Symposium EP08 : Ultra-Wide-Bandgap Materials and Devices

Monoclinic β-phase Ga₂O₃ has outstanding potential for power electronics, and high quality, large diameter bulk crystals and epitaxial layers of Ga₂O₃ are already available with a range of controllable n-type doping levels by edge-defined film-fed (EFG) growth using iridium crucibles, by Czochralski or by float zone. The direct energy bandgap of Ga₂O₃, ∼4.9 eV, yields a very high theoretical breakdown electric field (∼8 MV/cm). For power electronics, the Baliga figure-of-merit proportional carrier mobility, critical electric field and breakdown voltage, is almost four times higher for Ga₂O₃ than for GaN. Currently, the major limitation for Ga₂O₃ based device fabrication is the lack of low resistance Ohmic contacts, low damage dry etching process, and thermally stable Schottky contacts. In this work, we report a technique by employing Aluminum Zinc Oxide (AZO) to improve Ohmic contacts on Ga₂O₃, studies of etch rates and etching induced damages with Cl₂/Ar and BCl₃/Ar based discharge, surface treatment prior to Schottky metal deposition as well as Ni/Au and Pt/Au Schottky contacts, and demonstration of 2300 V breakdown voltage Ga₂O₃ Schottly diode.

Ga₂O₃ has emerged as a noteworthy ultrawide bandgap semiconductor in the past five years. Owing to excellent material properties based on an extremely large bandgap of over 4.5 eV and the commercial availability of native wafers produced from melt-grown bulk single crystals, Ga₂O₃-based electronic devices are promising candidates for various applications in power switching, RF, and harsh-environment electronics.
First, this presentation will give an overview of our state-of-the-art lateral depletion-mode (D-mode) Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs) [1]. The devices demonstrated an off-state breakdown voltage of over 750 V, a drain current on/off ratio of more than nine orders of magnitude, stable device operation at temperatures up to 300°C, and negligibly small DC–RF dispersion. Furthermore, the bulk Ga₂O₃ channel exhibited strong gamma-ray tolerance by virtue of showing very little on-resistance degradation and threshold voltage shift, thereby demonstrating the strong potential of Ga₂O₃ devices for radiation-hard electronics [2]. We will also present RF and thermal characteristics of the lateral MOSFETs [3-5].
In the second part, we will discuss our recent developments of vertical D-mode Ga₂O₃ MOSFETs [6, 7]. The devices had a current blocking layer formed by Mg- or N-ion implantation and successfully demonstrated drain current modulation by an applied gate bias. However, the devices were still at a primitive development stage and had some severe issues to be resolved.
This work was partially supported by Council for Science, Technology, and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO).

[1] M. H. Wong, M. Higashiwaki et al., IEEE Electron Device Lett. 37, 212 (2016).
[2] M. H. Wong, M. Higashiwaki et al., Appl. Phys. Lett. 112, 023503 (2018).
[3] M. H. Wong, M. Higashiwaki et al., Appl. Phys. Lett. 109, 193503 (2016).
[4] Manikant, M. Higashiwaki, M. Kuball et al., Compound Semiconductor Week 2018, May 2018.
[5] J. W. Pomeroy, M. Higashiwaki, M. Kuball et al., 60^th Electronic Materials Conference, June 2018.
[6] M. H. Wong, M. Higashiwaki et al., Appl. Phys. Express 11, 064102 (2018).
[7] M. H. Wong, M. Higashiwaki et al., Compound Semiconductor Week 2018, May 2018.

In this paper, we report performance comparisons between Silicon Dioxide (SiO₂) and Aluminum Oxide (Al₂O₃) as gate dielectrics for beta-Gallium Oxide (β-Ga₂O₃) metal insulator semiconductor transistors (MISFETs). To this end, we will present charge trapping results in MIS capacitors fabricated on commercial (-201) oriented β-Ga₂O₃ substrates using atomic layer deposition (low temperature process, for Al₂O₃ and SiO₂) and low-pressure chemical vapor deposition (high temperature process for SiO₂). Our findings indicate that the both interface trap density and ‘slow’ border trap density at ALD Al₂O₃/Ga₂O₃ interfaces are about 2x lower than that of LPCVD SiO₂/Ga₂O₃ interfaces. Despite this shortcoming, LPCVD SiO₂ gated devices have significantly lower leakage current and higher breakdown strength compared to different ALD Al₂O₃ processes, consistent with the higher conduction band offset in SiO₂/β-Ga₂O₃. For future β-Ga₂O₃ based high voltage power electronics, this factor makes SiO₂ more attractive. In addition, the significantly higher fixed/stable negative charge at SiO₂ interfaces, is conducive for normally-off MISFET channel design, which is highly desirable for power switching. To demonstrate this proof-of-concept, LPCVD SiO₂ was used as gate dielectric to fabricate lateral long channel MISFETs on a (010) oriented β-Ga₂O₃ homoepitaxial thin film grown using HVPE at Kyma Technologies (USA). While the devices were not optimized, the threshold voltages were ~10 V which indeed indicate normally-off operation. In this paper, these initial results will be complimented with results from ongoing experiments and the factors to be considered when choosing the gate dielectric in Ga₂O₃ MISFETs, will be discussed in detail.

The ultra-wide bandgap semiconductor β-Ga₂O₃ has received great attention in recent years due to its potential to become an attractive alternative to conventionally used materials for future power electronic applications. The estimated dielectric strength of 8 MV/cm in combination with the expected Baliga’s figure of merit are promising indicators to pave the way for the realization of power devices with even higher breakdown voltages and efficiencies than their SiC and GaN counterparts. Up to now several studies have demonstrated the successful realization of Schottky barrier diodes and field-effect transistors based on β-Ga₂O₃ with promising results for the development of high efficiency power converters. In order to electrically isolate such electronic devices from each other dry etching of mesa structures into the β-Ga₂O₃ is commonly carried out. On the other hand device isolation by ion implantation was shown in several reports on GaN-based devices to be an attractive alternative as it allows maintaining the planarity of the wafer surface and thus significantly improves the yield in fine-pitch lithography. However, the inter-device isolation by ion implantation of Ga₂O₃ has not yet been investigated and the applicability of this technology in terms of processing boundary conditions on β-Ga₂O₃ is still unknown to date. In this study we report on the application of multiple energy nitrogen ion implantation for the electrical isolation of electronic devices on monoclinic β-Ga₂O₃. By the introduction of uniformly distributed midgap damage-related levels in the β-Ga₂O₃ crystal lattice we are able to increase the sheet resistances by more than 9 orders of magnitude to ≥10¹³ Ω/sq which remains electrically stable up to annealing temperatures of 600 °C carried out for 60 seconds under nitrogen atmosphere. At higher annealing temperatures the damage-related trap levels are being removed causing a significant drop of the sheet resistance down to 4 x 10⁵ Ω/sq after annealing at 800 °C. This effect is preceded by a structural recovery of the implantation damages via the recrystallization of the crystal lattice at already 400 °C as verified by x-ray diffraction measurements. The extracted activation energies of the deep states responsible for the high resistive β-Ga₂O₃ layer after implantation is in the range of 0.7 eV. It shows a strong correlation with the annealing temperature dependence of the sheet resistance and thus supports the theory of a damage-induced isolation mechanism. The outcome of this work is an important step towards a more robust fabrication method of electronic devices based on β-Ga₂O₃ for high efficiency power electronics of the next generation.

We report on high performance solar blind vertical Schottky photodiode which utilizes high quality n β-Ga₂O₃ epitaxial film with an electron mobility of up to 176 cm²/Vs in the active area of the device. The growth of the β-Ga₂O₃ epitaxial layer was conducted by MOCVD on an n⁺β-Ga₂O₃(010) substrate. The layer was fabricated into a vertical Schottky photodiode with a Pt/n(-)Ga₂O₃/n⁽⁺⁾Ga₂O₃(010) structure. A 30Å semitransparent Pt metal was used on the active part of the photodiode to form good Schottky contact. The photovoltaic response of the devices showed a maximum responsivity of 0.16 A/W at 222 nm at zero bias with the corresponding external quantum efficiency (EQE) of ~87 %. The cutoff wavelength and the out of band rejection ratio were ~260 nm and ~10⁴, respectively, showing a true solar blind operation with an excellent selectivity of the device. The time response of the photodiode is in the millisecond range and has no long-time decay component which is common in the photoconductive wide bandgap devices. In this work, we will also discuss the rectifying characteristics, reverse bias leakage currents up to critical breakdown fields, and the temperature dependence of the Schottky photodiode showing its potential use for applications where high optical gains are required.

We report on the MOCVD growth of controllably doped device quality β-Ga₂O₃ and β-(Al_xGa_1-x)₂O₃ epitaxial layers with carrier concentration between 1E15 and 1E20 1/cm³. For the realization of β-Ga₂O₃ based high performance power electronics with high breakdown voltages and low on-resistance, the low background impurity concentration and high electron mobility are critical. However, one of the challenges in the MOCVD growth of β-Ga₂O₃ is the presence of unintentional background concentration which is mainly attributed to carbon and hydrogen impurities that incorporate into the epilayers primarily from the metalorganic precursors. Here, we present an in-situ and ex-situ methods used to reduce the incorporation of these impurities. Optimization of the MOCVD growth condition, including O₂ flow rate, chamber pressure, and temperature were found to be critical to control the incorporation of these impurities, hence the background concentration and electron mobility in the film. With optimal growth conditions, films with record electron mobility of up to 176 cm²/Vs and ~3500 cm²/Vs were measured at room temperature and 54 K, respectively [1]. Low background concentration of ~1.5E15 1/cm³ was obtained. A short post growth annealing of the β-Ga₂O₃ in an oxygen atmosphere was also found to reduce the background concentration in the films with no influence on the electron mobility. Si doping of β-Ga₂O₃ and β-(Al_xGa_1-x)₂O₃ alloys using silane and tetraethoxysilane (TEOS) was systematically investigated with the carrier concentration ranging between 1E15 and 1E20 1/cm³. The effect of using silane and TEOS on the incorporation of carbon and hydrogen impurities in the doped layers will be discussed. The growth of high-quality strained β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ heterostructures and superlattices on (010) β-Ga₂O₃ substrates will be presented. The structural quality, abruptness of hetero-interfaces, surface morphology and electrical properties of the heterostructures will be discussed.

[1]. Yuewei Zhang, Fikadu Alema, Akhil Mauze, James Speck, Andrei Osinsky, and Ross Miller, “MOCVD grown epitaxial β-Ga2O3 thin film with an electron mobility of 176 cm2/Vs” submitted for publication to APL materials.

In this work, we predict the formation of two-dimensional electron gas (2DEG) at ε-AlGaO₃/ε-Ga₂O₃- based heterostructures. β-Ga₂O₃ is an ultra-wide band gap semiconductor, with favorable material properties for high power applications. Being the most stable phase of Ga₂O₃ and its ease of bulk crystal growth β-Ga₂O₃ is the most studied phase of Ga₂O₃. Recently, ε-Ga₂O₃, a metastable phase, has been predicted to be polar with a large spontaneous polarization and a wide band gap. It has also been predicted that ε-Ga₂O₃ can be stabilized by epitaxial strain. Theoretical modeling of ε-Ga₂O₃ material properties is important to gauge the potential of this material. There is little information on the properties of ε-Ga₂O₃ and its alloys in literature. Ab-initio modeling of ε-Ga₂O₃ and alloys was done using density functional theory. Parameters such as spontaneous polarization, elastic constants, band offsets etc. were calculated for both disordered and ordered alloys of ε-Ga₂O₃. Calculations show that spontaneous polarization of ε-Ga₂O₃ to be 0.23 C/cm², which is much higher than other wurtzite semiconductors such as GaN and AlN¹. ε-Ga₂O₃ is also predicted to be a ferroelectric semiconductor with a switching barrier of 0.93 eV to switch the polarization field¹. Calculated band parameters indicate a large band offset at the interface, ideal for confining electrons. Band diagram calculations using Schrödinger -Poisson solver between ε-(AlGa)₂O₃(0001) /ε-Ga₂O₃(0001) show the formation of 2DEG at the heterojunction. The 2DEG carrier concentration was in the range of 1-5e13 cm^-2 at the interface. Spontaneous polarization was found to be the dominant contribution to the 2DEG formation. A 2DEG can also be formed by reversing the polarization between ε-(AlGa)₂O₃(000-1) /ε-Ga₂O₃(000-1) interface in this case, a graded back barrier with n-type doping to account for an electron source for 2DEG. The 2DEG density was found close to the ε-(AlGa)₂O₃(0001) /ε-Ga₂O₃(0001) interface. The polarization of the selective barrier layers can be flipped by applying a bias to the Schottky gate metal-2deg electrodes. The 2DEG density modulation using gate bias can be potentially used as a ferroelectric memory switch. This is first theoretical study which predicts 2DEG formation at ε-(AlGa)₂O₃(0001) /ε-Ga₂O₃(0001) interface. These results show that polar ε-Ga₂O₃ is a promising semiconductor for high frequency and high power and memory applications.

[1] SB Cho, R Mishra Applied Physics Letters 112 (16), 162101, 2018

GaN-based power devices are under intensive study in academy and industry because of superior physical properties of GaN materials, such as high critical electrical field, high electron saturation velocity. And lateral GaN HEMT or MOS-HEMT structures are favorable as the high mobility of 2DEG in AlGaN/GaN heterojunction can be transferred to low on-state resistance, many technology innovations such as p-Gate, multiple field plate, in-situ passivation have been implemented to enhance GaN power transistor performance. But till now, there is relatively large gap comparing to ideal device figure-of-merit in 1D junction case. The root cause can be traced to lack of selective area doping technology, which was used in conventional Si, and SiC power device fabrication to modulate electrical field in surface and bulk semiconductor materials. The lack of selective doping technique also makes the development of vertical GaN power devices is challenging. Since applying Magnesium ion implantation and annealing to form p-GaN is not very successful, we proposed selective trench refilling epitaxy to form p-n junction on GaN-on-Sapphire or free-standing GaN substrate, this technology involved deep GaN trench etch, and p-GaN refilling epitaxy with MOCVD. We have developed this process integration technology, and made significant progresses. Those highlighted results are listed here: 1) anisotropic trench refilling epitaxy characteristic was studied, and it was found lateral growth in (11-20) plane is much faster than the growth in (1-100) plane. 2) hexagonal device layout, instead of conventional circular shape layout, was modified to mitigate the anisotropic growth characteristic. 3) process optimizations, such as trench surface wet KOH etch, and pre-growth in-situ cleaning, were studied and high forward current density (100A/cm² at 5V forward bias) of p-n diode was achieved. 4) p-n junction diode Ion/Ioff ratio of 1x10⁶ (±5V bias) was achieved by separating regrowth interface and junction interface.

AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have emerged as promising candidates in high-power/high-frequency and sensing applications due to their excellent material properties. The tremendous success of this device technology is due to the presence of high density and high mobility two-dimensional electron gas (2DEG) at the AlGaN/GaN heterointerface. In the power switching domain, AlGaN/GaN based HEMTs are believed to make an impact similar to what Si MOSFETs did in 1970s in switching power supply applications by replacing BJTs. However, a major challenge with this device technology is the normally-ON nature which reduces their efficiency in power systems. There have been multiple instances of overcoming this issue by various (post growth) processing methods [1][2]. Here we show, for the first time, the integration of a body-diode based back-gate control in AlGaN/GaN HEMTs to shift the threshold voltage to normally-OFF regime and dynamically control the device performance with a fourth back-gate terminal. This approach is similar to the body-bias technique of threshold voltage control in CMOS [3].

We experimentally demonstrate the role of the back-gate voltage in controlling the performance of HEMTs in both ON and OFF states of the device. The HEMT structure used in this study has been epitaxially grown using metal organic chemical vapor deposition technique. To incorporate body-diode, conventional HEMT structure was epitaxially grown on p-type GaN. The role of Mg activation in the p-GaN layer on the electrical properties of the device has been studied and will be discussed. With the integration of body-diode, the 2DEG current shows modulation with the change in magnitude and polarity of the back-gate voltage. An increase in 2DEG current is measured with the application of a positive back-gate voltage and a decrease is measured under a negative back-gate bias. Moreover, a positive/negative shift in the threshold voltage is observed with the application of negative/positive back-gate voltage. The modulation of 2DEG density is attributed to the modulation of the body-diode depletion width. Along with extensive results of such modulation, a comprehensive study demonstrating 3-terminal and 4-terminal output/transfer characteristics and capacitance-voltage characteristic of this novel device structure will be presented.

References:
[1] W.-T. Lin, W.-C. Liao, Y.-N. Zhong, and Y. Hsin, “AlGaN/GaN HEMTs with 2DHG Back Gate Control,” MRS Adv., vol. 3, no. 3, pp. 137–141, Dec. 2017.
[2] M. Meneghini, O. Hilt, J. Wuerfl, and G. Meneghesso, “Technology and Reliability of Normally-Off GaN HEMTs with p-Type Gate,” Energies, vol. 10, no. 2, p. 153, Jan. 2017.
[3] V. Kursun and E. G. Friedman, “Multi-voltage CMOS circuit design, ch. 3, sec. 3.3.1, pp. 58-74,” Aug. 2006.

Current display technology is reaching its practical limitations as the Thin-Film-Transistors (TFTs) that make up displays are struggling to be reduced further in size. Emerging technologies such as Augmented and Virtual Reality are also challenged to find transparent and high-resolution displays. Nanowire (NW) Light Emitting Diodes (LEDs) are appearing as the solution due to higher efficiencies (70% vs. 5%), high reliability, and the ability to be manufactured at nanoscale. Though NW LEDs are being pursued, there are key issues in monolithically integrating Field Effect Transistors (FETs) with those LEDs. Common solutions include growth on Silicon to incorporate Complementary Metal Oxide Semiconductor (CMOS) technology, layer regrowth for High Electron Mobility Transistors (HEMTs), and deposition of classical TFTs on top of the LEDs. Nevertheless, all these approaches would sacrifice display area and device performance to introduce FETs to LEDs.

Presented here is a novel monolithic integration scheme to vertically combine GaN NW FETs with InGaN NW LEDs. Novel to this work, the layer of unintentionally doped GaN (u-GaN), serving as a template to LED growth, is used for FET channel. This makes the FET in series with an NW LED for switching. Our approach allows for straightforward fabrication, no loss of display area, and eliminates device degradation. Many different types of FETs can be realized, and here a Static Induction Transistor (SIT) is selected due to the ease of fabrication.

To start, a conventional LED structure is used consisting of sapphire, u-GaN, n-type GaN, InGaN quantum wells, and p-type GaN. The NWs are fabricated through a top-down method utilizing Reactive Ion Etching (RIE) to etch down into the u-GaN layer. The etched NWs are then immersed in a KOH solution to crystallographically wet etch the wires, shrinking the diameters along with removing etch damage. Titanium is next evaporated at the base of the wires and annealed. Annealing creates nitrogen vacancies which construct the n-i-n structure common to modern transistors. Following the anneal, Polydimethylsiloxane (PDMS) is deposited and etched back to provide an insulating layer. Nickel is then evaporated creating a 30 nm schottky gate on the u-GaN. A second layer of PDMS is then coated and etched back to reveal the NW tips, followed by a deposition of Indium Tin Oxide (ITO) for top contact. These preliminary devices show good gate control with the ability to switch on/off the NW LEDs. As the gate becomes reverse bias the depletion region of the gate pinches off the current flow through the u-GaN layer, switching off the LED. Silvaco simulations are additionally performed to investigate and model device operation. This monolithically integration of NW LED and SIT can open the door to the next generation of display technology with the ability to be fabricated at diameters smaller than 100 nm, along with the inherent ability to be fully transparent.

β-Ga₂O₃ is an emerging wide bandgap semiconductor for the next generation power devices to replace GaN and SiC. It has an ultra-wide bandgap of 4.8 eV and a corresponding high breakdown field of 8 MV/cm. However, the β-Ga₂O₃ bulk substrate has a low thermal conductivity (k) of 10-25 W/mK and thus severe self-heating effects (SHE) can be observed. In high-power electronic devices, the output power density and the maximum drain current can be significantly limited by the elevated channel temperature caused by SHE and it has become a key challenges in β-Ga₂O₃ research. To suppress severe self-heating at high powers, we herein demonstrate top-gate nano-membrane β-Ga₂O₃ field effect transistors on a high thermal conductivity diamond substrate. The devices exhibit enhanced performance, with a record high maximum drain current of 980 mA/mm for top-gate β-Ga₂O₃ field effect transistors and 60% less temperature increase from reduced self-heating, compared to the device on sapphire substrate operating at the same power. With Improved heat dissipation, β-Ga₂O₃ field effect transistors on a diamond substrate are validated using a newly developed ultrafast high-resolution thermoreflectance imaging technique, Raman themography, and thermal simulations. The work is in close collaborations with Dr. Marko Tadjer’s team at NRL and Prof. Ali Shakouri’s team at Purdue University.

Heat dissipation has become an increasingly critical technological challenge in modern electronics and photonics. To address this challenge, discovering new high thermal conductivity materials that can efficiently dissipate heat from hot spots and improve the device performance of gallium nitride based electronics are urgently needed. Recent theoretical work including ab initio has predicted a new class of thermal materials, however, experimental demonstration has been challenged by materials synthesis and thermal characterization. Here, we describe our current progress in developing and characterizing these emerging high thermal conductivity materials [1-3].

We have chemically synthesized high-quality boron phosphide single crystals and measured their thermal conductivity as a record-high 460 W/mK at room temperature[1]. We have, for the first time, experimentally measured the phonon mean free path spectra of boron phosphide and analyzed experimental results by solving three-dimensional and spectral-dependent phonon Boltzmann transport equation using the variance-reduced Monte Carlo method. The experimental results are in good agreement with that predicted by multiscale simulations and density functional theory, which together quantify the heat conduction through the phonon mode dependent scattering process.

Our finding underscores the promise of the emerging high thermal conductivity material for advanced thermal management and provides a microscopic-level understanding of the phonon spectra and thermal transport mechanisms. The study aims to enable a rational design of thermal materials and nano- to multiscale devices, including the heat management of wide-bandgap electronics.

References:
[1] J Kang, H Wu, and Y. Hu, Nano Letters 17, 7507 (2017).
[2] J. Kang, M. Ke, and Y. Hu, Nano Letters 17, 1431 (2017).
[3] Y. Hu et al, Nature Nanotechnology 10, 701 (2015).

β-Ga₂O₃, with a bandgap of 4.8 eV, is expected to be power semiconductor whose performance exceeds that of SiC and GaN. Recently, halide vapor-phase epitaxy (HVPE) has progressed rapidly and has led to the epitaxial growth of high-quality films at high growth rates. Previously, we reported the relationship between etch pits and leakage current in β-Ga₂O₃ crystals grown by edge-defined film-fed growth (EFG) Schottky barrier diodes (SBDs) on (010), (-201), and (001) and identified etch pits responsible for leakage current.[1] In this talk, we identify etch pits responsible for leakage current in HVPE-grown β-Ga₂O₃ SBDs.

The sample was a HVPE-grown 6- to 11-μm-thick epitaxial layer grown on an EFG-grown β-Ga₂O₃ (001) substrate. On the entire back face, Ti/ Au ohmic contacts were formed; on the surface side, a pixel array of Pt/ Ti/ Au Schottky contacts with a 400-μm diameter were formed. After current–voltage and capacitance–voltage measurements of the SBDs, we observed light emission inside the SBDs with a high reverse bias operation. After the measurements, the SBD contacts were etched and the etch pits were observed by differential interference contrast microscopy and atomic force microscopy.

On the HVPE-grown surface after etching, bullet-shaped etch pits were observed. They are similar to those on the EFG-grown (001) surface. [2] However, among the bullet-shaped etch pits, we found that some have a deep core near their point. In parallel, inside the SBD with a high leakage current, we observed emission spots under a high reverse bias condition. Further we have found that at the position of some emission spots, the bullet-shaped etch pits with a core at their point exist. This suggests that a bullet-shaped etch pit with a core at the point is one of leakage-current paths of the SBD.

[1] M. Kasu, K. Hanada, T. Moribayashi, et al, Jpn. J. Appl. Phys. 55, 1202BB (2016).
[2] T. Oshima, A. Hashiguchi, M. Kasu, et al. Jpn. J. Appl. Phys. 56, 086501 (2017).

Wide-band-gap semiconductors such as Ga₂O₃ and In₂O₃ are used in a wide range of technological application which include solar cells, OLEDs, high-power transistors, etc. Most of the oxides, including Ga₂O₃ and In₂O₃, show an unintentional n-type conductivity. This can be explained by their typical high electronic affinity, i.e., their conduction band are quite low with respect to the vacuum level, such that many impurities tend to act as shallow donors. On the other hand, their localized valence band is composed by O p-orbitals and lie very low in absolute energy, leading to high ionization potential. Thus, impurities with one less valence electrons than the host atoms tend to act as deep acceptors. In addition, holes tend to self-trap, forming small polarons, making it impossible to achieve p-type doping conductivity. To overcome this problem, we propose to rise the valence band by alloying Ga₂O₃ and In₂O₃ with post transition metals. Using the density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional (HSE) we calculate the alloy stability, formation enthalpies, and the band gap variation as a function of alloy concentration. We also address the band alignment between the different alloy compositions and the parent compounds and discuss the effects of alloying on the optical properties.

Ga₂O₃ is an important material for solar-blind UV photodetectors and high-power transistors. Alloying with Al, as in (Al_xGa_1-x)₂O₃, adds great flexibility to the design of Ga₂O₃-based devices through band engineering. Basic key parameters in the device design, such as band gap variation with alloy composition and band offset between Ga₂O₃ and (Al_xGa_1-x)₂O₃, are yet to be established. Using density functional theory with the HSE hybrid functional, we find that the formation enthalpies of (Al_xGa_1-x)₂O₃ alloys are significantly lower than that of (In_xGa_1-x)₂O₃, and the (Al_xGa_1-x)₂O₃ with x=0.5 can be considered as an ordered compound AlGaO₃ in the monoclinic phase, with Al occupying the octahedral sites and Ga occupying the tetrahedral sites. By adjusting Al composition, the direct band gaps of the alloys can be tuned from 4.69 to 7.03 eV for the monoclinic phase and from 5.25 to 8.56 eV for the corundum phase. The band offset of the (Al_xGa_1-x)₂O₃ alloy mainly arises from the discontinuity in the conduction band. Consequences for designing modulation-doped field effect transistors (MODFETs) based on (Al_xGa_1-x)₂O₃ /Ga₂O₃ are also discussed.

The preparation of nano-layer β-Ga₂O₃ flake by a mechanical exfoliation method and its fabrication into various types of electron and optoelectronic devices will be presented. Firstly, we will show a heterostructure n-channel depletion-mode β-Ga₂O₃ junction field-effect transistor through a van der Walls bonding with an exfoliated p-WSe₂ flake. β-Ga₂O₃ and hexagonal boron nitride heterostructure-based metal-insulator-semiconductor field-effect transistors (MISFETs) are achieved by integrating mechanical exfoliation of quasi-two-dimensional materials with dry transfer process, where nano-thin flakes of β-Ga₂O₃ and h-BN were utilized as the channel and gate dielectric, respectively, of the MISFET. We will present a β-Ga₂O₃ metal-semiconductor field-effect transistor with a high off-state breakdown voltage (344 V), based on a exfoliated β-Ga₂O₃ field-plated with hexagonal boron nitride. This heterostructured ultra-wide bandgap nanodevice shows a new route toward high power nano-electronic devices.

Zinc germanate (Zn₂GeO₄) and gallium oxide (Ga₂O₃) are wide band gap semiconductors (E_g = 4.5 and 4.9 eV, respectively) with promising applications due to their matching with UV radiation in UV photodetectors, phosphors in flat panel displays or photo catalysis. In addition, Ga₂O₃, in particular, is becoming an emergent material for high power devices while Zn₂GeO₄ is a good candidate to be used in Li-ion batteries. In both oxides, the electronic and optical properties have been investigated recently, and an active research is going on. For example, there is still controversy about the origin of the visible luminescence of these oxides, usually attributed to oxygen vacancies, which are mainstream defects in oxide materials. Furthermore, doping oxides to modify electronic conductivity or luminescence properties is also a challenge because of the interplay of impurities with native defects, which add more complexity to the physical properties. On the other hand, nanowire morphologies of a number of compounds have been recently studied on the view of particular applications, such as optical microcavities or vertical devices designs. Since optoelectronic properties are intrinsically related to the electronic levels in the band gap, high-energy electron and UV light are suitable probes to test these properties. In this work, we explore the optical properties of undoped Zn₂GeO₄ and Zn doped Ga₂O₃ and nanostructures synthetized by a thermal evaporation method. We will carry out polarization dependent luminescence and Raman measurements to get some insight into their luminescence features and their correlation with their chemical and structural configuration at atomic scale level.

Gallium oxide (Ga₂O₃) is a wide band gap semiconductor material that has been widely studied during the last three decades in the form of thin films for its high-temperature sensing properties towards oxygen and reducing gases. Its optical and sensing properties have been largely studied and several improvements for its use as gas sensor have been achieved introducing different modifications. These changes, that include surface functionalization, material doping or nanowire (NW) morphology, allow working at low temperatures where the sensing mechanisms are supposed to be deactivated. The high surface-to-volume ratio attributed to nanowire morphology decreases the power consumption of the devices, while allowing to sense at lower temperatures than thin films. In our study, devices containing a single Ga₂O₃ NW are studied as humidity sensors, working at room temperature.
β-Ga₂O₃ nanowires were fabricated via a metal-assisted vapor-liquid-solid process using a carbothermal reduction and the synthetized nanowires were structurally and optically characterized using X-ray diffraction, scanning and transmission electron microscopy and related techniques as well as photoluminescence and X-ray photoelectron spectroscopy. Measurements revealed a crystalline material, different photoluminescence emission peaks in the visible range, and a transition band gap of 4.2 eV that suggests the presence of a high density of intraband states.
Using Focused Electron Beam Induced Deposition techniques, nanowires were individually contacted for their use as gas sensors. The fabricated devices have been tested, from room temperature up to 200 C, in environments with different concentrations of relevant gases for air quality monitoring, such as nitrogen dioxide and carbon monoxide, as well as oxygen and water vapor,. Fast, stable and reproducible response was measured towards water vapor at room temperature (25 C) using power consumptions between 0.25 and 250 nW. Tests under nitrogen ambient revealed that, at room temperature, the pre-adsorbtion of oxygen ions at the NW surface is mandatory for the water vapor sensing and that oxygen, even at low concentrations, is rapidly re-adsorbed at the surface of the material in a lapse of one minute. Furthermore, the presence of carbon at the surface of the nanowires, result of the growth process, plays an important role in sensing capabilities and will be discussed.

Gas sensors with intelligent systems have become a core part of the complete Internet of Things (IoT) since they offer continuous information on the presence of specific gases in the ambient atmosphere. As a representative gas sensor, semiconducting type has attracted much attention due to their cost effectiveness, simplicity in fabrication, high response and easy integration with electronic circuits. Generally, the semiconducting gas sensors require a high operating temperature of 150–400°C for adsorption and desorption of target molecules. However, high temperature reduces sensor stability and life time due to thermally induced growth of grains, and can lead to risks of ignition when detecting flammable or explosive analytes. Furthermore, it can also affect interconnected electronics and requires high power consumption that is an important parameter for the new generation of battery-loaded wireless sensors, resulting in reluctant practical application. Hence, one of the most important challenge and issue in gas sensor society is to make a high-performance gas sensor that operates at room temperature for high stability and low-power consumption. Over the past decade, there are various approaches to enhance the sensing performance at room-temperature using metal additives or heterojunctions and two-dimensional materials. Despite these extensive efforts, there remain challenging including poor response and incomplete recovery because electronic conduction based sensing mechanism is limited by insufficient reaction energy between the analytical molecule and sensing material at room-temperature. Herein, we suggest a new strategy for a room temperature gas sensor using the ionic conduction based gas sensing mechanism. Glancing angle deposition (GLAD) method was used to fabricate highly porous SnO₂ nanorods. The ionic conduction induced by humidity was confirmed using impedance spectroscopy, I-V characteristic and XPS analysis. The relation between the sensing properties and relative humidity was systematically investigated. Our experimental results show that the response of SnO₂ nanorods to 5 ppm NO₂ has a maximum response over 1400 at RH 20%. Also, it is observed that the response rate and recovery rate are accelerated as relative humidity increases. The gas sensing mechanism can be demonstrated based on the principle of the ionic conduction, humidity sensor, and water splitting. We believe that the ionic conduction based SnO₂ nanorods open a new direction for developing the room temperature gas sensor.

Thin-film transistors (TFTs) that utilize oxide semiconductors as channel materials have become a key technology for various applications such as displays, sensors, and memory devices due to their superior characteristics, which include high mobility, low off-current, low processing temperature, and applicability to large-area production. Zn-O-N (ZnON) [1] has recently received attention as a channel material for high-mobility TFTs, especially those employed in demanding applications such as large ultra-high definition organic light-emitting diode displays. ZnON-TFTs exhibit much higher field-effect mobilities (>50 cm²/Vs) [2] than conventional oxide TFTs. However, ZnON-TFTs have some drawbacks that must be addressed, including a negatively shifted threshold voltage (V_th), a large subthreshold swing (SS), and V_th instability when stored in air. We have recently reported that Si doping of ZnON is effective for improving the switching characteristics and long-term stability of ZnON-TFTs [3].
In the present work, the effects of impurity doping on the electrical characteristics of ZnON-TFTs are further investigated by co-sputtering of Zn and other elements (Ta, Zr, or In). The results indicate that doping with Ta or Zr, both of which have a high bond-dissociation energy with nitrogen, is effective for overcoming the drawbacks of ZnON-TFTs, as in the case of Si doping [3]. In particular, Ta-doped ZnON-TFTs with an optimal doping level exhibited a high field-effect mobility of 49 cm²/Vs, improved switching behavior (less negative V_th and smaller SS), and better V_th stability than non-doped ZnON-TFTs. On the other hand, the use of In doping enhanced the mobility of ZnON-TFTs, and this could be attributed to the electron pathways formed by the broad 5s orbitals of In. In-doped ZnON-TFTs exhibited a high field-effect mobility of up to 59 cm²/Vs, although a more negative V_th was also observed. These results indicate that impurity doping is an effective approach to improving and enhancing the performance of ZnON-TFTs.

[1] Y. Ye et al., J. Appl. Phys. vol.106, p.074512 (2009).
[2] K.-C. Ok et al., IEEE Electron Device Lett. vol.36, p.38 (2015).
[3] H. Tsuji et al., AIP Adv. vol.7, p.065120 (2017).

Ga₂O₃, which is one of the ultra wide-bandgap (UWBG) semiconductors, has attracted attentions as a next-generation material of power devices [1]. Among their six different phases (α, β, γ, δ, ε, and κ), the α-phase is the most suitable for bandgap engineering due to its crystal structure of corundum, although it is the metastable phase. Alloys with α-Al₂O₃ and α-In₂O₃ enabled the bandgap engineering from 3.7 to 8.7 eV[2]. However, we must consider dislocations in α-Ga₂O₃ formed due to large lattice mismatch to sapphire substrates. In this study, epitaxial lateral overgrowth (ELO) of α-Ga₂O₃ was conducted on sapphire substrates, aiming at reducing the dislocation density in α-Ga₂O₃.
Stripe-patterned dielectric masks of SiO₂ were formed on sapphire substrates. The both widths of openings and masks were 5 μm. The various orientations of substrates (c-, a-, m- and r-planes) and openings were used. All the growth of α-Ga₂O₃ was conducted on the patterned substrates by using the mist-CVD method. GaCl₃ was adopted as a Ga source, which enables the growth rate of α-Ga₂O₃ as high as 7 μm/hour. The growth temperature was changed between 500 and 700 ^oC.
α-Ga₂O₃ was selectively grown on the openings at the temperatures higher than 550 ^oC, while the deposition on masks was observed at 500 ^oC irrespective of the orientation of sapphire substrates and openings.
When α-Ga₂O₃ was grown on c-plane sapphire substrates, the triangular stripes with {10-11} and {10-12} facets were formed for <11-20> openings, while the rectangular cross section with a (0001) top facet and {11-20} sidewalls was observed for <10-10> stripes. Although the shapes of the features depended on the orientations of substrates and openings, all the facets were formed by {10-11}, {10-12}, {11-20} and (0001), which are consistent with stacking of oxygen and gallium layers. These planes were considered to be stable under the oxygen rich growth condition by the mist-CVD method. The growth of α-Ga₂O₃ on a-plane sapphires with <10-10> stripes showed the largest ratio of the lateral and vertical growth rates, that is, 0.87. Under this condition, the coalescence of two adjacent wings was achieved. TEM observations were conducted to discuss dislocation structure in the α-Ga₂O₃. The cross sectional TEM images revealed that the dislocation density in the α-Ga₂O₃ was successfully reduced, while the dislocations on the openings were propagated without bending.
From these results, the ELO technique is useful to reduce the dislocation density in α-Ga₂O₃. We need to investigate the growth conditions with bending dislocations on openings to decrease the dislocations on openings effectively.
Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO).
[1] S. J. Peatron, et. al., Appl. Phys. Rev. 5, 01130 (2018)
[2] S. Fujita, et. al., Jpn. J. Appl. Phys. 55, 1202A3 (2016)

We report on growth studies and germanium doping of heteroepitaxial and homoepitaxial beta- Gallium Oxide using low pressure chemical vapor deposition technique. Gallium oxide has attracted a lot of attention because of its potential applications in high power devices. The main advantage of Ga₂O₃ over other wide band gap materials is the availability of a high quality commercial substrates and shallow n-type dopants. A variety of growth techniques exist for growth of Ga₂O₃ thin films including MBE, MOCVD, HVPE etc. Low pressure CVD [1] is a simple, low-cost technique to grow high-quality Gallium Oxide with sufficiently high growth rates and low impurity concentration, as the precursors are ultrapure Gallium, oxygen gas and Argon carrier gas.
To calibrate the growth rates and understand the growth regimes, we characterized gallium oxide thin films grown on sapphire substrates as a function of growth temperature, oxygen flow rate and Argon flow rate. The nominal growth rate (characterized using cross sectional SEM) varies from 2.5µm/hr to 3.5 µm/hr for temperature between 850 C – 950 C, for chamber pressure of 1.5 Torr. At low oxygen concentrations, the growth rate doesn’t show significant variation with oxygen flow rate indicating a mass transport limited growth, at the substrate temperature of 935 C. With flow rates higher than 6 sccm (Ar flow rate of 140 sccm), gallium oxide power formation was observed which resulted in much thinner films. Ga₂O₃ films were grown on both c-plane and vicinal sapphire substrates, thin films on vicinal sapphire were much smoother than c-plane wafers. Also, the roughness of these films dropped with increasing oxygen flow rate. Using a two-step growth technique, surface roughness was reduced to as low as 10 nm for a 10 micron thick LPCVD grown film. Using CV characterization on undoped films grown on Sn-doped n-type bulk substrates, background carrier concentration as low as 1e15 cm^-3 was obtained. Solid Ge source was used to explore n-type doping. Using transfer length method (TLM) structures fabricated using annealed Ti (50 nm)/ Au(100 nm)/ Ni(100 nm) metal stack, resistivity ranging from 0.1 to 7.5 ohm-cm was obtained. Temperature dependent hall mobility and carrier concentration, activation energy for Ge dopants in Ga2O3 will be presented. These results indicate the promise for controlled high quality growth of Ga2O3 with high growth rates for high performance power electronics.
References [1] S Rafique, MR Karim, JM Johnson, J Hwang, H Zhao Applied Physics Letters 112 (5), 052104

The metal-insulator- Wide- and ultra-wide-bandgap semiconductor field-effect transistors (MISFETs) are expected to realize good characteristic in terms of high breakdown voltage and low-loss features. However,it was difficult to form a thermally SiO₂ film as a gate insulating or passivation film on Wide- and ultra-wide-bandgap semiconductors. Various insulators have been investigated using atomic layer deposition (ALD), which forms films with unparalleled uniformity and reproducibility. Among those films, ALD-Al₂O₃ is an attractive candidate, having a wide bandgap of 7eV [1], a high dielectric constant of 9 [2], etc. A major challenge for the Al₂O₃ in practical applications is Al₂O₃-related bias instability (BI) needs to be reduced. The purpose of this study is to achieve this by performing high-temperature annealing after ALD-Al₂O₃.
Measurement of the bias stability is observed by shifting the flat band voltage before and after the constant-voltage. The flat-band voltage shift was estimated by alternately repeating capacitance−voltage (C-V) measurement and constant-voltage stressing of the MIS capacitors. The flat-band voltage shift of stressed Al₂O₃ MIS capacitors is approximately a Kohlrausch-type complementary extended exponential function of stress time [3], thereby enabling to estimate the maximum flat-band voltage shift (ΔV_fb,max) based on a finite-time data set. In this way, we compared theΔV_fb,max of post-deposition-anneal (PDA) temperature at 973K. This comparison was made for different substrates: Si, GaN and Ga₂O₃.
The Al₂O₃ films on Si and GaN can suppress the ΔV_fb,max by PDA at 973K better than the ΔV_fb,max of as-deposited Al₂O₃ on Si and GaN. These observations indicate that PDA at 973K caused a decrease the defects in the Al₂O₃ film and at the Al₂O₃/substrate-interface.Therefore, this Al₂O₃ is the most promising insulator for high-reliability gate insulation and passivation. In the Ga₂O₃ substrate, however, the ΔV_fb,max increases by PDA at 973K. We will investigate the optimum PDA condition and announce it on the day.
This research is supported by the “Program for research and development of next-generation semiconductor to realize energy-saving society” of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
[1] E.O.Filatova, et.al., J. Phys. Chem. C 119, 20755 (2015).
[2] A. Hiraiwa, et al., J. Appl. Phys. 117, 215304 (2015).
[3] R. Kohlrausch, Annal. Phys. 167, 179 (1854).

β-Ga₂O₃ has attracted a lot of interest for high power electronic applications [1]. Although the calculated electron mobility in Ga₂O₃ is much lower than that in GaN (300 cm²/vs vs 1200 cm²/vs), it has a four times larger Baliga figure of merit (FOM) (3400)[1] compared to GaN, due to its very large bandgap (4.8 eV). Moreover, high quality single crystal β-Ga₂O₃ can be grown economically using melt growth techniques such as edge-defined film-fed growth[2], the floating zone techniques[3] or Czochralski (Cz).[4]
Using plasma-assisted molecular beam epitaxy (PAMBE), we have demonstrated successful growth of β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ heterostructures with β-(Al_xGa_1-x)₂O₃ films having Al content up to 18% [5], [6]. We also studied Schottky diodes fabricated on these structures using Ni as the Schottky contact metal, and measured the barrier height, and the dependence of ideality factor on temperature and Al content [7].
We have also successfully achieved a wide range of electron concentration in β-Ga₂O₃ films using Sn [8] as the dopant. In addition, we have investigated Ge as n-type dopant in β-Ga₂O₃ (010) films, and obtained a wide range of electron concentration(1×10¹⁷ cm^-3 - 1×10²⁰ cm^-3)[8]. Mobility of 97 cm²/Vs was achieved for a charge density of 1.6×10¹⁸ cm^-3 using Ge as dopant. This mobility is two times higher than the mobility achieved for a similar charge density using Sn.
Using Ge as the intentional donor, we recently demonstrated modulation doping in β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ heterostructures. The formation of 2DEG was confirmed by capacitance-voltage measurements. Modulation doping field effect transistors were fabricated. A maximum current density of 20 mA/mm, with a pinch of voltage of -6V was achieved on the sample with a 2DEG sheet charge density of 1.2×10¹³ cm^-2.[9]
In this talk, I will discuss the above-mentioned progress in detail.

References
[1] M. Higashiwaki et al., Semicond. Sci. Technol., vol. 31, no. 3, p. 034001, Mar. 2016.
[2] H. Aida, K. Nishiguchi, H. Takeda, N. Aota, K. Sunakawa, and Y. Yaguchi, Jpn. J. Appl. Phys., vol. 47, no. 11, pp. 8506–8509, Nov. 2008.
[3] N. Ueda, H. Hosono, R. Waseda, and H. Kawazoe, Appl. Phys. Lett., vol. 70, no. 26, p. 3561, 1997.
[4] Y. Tomm, P. Reiche, D. Klimm, and T. Fukuda, J. Cryst. Growth, vol. 220, no. 4, pp. 510–514, 2000.
[5] S. W. Kaun, F. Wu, and J. S. Speck, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film., vol. 33, no. 4, p. 041508, Jul. 2015.
[6] Y. Oshima, E. Ahmadi, S. C. Badescu, F. Wu, and J. S. Speck, Appl. Phys. Express, vol. 9, no. 6, p. 061102, Jun. 2016.
[7] J. S. S. Elaheh Ahmadi, Yuichi Oshima, Feng Wu, Semicond. Sci. Technol., 2017.
[8] E. Ahmadi et al., Appl. Phys. Express, vol. 10, no. 4, p. 041102, Apr. 2017.
[9] E. Ahmadi et al., Appl. Phys. Express, vol. 10, no. 7, p. 071101, Jul. 2017.

Here, we present the results of a fundamental growth investigation into the effects of Al-alloying and delta doping using Si on the structural and electronic properties of β-(Al_xGa_1-x)₂O₃ heterostructures grown on semi-insulating Fe-doped β-Ga₂O₃ (010) single crystals. High quality β-(Al_xGa_1-x)₂O₃ and Si-doped β-(Al_xGa_1-x)₂O₃ epitaxial layers were grown using ultra-high vacuum pulsed laser epitaxy at 500 °C, using a KrF excimer laser (λ=248nm) operating at 4 Hz and a fluence of ~3 J/cm². Targets with an Al content ranging from x = 0.175 - 0.5 were used for undoped layers of β-(Al_xGa_1-x)₂O₃, while targets with an Al content of x = .175, .225 and a Si content of 0.1% were used for the doped heterostructures. The Al content was varied to increase the bandgap, and Si was used in the heterobarrier of β-(Al_xGa_1-x)₂O₃ for modulation doped field-effect transistors. High-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) confirm both β-(Al_xGa_1-x)₂O₃ and Si doped β-(Al_xGa_1-x)₂O₃ films. Both HRXRD and HRTEM showed we were able to obtain high quality epitaxial films of β-(Al_xGa_1-x)₂O₃ up to x = .225, which was confirmed by X-ray photoelectron spectroscopy (XPS). However, HRTEM suggests we were able to obtain β-(Al_xGa_1-x)₂O₃ up to 50%, but the stability of this phase is still under investigation. As a result of this research, we demonstrate that pulsed laser deposition can be used to grow both β-(Al_xGa_1-x)₂O₃/Ga₂O₃ and Si-doped β-(Al_xGa_1-x)₂O₃/Ga₂O₃ heterostructures for wide bandgap electronic devices and specifically, modulation doped transistors. Finally, Hall transport and capacitance-voltage will be used to characterize the heterojunction electrical properties, and XPS will be used to correlate Al content with band offset.

The intrinsic structural and electronic properties of Ga₂O₃ suggest that it may ultimately outperform existing wide-gap semiconductors such as SiC for certain metal oxide semiconductor (MOS) device applications, especially at very high power. However, as with SiC, interfacial defects at the Ga₂O₃ / gate oxide interface adversely affects the performance of Ga₂O₃-based MOS devices. The various choices of gate oxide material for Ga₂O₃ -based devices, such as SiO₂ and Al₂O₃, present their own trade-offs in terms of electrical properties (bandgap and breakdown strength) and interface abruptness and stability. Previous work on SiC has shown that a narrower transition layer at the SiC/ SiO₂ interface correlates to decreased interface trap density and enhanced channel mobility.

In this work, we discuss chemical and structural features of interfaces between Ga₂O₃ and SiO₂ and Al₂O₃ investigated using high resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning TEM (HAADF-STEM) combined with electron energy loss spectroscopy spectrum imaging (EELS SI). STEM and EELS measurements allow identification of the width, composition, and bonding characteristics of the interfacial region. Hyperspectral decomposition of EELS signals using machine learning techniques reveal components corresponding to Ga, O, and Si or Al. Gate dielectric deposition and post-deposition annealing (PDA) conditions are found to affect interface quality, with higher temperature processing correlated with interfacial roughness. The Ga₂O₃ / Al₂O₃ interface is not fully abrupt and contains an interfacial region likely corresponding to interdiffusion between Ga and Al. Additionally, rapid crystallization of the Al₂O₃ gate oxide layers, outward from the interface, was observed during TEM imaging despite remaining at a lower temperature than during the gate deposition or PDA processes. Contributions of thermal gradient, strain and radiolysis to this beam-induced crystallization effect will be presented.

Supported by ARL under Grant No. W911NF1420110.

We perform a microscopic investigation of ultra-wide bandgap (UWBG) β-Ga₂O₃ materials and interfaces using atomic resolution scanning transmission electron microscopy (STEM). Our goal is to establish fundamental understanding on the atomic to nanoscale structure and defects that directly affect the basic materials properties and device performance of β-Ga₂O₃, which is essential to advance β-Ga₂O₃ to many technologically important UWBG applications. Here, we present the details of our STEM investigation and the experimental results from various β-Ga₂O₃ materials and heterostructures. First, we will present the unique technical challenges in the STEM characterization of β-Ga₂O₃ due to its high structural anisotropy and TEM sample preparation. Second, we will show the detailed structure, formation, and dynamics of extended defects that can directly influence the properties of various homo- and hetero- β-Ga₂O₃ interfaces. Finally, we will present our ongoing development of the new STEM imaging mode that uses the electron channeling effect to image individual point defects (both intrinsic and extrinsic) in β-Ga₂O₃. The imaging mode can utilize the new-generation pixelated fast STEM detector, which can capture the channeling signals at a narrowly confined scattering angle and determine the position and structure of point defects with high precision.

This presentation is about ab initio calculations in the area of Ga₂O₃ and thereabouts. I will first discuss the anisotropy of absorption in the β phase of Ga₂O₃, and a revised mixing phase diagram of InGaO phases including strain. I will then concentrate on the newly-synthesised ε (epsilon) phase of Ga₂O₃, in particular in relation to its polarization properties (is this a pyro- or a ferro-electric ?), electronic structure (first ARPES spectra, Seebeck and Peltier cofficients), and phonons (dispersion, thermal conductivity, thermoelectric ZT, and core lineshapes).

Gallium oxide has emerged as a promising candidate for next-generation power electronics due to a number of favorable properties such as its large band gap, controllable conductivity and the availability of large single-crystal substrates grown from the melt. Despite the rapid surge of interest in this material, there are still a number of outstanding questions as to how various defects, i.e. native point defects and extrinsic impurities, influence the properties of this material. In this talk we use hybrid functional calculations to elucidate the role of common impurities that have been identified in Ga₂O₃ single crystals and epitaxial films. Particularly we discuss the role of Fe and Ir impurities, which we find to be highly soluble in Ga₂O₃ and both electrically and optically active defects depending on the conditions. Our results identify that Fe impurities act as a highly soluble deep acceptors that are commonly observed in Ga₂O₃ bulk samples, while Ir is a deep donor that can degrade optical performance in insulating samples. We discuss our results in the context of recent experimental evidence of properties correlated with the presence of these impurities.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Wide- and ultra-wide-bandgap semiconductors are promising power-device materials due to high blocking capability. The wide bandgap, however, poses a serious challenge of gate insulation and surface passivation, because the insulator barrier against carrier emission from the semiconductors is inevitably small requiring wide-bandgap insulators. Although having the widest bandgap and well established in Si industry, thermal SiO₂ is not amenable here. Therefore, atomic-layer-deposited (ALD) Al₂O₃ is the most promising insulator owing to wide bandgap (7 eV), relatively high dielectric constant (9), and good uniformity/reproducibility specific to ALD [1]. To promote the practical application of this Al₂O₃, Al₂O₃-related bias instability (BI) needs to be reduced. This study aims to achieve this through investigating the current conduction process in ALD Al₂O₃.
The current conduction in Al₂O₃ follows the space-charge-controlled field emission (SCC-FE) process [2], which, however, is hitherto only suitable for low-field analysis, imposing a limitation on its BI analysis. To solve this problem, the charged state of Al₂O₃ is assumed to dynamically change with bias voltage and is investigated by alternately measuring current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Al₂O₃ metal-insulator-semiconductor capacitors, with the stop voltage of the I–V measurements increasing successively. The Al₂O₃ charged states in the I-V measurements, except for the first one, remain unchanged until the stop voltage and, therefore, the barrier height is accurately extracted based on the static SCC-FE process. Then, the sheet of Al₂O₃ charge is estimated as a function of bias voltage and, using the flat-band voltages obtained from the C-V characteristics, the sheet of charge at the Al₂O₃/substrate interface is given as a function of bias voltage. In this way, we compared the charged states of four kinds of Al₂O₃ deposited by varying the oxidant (H₂O or O₃) and the temperature (200 or 450 °C). This comparison was made for different substrates: Si, GaN and Ga₂O₃.
The Al₂O₃ films on GaN and Ga₂O₃ have smaller barrier heights than those on Si but allow smaller leakage currents because of less Al₂O₃ positive charge. Despite a large leakage current, high-temperature H₂O-grown Al₂O₃ is found to exhibit the highest bias stability, irrespective of the substrates, due to the excellent Al₂O₃/substrate-interface stability against biasing. Therefore, together with the longest dielectric-breakdown lifetime [3], this Al₂O₃ is the most promising insulator for high-reliability gate insulation and passivation.
This research is supported by the “Program for research and development of next-generation semiconductor to realize energy-saving society” of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

[1] T. Suntola, Mater. Sci. Rep. 4, 261 (1989).
[2] A. Hiraiwa, et al., J. Appl. Phys. 119, 064505 (2016).
[3] A. Hiraiwa, et al., ibid. 123, 155303 (2018).

The recombination of photoexcited carriers with electrons or holes bound to point defects results in many experimentally observed optical phenomena in wide gap systems, such as ultra-violet, yellow, green, and red luminescence in GaN. Such processes are usually explained with reference to a particular defect state and often such states are studied systematically via computational techniques. The balance between different defect states, however, which may be metastable but relevant in the time-scale of optical processes, is frequently omitted from such analysis. Here we study how different configurations of electrons and holes, whether bound to defects in well-localised 'compact' states, or in extended 'diffuse' states, can alter the observed luminescence in GaN. For our calculations we employ the hybrid quantum mechanical/molecular mechanical embedded cluster method, which offers advantages over more commonly-applied supercell-based techniques when modelling defects in wide gap materials. The analysis regarding the balance between compact and diffuse states, however, is not dependent on the computational technique we employ. Our results allow us to account for various photoluminescence peaks observed routinely in doped and nominally undoped GaN samples. In particular, we attribute the 3.46 eV and 3.26 eV ultraviolet emission peaks to nitrogen vacancies binding compact and diffuse holes respectively, and describe processes related to gallium vacancy complexes that result in yellow, green and red luminescence. We demonstrate that the competition between these differently bound carrier states is key to understanding the luminescence properties of GaN, a point that also has implications for wide gap oxides. Indeed, we show that taking into account the diffuse states associated with oxygen vacancies in In₂O₃, ZnO and SnO₂ helps explain the different intrinsic conductivity properties of these transparent conductors.

Monoclinic gallium sesquioxide (β-Ga₂O₃) belongs to the transparent semiconducting oxides. It is distinguished by its large band gap of about 4.7 eV, which is the reason for an optical transparency range extending deep into the ultraviolet and for a high electrical break down field estimated at 8 MV/cm. Combined with the feasibility of n-type doping by Sn, Si, or Ge, β-Ga₂O₃ has great potential as a material for solar-blind photodetection and for power electronics where it might outperform GaN and SiC. To fully exploit the favorable properties of β-Ga₂O₃, single-crystalline material of high structural perfection and controlled electrical characteristics is a prerequisite. In particular, it is necessary to find out how the material can be doped in a controlled manner. Not only the selection of suitable dopants plays a role, but also the formation of compensating defects, including the introduction of unwanted impurities, must be understood.
Here we present a critical review of these doping issues. While p-type conduction might be impossible to be achieved due to intrinsic obstacles such as self-trapping of holes and large effective hole mass, intentional n-type doping has to cover wider ranges, must be reproducible and thermally stable. In this context, we discuss own and recently reported results of temperature dependent Hall effect measurements, deep level transient spectroscopy, local vibrational mode spectroscopy, electron paramagnetic resonance spectroscopy, and electronic Raman scattering investigations. Silicon, germanium, and tin are revealed to be effective-mass like shallow donors without any peculiarity such as DX behaviour. Compensation of shallow donors by gallium vacancies interacting with hydrogen is proposed as an alternative to other compensation mechanisms. In bulk crystals grown from the melt additionally transition metal impurities must be taken into account as compensating acceptors. Furthermore, we show that the deterioration of n-type doping in epitaxially grown layers may be due to extended defects rather than due to point defects and can be overcome under proper growth conditions. In cases of point defects, latest theoretical predictions of charge state transition levels and formation energies are compared with experimental values of energy levels and defect concentrations.

Beta-phase gallium oxide (β-Ga₂O₃) is attracting great interest for high power devices due to its ~4.8eV bandgap, projected ~8 MV/cm breakdown field, and the availability of native substrates. [1] Moreover, its predicted high radiation hardness makes it a prospective candidate for space applications. [2] However, due to the early stage of development, knowledge about its response in harsh radiation environment is very limited. Here, we investigate the presence and properties of defects introduced by high energy neutron irradiation throughout the bandgap of plasma-assisted molecular beam epitaxy (PAMBE)-grown Ge-doped (010) β-Ga₂O₃ homoepitaxy using deep level transient/optical spectroscopy (DLTS/DLOS). The introduction rates of individual defect states and their possible role in carrier compensation are explored as a function of neutron irradiation fluence.

The epitaxy structure consisted of a ~200 nm of n+ β-Ga₂O₃:Ge layer on Sn-doped substrate to support an Ohmic contact, followed by a lightly-doped, ~600 nm layer of β-Ga₂O₃:Ge (measured doping ~8×10¹⁶ cm^-3 ) to serve as the test layer, patterned with 8 nm Ni Schottky. The samples were irradiated with fast neutrons (> 0.5 eV) in the Ohio State Nuclear Reactor Laboratory with 2×10¹⁵ cm^-2 and 4×10¹⁵ cm^-2 fluences. The same diodes were characterized by current-voltage, capacitance-voltage, DLTS and DLOS measurements, before and after irradiation to allow convincing comparisons. The discussion below is focused on the 2×10¹⁵ cm^-2 fluence results.

The Schottky diodes showed similar leakage (on order of 10 µA/cm²) before and after irradiation. However, a significant carrier reduction of ~1.7×10¹⁶ cm^-3 was observed in the irradiated sample, indicating formation of compensating defects by irradiation which were explored by DLTS and DLOS. The pre-irradiation defect spectra consisted of states at E_C - 0.21-0.25 eV, 0.42 eV, 0.60 eV, 0.96 eV, 1.29 eV, 2.00 eV and 4.40 eV. After irradiation, DLTS detected a previously unobserved state at E_C-0.78 eV with ~10¹⁵ cm^-3 concentration. Earlier worked reported introduction of a similar state by proton irradiation in β-Ga₂O₃ materials [3]. There was also a noticeable increase of the pre-existing E_C – 1.29 eV and E_C – 2.00 eV states after irradiation. This implies that they are native, and likely responsible for carrier compensation. Theoretical studies predicted oxygen and gallium vacancies or hydrogenated-gallium complexes near these levels [4, 5]. We are currently investigating irradiation effects at the other fluence to create a comprehensive picture with respect to individual defects, which will be reported at the conference.

References: [1] M. Higashiwaki et al, Appl. Phys. Lett.100, 013504 (2012).
[2] E. Wendler et al., 11th Intl. Conf. Interact. of Rad. with Sol., 93, (2015).
[3] M. E. Ingebrigtsen et al, Appl. Phys. Lett. 112, 042104 (2018).
[4] J. B. Varley et al, J. Phys.: Cond. Mat. 23, 334212 (2011)
[5] J. B. Varley et al, Appl. Phys. Lett. 97, 142106, (2010)

Interfaces govern the behaviour of all electronic devices. Herbert Kroemer coined the famous phrase “the interface is the device” in his 2000 Nobel Prize lecture, and we are still applying tremendous effort to understand interfaces in new material generations, with wide-bandgap materials being no exception. If anything, wide bandgap materials are more vulnerable to defect states purely due to their larger bandgap. Understanding gained for the bulk behaviour of semiconductors can often not be extended to the behaviour of materials in structured film stacks were interfaces play a vital role. SiC/SiO₂ is a prototypical wide-bandgap semiconductor/dielectric interface, which represents the challenges faced by many such material systems. A multitude of different defects leads to unacceptably large defect densities exceeding 10¹³ cm^-2 eV^-1 in the vicinity of the conduction band of 4H-SiC. The management of interfacial defects still remains a topic of lively discussion and current interest.
The main reasons for the ongoing struggle in understanding and controlling such defects lies in the lack of direct probes for chemical states at interfaces. Interfaces present a special challenge for physical characterisation techniques due to the spatial confinement of defects in a narrow region, the fact that by their nature interfaces are buried beneath a variety of overlayers, and the starkly different behaviour of chemical species at interfaces compared to surfaces and bulk. Advanced X-ray spectroscopy methods can tackle some of these issues, and X-ray Photoelectron Spectroscopy (XPS) in particular can deliver great insight into interfaces as it combines both qualitative and quantitative information on elemental distributions, chemical environments, and valence states.
Here, we present a systematic study of the 4H-SiC/SiO₂ interface in industrially manufactured samples with a particular focus on the effects of nitridation in a variety of atmospheres, to reduce interface defect states. Clear differences are found in both spectroscopy and electrical behaviour after high temperature treatments in N₂, NO, NH₃ and NO+NH₃ atmospheres. Si 2p, C 1s, O 1s, and N 1s core level spectra are analysed to give a complete picture of chemical environments present in the oxide and carbide layers as well as at the interface. Several species are found only at the interface providing insight into defect states and how they are compensated by nitridation. Mixed silicon oxycarbides (SiO_xC_y) and oxynitrides (SiO_xN_y) as well as Si-C-N species from reaction of N with dangling C defects on the SiC side of the interface are identified. The findings from XPS are used to explain changes in the electrical behaviour of these device stacks.
Ultimately, the detailed understanding of advanced spectroscopy results in combination with electrical characterisation can be applied to a wide range of materials, particularly to wide- and ultra-wide-bandgap materials.

Characterization of SiC/SiO₂ interface is important for the improvement of device performance of silicon carbide metal–oxide–semiconductor field-effect transistors (SiC-MOSFETs).[1] While it is well known that conventional dry oxidation of SiC usually results in a high density of interface states and that post-oxidation annealing in a nitrogen-containing gas can improve the properties of SiC/SiO₂ interface, the mechanism of the nitridation is still poorly understood.
To investigate the density and energy depth of the interface states, deep level transient spectroscopy (DLTS) is a powerful technique.[2] While typical DLTS measurement usually employs a MOS capacitor and consequently is not capable of analyzing the spatial distribution of the interface states, the additional function to visualize the spatial distribution is hopeful to allow obtaining information on the properties of the interface states from another point of view. Based on this idea, we recently developed the measurement system to perform the DLTS measurement locally using a cantilever and visualized the distribution of interface states at nanoscales.[3] The current measurement system is based on time-resolved scanning nonlinear dielectric microscopy (tr-SNDM), which is a capacitance microscopy with a high capacitive sensitivity and an excellent time resolution. In the recent study, we reported that non-uniform contrasts with the scale of several hundreds of nanometres were observed in the map of the density of interface states (D_it) of SiO₂/SiC samples.[4] However, the spatial resolution reported in the study was not sufficiently-high compared to the expected sizes of the clusters of the excess atoms at the SiO₂/SiC interfaces.[1] Because the spatial resolution of tr-SNDM is influenced by factors such as the tip radius of the capacitance probe or the oxide thickness of the sample, refining these parameters are essential for visualizing finer structures in the distribution of interface states. In this work, we perform mapping of D_it with a higher spatial resolution using a cantilever with a tip radius of 25 nm and employing a SiO₂/SiC sample with an oxide thickness of 10 nm. The spatial resolution of tr-SNDM is also discussed based on the results obtained by TCAD simulation.
[1] V.V. Afanas' ev, A. Stesmans and C. I. Harris, Materials Science Forum, 264, 857 (1998).
[2] D. V. Lang, J. Appl. Phys. 45 (1974) 3023.
[3] Chinone et al., J. Appl. Phys. 122 (2017) 105701.
[4] Yamagishi et al., Appl. Phys. Lett. 111 (2017) 163103.

The information inside Silicon carbide (SiC) power devices such as temperature or leak current is required for reliable power electronic operations. Sensing using defects in wide bandgap materials has been intensively studied because of its potential for room-temperature quantum technologies [1]. The silicon vacancy (V_Si) in SiC has the possibility of detecting the magnetic field and temperature through the optically detected magnetic resonance (ODMR) method [2].
As a first step, we improved the sensitivity of the V_Si-based sensors due to the insignificant contrast of the observed ODMR signals in temperature sensing experiments. We exploited V_Si in 4H-SiC for measuring the magnetic field at room temperature and evaluated the spin coherence time dependence on the annealing temperature as a first step of improving the sensitivity.
We used a home-built microscope to perform measurements on 4H-SiC samples irradiated with 2 MeV electrons at a fluence of 10¹⁸ cm^-2 at room temperature. The 532 nm laser expanded by a beam expander was defocused onto the 4H-SiC sample by an oil objective. The fluorescence was collected through the same oil objective before transmitted through a beam splitter and a 900 nm long pass filter to a detector. In ODMR experiments the radio-frequency signal was generated by a signal generator and subsequently amplified by an amplifier. In annealing experiments, the 4H-SiC samples were thermally annealed in several steps from 200^oC to 800^oC.
In the magnetic sensing experiments, without applying the magnetic field, we observed the maximum intensity of the ODMR line around 70 MHz, corresponding to the zero-field splitting of V_Si in the ground state [1]. When an external magnetic field is applied parallel to the c-axis of 4H-SiC crystal, the ODMR line is split by the Zeeman effect. Additionally, the ODMR linewidth and contrast are strongly dependent on the RF power. We also observed that the revivals resulted from the spin-spin interactions between the vacancy and nearby nuclei appear in the echo modulation of V_Si by using the Hahn-echo sequence. In annealing experiments, increasing the annealing temperature resulted in the diminishing of the revivals and an increase of more than 2 times (from 1.6 µs to 3.5 µs) in the spin coherence time T₂ of V_Si corresponding to non-annealing and the annealing temperature of 800^oC. We confirmed that the ODMR of V_Si can be improved by an appropriate annealing process.
In conclusion, we proved that the magnetic field dependent ODMR of V_Si under 532 nm excitation agrees well with theoretical calculations. Associating with the temperature sensitive property of V_Si in 4H-SiC, the results presented in this report will open up an opportunity to realize highly sensitive temperature sensors.
Acknowledgments: This study was partially supported by JSP CREST Grant (No. JPMJCR1333).
[1] M. Niethammer et al., Physical Review Applied 6, 034001 (2016).
[2]A.N. Anisimov et al., Scientific Reports 6, 33301 (2016)

Diamond is a fascinating semiconductor with exceptional physical properties such as a wide band gap, a high breakdown electric field (10 MV/cm), an outstanding thermal conductivity (20 W/cm/K) and high carrier mobilities. These exceptional properties, or more precisely, the combination of some of these properties makes diamond an ideal semiconductor for high power and/or high frequency electronics which should surpass other materials like silicon, silicon carbide or gallium nitride. Numerous diamond field effect transistors are under investigation: H-terminated accumulation FET, O-terminated inversion channel FET, metal-semiconductor FET and junction FET.
In this work, we propose a new transistor concept in order to exploit the full potentialities of diamond material¹. The deep depletion concept will be described and proposed for MOSFET devices. A proof of concept of deep depletion diamond MOSFETs will be presented^1-3. Finally, the recent progresses achieved in terms of device performances will be discussed.
References
¹ T.T. Pham, N. Rouger, C. Masante, G. Chicot, F. Udrea, D. Eon, E. Gheeraert, and J. Pernot, Appl. Phys. Lett. 111, 173503 (2017).
² T.T. Pham, A. Maréchal, P. Muret, D. Eon, E. Gheeraert, N. Rouger, and J. Pernot, J. Appl. Phys. 123, 161523 (2017).
³ T.T. Pham, J. Pernot, G. Perez, D. Eon, E. Gheeraert, and N. Rouger, IEEE Electron Device Lett. 38, 1571 (2017).

Low SBH for 2DHG:
Diamond has superior properties as p-type conducting compared with other wide bandgap semiconductors. Among them P-type Schottky barrier height (SBH) is very low (< 0.1eV) on the hydrogen terminated (C-H) diamond surface in high work-function metal such as Au [1]. This property enables hole injection smoothly from metal to diamond subsurface when surface electron potential is high (surface band bends upward). It is realized by negatively charged surface or negative surface voltage bias. Then, 2 dimensional hole gas (2DHG) is produced by hole injection from metal. It is desirable for high speed FET operation [1].
High Frequency FET:
The first GHz operation in diamond [2] has been realized by metal semiconductor (MES) FET, where Au is used for source and drain contacts and Al for Schottky gate on the same C-H diamond surface. Al shows the SBH of 0.6 eV, because Al work function is lower than Au by 0.7 eV. The work function dependence of SBH indicates that C-H diamond surface has low surface states density, necessary for MOSFET. In addition to MESFET [3], MOSFETs [4,5] exhibited GHz operation up to 70 GHz [6] in f_T (cutoff frequency) and >100GHz [3] in f_max. The power density is now 3.8 Wmm^-1 [7] at 1GHz obtained by high bias voltage (~50V). Its electric field is above 2x10⁵ Vcm^-1, where the hole velocity is nearly saturated in an entire FET. The power density is lower than AlGaN/GaN HEMT, but higher voltage operation can enhance it much further.
High Voltage FET:
C-H diamond MOSFETs are uniquely designed for high-voltage (~1000V) and high-temperature (up to 400°C) operation using the high temperature (450°C) ALD Al₂O₃ as gate oxide and passivation of gate-drain (drift) region. The maximum breakdown voltages (V_B) are above 1500 V [8] obtained at long drift region (L_GD > 15 um). V_B/L_GD is ~1.0 MV/cm, which is equivalent to those of SiC MOSFET and AlGan/GaN HEMT. The drain current density is comparable to SiC, but 3-4 times lower than AlGaN/GaN at similar V_B. Conductivity in drift region must be improved by mobility enhancement.
Vertical FET:
The first vertical diamond MOSFETs have been developed using 2DHG layer on trench structure [9]. Hole conduction at the trench side wall (3-4 um depth) acts as drift region and is electrically connected to p+ substrate (drain). The drain current density of active planar area has reached to 5000 Acm^-2 [10], which is comparable to those of SiC or GaN MOSFET. Specific on resistance is 5 mΩcm² at present.

[1] Kawarada, Surf. Sci. Rep. 26 (1996) 205.
[2] Taniuchi, HK et al. EDL 22 (2001) 390.
[3] Ueda, Kasu et al. EDL 27 (2006) 570.
[4] Matsudaira, HK et al. EDL 25 (2004) 480.
[5] Russel, Moran, et al. EDL 33 (2012) 1471.
[6] Yu et al. EDL (2018).
[7] Imanishi, HK et al. MRS Fall 2018.
[8] Kawarada et al. Sci. Rep. 7 (2017) 42368.
[9] Oi, HK et al. Sci. Rep.8 (2018).
[10] Iwataki, HK et al. MRS Fall 2018.

As a semiconductor, diamond possesses many unique properties that make it attractive for the production of high performance devices such as robust, high power RF FETs. Such properties include a large bandgap of 5.5 eV, high thermal conductivity of up to 20 Wcm^-1K^-1 and high carrier saturation velocity of 2 × 10^7 cm^-1 for electrons and 0.8 × 10^7 cm^-1 for holes. Development of electronic diamond devices has been largely limited however by the immaturity of existing doping processes used to introduce mobile charge into its naturally insulating crystal structure. ‘Transfer doping’ of hydrogen-terminated diamond (H-diamond) presents a potential solution to this challenge which has allowed for the production of high performance FETs. Stability issues associated with traditional transfer doping, which relies on the presence of atmospheric species on the diamond surface, has limited the maturity of device technologies that exploit these doping techniques. More recently, various work has demonstrated the potential to improve the stability and efficiency of transfer doping in diamond utilising electron acceptor oxide materials on the diamond surface.
In this work, we apply the electron oxide acceptor material V2O5 into H-diamond FET technology and demonstrate substantial performance improvement in comparison with traditional atmosphere-exposed devices. These performance figures include the highest reported drain current and transconductance for a H-diamond FET to incorporate an electron acceptor oxide such as V2O5. Furthermore, a 400C anneal stage utilised in the process flow for devices (as required to ensure stability of the V2O5 layer) is also found to reduce the ohmic contact resistance and increase the carrier concentration beneath the gate of the devices, further improving device performance.
The potential mechanisms for this performance enhancement and future implementation of these techniques to enhance the performance and robust operation of H-diamond FET technology will be discussed.

Higher power, higher frequency RF transistors than those possible using the III-nitride semiconductors are desired for large data (high bandwidth) information transmission, highly advanced radar detection, and more efficient communication. We are developing hydrogenated diamond surface conduction field effect transistors (FETs) for use in these next generation RF systems accessing diamond’s wide bandgap (5.47 eV) extremely high thermal conductivity (> 20 W/cm) and impressive breakdown field (10 MV/cm). Our latest unpassivated, atmospheric transfer doped FETs have maintained high current density performance (up to 700 mA/mm at 10 V) with occasional testing over an 8 month period indicating very limited degradation in an indoor environment and the ability to maintain DC operation powers of ≈ 7 W/mm.

Optimizing RF surface channel FETs is most effectively done using device physics information obtained directly from the same device that the RF measurements are made on. This best connects the influence of material, fabrication steps and structure on performance. In this regard, we obtained the hole velocity in the FET channel using a delay time measurement and this is we believe the first ever discussion of velocity obtained in this way applied directly on a diamond RF FET. This will be related to carrier density and effective mobility obtained from RF FETs. RF small signal characteristics (current and power gain cutoff frequencies (Ft, Fmax)) and large signal characteristics (power, gain and efficiency) from load pull measurements will also be discussed for the same devices.

Transit time values obtained from a delay time measurement indicates these FETs have a hole velocity in the channel ≈ 5×10⁶ cm/s and drain depletion region delay ≈ 2.5 ps. On wafer RF measurements for intrinsic current gain cutoff frequency indicate an opposite trend with gate length of Ft = 70 GHz (Lg = 50 nm), 49 GHz, (100 nm), and 10 GHz, (500 nm) when Vds = 10V. With improved contact and access region resistances we expect the frequency bandwidth to increase. Load pull measurements indicate RF output power densities increased 30% as the drain and gate bias voltage pulse spacing increased (a duty cycle reduction from 5% to 0.5%) suggesting that RF output power is affected by heating of the transfer dopant in unpassivated FETs. These measurements also demonstrate a peak RF output power density of 0.66 W/mm at 2 GHz.

We have also observed that the Schottky barrier heights for the atmospheric transfer doped devices with good gate control are over 0.38 eV and range up to 0.63 eV, however, the ideality factor is quite high (between 1.5 and 7.3). This may be an indication of the roughness of the surface and challenge contacting a hydrogenated region that the gate finger metal sits on. As the gate length reduced from 3 micron down to 50 nm the FET current density increased six fold, and knee voltage reduced uniformly by 50%.

This paper will overview the status and prospects of diamond for power electronics applications. Both the potential and the current/future challenges will be discussed. The particular example of diamond Schottky diodes for power electronics will be explored in more detail. The authors have worked on diamond diodes by developing high quality substrates, low-defect doped epitaxial layers and diamond microfabrication processes. Two diode structures studied are the vertical Schottky diode and the pseudo-vertical diode. The vertical diode requires a thick (>250 µm) p⁺ doped substrate and the pseudo-vertical diode requires an undoped substrate with low dislocation defect density. Part of this effort is directed at providing substrates of p⁺ diamond to thicknesses >250 µm and providing p⁺ epi-layers for the pseudo-vertical devices. Improvement of the p⁺ epi-layer deposition was studied by reducing the particles landing on the surface during deposition and increasing the time the diamond CVD reactor can run before soot formed that required the run be terminated. The soot formation is a known problem for diamond deposition using microwave plasma-assisted CVD due to the high boron level added to the deposition process. The boron doped p^- layer is the region that provides the breakdown voltage of the Schottky diode. The p^- region needs to be deposited/grown with low dislocation defect density, controlled doping, controlled thickness and low compensation from impurities like nitrogen. The p^- layer was grown at 800C with a feedgas of hydrogen, methane (4%), oxygen and a small amount of diborane as needed for the desired p-type doping concentration. The addition of the oxygen helps to improve the quality of the epi-layer and reduce passivation of the boron doping by hydrogen. Schottky diodes fabricated showed breakdown voltages exceeding 1800 V. The 1800 V diodes showed forward current densities of up to 300 A/cm². Other diamond diode work will also be overviewed.

A diamond semiconductor with a bandgap of 5.47 eV is expected to be the ultimate power device because of its exceptional physical properties, such as a high breakdown field (>10 MV/cm), high mobility, and highest thermal conductivity [20 W/(cm K)]. We previously reported high radio-frequency (RF) power performance of diamond field-effect transistors with a power-gain cutoff frequency, f_MAX, of 120 GHz and an RF output power of 2.1 W/mm at 1 GHz. In this talk, we focus on very recent progress in two basic technologies related to diamond electronics: wafer technology and carrier doping technology.

To overcome the size limitation of diamond crystals, diamond heteroepitaxy technology has progressed rapidly. A 1-inch heteroepitaxial diamond has been demonstrated. In heteroepitaxy, epitaxial overgrowth technology is used to drastically decrease the dislocation density. Furthermore, microneedle technology has been proposed and demonstrated as a method to delaminate heteroepitaxial diamond from its substrate without cracking. [1] In the final process of heteroepitaxy technology, chemical mechanical planarization of the heteroepitaxial diamond surface is important for obtaining a damage-free and smooth surface. The full-width at half-maximum of the (004) plane was as low as 0.03°, and the curvature was 0.29 m, indicating the world’s highest quality heteroepitaxial diamond. The size and quality of diamond crystals is improving. We have fabricated diamond field-effect transistors on heteroepitaxial diamond, and the resultant device shows the same drain-current level as conventional homoepitaxial diamond on a HPHT substrate.

Concerning carrier doping technology in diamond, Kubovic and Kasu previously reported NO₂ p-type doping, which they used to fabricate FETs; that is, NO₂, O₃, NO, and SO₂ molecules adsorbed onto H-diamond generated hole carriers and the hole sheet concentration was as high as ~1 × 10¹⁴ cm⁻² at room temperature. [2] Shiraishi and Kasu explained this phenomenon as the LUMO/SOMO orbital energies in the inorganic molecules adsorbed onto H-diamond being below the valence-band top of H-diamond, resulting in electron transfer from H-diamond to these molecules. Recently, Geis and Wade at the Massachusetts Institute of Technology investigated the NO₂–H-diamond surface further by surface chemical techniques such as Fourier transform infrared spectroscopy and elucidated the surface reaction and states. Their reports surprisingly agree with our previous findings. Recently, we used synchrotron X-ray photoelectron spectroscopy/X-ray absorption near-edge spectroscopy and capacitance and conductance measurements to determine the energy band diagram and found oxygen-related states and boundary states in the metal-oxide-semiconductor interface.

References
[1] H. Aida, S. W. Kim, et. al. Appl. Phys. Express 9 (2016) 035504.
[2] M. Kasu. Jpn. J. Appl. Phys. 56 (2017) 01AA01.

Transfer doping of hydrogen terminated diamond (Diamond:H) with various molecular-like surface acceptors suffers from low efficiency and temperature instability. In contrast, high electron affinity transition-metal oxides (TMOs) (i.e. MoO₃, WO₃, V₂O₅ and ReO₃), when employed as surface acceptors for transfer doping on Diamond:H, have recently yielded improved p-type sheet conductivity and remarkable thermal stability even with only a few monolayers of coverage^1-4.
Despite these properties, the realization of Diamond:H electronic devices using TMOs remains very challenging^5,6. This is due to undesirable changes in the physical and electronic characteristics of the TMO caused by the device fabrication process. In particular, stoichiometry reduction, crystalline phase transitions and structural morphology aggregations take place.
In this work, we will discuss how different TMOs' physical parameters affect the electrical properties of the resulting diamond:H/TMO-based transistors, and how these undesirable effects can be minimized. Electrical and surface characterization monitored before and after transistor fabrication reveal TMO oxygen reduction and a change in its oxidation state leading to electrical conductivity degradation. Based on these findings, we propose and demonstrate a way to improve diamond:H/TMO transistor performance and stability.
References:
1. Russell et.al. Appl. Phys. Lett., vol. 103, no. 20, pp. 2–6, 2013.
2. Tordjman et.al. Adv. Mater. Interfaces, vol. 1, no. 3, pp. 1–6, 2014.
3. Crawford et.al. Appl. Phys. Lett., vol. 108, no. 4, pp. 1–5, 2016.
4. Tordjman et.al. Appl. Phys. Lett., pp. 1–13, 2017.
5. Vardi et.al. IEEE Electron Dev. Lett. 35, 12, 2014.
6. Yin & Tordjman et.al. IEEE Electron Dev. Lett. 39, 4, 2018.

H-terminated diamond with 2 dimensional hole gas (2DHG) provides a promising surface channel for high-power and high-frequency applications due to its excellent properties for FETs. From 2001, mile stones of RF performance of diamond FETs are first GHz operation in MESFETs [1], MISFETs with cutoff frequency (f_T) > 20 GHz [2], high power density > 2.0 W/mm [3], maximum oscillating frequency (f_max) > 100 GHz [4] and f_T > 50GHz [5]. Particularly, the power density of diamond FETs (up to 2.2 W/mm) [3, 6] is higher than those of GaAs and LDMOS. However, the operating voltage was as low as 15 ~ 20 V because of low breakdown voltage. Power density would be much more improved by realizing high voltage operation. Recently, we reported high average electric field [7] in MOSFETs with Al₂O₃ deposited as gate insulator [6, 8] and passivation layer [9] by high temperature atomic layer deposition (ALD) [10]. In this work, we fabricated ALD-Al₂O₃ 2DHG diamond MOSFETs, whose structure is capable of withstanding high voltage, and evaluated small signal and large signal performance at high voltage operation (|V_DS| ≦ 60 V). As a result, the highest power density of 3.8 W/mm was obtained in diamond.
We fabricated ALD-Al₂O₃ 2DHG diamond MOSFETs with 100 nm Al₂O₃ film on IIa-type polycrystalline diamond substrate with a <110> preferential growth surface. The source-gate length, gate length (L_G) and gate width were fixed to 0.5, 0.5 and 100 μm, respectively and gate-drain length (L_GD) was ranged from 1 to 3 μm.
The drain current density (I_DS) was −730 mA/mm at V_GS = −20 V and V_DS = −40 V and the transconductance was 15 mS/mm at V_GS = 12 V and V_DS = −40 V for L_GD = 1 μm. Extrinsic f_T and f_max, including parasitic pad capacitances and inductances, were 30 GHz and 27 GHz at V_GS = 16 V and V_DS = −60 V for L_GD = 3 μm. f_T = 30 GHz @ L_G = 0.5 μm corresponds the saturation velocity of 1x10⁷ cm/s. It is the first report that diamond FET reaches its saturation velocity. The large signal performance was evaluated using load pull system. The bias point for A-class operation were V_GS = 12 V, V_DS = −50 V and I_DS = −405 mA/mm, respectively. The power density reached 3.8 W/mm at 1 GHz with associated gain of 11.6 dB and power added efficiently of 23.1 % for L_GD = 2 μm. The power density is the highest ever reported for diamond FETs.
[1] H. Taniuchi, H. Kawarada et al: IEEE Electron Device Lett. 22 (2001) 390.
[2] H. Matsudaira, H. Kawarada et al: IEEE Electron Device Lett. 25 (2004) 480.
[3] M. Kasu et al: Electron. Lett. 41 (2005) 1249.
[4] K. Ueda, M. Kasu et al: IEEE Electron Device Lett. 25 (2004) 480.
[5] S.A. Russell, D. Moran et al: IEEE Electron Device Lett. 33 (2012) 1471.
[6] K. Hirama, H. Kawarada et al: IEDM (2007) 873.
[7] H. Kawarada et al: Scientific Reports 7 (2017) 42368.
[8] M. Kasu et al: Applied Physics Express 5 (2012) 025701.
[9] D. Kueck, E. Kohn et al: Diamond and Related Materials 18 (2009) 1306.
[10] A. Hiraiwa, H. Kawarada et al: J. Appl. Phys. 112 (2012) 124504.

In the field of power devices, diamond’s intrinsic physical properties suggest that they can facilitate ultimate device performance. Just as for other wide-bandgap materials, the availability of wafer-size diamond substrates with high single crystal quality is an indispensable prerequisite. There are two approaches which start from opposite points: First, homoepitaxial growth on carefully selected rather small single crystals with minimum dislocation densities can be performed in such a way that the available area is increased step by step while the dislocation density is kept low. By a similar approach, 4H-SiC has progressively been scaled over 20 years starting from small Acheson platelets to 6” wafer size [1]. The second alternative is based on heteroepitaxy which starts on large areas but with high dislocation densities (DDs). The challenge here consists in a controlled decrease of the DD by several orders of magnitude while preserving the initial size.
This presentation is focused on heteroepitaxy of diamond on Ir/YSZ/Si(001) which has recently provided the first single-crystal diamond wafer with a diameter > 3.5” and a total weight of 155 carat [2]. All relevant steps for the wafer preparation will be described. First applications of the material will be presented and the potential for further applications will be discussed.
[1] T. Straubinger, R. Eckstein, M. Vogel, A. Weber, presented at 7th International workshop on Crystal Growth Technology, Potsdam, July 02-06 2017.
[2] M. Schreck, S. Gsell, R. Brescia, M. Fischer, Sci. Rep. 7, 44462 (2017).

Diamond is a transparent wide band gap material with outstanding optical and electronic properties that are attracting a lot of attention for the development of the next generation of devices. Indeed single crystal diamond provides an ideal host material to incorporate different types of impurities that can drastically modify its properties. The use of dopants such as boron can for example allow tuning the electrical conductivity of the film up to the metallic conduction which could allow to produce highly boron doped substrates and develop vertical components whose design and architecture for the realization of more complex function is simpler. In addition nitrogen or silicon are some of the elements that can be introduced in the crystal in order to create optically active centres such as the well-known NV (nitrogen-vacancy) and SiV (silicon-vacancy). Both defects exist in different charge states that can be stabilized depending on the doping level of the diamond.
In this presentation, we will focus more specifically on the production aspects of doped monocrystalline diamond films by chemical vapour deposition assisted by microwave plasma with either boron or nitrogen, highlighting all the constraints inherent to the targeted field of application. In the case of boron doping, particular attention will be paid to showing the plasma conditions which it is essential to maintain in order to obtain a sufficiently thick and doped film leading to on state resistances compatible with their use in vertical components. It will be shown in particular the importance of the gas composition to inject high microwave power allowing coupling high material quality with high growth rate. With regard to nitrogen doping, the conditions for optimizing the formation and orientation of NV colour centres will be discussed and the role of temperature, substrate orientation and gas composition will be highlighted.

To realize next-generation power devices and highly sensitive quantum sensors, heteroepitaxy of diamond on Si substrates is a key technology from the viewpoint of scalability and Si CMOS hybrid system [1-3]. We utilize 3C-SiC as an intermediate layer between Si substrates and diamond films because it can be directly grown on the Si and the lattice constant and the surface energy of the SiC are close to the diamond.
We would like to introduce the heteroepitaxial growth of diamond on both Si (001) and (111) substrates by original antenna-edge type microwave plasma CVD with in-situ bias current monitoring during bias enhanced nucleation(BEN). We show the properties of the diamond films, and then the potentials for both power devices (Schottky barrier diodes) and quantum sensors.
Concerning the schottky barrier diodes (SBDs) on heteroepitaxial diamond (001) films, the specific on-resistance of 0.2 Ω−cm² and high rectification of 10⁸ (±5 V) were obtained which are comparable for SBDs on homoepitaxial diamond films.
The sensor devices using nitrogen-vacancy (NV) centers were formed in the heteroepitaxial diamond (111) films. The NV centers could be preferentially aligned of the NV axis to the one direction and be leading to improving the sensitivity was confirmed.
This work was supported in part by JST-CREST Grant No. JPMJCR1333, KAKENHI (17H01262 and 18H01472), and JSPS Bilateral Open Partnership Joint Research Projects.

[1] M. Schreck , et al., Sci. Rep.7, 44462 (2017).
[2] H. Kawarada, et al., JAP 81, 3490 (1997).
[2] J. C. Arnault, et al., APL 90, 044101 (2007).
[3] J. Yaita, et al., APEX 10, 045502 (2017).
[4] T. Suto, et al., APL 110, 062102 (2017).
[5] J. Yaita, et al., APEX11, 045501 (2018).

Diamond’s extreme properties make it a prime candidate for next-gen electronic devices, including high frequency and high power operation. While monolithic diamond devices based on doped monocyrstalline layers, enabling both unipolor as well as well as bipolar design schemes, have great potential, the currenly obtainable substrate size and quality leave several years of scientific and technological development. The fact that diamond has an extremely high thermal conductivity and can be deposited on non-diamond substrates, opens up an intermediate pathway for hybrid power devices that integrate diamond with other wide bandgap materials such as SiC or GaN. The main goal in such cases is to achieve an enhanced reliability of the latter active materials by providing superior cooling minimizing thermal hotspots and withstanding large break down electric fields.

Here, the case of diamond on GaN-based HEMT structures is considered. First, the deposition of thin CVD diamond layers will be discussed, including the surface chemistry governing the nanodiamond particle seeding, the deposition conditions, and the use of different microwave-based CVD techniques, including resonant cavity and linear antenna technology. Cross-sectional TEM and EELS mapping of the stacks are employed to discuss the structural and morphological properties and to extract possible changes in composition in the underlying Si₃N₄/AlGaN/GaN interfacial layers induced by the substrate temperture during diamond deposition. Finally, the evaluation of the effective thermal conductivity of the thin diamond layers is discussed. The contactless transient thermoreflectance technique, used to study the heat spreading capabilities, shed light on the thermal conductivity and interfacial thermal boundary resistance.

Semiconducting diamond is an attractive candidate for the next generation of high voltage and high frequency power devices, thanks to his exceptional properties in terms of wide bandgap, high breakdown field and thermal conductivity. In the literature, several diamond-based field-effect-transistors (FETs) have already revealed good on state performance and high blocking voltage capability (~2kV) in a wide range of operating temperatures. The possibility of generating an inversion regime in diamond metal-oxide-semiconductor FET (MOSFET), and the new Deep Depletion regime (D2MOSFET) specific to wide bandgap semiconductors pave the way for a new generation of power devices. The critical part of the transistor is the gate oxide, with electrical charge traps located within the oxide or at its interface with the semiconductor. These traps can screen the gate potential and shi