levels were determined. On the basis of formation energies vs Fermi level diagrams, relative stability of different charge states were predicted. We found that several complexes of phosphorus, vacancies and hydrogen atoms are energetically stable in diamond and therefore, might form unidentified phosphorus-related defects. Our theoretical results provide a valuable information on hydrogen interaction with PV centers and may be useful in identification of unknown phosphorus-related complexes in diamond

Gallium Nitride (GaN), a wide bandgap semiconductor, has been widely employed as the material for optoelectronic and electronic devices. Electron transport properties has great impact on the device performance. In this work, we directly compute electron-phonon scattering in GaN based on density functional theory and Boltzmann transport equation, and report carrier mobility based on first principles simulations. We will specifically discuss how the mobility and, especially, the electron mean free path as well as the relaxation time, is affected by the large electronegativity difference between Ga and N, which leads to a strong polar scattering as well as piezoelectric effect. The obtained results regarding the intrinsic electron transport properties will be useful for designing and optimizing device performance.

Polymorphism is the ability of a material to adopt a different crystal structure while maintaining stoichiometry. Titania (TiO₂) is a well-known transparent metal oxide with three primary polymorphs, rutile, anatase, and brookite. It is used in many applications ranging from photocatalysts, cosmetics, gas sensors, and the biomedical industry. We aim to understand the pathways by which TiO₂ transforms into the metastable brookite polymorph and how it is affected by the presence of the metastable anatase and stable rutile polymorphs. We study theoretically helper-ion incorporation, substrate matching, and chemical transformation to guide synthesis of brookite thin films. Amorphous thin films are deposited on SLS-glass, ThO_x on Si, and YSZ(110) substrates by pulsed laser deposition under different conditions to explore the nature of different amorphous precursors on the crystallization of the polymorphs. Structural characterization by X-ray diffraction is performed in-situ during rapid and conventional annealing and reveals the formation of brookite upon heating to 340°C and subsequent partial conversion to anatase upon cooling. Micro-Raman spectroscopy and atomic force microscopy together map the micron scale regions of the pure polymorphs. TEM is used to examine whether ion incorporation from the substrate contributes to the formation of brookite.

A series of (Dy-Al)³⁺ co-substituted SrFe_12-X(DyAl)_XO₁₉ (0 ≤ x ≤ 0.9 ) hexaferrite were prepared via auto-combustion method with subsequent heat treatment in air at 950^oC for 10 h. The phase identification of the powders performed using x-ray diffraction show presence of single phase hexaferrite for x<0.3 and presence of secondary phase, Fe₂O₃, for x≥0.3. Lattice parameter of SrFe_12-X(DyAl)_XO₁₉ remains unaffected upon doping mainly due to Fe³⁺ being replaced with a smaller size Al⁺³ and a larger size Dy⁺³ ions than Fe³⁺. (DyAl)⁺³ substitution modifies saturation magnetization, Ms, and coercivity, Hc. The room temperature Ms value decreased rather slowly with substitution while Hc increased to a maximum value of 5,572 Oe at x = 0.9, which is ~33% increase in coercivity as compare to that of pure Sr-ferrite. The Curie temperature, T_C, was observed to decreases upon (DyAl)³⁺ substitution. Dielectric permittivity was improved with (DyAl)³⁺ substitution in 800 MHz to 14 GHz frequency range. The DC electrical resistivity of all samples decreased with temperature showing semiconducting behavior. The room temperature Mossbauer spectral analysis indicate that Al³⁺ prefers 12k, 2a, and Dy³⁺ prefers 4f1 sites. The weighted average hyperfine field and isomer shift decreases with (DyAl)³⁺ content due to magnetic dilution effect and increased charged density per unit cell, respectively. These results indicate that co-doping rare-earth and non-magnetic ion can together maintain high coercivity with fairly high magnetization value of the doped Sr-ferrite.

The influence of indium concentration on the misfit relaxation processes that take place in high alloy content InGaN thin films was considered. Such films are promising for high efficiency photovoltaics applications. We have considered In_xGa_1-xN films grown by molecular beam epitaxy (MBE) across the x = 0.1-0.6 compositional range. The films were deposited on (0001) GaN/sapphire templates using the temperature as variable under constant element fluxes, and had thicknesses up to 500 nm. Experimental observations were performed using transmission electron microscopy (TEM), high resolution TEM (HRTEM), scanning TEM (STEM) and energy dispersive x-ray spectroscopy, in combination with high resolution X-ray diffraction (HRXRD). Films grown at lower temperatures exhibited higher alloy contents as a result of reduced indium desorption. Although the reduced growth temperature promoted film mosaicity, they relaxed in a classical manner through misfit dislocation segments. However, upon increasing the growth temperature, indium mobility and segregation at the growth front promoted the compositional pulling phenomenon in response to the misfit strain, thus resulting in compositional grading in the initial stages of growth. In such films, only a small part of the overall misfit was plastically relaxed at the heteroepitaxial interface through misfit dislocations. The relaxation of the residual elastic strain was delayed until a critical thickness was attained for the introduction of multiple basal stacking faults and associated partial dislocations. This resulted in sequestration layering of the films. The sequestration interface was found to constitute a primary source of a-type threading dislocations. Further increase of the growth temperature suppressed the film sequestration but led to an alternative strain relaxation mechanism through V-pit formation attributed to preferential indium segregation at the cores of ascending threading dislocations with c-type Burgers vector components.
Acknowledgement: Research co-financed by the EU (ESF) and Greek national funds - Research Funding Program: THALES, project NITPHOTO.
[1] C.Bazioti, E. Papadomanolaki, Th. Kehagias, T. Walther, J. Smalc-Koziorowska, E. Pavlidou, Ph. Komninou, Th. Karakostas, E. Iliopoulos, G. P. Dimitrakopulos, J. Appl. Phys. 118 (2015) 155301

Relaxed silicon germanium (Si_1-xGe_x) layers have been extensively studied for use as virtual substrates to create strained silicon layers. While these advances have enabled constant innovation in Si based technologies, the use of Si_1-xGe_x layers in III-Nitride structures has been quite rare. Due to the large wafer size and low cost of Si substrates, there has been increasing interest in the growth of GaN on Si for use in high power devices and LED applications. However, GaN on Si technology has faced many challenges, particularly in regards to the large lattice mismatch and thermal expansion mismatch between GaN and Si, which often lead to cracking and large threading dislocation densities in the GaN layer. GaN on Si (111) epitaxy often uses an AlN buffer layer followed by an AlGaN transition layer in order to build compressive stress into the GaN layer to prevent cracking and reduce threading dislocation density. However, it is also well known that Si_1-xGe_x layers on Si are compressively strained due to the larger lattice constant of Ge. By growing pseudomorphically strained Si_1-xGe_x layers on Si (111) and other substrate orientations, compressive stress can be incorporated into the structure prior to AlN and GaN growth. So long as these layers are below a critical thickness, compressive stress incorporation can be accomplished without changing the in-plane lattice constant of the substrate, maintaining the mismatch between the Si/Si_1-xGe_x and AlN buffer layers.

In this study, the use of pseudomorphic Si_1-xGe_x layers for strain mitigation in AlN/GaN epitaxy was investigated. The Si/Si_1-xGe_x layers were grown in a cold-wall MOCVD system used for group III-nitride epitaxy using SiH₄ and GeH₄ at 1150°C and 10 Torr total pressure . An in-situ stress monitoring system, a Multibeam Optical Sensor (MOS) from k-Space technologies, was used to monitor wafer curvature during growth. The MOS system was used to monitor stress relaxation in the Si_1-xGe_x layers in real time as a function of Ge composition and layer thickness. MOS data was combined with post-growth high resolution X-ray diffraction (HRXRD) information to determine the maximum amount of stress which can be incorporated without relaxing the structure. By combining theoretical critical thickness estimations with both in-situ and ex-situ characterization techniques, we investigate how incorporation of compressive stress prior to AlN/GaN growth affects the quality of the grown layers. Furthermore, we explore using thin, strained Si layers grown on relaxed Si_1-xGe_x layers on thick Si (001) substrates to explore how strain affects the surface of Si (001), and to explore the effect of surface strain in Si (001) on AlN/GaN epitaxy.

Recently, TiO₂ is paid much attention for many applications such as photocatalysis , chemical sensor, ferroelectrical devices , opto-electric devices and solar cells because of its ferroelectrical, photocatalytic and optical properties. In particular, for photocatalysis, sensor and opto-electric devices, the photocatalytic property and crystallinity of TiO₂ film need be improved in order to develop the TiO₂ devices with high-performance. Many Authors reported that the photocatalytic activity of TiO₂ strongly depended on the structure property and crystal size of TiO₂^.
Recently, this author reported that polycrystalline TiO₂ film was electrochemically obtained on conductive substrate by adding hydroxylamine (NH₂OH) into titanium potassium oxalate dehydrate (K₂[TiO(C₂O₄)₂]) aqueous solution. NH₂OH played an important role to grow the crystalline TiO₂ film and to state Ti4+ ion into this electrolyte. However, Structural property of TiO₂ film was not controlled by electrochemical parameters such as pH, deposition temperature and cathodic potential.
In order to grow TiO₂ film with high photocatalytic activity and high corrosion resistance, structural property of TiO₂ film need to be controlled by electrochemical parameters (high stability of NH₂OH groups). In this investigation, the structure property and photocatalysis of TiO₂ film will be controlled by adding N-Methylhydroxyl amine into the titanium dioxides electrolyte. In this presentation, the influence of N-Methylhydroxyl amine on the properties of TiO₂ films is discussed in detial.

Transition metal (TM)-doped II-VI semiconductor thin films have shown to be promising materials for mid-infrared (mid-IR) laser sources. Although these sources are not used in solid state lighting or energy applications, these lasers represent a major new appliation for wide-band gap semiconductor materials. When a II-VI semiconductor such as ZnSe is doped with TM ions such as Cr²⁺, the resulting broadband emission characteristics in the 2-3 µm spectral range (due to emission from the impurity) creates the potential for tunable lasing in the mid-IR. Mid-IR lasing under optical excitation has been demonstrated in these systems and a compact, electrically pumped tunable laser operable at room temperature in this spectral region is now highly sought after. Such an intense laser source could be integrated into numerous platforms civilian and defense applications. Achieving such an efficient electrically-pumped laser source requires the integration of the mid-IR active layer (Cr²⁺:ZnSe) with high-quality layers that allow charge injection and confinement of the optical field for optical amplification. The ternary alloy ZnS_xSe_1-x was chosen as a waveguide material due to its lattice-matching ability and lower index of refraction with respect to the Cr²⁺:ZnSe active material. Epitaxial growth of each layer is desired to achieve the electronic and optical properties necessary for successful integration into a multilayered lasing device, so a careful study was conducted on the effects of increasing sulfur content and growth temperature on the crystal quality of the resulting thin films. Several films of ZnS_xSe_1-x with a thickness of approximately 1 μm were deposited at various temperatures (400°C, 425°C, and 450°C) and with various compositional parameters (x=0.02, 0.06, and 0.10) on (100) GaAs substrates by pulsed laser deposition. The thin films and integrated structures were then analyzed via x-ray diffraction (XRD) to investigate the crystal quality and impact of sulfur incorporation on epitaxy of the Cr²⁺-doped layer for effective device integration. Structures featuring epitaxial thin films are achieved with dislocation densities on the order of 10¹⁰ cm^-2 for several regimes of compositional parameter and growth temperature. Analysis of XRD rocking curves of ZnS_xSe_1-x indicates relaxed films with shifts in the film peak relative to the GaAs substrate peak as sulfur content is varied with the smallest shift obtained for a substrate temperature of 450°C and compositional parameter of x=0.06. Analysis of the full width at half maximum of the XRD peaks reveals a decrease in dislocation densities with increasing sulfur content with the lowest defect density obtained for a substrate temperature of 425°C and compositional parameter of x=0.10. Mid-IR emission from Cr²⁺:ZnSe in these waveguiding structures will be presented and several routes for their integration into application systems will be discussed.

The development of ultraviolet light emitting diodes (UV-LEDs) in the UVC (280 nm - 200 nm) and UVB (320 mn - 280 nm) spectral range opens up new opportunities for the realization of compact, selective, and highly sensitive optical sensors [1]. For example, many amino acids and organic compounds, like tryptophan, tyrosine, and DNA, which are common in living organisms exhibit absorption bands in the UVC spectral range. The same is true for a number of gas molecules, like ammonia and NO_x. Based on optical absorption measurements or fluorescence excitation spectroscopy, deep UV-LEDs can be applied to the detection of dangerous multi-drug-resistant germs or to the monitoring of medical conditions. This presentation will provide an overview of recent advances in the growth of high quality AlGaN materials by metalorganic vapour phase epitaxy (MOVPE) as well as deep UV-LED device technologies. We will discuss novel approaches to improve the external quantum efficiency of UV light emitters, including the growth of low defect density AlN layers on sapphire substrates by epitaxial lateral overgrowth (ELO) as well as low resistance Si-doped AlGaN current spreading layers at aluminium mole fractions exceeding 80%. We will present efficient carrier injection into AlGaN-based LEDs emitting below 240 nm based on Mg-doped Al(Ga)N/AlGaN electron blocking heterostructures and examine advanced methods for light extraction, e.g. by controlling the optical polarization of the emission from deep UV AlGaN quantum wells or by utilizing UV-transparent graphene electrodes. We have been able to realize (In)AlGaN multiple quantum well LEDs in the wavelength range between 340 nm and 230 nm and demonstrate UVB-LED arrays near 310 nm with output powers of more than 50 mW and estimated lifetimes of 10,000 hours. Finally, we will illustrate how today’s UV-LEDs are being already successfully applied in plant growth lighting and gas sensing, and how they may impact other applications in the near future like medical diagnostics and phototherapy.

[1] M. Kneissl, J. Rass (Eds.), “III-Nitride Ultraviolet Emitters - Technology & Applications” (Series on Material Science, Vol. 227), Springer (2015).

Aluminum nitride (AlN) crystals, grown using a Physical Vapor Transport (PVT) method, are used by Crystal IS, Inc., to make single crystal AlN wafers which are then used in the commercial manufacturing of UVC LEDs. In order to obtain two-inch AlN wafers, the AlN crystals were grown with diameter slightly larger than 2”. The wafers show excellent quality as measured by XRD rocking curves with FWHM < 30 arcsec and low UV absorption that is needed to produce high power UVC LEDs.

The AlN substrates are used to fabricate high performance, long lifetime UVC LEDs for air purification, water disinfection, and environmental sensing. High quality pseudomorphic epitaxial layers are grown using an MOCVD process to form the device structure. UVC LEDs for instrumentation have been released with wavelengths between 250 and 280 nm with lifetimes greater than 3,000 hours. Recently released SMD product (Klaran™) achieve germicidal power outputs of 15 mW to 30 mW at a nominal wavelength of 265nm with drive currents up to 400 mA.

The substrate purities measured by GDMS and SIMS and growth regimes are linked to the high UV transmission. One correlation found is between carbon concentration and UV absorption in the 2-inch AlN substrates. Carbon-to-oxygen elemental ratio is the other factor which is linked to UV absorption. Such correlations were obtained through careful preparation of conditions in crystal growth. Similar correlations were previously reported by Hartmann et al. [1].


[1] C. Hartmann, J. Wollweber S. Sintonen A. Dittmar, L. Kirste, S. Kollowa, K. Irmscher and M. Bickermann, CrystEngComm, 2016, 18, 3488-3497

AlGaN based multiple quantum wells (MQWs) were grown on vicinal 4H p-SiC substrates by plasma-assisted molecular beam epitaxy. The MQWs were designed to emit near 300 nm using the k●p model. The MQW periodicity and strain state were measured with X-ray diffraction. The optical properties were characterized with temperature dependent photoluminescence (PL) and room temperature photocurrent measurements. The internal quantum efficiency (IQE) was estimated from the ratio of room temperature to 18K integrated PL intensity. One MQW design showed an estimated 10% IQE due to reduced defect density and deep potential fluctuations. These structures show promise for integration in vertical and inverted ultraviolet light-emitting diodes, grown on p-SiC substrates.

Ultraviolet sources are ubiquitous in many applications and necessitate the development of wavelength-selective photodetectors for dosimetry, that can operate under ambient conditions. Traditional UV detectors in the PIN configuration, such as those employed in solar-blind applications, achieve wavelength selectivity by employing a vertical device geometry. They are typically illuminated from the substrate side, employ appropriate n-AlGaN layer to ensure a shorter wavelength cut off, while the longer wavelength edge is given by the band-gap of the active layer. However, these structures are limited by the difficulty in p-type doping and Ohmic contact formation; presence of band-tails etc., all of which are exacerbated at high Al content. In this work we present an alternative route to achieving wavelength selectivity in UV photodetection, based on lateral current transport in AlGaN Quantum wells. We also evaluate their device performance and its dependence on the presence of complex alloy phenomena in these materials.
Al_0.35Ga_0.65N/Al_0.55Ga_0.45N Multiple Quantum Well structures with 1.5nm wells and 2nm barriers were grown by Plasma Assisted Molecular Beam Epitaxy on c-plane sapphire substrates. Lateral current transport in these structures was determined using Indium contacts. Steady state and transient photocurrent measurements were carried out at room and low temperatures.
The excitonic nature of photocurrent and photoluminescence processes is more prominent in quantum wells due to confinement of the photogenerated carriers, which impede their diffusion. However, the generation of photocurrent necessitates the splitting of the exciton by an applied electric field, a process which is strongly affected by their spatial localization. Our results indicate that for AlGaN alloys, an additional in-plane carrier localization can be achieved by generation of compositional inhomogeneities. Growth by MBE takes place at a temperature range where there is strong disparity of desorption rates and surface mobilities of gallium and aluminium adatoms. This phenomenon is useful in achieving a degree of control over the nature and intensity of band-gap fluctuations, where carriers are spatially localized.
Optimization of the surface diffusion of adatoms during growth can be controlled by several parameters, such as growth temperature, group III/V flux ratio, and the use of a surfactant. Growth of Al_0.35Ga_0.65N/Al_0.55Ga_0.45N MQWs under optimal conditions lead to strong wavelength selectivity, due to the critical formation rate as well as stability of the excitons formed. A room temperature photocurrent spectrum consisting of a single near-Gaussian peak at 300nm was observed with an FWHM of 17nm. We determine the formation and splitting mechanism of the exciton by varying the temperature and applied electric fields. Modification of alloy compositions and thicknesses can generate a range of wavelength-specific ultraviolet photodetectors on non-transparent substrates.

Transparent conductive electrodes in the deep ultraviolet wavelength region are attracting a great attention to use with UV LEDs for a various application, such as sterilization, water purification, medicine and light sources for high density optical recording. In this study, the high-performance Zn_1-xMg_0.2Be_xO/metal/Zn_1-xMg_0.2Be_xO multilayer structures were prepared on the quartz substrate by RF magnetron sputtering, with the energy band gap of ~ 4.5 eV, the optical transmittance of ~ 90% over the visible wavelength range and ~ 75% over the UV wavelength range, and the resistivity of ~ 6.5×10^-5 Ωcm. Furthermore, it was designed and realized that an ultra large band gap (~ 6 eV) transparent conductive electrode can be obtained by increasing the band gap of the ZnMgBeO layer and utilizing very thin inter-metal layer. High resistivity of the ZnMgBeO layer did not critically affect the conductivity of the multilayer, because the metal layer acted as the conducting path. Results with optimized multilayer structures with high transmission and low resistivity as well as the mechanisms operating will be further discussed in the presentation.