Symposium S.EL15—Ultra-Wide Bandgap Materials, Devices and Systems

Ga2O3, as a very wide bandgap semiconductor, promises a very high critical breakdown field. The theoretically estimated breakdown field is about 8 MV/cm, and the experimetally observed values are as high as 5.4 to 5.9 MV/cm to date. To take full advantage of the bandgap of Ga2O3, it is necessary to manage the electric field distribution in a Ga2O3 device to be as slow-changing as possible. In this talk, I will review various strategies and their underlying physical principles to manage the field distribution. With these strategies, record high figure-of-merits BV^2/Ron have been demonstrated in Ga2O3 Schottky barrier diodes and fin-transistors [1,2].
[1] Wenshen Li, Kazuki Nomoto, Zongyang Hu, Debdeep Jena, and Huil Grace Xing. Field-plated Ga2O3 trench Schottky barrier diodes with a figure-of-merit of up to 0.95 GW/cm2. IEEE Electron Device Letters (2019). [2] Wenshen Li, K. Nomoto, Z. Hu, T. Nakamura, D. Jena and H. G. Xing. Single and multi-fin normally-off Ga2O3 vertical transistors with a breakdown voltage over 2.6 kV. IEEE International Electron Device Meeting (IEDM) 2019.

Gallium oxide has many compelling material properties that have generated a large and growing interest for applications in opto-electronics, power devices, and RF devices. The wide bandgap of ~4.6-4.8 eV leads to a large breakdown field, which increases the Baliga and Johnson figures of merit, indicating potentially superior performance in high-power and high-frequency devices. Additionally, wide bandgap materials have a propensity for better radiation hardness due to higher required displacement energies. As such, there are many applications for β-Ga₂O₃ based devices within space technology, where harsh radiation environments may be present. There has been significant work to understand the impact of radiation on the GaN based devices, which degrades device performance due to the introduction of crystal defects. Ga₂O₃ has been shown to have higher displacement energies compared to GaN, so it may have improved radiation tolerance. The device studied here is a Si δ-doped β-Ga₂O₃ MESFET, which has already been demonstrated in separate work to be highly scalable and has the potential for RF performance with an experimentally measured cutoff frequency of 27 GHz [1]. The impact of neutron radiation on this device design is explored and monitored by changes in the device characteristics, such as maximum drain current (I_D,max), on-resistance (R_ON), threshold voltage (V_T), transconductance (g_m), and effective mobility (μ_eff). The devices are simulated using Silvaco Atlas, informed through bulk radiation studies on Ga₂O₃, to model how the radiation damage is influencing the device performance.

The MESFETs were grown by plasma-assisted molecular beam epitaxy (PAMBE) on an (010) EFG Fe-doped substrate. The growth was done at a substrate temperature of 630°C (pyrometer calibrated to Si emissivity), with Si being used as the channel dopant. The device has a 550 nm thick buffer to reduce any trapping effects due to the Fe doped substrate, with a 30 nm top layer above the channel. This led to a charge density of 1.19×10¹³ cm^-2 with a μ_eff of 73 cm²/V-s. The process was done using a regrown ohmic contact to reduce the Ti/Au source and drain contact resistance. A Ni Schottky contact is used for the gate to provide control over the charge in the channel.

The device terminal characteristics were measured using a Keithley 4200 SCS both before and after radiation. The drain current with a gate bias of zero was reduced by approximately 36%. C-V analysis shows that the charge in the channel (n_s) dropped from 1.19×10¹³ cm^-2 to 7.78×10¹² cm^-2. The effective mobility vs. n_s curve is measured through a gated transfer length method structure, which shows a reduction in mobility over the range of n_s. The zero bias condition n_s showed a 24% drop in μ_eff from 74 cm²/V-s to 56 cm²/V-s. The mobility is currently being modeled to understand the driving scattering mechanism that caused the reduction. The charge is expected to be compensated through the introduction of new defect states in Ga₂O₃, which trap the free electrons in the channel. These defects cause dispersion in both the V_T and R_ON as shown in prior work [2-3], and the change in the defect spectrum from radiation is being investigated in these devices. Our previous investigations on the radiation-induced trap introduction in bulk Ga₂O₃ materials studies [4] is currently being used to inform Silvaco simulations to match the pre- and post-radiation device characteristics. The Silvaco modeling will provide an understanding of how the generated defects caused the reduction in charge, as well as how the device design can be modified to be more tolerate to such defects.

[1] Z. Xia et al., IEEE Electron Device Lett., vol. 40, no. 7, pp. 1052–1055, Jul. 2019.
[2] J. F. McGlone et al., IEEE Electron Device Lett., vol. 39, no. 7, pp. 1042–1045, Jul. 2018.
[3] J. F. McGlone et al., Appl. Phys. Lett., vol. 115, no. 15, p. 153501, Oct. 2019
[4] E. Farzana et al., APL Mater., vol. 7, no. 2, p. 022502, Dec. 2018

β-Ga₂O₃ with a bandgap of 4.8 eV is expected to be a high-power semiconductor surpassing the capabilities of SiC and GaN. Recently we demonstrated 20 ampere-class β-Ga₂O₃ Schottky barrier diodes (SBDs). However, the reliability of these SBDs is quite important for their commercialization in power system circuits.
In this study, we used an emission microscopy to observe SBDs with various bias conditions in operation. The emission microscopy comprised a photon-sensitive electron- multiplying CCD camera and a probe station enabling the observation of light emission patterns of an SBD in real time. The sample was HVPE-grown ~10 μm-thick epitaxial layer grown on EFG-grownβ-Ga₂O₃ (001) substrate. The Pt/Ti/Au Schottky electrodes with diameters of 50, 100, 200, 500, 800, and 1000 μm were formed on the surface, and Ti/Au ohmic contacts were formed on the back surface.
In the reverse bias conditions, we observed light emission patterns from SBD from the back surface. As reverse bias increased, leakage current increased, and the number of emission spots increased. We confirmed clearly a relation between the number of emission spots and the leakage current. When additional high reverse voltage was applied, new emission spots appeared, and at a certain voltage catastrophic breakdown occurred at the new emission spot. Thus, emission spots could be classified into leakage-current related emission spots and catastrophic-breakdown related emission spots. Finally, we studied the crystal structure of the observed emission spots using the etch pit, AFM, and cross-sectional TEM methods.
A part of this work is supported by New Energy Development Organization Project of Japan.

β-Ga₂O₃ has an ultrawide bandgap (~ 4.8 eV), high critical electric field strength ( ~8 MV/cm), and significantly higher Baliga’s and Johnson’s figure of merit compared to GaN and SiC, which makes it promising for deep UV solar blind detectors, radio frequency applications, power rectifiers. β-Ga₂O₃ power MOSFETs emerged as promising candidates for the future high power/high voltage market. However, Ga₂O₃ has a much lower thermal conductivity (~10 - 30 W/mK) than GaN or Si. The heat dissipation at high voltages can significantly affect the performance and reliability of these devices. It is necessary to understand the thermal characteristics of β-Ga₂O₃ based electronics for design of packaging and thermal solutions. In this work, we are using Transient Thermoreflectance Imaging (TTI) to perform ultrafast thermal measurements of β-Ga₂O₃ FETs. TTI is a promising technique to do temporal and spatial measurements with submicron-scale resolution. A thermoreflectance coefficient was first calculated at the top surface of electrode pads of FETs. The selection of LED of appropriate wavelength is important for high signal-to-noise ratio and accurate measurements. Then, this coefficient is used to calculate the temperature over the electrode pads with a spatial and temporal resolutions of nanoscales (approximately up to 50 nm/pixel and 400 ns). We have calculated the Joule heat generation profiles and temperature distribution at different gate voltages using TCAD Sentaurus. The simulations are compared against the experiments and highlighted that peak temperatures are different at different gate voltages even though the power dissipation is same. We have performed thermal measurements at different gate voltages and calculated thermal time constants of a device to better understand the device’s thermal dynamics.

Metal-assisted Chemical Etching (MacEtch), discovered in 2000 for porous Si generation originally, is a local (open-circuit) electrochemical etching method capable of producing anisotropic high aspect ratio semiconductor structures with a simple wet etching process catalyzed by a patterned metal film. MacEtch eliminates plasma and high-energy ion induced damage typically occur in conventional reactive ion etching. Nanowires, vias, trenches, and numerous other patterns have been demonstrated using MacEtch for silicon (Si), germanium (Ge), and compound semiconductors (including GaAs, InGaAs, InP, GaP, SiC, GaN, β-Ga2O3), with unprecedented aspect ratio and sidewall quality. In this talk, we present the MacEtch process and characterization of wide bandgap semiconductors including β-Ga2O3, GaN, and SiC.
Acknowledgement: NSF ECCS 18-09946.

β-Ga₂O₃ with an ultra-wide bandgap of 4.5-4.9 eV and capability of n-doping promises its applications for high power electronics. β-Ga₂O₃ is predicted to have high breakdown field (6-8 MV/cm) with room temperature mobility of ~200 cm²/Vs. The Baliga figure of merit (BFOM) of β-Ga₂O₃ for power electronics is predicted to be 2 to 3 times higher than that of GaN and SiC. More advantageously, the availability of high-quality native Ga₂O₃ substrates produced from melt growth techniques is critical for large scale production. High voltage (>1 kV) devices, as well as RF devices with 27 GHz cut-off frequency, have been demonstrated recently [1, 2]. Nevertheless, development of high-quality β-Ga₂O₃ thin film growth technology is the cornerstone for high-performance device applications. Epitaxy of β-Ga₂O₃ has been investigated via different methods, including MBE, MOCVD, LPCVD, PLD, ALD and etc. Recently, MOCVD grown β-Ga₂O₃ has exhibited record high electron mobilities in both unintentionally doped (UID) [3] and Si-doped films [4].
In this work, we continue optimizing the MOCVD β-Ga₂O₃ homoepitaxial process on (010) Ga₂O₃ crystal orientation, and expand our investigation of β-Ga₂O₃ MOCVD growth along different orientations including (-201), (001) and (100). Key growth parameters, including growth temperature, growth pressure, Ga/O molar ratio, and substrate preparation, were investigated. For films grown on semi-insulating Fe doped (010) Ga₂O₃, the epi-film exhibit low background doping. From secondary ions mass spectroscopy (SIMS) depth profile, impurities such as hydrogen (H), carbon (C), chlorine (Cl), iron (Fe) were all below the detection limit. With low intentional Si doping, we demonstrated (010) β-Ga₂O₃ films with controllable doping between 10¹⁶ to 10¹⁹ cm^-3. From temperature dependent Hall measurements and analysis taking into account various carrier scattering mechanisms, we extracted a very low compensation level of N_a < 1×10¹⁵ cm^-3. Peak electron mobility reaches ~9500 cm²/Vs at T~45 K for an unintentionally doping (010) β-Ga₂O₃ film. We demonstrated record high room temperature mobility of ~194 cm²/Vs with n = 8×10¹⁵ cm^-3. The superior transport properties of the MOCVD grown (010) β-Ga₂O₃ films demonstrated high purity MOCVD epitaxy process with low defects densities.
For films grown along different crystal orientations, we use scanning electron microscopy (SEM) and atomic force microscopy (AFM) to characterize the surface morphologies, which have shown significant dependence on substrate orientation. Film growth rate, doping incorporation and transport properties are investigated.
In summary, we demonstrated superb electrical transport properties from MOCVD grown (010) β-Ga₂O₃ thin films with high purity and low defect densities. Growth mechanisms will be investigated for films grown along other orientations. The results from this study will provide fundamental understanding of the material properties of β-Ga₂O₃, which determines its potential for power device applications.

Acknowledgment: The authors acknowledge the funding support from the Air Force Office of Scientific Research No. FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir).

References:
1. Z. Hu, K. Nomoto, W. Li, N. Tanen, K. Sasaki, A. Kuramata, T. Nakamura, D. Jena and H. G. Xing, IEEE Electron Device Lett. 39, 869 (2018).
2. Z. Xia, H. Xue, C. Joishi, J. F. McGlone, N. K. Kalarickal, S. H. Sohel, M. Brenner, A. Arehart, S. Ringel, S. Lodha, W. Lu, and S. Rajan, IEEE Electron Device Lett. 40, 1052 (2019).
3. Y. Zhang, F. Alema, A. Mauze, O. S. Koksaldi, R. Miller, A. Osinsky, and J. S. Speck, APL Materials 7, 022506 (2019).
4. Z. Feng, A F M A. U. Bhuiyan, M. R. Karim, H. Zhao, Appl. Phys. Lett., 114, 250601 (2019).

To achieve the full potential of high-power, high-frequency Ga₂O₃ power devices, a low-resistance ohmic contact with good stability is needed to minimize conduction loss. The location of the β-Ga₂O₃ charge neutrality level within the ultra-wide bandgap (~4.8 eV) causes upward band bending and a surface depletion layer. This facilitates the formation of rectifying Schottky contacts, but the lack of Schottky-Mott metal-semiconductor band alignment makes ohmic contact formation difficult. One well-known way to improve ohmic contacts is to degenerately dope the semiconductor to enable tunneling across the junction. For the emerging β-Ga₂O₃ system, this often has been done by performing silicon donor ion implantation, post-implant anneal, and reactive ion etch (RIE) before Ti/Au metallization. Here, we quantitatively analyze the effect of silicon ion implantation, activation, and RIE on the charge transport mechanisms across ohmic Ti/Au - Ga₂O₃ junctions. Silicon-doped n+ conduction layers (~1.8 × 10¹⁸ cm^-3) were grown by molecular beam epitaxy on bulk semi-insulating Fe-doped (010) Ga₂O₃. The substrates were solvent cleaned before Ti/Au deposition followed by a 470°C 1-min N₂ post metallization anneal. Temperature dependent current-voltage measurements were performed on Circular Transmission Line Model (CTLM) structures, and the results were analyzed to determine the specific contact resistivity versus temperature. We found that Ti/Au metallization on this heavily n-type doped (010) Ga₂O₃ epi-layer has a specific contact resistivity of 3.29 × 10^-3 Ω cm^-2 at room temperature. At an elevated temperature of 175°C, the specific contact resistivity decreases significantly to 6.35 × 10^-5 Ω cm^-2. The temperature-dependent data fit the thermionic emission (TE) model, in which the barrier height of the pseudo-ohmic junction is ~0.27 eV. Surprisingly, this value agrees with the barrier height predicted from the difference between metal work function and semiconductor electron affinity. To analyze the impact of n++ doping on contact properties, a second sample was made using the same epitaxial substrate, this time with a box-shaped n++ profile of ~3 × 10¹⁹ cm^-3 by Si ion implantation, a post-implant anneal to remove implant damage, and a RIE process prior to metallization. CTLM measurements show that the ion implant significantly reduces the contact resistance to 1.51 × 10^-4 Ω cm^-2 at room temperature. The contact resistance of the ion-implanted structures is only slightly temperature dependent, with a contact resistance at 175°C of 6.16 × 10^-5 Ω cm^-2. The thermionic-field emission (TFE) model can be used to describe this Ti/Au – implanted-Ga₂O₃ junction. We will furthermore report our latest results on optimizing the ohmic contacts to gallium oxide and assessing their microstructure and stability.

Power electronic devices require ohmic contacts with low specific contact resistance to ensure low conduction loss and high-frequency operation. Gallium oxide, unlike other oxide semiconductors, has a charge neutrality level that falls within its ultra-wide bandgap of ~4.8 eV, causing upward band bending at the semiconductor surface. This upward band bending has enabled easy formation of Schottky rectifiers using a variety of metals and metal oxides. Such Schottky diodes are remarkably stable, with operation recently demonstrated at high temperatures up to 500°C. However the surface electron depletion layer of gallium oxide makes formation of ohmic contacts more challenging. Most of the existing work on ohmic contacts uses titanium as the metal layer directly in contact with gallium oxide. However our thermodynamic calculations predict that titanium will react with gallium oxide even at room temperature, causing formation of various titanium oxides. In this talk, we describe our observation of reactions at the interface between beta-phase (010) surface bulk gallium oxide and titanium/gold contact layers as a function of gallium oxide doping, metallization layer thickness, and post-metallization anneal time. Using transmission electron microscopies such as TEM, HR-TEM, and STEM with EDX and EELS, we observe the formation of a complex multi-layer interfacial structure that includes a defective gallium oxide layer, an approximately 5-nm thick Ti/TiO_x interfacial layer that - under certain processing conditions - is lattice-matched to gallium oxide, and a mixed-metal Ti/Au contact layer containing titanium-rich nanocrystals. The same layers form on both unintentionally doped (UID) and heavily tin-doped gallium oxide. In addition, the same layers form whether the titanium layer is thin (20-nm) or thick (50-nm), however a longer anneal time is required for full reaction of the thick metallization layer. We hypothesize that the thin Ti/TiO_x layer that forms upon annealing acts as an in situ barrier layer preventing further exchange of titanium and gallium across the contact interface. In addition, Sn-doping appears to play a minor role in reducing the amount of gallium out-diffusion from the gallium oxide bulk into the mixed metal layer. We will also outline some of the future challenges for ohmic contacts to gallium oxide. For the future success of gallium oxide power devices, further work is needed to understand the mechanisms of charge transport across the contact and to assess the stability of the Ti/Au ohmic contact to subsequent anneals and operation at elevated temperatures under bias or current stress. In addition, new methods may be needed to form reliable and stable ohmic contacts to lightly-doped beta-gallium oxide and gallium oxide with different orientations or crystal phases.

β-Ga₂O₃ has attracted significant attention due to its promising advantages such as large energy bandgap (~4.8 eV), controllable n-type doping and high predicted breakdown field strength (6-8 MV/cm). Energy bandgap engineering by alloying Ga₂O₃ with Al₂O₃ can expand the accessible bandgap up to 8.8 eV. Lateral device structures based on AlGaO/GaO heterostructures forming two dimensional electron gas (2DEG) has been demonstrated recently [1]. In order to maximize the advantages provided by the AlGaO/GaO heterostructures, the development of high quality AlGaO with controllable Al composition and n-type doping is critical. In this work, β-(Al_xGa_1−x)₂O₃ thin film growth on (010) Ga₂O₃ substrates via metalorganic chemical vapor deposition (MOCVD) were investigated [2]. Triethylgallium (TEGa), Trimethylaluminum (TMAl) and pure O₂ were used as Ga, Al and O precursors, respectively. Argon (Ar) was used as the carrier gas. The growth temperature was varied between 825°C and 920°C [3].
β-(Al_xGa_1−x)₂O₃ thin films with Al composition x ≤ 40% were achieved by the systematical tuning of TEGa/TMAl molar flow ratio. For x<27%, high quality β-(Al_xGa_1−x)₂O₃ films were achieved, while the XRD spectra showed phase segregation when Al composition x > 27%. SEM and AFM characterization show smooth and featureless surface morphology with RMS roughness of 0.51 nm for a β-(Al_0.1Ga_0.9)₂O₃ film with thickness of ~800 nm. As Al composition increases, the films show increased roughness which can be related to the phase separation observed in high-Al AlGaO films. The high resolution scanning transmission electron microscopy (STEM) imaging of β-(Al_0.15Ga_0.85)₂O₃ thin film demonstrated high crystalline quality without extended defects or dislocations. Atomic resolution STEM images showed coherent growth of 8-periods of (Al_0.23Ga_0.77)₂O₃/Ga₂O₃ superlattice (SL) structure with uniform Al distribution in AlGaO layer. The nonuniformity of the layer thicknesses were observed for superlattice structure with 40% of Al content as the growth proceeds, although the β-phase lattice structure was maintained in the first few layers. The XRD spectrum also showed sharp and distinguishable high order satellite peaks along with strong 0^th order peak for SL structure with 23% Al, indicating sharp interfaces and uniform alloy composition in the AlGaO layers. The decrease of the XRD satellite peak intensities and periodicities for SL structure with 40% Al content indicated inhomogeneous alloy compositions especially in the upper layers. N-type doping of β-(Al_xGa_1−x)₂O₃ from 6.3% to 33.4% of Al content was demonstrated using Si as n-type dopant. By tuning the Silane flow rate, controllable n-type doping concentrations from low-10¹⁷ cm^-3 to low-10¹⁸ cm^-3 were achieved at room temperature (RT). For x = 23.3%, the RT mobility of 108 cm²/V.s was measured with carrier concentration of 1.36x10¹⁷ cm^-3. A low temperature peak mobility of ~575 cm²/V.s at T = 65K with carrier concentration of 1.2x10¹⁷ cm^-3 were measured for x = 17%. As the Al content reaches 33.4%, the RT mobility of 82 cm²/V.s was measured with carrier concentration of 2.42x10¹⁷ cm^-3, while the corresponding low temperature peak mobility of ~200 cm²/V.s was recorded at T = 90K with carrier concentration of 1.0x10¹⁷ cm^-3.
In summary, we focus on the MOCVD development of AlGaO with high-Al composition, and understanding the phase segregation and n-type doping in AlGaO with varied Al compositions. Comprehensive physical/structural/chemical characterization of the MOCVD grown AlGaO films will provide in-depth understanding of this emerging UWBG material.

Acknowledgment: The authors acknowledge the funding support from the Air Force Office of Scientific Research No. FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir).

References:
1. Krishnamoorthy et al., Appl. Phys. Lett. 111, 023502 (2017).
2. Bhuiyan et al., Appl. Phys. Lett. 115, 120602 (2019).
3. Feng et al., Appl. Phys. Lett., 114, 250601 (2019).

(Al_xGa_1-x)₂O₃ is an ultra-wide bandgap semiconductor with a bandgap tunability ranging from 4.8 eV (β-Ga₂O₃) to 8.7 eV (α-Al₂O₃), highly promising for future high power transistors and deep ultraviolet (DUV) photodetectors [1]. The bandgap of (Al_xGa_1-x)₂O₃ increases monotonically with Al content that could be beneficial for device applications. Surprisingly degraded device efficiency was reported at Al content, x>0.25 [2]. It is crucial to understand the structural and chemical evolution of (Al_xGa_1-x)₂O₃ as the Al content changes from low (x=0.10) to high (x>0.25) that will provide researchers an insight about the structural transformation of this material. Atom probe tomography (APT) is a nanoscale characterization tool capable of providing atomic level information of material’s structure and chemistry in sub-nanometer resolution [3]. However, due to massive data points collected in APT, some material’s features are lost as noise along with uncertainty of data arising from multi-hit events. This hinders the exact quantification of chemical heterogeneity within the material. A data mining approach, based on principal component analysis (PCA), can decipher patterns in APT data to understand material’s structural chemistry. Here PCA was conducted on time-of-flight (TOF) of atoms collected by APT to study the structural and chemical evolution of (Al_xGa_1-x)₂O₃ with varying Al compositions.

β-(Al_xGa_1-x)₂O₃ samples with Al content of x=0.10-1.0 was grown on (010) β-Ga₂O₃ substrate by metal organic chemical vapor deposition (MOCVD) [4]. The controlled removal of atoms via field evaporation was achieved using a UV laser with pulse energy of 30 pJ, employing CAMECA LEAP 5000X HR atom probe system. The location of each atom was traced back using its TOF and position on the detector. PCA was performed on the TOF of ions to determine the pattern in structural chemistry of (Al_xGa_1-x)₂O₃ layers with x=0.1-1.0. Typical APT output is a single TOF spectrum. Our approach converts this spectrum into ten TOF spectra with 4 million ions in each. The count of ions vs. TOF in these spectra were used as input variables for PCA. Principal component 1 (PC1) captures the change in TOF values of the ions while principal component 2 (PC2) provides the randomness of elemental distribution in lateral planes, perpendicular to the growth direction. When PC2<0, the layer is homogeneous due to random Al distribution, while PC2>0, the (Al_xGa_1-x)₂O₃ layers are inhomogeneous. For Al composition of x=0.10-0.20, PC2<0, indicating the layers are homogeneous. As the Al content increases, the increasing trend of PC2 suggests inhomogeneity starts to appear. For x=0.30, PC2>0, elemental segregation is present in the layer resulting in degraded crystallinity. For x=0.40-0.60, PC2 remains greater than zero, implying chemically inhomogeneous layers. As the Al content continues to increase towards x=0.80, a decreasing trend of PC2 suggests a gradual reduction of chemical heterogeneity. At high Al content (x=0.80-1.0), PC2<0, an indication of (Al_xGa_1-x)₂O₃ layers regains homogeneity.

Here we investigated the structural and chemical transformation of (Al_xGa_1-x)₂O₃ as the Al content varies from low (10%) to high (100%) by employing data mining incorporating PCA in APT data analysis. This approach provides information about structural and compositional alteration of alloy directly by quantifying the degree of layer inhomogeneity correlated to ion’s TOFs. The information thus obtained will be significant for designing of β-(Al_xGa_1-x)₂O₃ devices.

Acknowledgement: Bhuiyan, Feng and Zhao acknowledge the funding support from the Air Force Office of Scientific Research No. FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir)

References:
1. B. Mazumder et. al., Appl. Phys. Lett. 115, 132105 (2019)
2. S.-H. Yuan et. al., IEEE Electron Device Lett., 39 (2), 220 (2018)
3. T. F. Kelly et. al., Annu. Rev. Matter. Res., 42, 1 (2012)
4. A F M A. U. Bhuiyan et. al., Appl. Phys. Lett. 115, 120602 (2019)

Diamond is a candidate material for next-generation power electronics and integrated circuit (IC) and micro-electro mechanical systems (MEMS) devices which operate under extreme environment. In order to use an advantage of high-density hole channel of hydrogenated diamond (H-diamond) surface, we have developed the high-k stack gate dielectrics and AlN heterojuction gate for H-diamond MOSFETs, such as HfO₂/HfO₂, LaAlO₃/Al₂O₃ Ta₂O₅/Al₂O₃, and ZrO₂/Al₂O₃, AlN/Al₂O₃ prepared by a combination of sputter-deposition (SD) and atomic layer deposition (ALD) techniques. In addition, we developed the routine ion-implantation process for preparing the diamond cantilever with a resonant frequency quality factor as high as one-million. Recently, we demonstrated the artificial diamond Fin-FETs with high-current level and the nanolaminate insulator gate MOSFET with k value as high as 100, and the new transistor concept named by metal-insulator-metal-semiconductor field-effect transistor (MIMS-FET) to achieve normally-off operation by combining the advantages of MOSFET and metal-semiconductor FET. In this presentation, we will show the comprehensive work on the diamond FET and MEMS devices.
Acknowledgements: This work was in collaboration with J-W. Liu, M. Imura, and M-Y. Liao in NIMS and partly supported by JSPS KAKENHI Grant Number JP16H06419.

Electron sources are widely deployed in communications through travelling wavetubes, industrial applications through magnetrons, and analytical instruments like electron microscopes, x-rays in industrial/medical/security settings and scientific fields, foremost free electron lasers (FELs). These apparatus, in general, utilize electrons from a thermionic cathode where the emission current is determined by the operating temperature as described by the Richardson-Dushman formalism. A figure of merit can be defined by the emission current (A) in terms of the power (W) dissipated in the device, i.e. [A/W]. A novel approach for electron sources exploits the negative electron affinity (NEA) of a diamond p-i-n diode in a mesa etched design. Operating the device under forward bias allows a fraction of the diode current to be released from the NEA sidewall of the diode. We utilized plasma-enhanced CVD to prepare modified diamond p-i-n diodes using boron-doped (111) oriented substrates and a nitrogen-incorporated nano-structured (nanoC) contact layer. Dedicated plasma deposition systems were used for the intrinsic, phosphorus-doped n-layer, and nitrogen-doped nano-structured carbon layer. Photo-lithography was employed to etch mesa structures with varying geometries and Ti/Pt/Au contacts. We present results from diamond p-i-n-nanoC diodes with an intrinsic layer thicknesses of 10μm. To control ohmic losses in the device the reduced thicknesses of the n-layer and nanoC layer, were 300nm and 150nm, respectively. At an applied bias of 2.2V a diode forward current of 1.15x10^-5A was established and an emission current of 2.14x10^-7A presented an electronic emission efficiency of 1.8% which is significant for electron emitters. With an increase in the bias to 2.7V the electronic emission efficiency was somewhat reduced to 1.5%. Operating the device at 2.7V shifted the power efficiency to 0.015 [A/W] which exceeds efficiencies for cathodes used in commercially available travelling wavetubes (TWTs) and magnetrons. We will discuss the p-i-n-nanoC diode operation and emission efficiency in terms of doping concentrations and layer structure and elaborate on the contribution of excitons to the electron emission.
This research was support by the Office of Naval Research under grant #N00014-17-1-3002.

Diamond based Schottky barrier and PIN diodes are currently of great interest for high power high temperature applications. To improve performance in terms of device metrics such as the on resistance and breakdown voltage, it is critically important to understand the current conduction mechanisms in the different voltage regions of operation of such diodes. Current in diamond diodes has been shown to exhibit space charge limited Mott-Gurney behavior some research groups. However, there has never been a systematic study of the various voltage and current regimes in forward biased diamond diodes. We perform Silvaco Atlas drift-diffusion simulations of Schottky PIN (SPIN) and PIN diamond diodes in comparison to experimentally measured J-V characteristics on the same structures. Incomplete ionization of dopants and nearest-neighbor/variable range hopping mobility models are included in the Silvaco simulations. Space charge limited current (SCLC) with single carrier injection is dominant in Schottky PIN (SPIN) diamond diodes with a relatively low doped and thin n-layer whereas SCLC with double carrier high injection limits current in PIN diamond diodes with a high doped and thicker n-layers. Measurements of SPIN and PIN diamond diode with i-layer >~1.5 μm show evidence of a Mott-Gurney region where J ∝ V² within a wide operating voltage range. For SPIN and PIN diamond diodes with thinner i-layers (< ~1 μm) the Mott-Gurney region is short lived only for a small voltage range and the current transitions back into a linear region limited by the resistivity of the quasi-neutral region. Furthermore, temperature dependent simulations and measurements were also performed to observe a “cross-over voltage” beyond which the increasing trend of diode forward current as a function of temperature is reversed, which is additional evidence of space charge limited current as corroborated by temperature dependent analytical curves of the Mott-Gurney law.
This research is supported by the NASA HOTTech program.

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[5] Suzuki, Mariko, et al. "Electrical characterization of diamond Pi N diodes for high voltage applications." physica status solidi (a) 210.10 (2013): 2035-2039.
[6] Brezeanu, Mihai, et al. "Single crystal diamond M–i–P diodes for power electronics." IET Circuits, Devices & Systems 1.5 (2007): 380-386.
[7] Ashok, S., et al. "Space charge limited current in thin film diamond." Applied physics letters 50.12 (1987): 763-765.

Diamond is a wide band gap semiconductor with outstanding semiconductor properties and the highest known thermal conductivity. The high electron and hole mobilities of diamond are unusual compared to all other wide band gap semiconductors, and support both high power and high frequency applications. PIN diodes have been used for microwave switching where the high power capability of diamond diodes may have a distinct advantage over other semiconductors. In this project, PIN diamond diodes were fabricated on (111) diamond substrates. High performance diodes show light emission during forward bias operation confirming bipolar operation. These diodes show a reduction in on-resistance as the forward voltage is increased beyond 10V. The reduction of the on-resistance is achieved with a phosphorus doped n-type layer and a nanocarbon interfacial layer between the metal contact and n-type diamond. This presentation discusses the role of the n-type contact resistance in achieving bipolar device characteristics and an exceedingly low differential on-resistance for high power microwave switching applications.

Diamond, when boron doped above the metal-insulator transition, is known to enter the superconducting state below a transition temperature of approximately 4 K for nanocrystalline films. The high normal state resistance of boron doped nanocrystalline diamond is particularly attractive for application to Josephson junction-based devices, while the high Young’s modulus is well suited to nanoelectromechanical devices. A severely limiting factor – with some similarity to many high temperature superconductors – is the short coherence length ξ ~ 10 nm, coupled with the naturally top-down nature of the device fabrication.

Here we describe the fabrication and measurement of nanoscale superconducting diamond devices, highlighting the technical challenges involved. We discuss the impact of the morphology on the results, and present some surprising results given the length scales involved.

Diamond semiconductor with a bandgap of 5.47 eV can be used in high-power devices beyond SiC and GaN. Kubovic and Kasu [1] demonstrated that NO₂ p-type doping on H-diamond produced a high hole sheet concentration of 1.4 × 10¹⁴ cm^−2,. Moreover, Hirama and Kasu et al. [2] fabricated a diamond p-type MOSFET with NO₂ p-type doping and obtained a high drain current of 1.3 A/mm for a gate length of 0.4-µm length by using EB lithography.
In this paper, we demonstrate a drain current of 0.7 A/mm with a gate length of 1.4-µm length using all photolithography, except EB lithography.
Diamond MOSFETs were fabricated on a 1-inch heteroepitaxial diamond. In heteroepitaxy, epitaxial overgrowth (ELO) technology is used to drastically decrease the dislocation density. Furthermore, microneedle technology has been proposed and used to delaminate heteroepitaxial diamond from the substrate without cracking[Editor4] it. In the final process of heteroepitaxy, chemical mechanical planarization process is important to obtain a damage-free and smooth heteroepitaxial diamond surface. The FWHM of (004) was as low as 0.03°, indicating the world’s highest quality level of heteroepitaxial diamond. NO₂ p-type doping was performed on H-diamond. Al₂O₃ as gate insulator and passivation layers were deposited by ALD. All pattern processes including gate formation were made by photolithography, not EB lithography. The gate electrode was deposited overhang to reduce the source resistance. In the DC drain-voltage characteristics, the drain current of 0.7 A/mm was obtained for FET with a gate length of 1.4-µm.
[1] M. Kubovic, M. Kasu, Appl. Phys. Express 2 (2009) 086502.
[2] K. Hirama, M. Kasu, et al., Jpn. J. Appl. Phys. 51 (2012) 090112.

Molybednum trioxide (MoO₃) is a widely used transition metal oxide with high electron affinity (EA) and work function (WF) and frequently employed as electron acceptor layers in surface-doped diamond field effect transistors (SDFET). The process of electron transfer from the hydrogen-terminated diamond surface to the acceptor layer i.e. MoO₃, while creating a hole channel in the surface, is a central mechanism that governs the operating principle of these devices. In addition, the electron extraction strength of the MoO3 layer is defined by its molecular chemical properties such as acidic strength and band gap. In order to understand the role of chemical composition and local structural configuration in modulating the acidic properties of the MoO3 layer, we have performed a density functional theory study and probed the acidic properties of the MoO₃ (010) surface doped with other transition metals (Zr, Nb, V, Re, and W). A suitable doping site for each of the dopant is accelerated using a machine learning (ML) approach. Most of the dopants tend to prefer the first few layers of the surface as their preferred sites. Using the optimized doped configuration, the acidic properties of the surface was evaluated by calculating the adsorption potential energy (APE) of ammonia on the surface because the Lewis acidity of a surface is related to its ability to act as an electron acceptor. A correlation between the electronegativity of the dopant and the Lewis acidity of the surface was observed. Specifically, metals with an electronegativity lower than Mo increased the acidity of the surface while metal dopants with an electronegativity higher than Mo tended to decrease the acidity of the surface. In addition, the size and the electronegativity of the dopants significantly influences the overall electronic properties of the host oxide. This result will help guide the development of SDFETs with properties tuned to desired specifications for high-power RF applications.

With its high avalanche breakdown electric field and resulting high figure-of-merit, Gallium Nitride (GaN) represents today’s state-of-the-art semiconductor for high-performance power electronics. However, the most prevalent GaN-based power device, the High Electron Mobility Transistor (HEMT), has not realized the full potential of GaN. This is partly due to the epitaxial growth of GaN on Silicon substrates for such devices, which results in a high density of defects in the material. Further, the HEMT typically shows breakdown at a voltage lower than ideal, due to a complex electric field distribution resulting from its lateral structure, as well as the related problem of the proximity of the high electric field to the surface of the device. Thus, HEMTs not only need to be over-designed for their rated voltage, but typically show catastrophic destructive breakdown (often dominated by surface effects) and do not have avalanche ruggedness. While this is acceptable for many lower-voltage applications (typically 650 V or less), it is not suitable for higher-voltage applications. In contrast, vertical GaN power devices grown on native GaN substrates do not possess these limitations. Breakdown voltages of nearly 5 kV have been reported, and several groups have shown evidence for avalanche breakdown in such structures. However, vertical GaN power devices are immature relative to GaN HEMTs as well as other wide-bandgap power semiconductor devices such as those based on Silicon Carbide. Fundamental materials challenges for vertical GaN in areas such as substrate quality and the epitaxial growth of thick (of order 100 um), low-doped (less than 10¹⁶ cm^-3 n-type) drift layers are topics of current research. Additionally, challenges exist in the processing of epitaxial layers into vertical device structures. One notable challenge is the realization of selective-area doping, which is needed not only for the functionality of many devices, but also for the edge-termination structures needed to prevent premature breakdown. Finally, minimal manufacturing infrastructure currently exists that can produce devices in quantity with good yield and good long-term reliability. In this talk, we will describe our team’s progress on the design, fabrication, and characterization of vertical GaN PiN diodes ultimately targeted at voltage ratings as high as 20 kV. We will discuss the growth of thick, low-doped drift layers by Metal-Organic Chemical Vapor Deposition as well as the design and processing of these layers into high-voltage devices. The processing effort includes the establishment of a foundry capability to demonstrate pilot production of these vertical GaN diodes, which encompasses evaluation of yield as well as extensive reliability testing and failure analysis. We will also describe methods to test the avalanche ruggedness of the devices, including the time response of the avalanche breakdown process. Finally, we will discuss some potential applications of the devices, including protection of the electric grid. This work was supported by the ARPA-E OPEN+ kilovolt devices cohort managed by Dr. Isik Kizilyalli. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

GaN transistors are very promising for power switching applications taking advantages of the material’s superior properties. Successful demonstration of the epitaxial growth of GaN on large diameter Si substrates has made the GaN devices emerged as a viable alternative with expectation of the low cost fabrication. Use of AlGaN/GaN hetero-junction as the current channel is one of the most notable features of the GaN transistor. Two dimensional electron gas (2DEG) with extremely high carrier concentration originated from the polarization-induced electric fields and the high electron mobility greatly help to reduce the on-state resistances. Lateral GaN transistors with the hetero-junction formed on the insulating buffer layers exhibit extremely low parasitic capacitances as well. These features of the GaN transistor enable high frequency switching with low operating losses. Thus, the GaN transistors can make power switching systems highly efficient and very compact by increasing the switching frequencies. Although achieving normally-off operation overcoming the high carrier concentration of the 2DEG had been the most critical issue for the practical use of GaN transistors, a newly proposed Gate Injection Transistor (GIT) with p-type gate has successfully demonstrated the normally-off operation with low on-state resistance.
In this presentation, current status of the GaN power devices on Si is reviewed. Here, the updated structure of the GIT called as Hybrid Drain-embedded GIT (HD-GIT) is described. Holes are injected from the p-type region formed as a part of the drain. The holes fill the trapping states in the GaN, which suppresses the current collapse even at higher drain voltages. Switching lifetime tests using an inductive load that can be called as dynamic high-temperature operating life (D-HTOL) test are proposed as the severe reliability test considering the practical application of GaN transistors. The HD-GITs ensure sufficient long lifetime under the D-HTOL test. Based on acceleration factors extracted from the Weibull plots of the D-HTOL test results, the D-HTOL lifetime of the 3kW totem-pole PFC (Power Factor Correction) is estimated to be 23.8 years. At present, the above-mentioned GITs are commercially available and the wide-spread use is expected as a game-changing power device enabling compact and energy-saving switching systems.

High Al-composition Al_xGa_1-xN are estimated to have high critical breakdown field, F_BR, (~9 MV/cm for x=0.5) and high electron saturated velocity, v_sat, which are attractive material properties for high voltage and high frequency power applications. [1] The extremely high F_BR is especially attractive, since this can potentially enable ultra-scaled devices with power density scaling which incumbent technologies cannot match. However, demonstration of electric fields approaching the material breakdown limit in an actual device remains a standing challenge. While PN junctions have achieved material breakdown limits for wide-band gap semiconductors, such demonstration for lateral diodes and FETs is challenging due to non-uniform electric field distribution in the depletion region (which causes electric field peaking), and is further limited by tunneling breakdown at the Schottky gate/anode electrode. A potential solution to this problem can be achieved by using extreme permittivity oxides inserted between the gate/anode metal and semiconductor and in the gate-drain region. [2, 3] Such extreme permittivity dielectrics enable improved breakdown by: a) reducing the electric field peaking in lateral electric field and b) reducing gate/anode leakage currents which suppresses gate/anode leakage related breakdowns. In this work, we demonstrate a BaTiO₃/Al_0.58Ga_0.42N lateral heterojunction diodes (HJDs) with significantly enhanced breakdown characteristics.
The epilayers were grown by LP-MOCVD and consisted of a 500 nm thick undoped i-Al_0.58Ga_0.42N buffer layer followed by 60 nm thick [Si⁺]-doped n-type Al_0.58Ga_0.42N layer ([Si⁺]=4×10¹⁸ cm^-3). Ohmic contact was achieved by employing selective area regrowth by MBE of reverse Al-composition graded contact layers on these MOCVD grown epilayers. A Ti-based metal stack was then deposited on the MBE-regrown contact regions via an e-beam evaporator followed by device isolation by ICP-RIE plasma etching. BaTiO3 was then deposited via RF sputtering in an oxygen ambient. BaTiO₃ was etched away using SF₆ plasma ICP-RIE etching under the anode regions, access regions and the region between the anode and access regions for control Schottky barrier diodes (SBDs) while BaTiO₃ was etched away only from the access regions for the HJDs. Finally, a Pt-based metal stack was used as anodes.
Breakdown characteristics measured with an Agilent B1500 parameter analyzer on randomly selected devices showed significant average breakdown fields improvement for BaTiO₃/AlGaN HJDs. In general, for the HJDs, for an anode to cathode spacing in the range of 0.2-0.3 µm, the minimum breakdown fields observed were in the range of 6-8 MV/cm. In contrast, the control SBDs displayed an average breakdown field ~4 MV/cm for devices with similar dimensions. The highest average breakdown field observed in this study, 8.5 MV/cm, is the highest observed experimental breakdown field for any semiconductor material to date. This demonstration provides a framework to realize ultra-scaled lateral devices with improved breakdown characteristics.

This work is sponsored by AFOSR (Kenneth Goretta)

[1] JL Hudgins et al., IEEE Transactions on Power Electronics 18 (3), 907 (2003).
[2] Z Xia et al., presented at the 2019 76th Device Research Conference (DRC) (2018)
[3] Z Xia et al., IEEE Transactions on Electron Devices 66 (2), 896 (2019)

High crystal quality AlGaN epitaxial films are highly desirable for near to deep UV optoelectronics applications due to a tunable wide bandgap ranging from 3.4 to 6.0 eV. For many years, the use of foreign substrates for AlGaN epitaxy resulted in highly-mismatched heterostructures with high dislocation densities. The use of recently available AlN single crystals allowed for close matching to AlGaN lattice parameters and thermal expansion coefficients. This lead to high crystalline quality AlGaN epitaxial films exhibiting a significantly reduced dislocation density resultant in improved device performance. However, the lattice mismatch between AlGaN and AlN results in compressively strained alloy films, varying in strain magnitude with composition. This compressive misfit strain can consequentially affect the properties of AlGaN-based devices leading to undesirable compositional non-uniformities to wavelength shifts or unreliable performance. This would require novel strain management schemes to minimize the influence of this strain. Nevertheless, there are no current models to describe AlGaN relaxation mechanisms on c-plane AlN as it is expected that these films are pseudomorphic on these substrates. This is expected as there is no resolved shear stress to activate the primary slip systems in hexagonal wurtzite structure under biaxial strain. As such, in this work, the strain relaxation of AlGaN on AlN single crystal substrate with relatively large lattice misfit is investigated.
Mismatch strain relaxation could happened by dislocation nucleation, dislocation bending, crystallographic tilting or surface roughening. In the case of preexisting dislocations in the substrate, their propagation through the film would play an important role in strain relaxation through dislocation bending. Al_xGa_1-xN layers were grown by MOCVD either on a c-plane AlN single crystal substrate or an AlN template on sapphire substrate to elucidate the role of these preexisting dislocations. The dislocation density in an AlN substrate is below 10³ cm^-2, whereas it's around 2×10¹⁰ cm^-2 in an AlN layer grown on sapphire. Al mole fraction was varied from 30% to 90% by varying metalorganic precursor flow rates. High resolution X-ray diffraction (HRXRD) was employed to determine the composition and strain relaxation. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were carried out to characterize the surface morphology and defect structures.
HRXRD symmetric (002) and asymmetric (105) reciprocal space mappings (RSMs) revealed that Al_xGa_1-xN layers grown on AlN substrate were pseudomorphic, regardless of alloy composition (0.5<x<0.9). Instead of plastic relaxation, the AlGaN films exhibited so-called “epilayer tilt”. The crystallographic tilt increased linearly with substrate miscut, as expected from the Nagai tilt model. In contrast, full relaxation of the in-plane lattice parameter was observed in Ga-rich 1 μm thick Al_0.3Ga_0.7N layers grown on AlN substrates. The relaxed Al_0.3Ga_0.7N layer did not show a strain gradient. This is in contrast to the strain gradient that was observed in AlGaN on AlN/sapphire, which is believed to result from dislocation inclination. The comparison between Al_0.3Ga_0.7N/AlN and Al_0.3Ga_0.7N/AlN/sapphire indicates that Al_0.3Ga_0.7N layer was abruptly relaxed due to a lack of the preexisting dislocations in AlN substrate. In addition, AFM revealed high density of growth spirals on the relaxed Al_0.3Ga_0.7N layer. X-ray rocking curve measurement also showed much broader AlGaN peaks compared the AlN bulk crystal. Therefore, Ga-rich AlGaN is plastically relaxed on AlN by generating dislocations seemingly contradicting the hypothesis that there is no resolved shear stress activating the primary slip system due to the expected biaxial strain. The dislocation structure in the relaxed AlGaN layer will be presented as well as possible relaxation mechanism including the resulting critical layer thickness.

Significant challenges in point defect control in AlGaN epitaxy has precluded commercialization of AlGaN based devices. Si and recently Ge are typically employed as n-type dopant in AlGaN and exhibit a low activation energy (<50 meV) in Al_xGa_1-xN with x<0.8 and x<0.5 respectively. However, Si doped AlGaN exhibits a “knee behavior” resulting in a conductivity and carrier concentration maxima at a specific Si concentration. Hence a high doping limit exists for Si in AlGaN that lowers the maximum achievable carrier concentrations that are necessary for AlGaN based optoelectronics. Similarly, a low doping limit (a minimum achievable carrier concentration with a corresponding maximum mobility) exists in AlGaN similar to that in GaN which precludes implementation of AlGaN power electronics that require low doped drift regions. Hence a major “point defect problem” exists in AlGaN that needs to be solved for implementation of AlGaN technology.
In this work, we demonstrate a systematic chemical potential control (CPC) based point defect control (PDC) where we relate the growth environment variables to the defect formation energy by determining and controlling the impurity chemical potentials and optimize the growth environment accordingly for minimal point defect incorporation or generation. Here, we employed this framework to provides a quantitative relationship between point defect formation energies and growth process parameters for the case of Si and Ge doping over the whole Al composition range of AlGaN. This allowed for classifying the difference between these two dopants in terms of incorporation and major compensating defect formation.
Accordingly, the V_III-nSi_III and V_III-nGe_III complexes and C_N were identified as the primary defects responsible for the doping limits in AlGaN with different compositions grown by metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrate and AlN single crystal substrates. We demonstrate control over the knee behavior (improving the peak carrier concentration by impeding the formation of complexes) and low doping limit (achieving lower carrier concentrations by reducing the compensating impurity (C_N) density) by controlling the chemical potentials of III/N and growth temperature for both types of dopants. Si was more effective for doping Al-rich (>40%) AlGaN while Ge was more effective for Al-poor (<40%) AlGaN. For the case of Ge doped AlGaN with Al composition of 40%, a plateau with constant carrier concentration (1-2×10¹⁹ cm^-3) followed by a sharp drop (one order of magnitude to 10¹⁸ cm^-3) in carrier concentration is observed which differs from the Si “sharp knee” doping behavior. This behavior is explained by formation of different types of complexes in Ge doping than Si. Moreover, two orders of magnitude increase in conductivity is observed by decreasing NH₃ flow rate from 3 slm to 0.3 slm, which results in III-richer growth environment and the resulting high formation energy of cation vacancy-related point defects. We also demonstrate the strong increase in vacancy formation with increase in growth temperature and consequently higher peak conductivity at lower growth temperatures.

GaN, AlN, and its ternary alloy have had significant development and implementation in commercial devices due to its wide range of applications in UV optoelectronics and power devices. However, it is subject to various point defects that limit their electrical and optical properties. Correct identification of point defects can aid in developing methods to reduce or eliminate undesirable defects that will lead to the properties needed for the devices exploiting the intrinsic material properties. The defects discussed here include self-compensating native defects, unintentional impurities, and various defect complexes. These are mainly point defects that are commonly present in undoped and doped Al_xGa_1-xN of different compositions with various doping levels of n and p-type dopants such as Si, Ge, and Mg.
Photoluminescence (PL) is a widely used characterization technique for the identification of point defects in GaN, AlN, and its ternary alloy. While PL is a commonly used technique that provides significant insight into various materials, analyses of the spectra are a major topic of debate in the community. Point defect identification has been a contentious issue especially due to previous DFT analyses underestimating the bandgap. Therefore, with the development of photoluminescence techniques along with updated hybrid functional studies, there have been many corrections and updates to previous statements regarding identification of point defects. However, hybrid functional studies still have a limitation when it comes to ternary alloys. This study utilizes defect energy values of GaN and AlN identified via hybrid functional studies and confirmed with PL as end points to identify the energy values of the defects in AlGaN experimentally via PL. The samples were grown via metal organic chemical vapor deposition at distinct growth conditions to ensure the presence of the point defect of interest. The measured PL spectra of the samples grown with precise growth parameters elucidate the energy of the defect over the entire Al composition of AlGaN. As expected, the defect energies increase with the increase of Al content. The energy values of the point defects follow a similar trend seen by the bandgap of AlGaN as a function of Al content as described by Vegard’s law with a bowing parameter. With this defined relationship, this work aims to provide up-to-date identifications of common point defects and their energies for the entire Al composition range of Al_xGa_1-xN for future development of AlGaN based technologies.

There is growing interest in wide and ultra-wide bandgap 2D materials such as hexagonal boron nitride (hBN) to serve as dielectric encapsulation layers for 2D heterostructures, as a host matrix for single photon emitters and as an integral part of III-nitride heterostructure devices. In the case of hBN, flakes and films have been formed by exfoliation of bulk crystals and CVD growth on metal substrates, respectively, but further advances are needed to realize wafer-scale single crystal films and expand the available range of wide bandgap 2D materials.
Our studies have focused on epitaxial growth of hBN and other 2D nitride layers by metalorganic chemical vapor deposition (MOCVD). High temperature epitaxial growth of hBN on c-plane sapphire was investigated using B₂H₆ and NH₃ precursors to achieve high purity films. Gas phase pre-reactions between B₂H₆ and NH₃ result in a significantly reduced growth rate for hBN at 1350^oC and 50 Torr in a H₂ carrier gas. Sequential precursor pulsing was used to minimize gas phase mixing and enable the growth of films with controlled thicknesses ranging from 5-100 nm. The hBN films on sapphire were subsequently used as templates for epitaxial growth of 2D transitional metal dichalcogenides such as MoS₂ and WSe₂. We have also investigated novel methods to stabilize 2D forms of conventional III-nitride materials such as GaN. This process uses controlled intercalation and reaction of group III and V precursors within the interfacial region of quasi-free standing epitaxial graphene formed on SiC to form stable ultrathin GaNx films. The graphene capping layer provides thermodynamic stabilization of a unique R3m structure as identified by aberration-corrected scanning transmission electron microscopy. Density functional theory predicts a bandgap energy in the range of 4.79-4.89 eV for this structure which correlates well with experimental results from UV-visible reflectance and absorption measurements. The realization of few layer films of group III-nitrides would broaden the range of accessible bandgap energies of 2D materials providing new avenues for scientific exploration and electronic/optoelectronic device development.

While high-performance n-type transparent conductive materials (TCMs) have existed for decades, heavy p-type doping of wide band-gap materials has proven much more challenging. The simple cubic compound CuI (band gap 3.1 eV) was recently rediscovered for this application and is currently the p-type TCM with the highest figure of merit. However, the native defects responsible for p-type conductivity in CuI, as well as compensating defects limiting the maximum achievable doping levels, have not yet been identified experimentally. Furthermore, there is disagreement in the literature on the work function and absolute band positions of CuI relative to vacuum. In this experimental study, we employ temperature- and intensity-dependent photoluminescence to draw new conclusions on the defect landscape of CuI. We then employ a combined photoemission spectroscopy-Kelvin probe system to show that the measured band positions depend critically on surface phenomena related to air exposure, with variations in work function up to 900 meV. Finally, we demonstrate that terahertz spectroscopy is an ideal tool for characterizing the electrical properties of CuI reliably and non-destructively.

Ultra-wide gap (UWG) semiconductors with band gap, E_G, in the range of 4-6 eV can offer significant device performance improvements over their narrow-gap counterparts in high-power RF electronics, and provide unique research opportunities including deep-UV optoelectronics, extreme-environment electronics, and exploration of ultra-high-field transport [1]. SrSnO₃ (SSO), a perovskite oxide, is an emerging UWG semiconductor with E_G in the range of 4-5 eV [2] that offers particular advantages over other UWG materials including potential p-type doping, and a greater range of device engineering achievable via integration with other functional perovskites including ferroelectrics. High-quality epitaxial SSO films have been grown using hybrid molecular beam epitaxy (MBE) [2], and long-channel field effect transistors (FETs) with promising performance have recently been demonstrated [3]. However, RF-compatible FETs that validate the potential of this material system have not yet been reported. This is due in part to the high contact resistance, R_c, of ~38 Ω-mm for Sc contacts used in previous studies [3]. Therefore, identifying Ohmic contacts with low R_c to doped SSO is essential before high-performance SSO FETs can be realized. In this study, we report the results of contact work-function engineering on Nd-doped n-type SSO thin films. Specific contact resistance, R_c-sp, in the range of 0.1-0.5 mΩ-cm² were achieved.
A controlled experiment was performed on heavily Nd-doped n-type SSO thin films and contact passivation was explored. The details of the MBE growth technique can be found elsewhere [4]. A 10-nm-thick unintentionally-doped SSO buffer layer was first grown on a 5 mm × 5 mm GdScO₃ insulating substrate followed by a 25-nm-thick n-SSO active layer. The concentration of Nd was controlled in the active layer by setting the Nd effusion cell temperature to 940 °C during growth. A sheet resistance, R_s, of 1874 Ω/■, carrier concentration of 3.5 × 10¹⁹ cm^-3, and a mobility of 37 cm²/Vs was obtained using Van der Pauw Hall measurements. A combination of electron-beam lithography, Ar plasma etch, evaporation and lift-off were used to pattern transfer length measurement (TLM) structures. A pre-treatment using O₂ plasma clean and a 45 s Ar plasma etch was performed prior to the contact formation. Sc, Mn, Ti, Al, and Cr metals whose work functions are in the range of 3.5 to 4.6 eV were explored, and all contact metals were capped with additional Ti/Au to form the metal lines in the TLM structures. Al and Cr contacts were found to be highly resistive when measured immediately after lift-off and were not considered for further studies. Sc, Mn, and Ti were less resistive, with Mn having the lowest R_c of 11 ± 3 Ω-mm, immediately after lift-off. To test the effect of annealing, contact passivation was performed by depositing 55 nm of Al₂O₃ via atomic layer deposition at 200 °C. After this process, all three metals showed strong Ohmic behavior. The Ti contacts had the lowest R_c of 2.3 ± 0.3 Ω-mm after passivation, with a resultant R_s of 1687 ± 71 Ω/■ obtained from the TLM measurements. Annealing at 300 °C did not result in any significant change in the R_c. R_c-sp values of 0.1, 0.26 and 0.5 mΩ-cm² were determined for Ti, Sc, and Mn respectively. In conclusion, we have performed a thorough study of metal contacts to n-doped SSO and the results show that Ti provides the lowest R_c among the metals analyzed. This work is an important step for evaluating SSO in electronic applications and can pave the way for demonstration of high-performance RF FETs using this emerging high-mobility UWG perovskite oxide.
The work was supported by the AFOSR through award number FA9550-19-1-0245.
[1] Tsao, J.Y., et al., Adv. Electron. Mat., 4.1, 1600501, 2018. [2] Wang, T., et al., ACS App. Mater. & Interf., 10.50, 43802, 2018. [3] Chaganti, V.R.S.K., et al., IEEE Electron Dev. Lett., 39.9, 1381, 2018. [4] Truttmann, T., et al., App. Phys. Lett., 115.15, 152103, 2019.

In perovskite oxides, defects are present in various dimensions and structures, where each defect forms unique and localized atomic bondings different from the host material. Crystalline defects not only modify physical and chemical properties of the host materials locally, but also create specific local electronic and magnetic structures. Defects often exhibit distinct properties such as enhanced chemical activity [1], different bandgaps [2], etc, which provides an opportunity to exploit defects as functional nanostructures. In recent years, new types of line defects have been discovered in oxides thanks to atomic-resolution imaging with scanning transmission electron microscopy (STEM) [2,3]. These defects are reported to alter the local electronic structures showing potential for defect-based nano-devices. Here, we present a new metallic-like line defect in La-doped BaSnO₃ thin films. The line defect is aligned along the film growth direction and exhibits a unique atomic arrangement. Structural and compositional analyses are performed using STEM-energy dispersive X-ray (EDX) spectroscopy. Ab-initio calculations including structure optimization and ground-state electronic structure calculations are also carried out. STEM-electron energy-loss spectroscopy (EELS) is employed to test the calculated electronic structures of the line defect.
La-doped BaSnO₃ films studied here were grown on various substrates by DC sputter deposition [4] and hybrid molecular beam epitaxy [5]. Plan-view STEM samples were prepared by mechanical polishing. STEM experiments were carried out using an FEI Titan G2 60-300 (S)TEM equipped with EDX and EELS. Ab-initio calculations were performed using the Quantum Espresso package [6] for structural relaxation and WIEN2K code [7] for ground-state calculations and EELS cord-edge simulations.

[1] Y. Jia, J. Chen, and X. Yao, Mater. Chem. Front. 2, 1250 (2018)
[2] J. S. Jeong et al., Nano Lett. 16, 6816 (2016)
[3] J. T. Yang et al., J. Mater. Chem. C 5, 11694 (2017)
[4] K. Ganguly, A. Prakash, B. Jalan, and C. Leighton, APL Mater. 5, 056102 (2017)
[5] A. Prakash et al., J. Vac. Sci. Technol. A 33, 060608 (2015)
[6] P. Giannozzi et al., J. Phys. Condens. Matter. 21, 395502 (2009)
[7] K. Schwarz, P. Blaha, and G. K. H. Madsen, Comput. Phys. Commun. 147, 71 (2002)

Group-III sesquioxides such as Ga₂O₃ and In₂O₃ are ultra-wide bandgap semiconductor materials. It is well known that bulk single crystals of thermally stable phases β-Ga₂O₃ (E_g = 4.5 eV) and c-In₂O₃ (E_g = 3.7 eV) can be grown by melt growth methods [1,2]. At present, high-speed growth of conductivity controlled homoepitaxial layers of β-Ga₂O₃ and c-In₂O₃ has attracted much attention to apply both materials for fabrication of vertical high-power electronic devices. In this presentation, the authors introduce high-temperature and high-speed growth of β-Ga₂O₃ and c-In₂O₃ by halide vapor phase epitaxy (HVPE) [3-7].
First, source gases and their carrier gases for HVPE growth of Ga₂O₃ and In₂O₃ were examined by thermodynamic analyses. The results clarified that by removing hydrogen atoms (H) from the system and using an inert carrier gas such as N₂, He and Ar, HVPE growth of high-quality Ga₂O₃ and In₂O₃ films becomes possible at high temperatures exceeding 1000 °C using GaCl-O₂ and InCl-O₂ systems, respectively.
Then, based on the results of thermodynamic analyses, HVPE systems for growing Ga₂O₃ and In₂O₃ were constructed. GaCl and InCl were generated in the upstream region of the reactor maintained at 800-850 °C by introducing Cl₂ gas over Ga and In metal, respectively. It was clarified that homoepitaxial growth at 1000 °C on bulk β-Ga₂O₃(001) substrates is possible at a growth rate above 10 μm/h without deterioration of crystalline quality. The unintentionally doped layer showed a low effective donor concentration (N_d - N_a) of less than 10¹³ cm^-3, and it was found that n-type carrier density can be controlled in the range of 10¹⁵ to 10¹⁹ cm^-3 by intentional Si-doping using SiCl₄. These results indicate that the HVPE method is a suitable method for producing homoepitaxial wafers for Ga₂O₃-based vertical power devices. Also in the In₂O₃ growth, heteroepitaxy at 1000 °C on (0001) sapphire substrates showed c-In₂O₃ growth at a rate of several μm/h. These results suggest that high-temperature and high-speed growth of (In_xGa_1-x)₂O₃ alloy by HVPE is also possible.
This work was partially supported by the Council for Science, Technology, and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO) and Grant-in-Aid for Scientific Research on Innovative Areas (No. 16H06417) from JSPS.

[1] A. Kuramata et al., Jpn. J. Appl. Phys. 55, 1202A2 (2016).
[2] Z. Galazka, R. Fornari et al., J. Cryst. Growth 362, 349 (2013).
[3] K. Nomura, Y. Kumagai et al., J. Cryst. Growth 405, 19 (2014).
[4] H. Murakami, Y. Kumagai et al., Appl. Phys. Express 8, 015503 (2015).
[5] R. Togashi, Y. Kumagai et al., Jpn. J. Appl. Phys. 55, 1202B3 (2016).
[6] K. Konishi, Y. Kumagai et al., J. Cryst. Growth 492, 39 (2018).
[7] K. Goto, Y. Kumagai et al., Thin Solid Films 666, 182 (2018).

Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with increasing band gap. Using hybrid density functional theory, we demonstrate rutile germanium oxide (r-GeO₂) to be an alternative UWBG (4.68 eV) material that can be ambipolarly doped. We identify Sb_Ge, As_Ge, and F_O as possible donors with low ionization energies and propose growth conditions to avoid charge compensation by native acceptor-type defects. Acceptors such as Al_Ge have relatively large ionization energies (0.45 eV) due to the formation of localized hole polarons. Yet, we find that the co-incorporation of Al_Ge with interstitial H can increase the solubility limit of Al and enable hole conduction in the impurity band. We also calculate electron (153.6 cm²V^-1s^-1) and hole mobilities (4.7 cm²V^-1s^-1) of r-GeO₂ at 300 K, suggesting r-GeO₂ has outstanding electronic properties that can compete with the state-of-the-art UWBG semiconductors such as β-Ga₂O₃. We will also discuss on our recent experimental progress on thin-film growth and electrical characterization of r-GeO₂. This work was supported by the Designing Materials to Revolutionize and Engineer our Future (DMREF) Program under Award No. 1534221, funded by the National Science Foundation. It used resources of the National Energy Research Scientific Computing Center, a DOE office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

S. Chae, J. Lee, K. A. Mengle, J. T. Heron, and E. Kioupakis, Rutile GeO2 : An Ultrawide-Band-Gap Semiconductor with Ambipolar Doping. Applied Physics Letters 114, 102104 (2019). doi:10.1063/1.5088370

The perovskite alkaline-earth stannates are gaining research attention due to their unparalleled room-temperature mobility among perovskite oxides and as wide-bandgap semiconductors. Most research attention is invested in the cubic aristotype member BaSnO₃ despite its unsolved challenges of large substrate-film misfit and consequent mobility disparity between thin films and bulk single crystals. The less-studied non-cubic member SrSnO₃ (SSO) has a smaller lattice parameter—closer to commercially available substrates—and thus has the potential to overcome the challenges associated with thin-film BaSnO₃.
First, by using a novel radical-based hybrid molecular beam epitaxy (MBE) approach, we show that SSO can be grown coherently—free of any misfit or threading dislocations—on commercially available substrates, hence solving a long-standing challenge in the stannate community [1]. Secondly, we show—in the first strain engineering study of stannates—that epitaxial compressive strain can control the octahedral rotations in SSO, hence stabilizing a high-temperature tetragonal polymorph at room temperature by more than 700 K below its stability range in bulk. Thirdly, by combining controlled doping, Hall measurements and secondary ion mass spectroscopy (SIMS), we show that SSO can be successfully doped n-type with rare-earth elements showing 100% dopant activation [2]. A record-high room-temperature mobility of 70 cm²V⁻¹s⁻¹ at 1 × 10²⁰ cm⁻³ was obtained in a 12 nm La-doped SSO film, making this the thinnest perovskite oxide semiconductor with electron mobility exceeding 25 cm²V⁻¹s⁻¹ at room temperature. By suppressing electron scattering using heterostructure design, we further boosted the room-temperature mobility to 80 cm²V⁻¹s⁻¹ in doped SSO—the highest reported value to-date among any distorted perovskite oxide.
Finally, we discuss that the electron-electron interaction effect plays an important role in determining low-temperature transport. The quantitative analysis of the magnetoresistance data yielded a large phase coherence length of electrons exceeding 450 nm at 1.8 K and revealed the electron-electron interaction to be accountable for the breaking of electron phase coherence in La-doped SSO films. These results, while providing critical insights into the fundamental transport behavior in doped stannates, also suggest the potential application of stannates in quantum coherent electronic devices owing to their large phase coherence length [3].

[1] T. Wang, A. Prakash, Y. Dong, T. Truttmann, A. Bucsek, R.D. James, D.D. Fong, J.-W. Kim, P.J. Ryan, H. Zhou, T. Birol, and B. Jalan, ACS Appl. Mater. Interfaces 10, 43802 (2018).
[2] T. Truttmann, A. Prakash, J. Yue, T.E. Mates, and B. Jalan, Appl. Phys. Lett. 115, 152103 (2019).
[3] J. Yue, L. R. Thoutam, A. Prakash, T. Wang, and B. Jalan, Appl. Phys. Lett. 115, 082102 (2019)

In recent years, β-Ga₂O₃ has drawn much attention in power electronic technology, attributed to the advantages of ultrawide bandgap (~4.9 eV), stability, and affordable large-diameter wafer. The ultrawide bandgap enables β-Ga₂O₃-based devices to exhibit high breakdown field, while there is limitation including the p-type doping difficulty and low thermal conductivity issue. In addition, heterostructure design in III-Oxide based materials is still limited, even though it is important for carrier confinement as demonstrated in the Si and III-V based technology. Extensive research works on III-Oxide binary and ternary alloys (In₂O₃, Ga₂O₃ and InGaO, AlGaO) have been reported, but there has been no literature on the β-Ga₂O₃-based quaternary material. Thus, investigating the optoelectronic properties of the β-Ga₂O₃-based quaternary materials is critical to gauge their potential use in device applications.
In this work, β-(Al_xIn_yGa_1-x-y)₂O₃ alloys with Al- and In-content each up to 18.75% are investigated using Density function theory (DFT) calculations. The effect of Al and In atoms on the electronic band structures of β-Ga₂O₃ quaternary alloys is studied combining with density of states analysis. Additionally, the lattice constants of β-(Al_xIn_yGa_1-x-y)₂O₃ alloys are analyzed and presented. The band alignment between β-Ga₂O₃ and (Al_xIn_yGa_1-x-y)₂O₃ is also investigated to explore the potential use of β-Ga₂O₃ / (Al_xIn_yGa_1-x-y)₂O₃ materials system in electronic devices. Preliminary experimental work is also discussed.
An appropriate crystal model for β-(Al_xIn_yGa_1-x-y)₂O₃ alloys is constructed for the DFT calculations using the supercell approach implemented in atomistic simulation package MedeA-VASP software. For example, in an 80-atom supercell, some Ga can be replaced with Al and In atoms, leading to β-(Al_xIn_yGa_1-x-y)₂O₃ alloys with various Al and In-content. β-(Al_xIn_yGa_1-x-y)₂O₃ alloys with high Al and In-content are not considered in this study to avoid the known phase separation issue happened to the β-Ga₂O₃-based ternary alloys. The band structure calculations for β-(Al_xIn_yGa_1-x-y)₂O₃ alloys were performed using the projector augmented wave (PAW) method. The semilocal generalized gradient approximation (GGA-PBE) is applied to treat the exchange-correlation potential in the calculations. Other computational details have also been carefully chosen to optimize the DFT calculations.
Analysis from the DFT-calculated band structures of β-(Al_xIn_yGa_1-x-y)2O3 alloys with different Al/In-content indicate that the alloys exhibit indirect band gaps, which is similar to β-Ga₂O₃. The direct bandgap of β-(Al_xGa_1-x)₂O₃ increases from 4.835eV to 5.171eV when the Al-content increases from 0% to 18.75%, while the direct bandgap of β-(In_xGa_1-x)₂O₃ alloys decreases from 4.835eV to 4.432eV when the In-content go up to 18.75%. In addition, the lattice parameters of the AlInGaO alloys have been investigated and analyzed. Our analysis shows that adding Al and In will reduce and increase the lattice of Ga₂O₃ alloys respectively. As a result, the quaternary β-(Al_xIn_yGa_1-x-y)₂O₃ alloys can be designed to be lattice-matching with β-Ga₂O₃ alloy, implying the possibility of adjusting the band properties of β-Ga₂O₃-based material without creating compressive or tensile strain within the material systems. In addition, electron effective mass is calculated using the energy dispersions of β-(Al_xIn_yGa_1-x-y)₂O₃ alloys. The average effective mass of electrons of β-(Al_xIn_yGa_1-x-y)₂O₃ alloys are obtained by taking the geometric mean of effective masses in different directions since the alloys exhibit anisotropy property. The electron effective mass values are generally within 0.22~0.28m₀, while the addition of Al and In has an obvious effect on the effective mass changing. Our findings could be valuable for carrier transport related applications such as power electronic and deep UV photodetector devices. Experimental details on AlInGaO alloys will be further discussed.

An accurate understanding of the critical electric breakdown field () characterizing semiconductor materials is necessary for the design of power switches, power diodes, and RF power transistors. It is particularly important to understand the dependence of on bandgap (E_g) as new ultrawide-bandgap materials are developed. Unfortunately, the reported dependencies of on E_g cover a surprisingly wide range in the literature. Moreover, while is often assumed to be constant for a given material, it is more accurately a function of the device depletion-region width and doping. Further, there are wide discrepancies in the literature where values for punch-through and non-punchthrough structures are compared without regard for these differences. We report a new normalization procedure that enables an equivalent comparison of values across materials, doping, and punch-through/non-punch-through device types. An extensive examination of many experimental avalanche-breakdown and ionization references reveals that the dependence ~ E_g^1.86 best fits the most reliable and newest data for both direct and indirect semiconductors over the range from E_g = 0.66 to 5.5 eV (comprising Ge, Si, InP, GaAs, 4H-SiC, GaN, and diamond). It may therefore be reasonable to use this ~ E_g^1.86 dependence as an approximate rule of thumb for predicting the critical electric fields of novel ultrawide-bandgap materials until precise measurements are made. Based on the ~ E_g^1.86 dependence, the relationship between specific on-resistance (R_ON,sp), breakdown voltage (V_BD), and E_g for power switches over this bandgap range is best described by R_ON,sp ~ V_BD² E_g^-5.58 for both direct- and indirect-gap semiconductors.

Diamond has the highest breakdown field and carrier mobility among all wide band-gap semiconductors making it attractive for use in next generation high-speed and high-power electronic devices [1]. Unfortunately, unlike other semiconductors such as Silicon, diamond cannot be doped with shallow dopants [2], and so other techniques such as surface doping and delta doping are required for doping diamond. Particularly, for the surface-doping technique, a thin-layer of acceptor layer is interfaced with the hydrogenated diamond surface which creates high-mobility hole channel at the interface. Motivated by this, we explore the possibility of using 2D materials such as graphene or hexagonal boron nitride (hBN) as acceptor layers. Though the surface lattice constants of (111) surface is nearly lattice matched and easy to create epitaxial heterostructure with h-BN, surface doped diamond field effect transistors (SDFETs) with diamond (100) surfaces are widely proposed as a next generated in p-type RF-power electronic devices [3]. Herein, we perform a first-principle study to investigate the structural alignment and electronic properties of h-BN/H-diamond (100) heterostructures. As a comparison, we also study the graphene/H-diamond (100) heterostructures. We perform a full analysis of the structural alignments of the 2D layers on Diamond (100) surface and identify optimal strain-compensated h-BN (graphene)/H-diamond (100) heterostructure. Using the optimized heterostructure, we perform electronic structure calculations and compare charge transfer through the van-der-Waal (vdW) gaps between the constituent layers in the heterostructures. Our results show that in spite of a relatively larger vdW-gap, the degree of charge transfer in h-BN and Diamond (100) is higher than that of graphene and H-diamond (100). We believe that our theoretical observations will help the fabrication of novel high-frequency and high-power electronic devices.

[1]. Chris J. H. Wort and et al., Materials Today, Vol 11, No. 1-2, 2008.
[2]. Erhard Kohn, Andrej Denisenko, Thin Solid Films, Vol 515, No 10, 2007
[3]. P.B. Shah, J. Weil, A.G. Birdwell, and T. Ivanov, MRS Advances, Vol 2, No. 41, 2017

High power and high temperature electronics and sensing devices that would perform reliably on a long term basis are mostly implemented in wide bandgap (WBG) and ultra-WBG (>3 eV) semiconductors. However, critical factors that are central to their successful implementation and reliable field operations remain technologically challenging. These include the availability of relevant substrates and doped epitaxies, packaging, contact metallization (Ohmic and Schottky) and diffusion barrier systems (DBS). The DBS essentially protects the ohmic contacts to the WBG semiconductors from the inward migration of oxygen (O) from the atmosphere and gold (Au) that is typically the top bond pad layer. Reaction and diffusion kinetics are generally accelerated with increasing temperatures, resulting in corresponding short lifetime to failure of these devices. It is, therefore, imperative to develop the appropriate metallization schemes that would provide effective ohmic contact and DBS to the semiconductor.
In this work, we report the results of the material analysis and electrical characterization of Ti (100 nm)/TaSi₂ (300 nm)/Ti (100 nm)/Pt (300 nm) ohmic contacts and Ti (100 nm)/Pt (300 nm) DBS on n-type 4H-SiC epilayer up to 800 °C. Device level evaluation was demonstrated on 4H-SiC micro pressure sensors to operate reliably for over 100 hours at 800 °C. Key sensor performance parameters [zero offset voltage (ZPO) and bridge resistance] that are critical to sensor measurement reliability were extracted. The results showed a strong dependency of the electrical parameters on the microstructures of the ohmic contact and DBS.
The ohmic contact sputter deposition and patterning was followed by rapid thermal anneal (RTA) at 800 °C for 10 seconds in near vacuum condition. The subsequent diffusion barrier metallization was similarly annealed. Finally, a TaSi₂ (20 nm)/Pt (100 nm)/Au (1 μm) bond pad metallization was deposited and furnace annealed at 800 °C in Ar for 30 minutes. This final layer also reinforced the underlying DBS. The in-situ measured sensor resistance and output voltage during atmospheric heating up to 800 °C was matched with corresponding periodic Focused Ion Beam-Field Emission Scanning Electron Microscopy (FIB-FESEM), Energy Dispersive Spectroscopy (EDS), and Auger Electron Spectroscopy (AES) depth profiling. The depth-resolved AES and EDS revealed three prominent intermixing and reaction zones within the diffusion barrier and metal/SiC ohmic interface. The reaction products were primarily silicides and carbides of Ti, which had combined to form the ohmic contact on the n-type SiC surface. The diffusion barrier zones consisted of intermixed Ti/Pt and silicide complexes of Pt while the top zone comprised of intermixes of Pt-rich/Ti that effectively prevented O and the bond pad Au from diffusing toward the ohmic contact. The overall stability of the bridge resistance and output voltage were predicated upon the effectiveness of the diffusion barrier scheme in preventing the migration of O and Au.
During the first 100 hours of atmospheric heating at 800 °C, the changes in the resistance correlated well with the diffusion of Au and O into the Ti/Pt layer. Both the bridge resistance and the ZPO simultaneously leveled off thereafter and remained relatively stable for the remaining duration of thermal treatment. The before and after comparisons of the results of thermal activities within the metallization showed direct correlation between the ZPO characteristic trend and the microstructural changes as a result of O and Au migration and new reaction products.
This work is very significant in that future WBG based sensors and electronics can be inserted further into the hotter section of engines, thereby eliminating the need for costly cooling and packaging schemes. It would help capture wider thermoacoustic frequency bandwidth, thereby improving the measurement needed for accurate validation of computational fluid dynamics codes.

Diamond semiconductor with a bandgap of 5.47 eV is expected to be used in high-power devices surpassing the capabilities of SiC and GaN. Diamond heteroepitaxial growth has been successfully demonstrated on cubic (001) MgO substrate [1] and yttria-stabilized zirconia / (001) Si substrate [2]. However, recently, S. -W Kim et al. demonstrated the highest quality heteroepitaxial diamond growth on a sapphire substrate. The FWHMs of (004) and (311) rocking curves were as low as 120 arcsec (0.03°) and 269 arcsec (0.07°) respectively, which are the world’s best values in heteroepitaxial diamond. Heteroepiatxial diamond growth on sapphire [3.4] has previously been reported, but the crystal growth mechanism remains unclear. Therefore, the present research focuses on revealing the growth mechanism.
We investigated initial diamond growth initiated by bias-enhanced nucleation on an Ir buffer layer deposited on (11–20) sapphire substrate. First, we revealed the crystal orientations of diamond, Ir buffer, and sapphire substrates using X-ray diffraction. Next, we observed diamond nucleation, nucleus coalescence, and layer growth processes sequentially by using atomic force microscopy and scanning electron microscopy . In particular, using cross-sectional transmission electron microscopy, we studied the interface of diamond nuclei with the Ir buffer layer. The mechanism would be different from Ref. 2. We documented the Ir buffer layer epitaxial growth on sapphire substrate and diamond nuclei growth on the Ir buffer layer.
References
[1] S. Washiyama, A. Sawabe et al., Appl. Phys. Express 4 (2011) 095502.
[2] F. Hormann, M. Schreck et al., Diamond Rel. Matter. 9 (2000) 256.
[3] Z. Dai, C. Bednarski-Meinke, B. Golding, Diamond Rel. Matter. 13 (2004) 552.
[4] A. Samoto, Y. Ando, A. Sawabe, T. Suzuki et al., Diamond Rel. Matter. 17 (2008) 1039.

Ultra-wide bandgap (UWBG) semiconductors have extended the opportunity for exploring novel application fields in electronics and optics. Ga₂O₃, being supported by its bandgap as wide as ~5 eV, is gaining increasing interests as a material for power electronics devices. Progress of monoclinic-structured Ga₂O₃ (β-Ga₂O₃) bulks and substrates, which is the thermodynamically most stable phase in a variety of polymorphous of Ga₂O₃, grown by solution-methods has accelerated the evolutional research on high-performance power devices. On the other hand, our research has directed on semistable corundum-structured Ga₂O₃ (α-Ga₂O₃) grown on sapphire (α-Al₂O₃). One of the reasons for paying attention to α-Ga₂O₃ is that there are a variety of corundum-structured oxide materials, which can be combined with α-Ga₂O₃ to establish novel devices. Alloys and heterostructures of Al₂O₃, Ga₂O₃, and In₂O₃ can achieve heterostructure devices by arbitral tuning of the bandgaps . We have succeeded in the bandgap tuning from 3.7 to 8.8 eV, and the n-type conductivity control of α-Ga₂O₃ with doping Sn and Si, leading to the device-oriented research. A start-up company, FLOSFIA, Inc., has demonstrated Schottky barrier diodes with on-resistance as low as 0.1 mΩcm², their reasonable heat resistivity, and the low-loss power conversion. Heterojunctions of α-(Al,Ga)₂O₃ alloys show the type-I band lineup, and this allows fabrication of a variety of heterojunctions and quantum wells for novel devices. α-In₂O₃ showed the higher electron mobility compared to that of α-Ga₂O₃, and we demonstrated the operation of an MOSFET with the field-effect mobility of 187 cm²/Vs. The bandgap of α-In₂O₃ (3.7 eV) is still wider than that of GaN and SiC, suggesting that α-In₂O₃ can also be a candidate material for power devices.

A various combination of corundum structured oxides allows unique materials properties. One of the examples is above-room temperature ferromagnetic properties of α-(Ga,Fe)₂O₃ and α-(In,Fe)₂O₃. α-Rh₂O₃ and α-Ir₂O₃ are native p-type materials. Especially, α-Ir₂O₃ is closely lattice-matched to α-Ga₂O₃. We demonstrated p-type conductivity of α-Ir₂O₃, followed by fabrication of α-Ir₂O₃/Ga₂O₃ pn junction. Efforts are continueing to control the bandngap and the hole concentration of p-type α-(Ir,Ga)₂O₃.

For optical applications of UWBG oxide semiconductors, we have focused on UWBG Mg-rich (x>0.7) rocksalt-structured Mg_xZn_1-xO, whose maximum band gap can be ~7.8 eV (at x=1). This attracts its potential as deep UV light emitters in the wavelength region which cannot be reached by III-nitrides, whose maximum band gap is ~6.0 eV of AlN. We used mist CVD method for the growth of MgZnO with the carbon-free source precursors. The x-ray and electron-beam diffraction evidenced rocksalt structure without noticeable incorporation of other phases. In the TEM images, it was revealed that dislocation defects were formed in the MgZnO layer originating from the defects on the MgO substrate surface, that is, the dislocation defects density in the MgZnO layer at present is dominated by the quality of the MgO substrate. Mg_xZn_1-xO films with x=0.95 and 0.92 showed the CL peaking at 199 and 212 nm at 6 K and at 205 and 217 nm at 300 K, respectively, without noticeable luminescence at the longer wavelengths. However, the luminescence peak energies were lower by 0.5-0.7 eV than the band gap energies. This large Stokes-like shift is attributed to the local band gap fluctuation. Efforts were continued for Mg_xZn_1-xO/MgO quantum wells. For the sample with x=0.92, the CL peak blue-shifted by 1-8 nm (0.08-0.21 eV) at 6 K compared to the single MgZnO layer with decreasing the thickness of the MgZnO layer. The spectrally integrated CL intensity at 300 K over that at 6 K (I₃₀₀/I₆), which is supposed to be the internal quantum efficiency at 300 K, reached ~13%, which was apparently higher than that of the single layer. The overall results suggest that MgZnO can pave the way for deep UV photonics.

Hafnium oxide (HfO₂) is an insulator that has a wide range of applications due to its high dielectric constant and wide band gap. Recent discoveries from 2011 on emergent ferroelectricity in doped HfO₂ thin films, have enabled it as a candidate for ferroelectric random-access memory, ferroelectric field-effect transistors, and ferroelectric tunnel junctions. However, the origin of the ferroelectricity of HfO₂ and doped versions has not been systematically studied yet. Furthermore, a significant amount of work has been done only on polycrystalline thin films. We are exploring the synthesis of epitaxial Hf-Zr-O₂ as a function of thickness with epitaxial oxide top and bottom electrodes by pulsed laser deposition to explore these ferroelectric transitions. We utilize high-resolution transmission electron microscopy to explore the atomic structure of epitaxial HfO₂ thin films and identify the ferroelectric phase transition with multislice simulation. The robust ferroelectric phase of HfO₂ can be found at a thickness below 10nm, even below 5nm, which is identified as the non-centrosymmetric orthorhombic phase with space group Pca2₁. The oxygen atom plays an important role in allowing a phase transition from the monoclinic to the orthorhombic phase.

Titania polymorphs have been shown to crystallize when thin-film amorphous precursors deposited by pulsed laser deposition are annealed [1]. This non-equilibrium deposition method, coupled with particular oxygen partial pressure ranges [2] and film thicknesses defined a region of phase space where particular polymorphs formed.

We have extended this study to investigate amorphous TiO₂ precursors deposited by RF-sputtering from a Ti target with an Ar/O₂ sputter mix of different ratios and find broadly similar trends. We can produce uniform thickness high-fraction brookite, anatase and rutile films by changing the oxygen deficiency in the precursor films and controlling the thickness in the range 30 – 200 nm. Synchrotron XRD analysis of a series of precursor and annealed films shows that the precursor films are fully amorphous and the crystallized films confirm the phases identified by the Raman analysis.

We observe the growth morphology and growth rates of the three major TiO₂ polymorphs. We anneal amorphous precursor films in-situ in flowing nitrogen at 300 – 450°C in a heating stage mounted on a microscope attached to a Raman spectrometer. We record the crystallization on video and distinguish the phases optically due to their different refractive indices and different morphologies, confirming phase identification with Raman spectroscopy. We find a particular precursor film yields the same polymorph when annealed at different temperatures, only the rate of formation changes. With in-situ analysis we found that brookite crystallizes faster than rutile and anatase. We also observed that RF-sputtered films have more crystallization centers than pulsed laser deposited films and crystallize slower. Brookite tends to grow anisotropically, while the rutile and anatase growth habits are more symmetric. The growth morphologies are complex in mixed-phase films. Photocatalysis measurements indicate that photo-degradation of methylene blue by the polymorphs differ from each other. Some photoactive films also revealed unusual XRD spectra.

[1] J.E.S. Haggerty, et al., Sci. Rep. 7, 15232 (2017).
[2] J.S. Mangum, et al., J. Non-Cryst. Solids 505, 109 (2019).

Metal-fluoride thin films have previously been investigated as gate insulators in MISFETs (CaF₂/Au/Diamond) and this work expands upon that investigation. AlF₃ is an ultra-wide-bandgap (10.8 eV) material with a low dielectric constant (2.2). These properties indicate that AlF₃ may be a suitable insulator for high power high frequency devices. AlF₃ films have been deposited on hydrogen terminated boron-doped polycrystalline diamond in a plasma enhanced atomic layer deposition (PEALD) process. PEALD is an emerging energy enhanced ALD technique that utilizes plasma radicals to drive surface reactions rather than thermal energy. PEALD allows for lower impurities, increased growth rates, improved stoichiometry, and lower deposition temperatures. The reactants used were trimethylaluminum, pyridine-hydrogen fluoride, and a hydrogen plasma. In-situ X-ray photoelectron spectroscopy was used to determine the band alignment of AlF₃ films on polycrystalline diamond. The valence band maxima are measured at 9.15 eV and 0.05 eV for AlF₃ and diamond, respectively. A type II staggered gap is formed with a conduction band offset of 3.77 eV and a valence band offset of 9.1 eV. The results indicate that AlF₃ should be appropriate as a dielectric on p-type diamond.
This research supported by the NSF under grant DMR-1710551.

Recent progress in wide band gap semiconductor based power electronics technology sparked humongous interest in the development of the semiconductor materials. With the aim of further reducing the cost, size, compactness while improving energy efficiency and performance of electrical systems, semiconductor materials of larger energy bandgap have been highly sought after. Recent studies show that ternary AlInN material grown lattice-matched with GaN provides access of energy bandgap of ~4.5 eV, allowing theoretically higher Baliga figure of merit than GaN-based devices and similar to that of β-Ga₂O₃ based devices. However, it also poses some difficulty in controlling the field effect since insulating material with larger energy bandgap is required to create conducting channel and block electron leakage from devices. Finding an insulator material with energy bandgap larger than 4.5eV that can be grown on AlInN material is thus important towards realizing the AlInN / GaN field effect transistor based devices. Most recently, AlInO material has been proposed as a viable material that can serve as the insulator through thermal oxidation of the AlInN material. While the AlInO material is expected to exhibit chemical formula of (Al_1-xIn_x)₂O₃, it is unclear if the AlInO material is thermodynamically stable and the information of AlInO material is extremely limited to date. Understanding the material properties of AlInO material from the viewpoint of structural and electronic properties is therefore critical for AlInN / GaN devices.
In this work, (Al_1-xIn_x)₂O₃ alloys covering complete In compositions are investigated using Density Functional Theory (DFT) calculations. Two different phases of AlInO alloys will be investigated: corundum and hexagonal. The effect of In atoms on the electronic properties of AlInO alloys is investigated. For structural properties, lattice constants and equilibrium volume of (Al_1-xIn_x)₂O₃ alloys are analyzed. Moreover, band alignment between AlInO and AlInN is studied, exploring possible heterojunction lineups in various orientations.
For DFT calculation, supercell approach is implemented in which a 60-atom AlInO crystal is constructed. The In composition is varied through the number of Al atoms being replaced in the crystal structure. DFT calculations were carried out using commercial package MedeA-VASP software. Band structure calculations were performed by using projector augmented wave (PAW) method, with the use of semilocal generalized gradient approximation (GGA-PBE) functional to treat the exchange correlation potential in the system. Other computational details such as the k-spacing and cutoff energy have been optimized to ensure reasonable DFT results. Surface supercells for both AlInO and AlInN alloys were also created in order to calculate the band alignment, and the parameters are set to be similar to bulk DFT calculations.
Our DFT-calculated band structures of AlInO alloys indicate the possibility of direct bandgap property crossing over to indirect bandgap property when In-content is larger than 20%. Preliminary analysis of the DFT-calculated band structures show that the energy bandgap of Al₂O₃ reduces significantly when In content is added into the alloy. The energy bandgap of AlInN with 20%-In is less than 5eV. The results are expected since In atoms are much larger than the Al atoms, and significant bowing occurs in atomic size mismatch condition. Further studies on density of states are required to understand the contribution of each orbital in the bandgap reduction. Note that Al₂O₃ and In₂O₃ have bandgap of ~8.8eV and ~3eV, respectively. Adding In in Al₂O₃ also results in significant reduction of lattice constants, but lattice matching condition with AlInN is possible with proper tuning of In-content. From the formation energy analysis, it is also expected that more than 20% In will result in phase separation issue in the AlInN alloys. Details on AlInN alloys will be further discussed.

In this work an experimental study to characterize titanium (Ti) doped amorphous-Gallium Oxide (Ga₂O₃) nano-films (a-Ga₂O₃:Ti) produced by magnetron sputtering at 300K is reported. The nano-films were deposited by the co-sputtering process using Ti and Gallium oxide (Ga₂O₃) targets with a two magnetron system; the films were deposited over glass substrates covered with a 20 nm of non-doped buffer Ga₂O₃ layer grown by the same process. The Ga₂O₃ concentration was changed by varying the RF-power of the Ga₂O₃ magnetron at 75 or 150 W, while the DC-power for the Ti target was set at 80W. Due to the non-equilibrium character of the sputtering technique the grown films can be considered as a distribution of Ti clusters embedded into an a-Ga₂O₃ matrix when the growth process is done at low temperatures, therefore post-grown annealing processes can be used to adjust the film properties fixing appropriately the annealing conditions, temperature or atmosphere composition. The annealing temperatures were 400, 450, 500 and 550 °C by periods of 10 min, in a high purity N₂.atmosphere. When the samples were annealed at low temperatures the resistivity of the films were varied from 5.74x10^-4 to 134.63 Ω-cm, electron concentration from 10²² to 10¹⁶ cm^-3 and mobility of 0.475 to 6.689 cm²/V-s. by annealing the samples at 550°C electrical insulating nano-films were produced by the complete oxidation of the Ti clusters. The relatively low mobility of the doped nano-films suggests the Ti dopant produces carrier dispersion suggesting the presence of Ti deep levels in the material. According to the X-ray diffraction analysis the Ti clusters in a-Ga₂O₃ matrix helps in reducing the required temperature to transform amorphous films to β-Ga₂O₃ at ~400 °C. The required temperature to produce the phase change was confirmed by SEM studies producing octahedral shape nanocrystallites with sizes from ~5 to 50 nm.

The intrinsic properties of diamond are attractive for use in high power receiver protector (RP) systems such as those required at the input of radar systems. At low input power, the RP device presents a low capacitance and high resistance so that it adds negligible insertion loss to the desired signals. However, at high input power levels the RP turns on with a resistance much smaller than the 50 Ω characteristic impedance and the majority of the input power is reflected away from the receiver input. P-I-N diodes made of Si and GaAs used in today’s conventional RP systems have limitations at high-power. The wide bandgap of diamond coupled with its higher thermal conductivity give it a superior RF power handling capability that can protect sensitive RF front-end components from high power incident signals.
Vertical diamond P-I-N diodes were fabricated with an n⁺-i-p⁺⁺ structure consisting of: a heavily phosphorus-doped n-type diamond layer with a doping concentration >1x10¹⁹ cm^-3 and a thickness of approximately 300 nm; an intrinsic diamond layer of thickness approximately 0.5 µm; and a heavily boron-doped diamond <111> substrate with doping concentration of 5x10²⁰ cm^-3 and a thickness of 300 µm. The sample was patterned by photolithography and Ti/Pt/Au contacts were deposited on both top and bottom surfaces of the diode. The PIN test structures consist of a total of 144 diodes with diameters of 50 µm, 100 µm, 200 µm, and 400 µm. A SiO₂ hard mask of thickness 2 µm was deposited on the diamond surface prior to the isolation etch step. Reactive ion etching was used to define electrically isolated mesa regions etched to a depth of ~600 nm. The SiO₂ etching was carried out using fluorine RIE with 25 sccm of CHF₃ and Ar at a chamber pressure of 30 mTorr at 200 W. Diamond etching was done in the chamber with 38 sccm of O₂ and 2 sccm of SF₆ at a chamber pressure of 15 mTorr at 300 W.
The electrical characteristics of the P-I-N diodes were measured using DC and RF probe stations. The diodes show significant forward current starting at around 10 V. The lowest specific on-resistivity at high forward bias was measured using the DC probe station and found to be 4.5x10^-4 Ω.cm². The capacitance of the diodes at zero bias was extracted from small-signal S-parameter measurements in the frequency range 0.1 – 10 GHz and found to be 7.6 nF/cm². Taken together these values suggest a figure of merit, FOM = R_on.C_off = 6.81 ps corresponding to a frequency of 23.37 GHz.
A 200 µm diameter PIN diode was attached to a coplanar waveguide (CPW) for RF small-signal measurements as follows. The p-type substrate of the diode was soldered onto the ground plane of the ground-signal-ground strip-line of the CPW while the n-type top contact was wire bonded to the signal line. The diode was DC biased with a bias-T connected to the input of the CPW. S-parameters were measured at no bias and 12 V of forward bias, over a frequency range of 0.5 to 5 GHz at 10 dBm power. S₁₁ and S₂₁ were found to be -15 dB and -0.4 dB respectively with no bias at 500 MHz, confirming negligible insertion loss with no forward bias applied. At 12 V of forward bias, S₁₁ and S₂₁ were found to be -2.5 dB and -13 dB respectively at the same frequency, confirming that up to 95% of the input power is reflected by the forward biased PIN diode. With further optimization of the diode growth and fabrication it will be possible to reduce the on-resistance and the associated R_on.C_off figure of merit with the goal of achieving PIN diodes that provide > 30 dB isolation for high power receiver protection applications.
This research is supported by a contract from Northrop Grumman Corporation. One of us (Mohammad Faizan Ahmad) acknowledges support from NSF contract ECCS-1542160.

Epitaxial Ga₂O₃ film was grown on the Al₂O₃ substrate using magnetron sputtering. Because of the lattice mismatch between Ga₂O₃ and substrate, epitaxial films reveal mosaic domain structure with high number density of interfacial dislocations. Interfacial Domain size has an average diameter of ~ 40 nm. On the top of the Ga₂O₃, without additional treatment, semiconducting indium gallium zinc oxide (IGZO) was stacked to obtain the oxide/semiconductor structure. An external electron beam control system for SEM and TEM was designed and built in house to acquire position specific physical properties – direct measurement for the electron density and luminescence characteristics from the optical energy states. External control system was successfully adopted to Electron Beam-Induced Current Imaging (EBIC) to visualize 2-dimensional distribution of electrons at the interface, which can be indirect indication of preferred electron conduction paths. Cathodoluminescence in TEM was also adopted to identify the 2-dimensional distribution of optical bandgap. Defects, originated from the interface, works as dead centers of luminescence, not wavelengths shifters as can be typically seen from the partial dislocation in GaN/InGaN lighting emitting diode structure.

The wide band gap and high thermal conductivity of gallium nitride semiconductors makes them ideal for high power and high frequency applications. These properties however are dependent upon the order and impurity concentration of the crystalline structure. In this study, we use Time Domain Thermoreflectance (TDTR) to investigate the role of defects on the thermal performance of GaN semiconductors. He atoms were implanted into GaN, with dosing concentrations varying between 1x10¹³ and 1x10¹⁹ cm^-2 and implantation energies at both 400eV and 400keV. TDTR is a pump-probe technique that measures the change in thermoreflectance on the surface of a sample as a function of pump-probe delay time from picoseconds to nanoseconds; this change in thermoreflectance is related to both the temporal temperature decay from impulse heating driven by the sub-picosecond pump pulse and the frequency-dependent temperature rise induced from the modulated pump pulse train. Thus, TDTR is well suited to measure both the thermal conductance across near surface interfaces along with thermal property changes of buried damaged regions from the ion irradiation. Our TDTR apparatus includes an 800nm 80MHz femtosecond pulsed laser, the pump is modulated at 8.4MHz, and all samples included an aluminum transducer deposited on the surface. Ions implanted at low powers localize near the surface of the GaN, and their effect can be observed when measuring the Aluminum-GaN thermal boundary conductance. Ions implanted at higher powers penetrate to a depth greatly exceeded the thermal penetration depth of the TDTR measurement technique. Here, the role of defects due to the passage of the ions is observed and measured. We believe the results and trends observed in this study are not limited to GaN, and a deeper understanding of the effects of doping on thermal properties could assist in cooling of doped semiconductor devices.

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects paly on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of Diamond and GaN power semiconductor material technology, and the potential impact crystal defects may have on power switching electronics. UWBG Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. Diamond faces the challenge of achieving n type doping. We report our results on delta doped channels in diamond CVD epitaxial layers and the achievement of delta doped field effect transistors. Finally, our recent experiments of proton and gamma radiation effects show that diamond transistors transfer characteristics do degrade as a result of radiation induced defects.

Recently, incorporation of MgxZn1-xO (MZO) as the emitter in CdSeTe solar cells has resulted in improved open circuit voltage and efficiency. However, the MZO layer appears to be sensitive to subsequent device processing, possibly due to unoptimized conduction band alignment or insufficient doping in the MZO layer. Unfortunately, the carrier concentration in the MZO is dominated by oxygen vacancies, which is difficult to control. Here we investigate how doping MZO films with F affect the CdSeTe properties. We deposit the MZO:F in two ways. The first method is by depositing an MZO/MgF stack followed by high temperature anneal, while the second method will be cosputtering ZnO, MgO, and MgF. Our preliminary results on the bilayer stack show that, peak PL intensity and bulk carrier lifetime of MZO:F/CdTe device increased after annealing. There is evidence of intermixing of the MZO and MgF after annealing, suggesting that the Fermi level of the MZO is closer to the conduction band at the film interface. If this is the case, band bending in the CdSeTe layer will be enhanced, leading to reduced interface recombination. Further investigation into the cause of these results will be presented, as will the results obtained when a cosputtered MZO:F emitter is used. The findings of this study may lead to higher efficient CdTe devices.

Interest in β-phase gallium oxide (β-Ga₂O₃) has been growing tremendously in recent years due to its great potential for power devices and RF electronics. The major factors causing this surge are its ultra wide direct bandgap (~4.6-4.8 eV), large breakdown fields, ease of n-type doping, availability of large area, melt-grown substrates and high predicted device figures of merit compared with GaN and SiC. Most epitaxial growth efforts are focused on molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) but low pressure chemical vapor deposition (LPCVD) allows for high growth rates in a high temperature growth regime and with additional features like large area scalability. LPCVD can achieve growth rates as high as 10 µm/hr and growth temperatures up to 1100°C using economical and eco-friendly high purity precursors or sources. LPCVD is recently gaining significant attention due to its ability to grow β-Ga₂O₃ films with thicknesses as high as 40 µm, with uniform and controllable Si-doping on β-Ga₂O₃ substrates or sapphire. LPCVD grown β-Ga₂O₃ films exhibit room temperature mobilities as high as 150 cm²/Vs. However, recent studies indicate the LPCVD grown β-Ga₂O₃ films are still unable to achieve high mobility at low temperatures with higher background acceptor compensation compared to MOCVD or MBE grown material. This study provides the first known full-bandgap evaluation of defect states of LPCVD-grown β-Ga₂O₃. Here we use deep level optical spectroscopy (DLOS), deep level transient (thermal) spectroscopy (DLTS), and admittance spectroscopy (AS) to identify bandgap states in LPCVD material. By comparing with our earlier work on β-Ga₂O₃ grown by other methods, the goal of this work is to identify the critical defects in LPCVD material and develop correlations with possible sources in order to guide strategies to suppress them.
A 1.2 µm thick layer of Si-doped β-Ga₂O₃ was homoepitaxially grown at 1050°C on a Sn-doped (010) Tamura Ga₂O₃ substrate using an in-house built LPCVD system using metallic gallium and O₂ sources [1]. Thin Ni Schottky diodes were fabricated to facilitate defect spectroscopy. The net doping from C-V was measured to be 2.3×10¹⁷ cm^-3 and Hall measurements on separate samples revealed electron mobilities of ~120 cm²/Vs at 300 K. Admittance spectroscopy (AS) detected a donor state at E_C-0.04 eV with a concentration of 2.3×10¹⁷ cm^-3, which matches the doping from C-V and is the likely Si donor state. AS also revealed another state at E_C-0.12 eV with a lower concentration of 9.1×10¹⁵ cm^-3 that was also seen by DLTS. Three other states were observed by DLTS, at E_C-0.18 eV, E_C-0.21 eV, and E_C-0.78 eV with trap concentrations of 2.1×10¹⁴ cm^-3, 5.7×10¹⁴ cm^-3, and 2×10¹⁶ cm^-3, respectively. The state at E_C-0.78 eV matches prior reports attributing this to Fe impurities [2] and dominates the DTLS spectrum. States deeper in the bandgap, which cannot be detected by AS or DLTS, were seen from DLOS measurements at E_C-2.0 eV, E_C-3.7 eV and E_C-4.4 eV with concentrations of 1.6×10¹⁵ cm^-3, 9×10¹⁵ cm^-3, and 1.5×10¹⁶ cm^-3, respectively. Of all six observed traps, only the deep level at E_C-3.7 eV appears to be unique in the LPCVD material compared to prior work on MBE, EFG and MOCVD-grown b-Ga₂O₃ [3,4] material, and is under specific investigation. Lighted C-V measurements are ongoing to correlate specific traps with the high degree of carrier compensation observed in this material, which will be compared with similar studies made on b-Ga₂O₃ grown by other methods, along with a detailed comparison of trap properties and concentration distributions from our prior work [3.4].
[1]. Z. Feng et al., APL Mater. 7, 022514 (2019).
[2]. J.F. Mcglone et al., IEEE Electron Device Lett. 39, 1042 (2018).
[3]. E. Farzana et al., J. Appl. Phys. 123, 161410 (2018).
[4]. Z. Zhang et al., Appl. Phys. Lett. 108, 052105 (2016).

Fe-doped Ga₂O₃ is the only commercially available but also highly desirable semi-insulating subtrates/crystal. In this work, optical properties of Fe-doped Ga₂O₃ were studied by polarized transmittance and photo-luminescence spectroscopy. The optical bandgaps of electrical field vector parallel to a* and c were 4.46 ev and 4.40 eV of (010) sample while 4.74 eV and 4.50 eV for b and c axes of (100) sample. An absorption band around 4.0 eV was observed when electrical field vector was parallel to b and a* axes. Two sharp peaks at 689nm and 697nm superimposing a broad peak at 710nm were emitted from Fe-doped Ga₂O₃ under UV laser excitation (233nm-270nm) at room temperature, while the almost zero PL intersity between 300 nm and 600nm. Temperature dependent PL showed that the broad peak diminished while two sharp peaks intensified with temperature decreasing, which matches the emission feature from transition metal Cr. ICP-MS was employing to check the existence of Cr impurity. PLE showed the 689 nm peak intensity reached its maximum when excitation wavelength was near the band gap, indicating energy transfer from electron at Ga site to this impurity site. This work demonstrates that Fe-doped Ga2O3 substrate is promising to provide zero PL background for ultra-thin epi-layer as well as to probe trace impurites.

Ga₂O₃ is an ultra-wide band gap oxide material, which promises great improvements in a range of applications, including power electronics, solar blind UV photodetectors, and gas sensing devices. Its high conductivity, high breakdown field, and large band gap have led to a flurry of research in the past few years. Most of the work has focused on its most stable form, monoclinic β-Ga₂O₃. However, Ga₂O₃ presents pronounced polymorphism and a number of other polymorphs beyond β exist, including hexagonal α-Ga₂O₃, cubic γ-Ga₂O₃, and orthorhombic ε-Ga₂O_3. Although this wealth of possible structures opens up opportunities to control and tune structure, electronic structure and ultimately physical characteristics, the polymorphs beyond β-Ga₂O₃ are comparatively unexplored. In particular experimental results are scare due to the general difficulty in producing high quality materials and the lack of theoretical results for the more structurally complex polymorphs.

Here, we present an in-depth study of the electronic structure of the α, β, γ, and ε polymorphs of Ga₂O₃. The samples investigated are either bulk single crystals or epitaxial films grown using molecular beam epitaxy (MBE) or atomic layer deposition (ALD), selecting the highest quality samples available for each of the polymorphs. We report high-resolution valence bands from hard and soft X-ray photoelectron spectroscopy (SXPS and HAXPES) which are directly compared to theoretical partial and total electronic densities of states as calculated within the framework of density functional theory (DFT). Both deep and shallow core level spectra are compared to DFT results to explore the influence of structure, rather than solely the oxidation state, on the core level behaviour. X-ray absorption spectroscopy (XAS) is used to probe the unoccupied states and in combination with SXPS is used to gain an estimate of the changes in the band gaps of the polymorphs. Ultimately, this work presents a systematic and comprehensive study of the electronic structure of Ga₂O₃ polymorphs, providing an insight into electronic trends and their relationship to crystal structure. This comparative study helps to discern trends between the different structures and advances our understanding of this polymorphic material. It lays the foundation for further exploration of Ga₂O₃ in applications beyond its β phase.

Ga2O3, as a very wide bandgap semiconductor, promises a very high critical breakdown field. The theoretically estimated breakdown field is about 8 MV/cm, and the experimetally observed values are as high as 5.4 to 5.9 MV/cm to date. To take full advantage of the bandgap of Ga2O3, it is necessary to manage the electric field distribution in a Ga2O3 device to be as slow-changing as possible. In this talk, I will review various strategies and their underlying physical principles to manage the field distribution. With these strategies, record high figure-of-merits BV^2/Ron have been demonstrated in Ga2O3 Schottky barrier diodes and fin-transistors [1,2].
[1] Wenshen Li, Kazuki Nomoto, Zongyang Hu, Debdeep Jena, and Huil Grace Xing. Field-plated Ga2O3 trench Schottky barrier diodes with a figure-of-merit of up to 0.95 GW/cm2. IEEE Electron Device Letters (2019). [2] Wenshen Li, K. Nomoto, Z. Hu, T. Nakamura, D. Jena and H. G. Xing. Single and multi-fin normally-off Ga2O3 vertical transistors with a breakdown voltage over 2.6 kV. IEEE International Electron Device Meeting (IEDM) 2019.

There is growing interest in wide and ultra-wide bandgap 2D materials such as hexagonal boron nitride (hBN) to serve as dielectric encapsulation layers for 2D heterostructures, as a host matrix for single photon emitters and as an integral part of III-nitride heterostructure devices. In the case of hBN, flakes and films have been formed by exfoliation of bulk crystals and CVD growth on metal substrates, respectively, but further advances are needed to realize wafer-scale single crystal films and expand the available range of wide bandgap 2D materials.
Our studies have focused on epitaxial growth of hBN and other 2D nitride layers by metalorganic chemical vapor deposition (MOCVD). High temperature epitaxial growth of hBN on c-plane sapphire was investigated using B₂H₆ and NH₃ precursors to achieve high purity films. Gas phase pre-reactions between B₂H₆ and NH₃ result in a significantly reduced growth rate for hBN at 1350^oC and 50 Torr in a H₂ carrier gas. Sequential precursor pulsing was used to minimize gas phase mixing and enable the growth of films with controlled thicknesses ranging from 5-100 nm. The hBN films on sapphire were subsequently used as templates for epitaxial growth of 2D transitional metal dichalcogenides such as MoS₂ and WSe₂. We have also investigated novel methods to stabilize 2D forms of conventional III-nitride materials such as GaN. This process uses controlled intercalation and reaction of group III and V precursors within the interfacial region of quasi-free standing epitaxial graphene formed on SiC to form stable ultrathin GaNx films. The graphene capping layer provides thermodynamic stabilization of a unique R3m structure as identified by aberration-corrected scanning transmission electron microscopy. Density functional theory predicts a bandgap energy in the range of 4.79-4.89 eV for this structure which correlates well with experimental results from UV-visible reflectance and absorption measurements. The realization of few layer films of group III-nitrides would broaden the range of accessible bandgap energies of 2D materials providing new avenues for scientific exploration and electronic/optoelectronic device development.

An accurate understanding of the critical electric breakdown field () characterizing semiconductor materials is necessary for the design of power switches, power diodes, and RF power transistors. It is particularly important to understand the dependence of on bandgap (E_g) as new ultrawide-bandgap materials are developed. Unfortunately, the reported dependencies of on E_g cover a surprisingly wide range in the literature. Moreover, while is often assumed to be constant for a given material, it is more accurately a function of the device depletion-region width and doping. Further, there are wide discrepancies in the literature where values for punch-through and non-punchthrough structures are compared without regard for these differences. We report a new normalization procedure that enables an equivalent comparison of values across materials, doping, and punch-through/non-punch-through device types. An extensive examination of many experimental avalanche-breakdown and ionization references reveals that the dependence ~ E_g^1.86 best fits the most reliable and newest data for both direct and indirect semiconductors over the range from E_g = 0.66 to 5.5 eV (comprising Ge, Si, InP, GaAs, 4H-SiC, GaN, and diamond). It may therefore be reasonable to use this ~ E_g^1.86 dependence as an approximate rule of thumb for predicting the critical electric fields of novel ultrawide-bandgap materials until precise measurements are made. Based on the ~ E_g^1.86 dependence, the relationship between specific on-resistance (R_ON,sp), breakdown voltage (V_BD), and E_g for power switches over this bandgap range is best described by R_ON,sp ~ V_BD² E_g^-5.58 for both direct- and indirect-gap semiconductors.

Ultra-wide bandgap (UWBG) semiconductors have extended the opportunity for exploring novel application fields in electronics and optics. Ga₂O₃, being supported by its bandgap as wide as ~5 eV, is gaining increasing interests as a material for power electronics devices. Progress of monoclinic-structured Ga₂O₃ (β-Ga₂O₃) bulks and substrates, which is the thermodynamically most stable phase in a variety of polymorphous of Ga₂O₃, grown by solution-methods has accelerated the evolutional research on high-performance power devices. On the other hand, our research has directed on semistable corundum-structured Ga₂O₃ (α-Ga₂O₃) grown on sapphire (α-Al₂O₃). One of the reasons for paying attention to α-Ga₂O₃ is that there are a variety of corundum-structured oxide materials, which can be combined with α-Ga₂O₃ to establish novel devices. Alloys and heterostructures of Al₂O₃, Ga₂O₃, and In₂O₃ can achieve heterostructure devices by arbitral tuning of the bandgaps . We have succeeded in the bandgap tuning from 3.7 to 8.8 eV, and the n-type conductivity control of α-Ga₂O₃ with doping Sn and Si, leading to the device-oriented research. A start-up company, FLOSFIA, Inc., has demonstrated Schottky barrier diodes with on-resistance as low as 0.1 mΩcm², their reasonable heat resistivity, and the low-loss power conversion. Heterojunctions of α-(Al,Ga)₂O₃ alloys show the type-I band lineup, and this allows fabrication of a variety of heterojunctions and quantum wells for novel devices. α-In₂O₃ showed the higher electron mobility compared to that of α-Ga₂O₃, and we demonstrated the operation of an MOSFET with the field-effect mobility of 187 cm²/Vs. The bandgap of α-In₂O₃ (3.7 eV) is still wider than that of GaN and SiC, suggesting that α-In₂O₃ can also be a candidate material for power devices.

A various combination of corundum structured oxides allows unique materials properties. One of the examples is above-room temperature ferromagnetic properties of α-(Ga,Fe)₂O₃ and α-(In,Fe)₂O₃. α-Rh₂O₃ and α-Ir₂O₃ are native p-type materials. Especially, α-Ir₂O₃ is closely lattice-matched to α-Ga₂O₃. We demonstrated p-type conductivity of α-Ir₂O₃, followed by fabrication of α-Ir₂O₃/Ga₂O₃ pn junction. Efforts are continueing to control the bandngap and the hole concentration of p-type α-(Ir,Ga)₂O₃.

For optical applications of UWBG oxide semiconductors, we have focused on UWBG Mg-rich (x>0.7) rocksalt-structured Mg_xZn_1-xO, whose maximum band gap can be ~7.8 eV (at x=1). This attracts its potential as deep UV light emitters in the wavelength region which cannot be reached by III-nitrides, whose maximum band gap is ~6.0 eV of AlN. We used mist CVD method for the growth of MgZnO with the carbon-free source precursors. The x-ray and electron-beam diffraction evidenced rocksalt structure without noticeable incorporation of other phases. In the TEM images, it was revealed that dislocation defects were formed in the MgZnO layer originating from the defects on the MgO substrate surface, that is, the dislocation defects density in the MgZnO layer at present is dominated by the quality of the MgO substrate. Mg_xZn_1-xO films with x=0.95 and 0.92 showed the CL peaking at 199 and 212 nm at 6 K and at 205 and 217 nm at 300 K, respectively, without noticeable luminescence at the longer wavelengths. However, the luminescence peak energies were lower by 0.5-0.7 eV than the band gap energies. This large Stokes-like shift is attributed to the local band gap fluctuation. Efforts were continued for Mg_xZn_1-xO/MgO quantum wells. For the sample with x=0.92, the CL peak blue-shifted by 1-8 nm (0.08-0.21 eV) at 6 K compared to the single MgZnO layer with decreasing the thickness of the MgZnO layer. The spectrally integrated CL intensity at 300 K over that at 6 K (I₃₀₀/I₆), which is supposed to be the internal quantum efficiency at 300 K, reached ~13%, which was apparently higher than that of the single layer. The overall results suggest that MgZnO can pave the way for deep UV photonics.

Group-III sesquioxides such as Ga₂O₃ and In₂O₃ are ultra-wide bandgap semiconductor materials. It is well known that bulk single crystals of thermally stable phases β-Ga₂O₃ (E_g = 4.5 eV) and c-In₂O₃ (E_g = 3.7 eV) can be grown by melt growth methods [1,2]. At present, high-speed growth of conductivity controlled homoepitaxial layers of β-Ga₂O₃ and c-In₂O₃ has attracted much attention to apply both materials for fabrication of vertical high-power electronic devices. In this presentation, the authors introduce high-temperature and high-speed growth of β-Ga₂O₃ and c-In₂O₃ by halide vapor phase epitaxy (HVPE) [3-7].
First, source gases and their carrier gases for HVPE growth of Ga₂O₃ and In₂O₃ were examined by thermodynamic analyses. The results clarified that by removing hydrogen atoms (H) from the system and using an inert carrier gas such as N₂, He and Ar, HVPE growth of high-quality Ga₂O₃ and In₂O₃ films becomes possible at high temperatures exceeding 1000 °C using GaCl-O₂ and InCl-O₂ systems, respectively.
Then, based on the results of thermodynamic analyses, HVPE systems for growing Ga₂O₃ and In₂O₃ were constructed. GaCl and InCl were generated in the upstream region of the reactor maintained at 800-850 °C by introducing Cl₂ gas over Ga and In metal, respectively. It was clarified that homoepitaxial growth at 1000 °C on bulk β-Ga₂O₃(001) substrates is possible at a growth rate above 10 μm/h without deterioration of crystalline quality. The unintentionally doped layer showed a low effective donor concentration (N_d - N_a) of less than 10¹³ cm^-3, and it was found that n-type carrier density can be controlled in the range of 10¹⁵ to 10¹⁹ cm^-3 by intentional Si-doping using SiCl₄. These results indicate that the HVPE method is a suitable method for producing homoepitaxial wafers for Ga₂O₃-based vertical power devices. Also in the In₂O₃ growth, heteroepitaxy at 1000 °C on (0001) sapphire substrates showed c-In₂O₃ growth at a rate of several μm/h. These results suggest that high-temperature and high-speed growth of (In_xGa_1-x)₂O₃ alloy by HVPE is also possible.
This work was partially supported by the Council for Science, Technology, and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO) and Grant-in-Aid for Scientific Research on Innovative Areas (No. 16H06417) from JSPS.

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[3] K. Nomura, Y. Kumagai et al., J. Cryst. Growth 405, 19 (2014).
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Diamond is a candidate material for next-generation power electronics and integrated circuit (IC) and micro-electro mechanical systems (MEMS) devices which operate under extreme environment. In order to use an advantage of high-density hole channel of hydrogenated diamond (H-diamond) surface, we have developed the high-k stack gate dielectrics and AlN heterojuction gate for H-diamond MOSFETs, such as HfO₂/HfO₂, LaAlO₃/Al₂O₃ Ta₂O₅/Al₂O₃, and ZrO₂/Al₂O₃, AlN/Al₂O₃ prepared by a combination of sputter-deposition (SD) and atomic layer deposition (ALD) techniques. In addition, we developed the routine ion-implantation process for preparing the diamond cantilever with a resonant frequency quality factor as high as one-million. Recently, we demonstrated the artificial diamond Fin-FETs with high-current level and the nanolaminate insulator gate MOSFET with k value as high as 100, and the new transistor concept named by metal-insulator-metal-semiconductor field-effect transistor (MIMS-FET) to achieve normally-off operation by combining the advantages of MOSFET and metal-semiconductor FET. In this presentation, we will show the comprehensive work on the diamond FET and MEMS devices.
Acknowledgements: This work was in collaboration with J-W. Liu, M. Imura, and M-Y. Liao in NIMS and partly supported by JSPS KAKENHI Grant Number JP16H06419.