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Thermodynamic analyses for the growth of group-III sesquioxides, including α-Al2O3, β-Ga2O3, and c-In2O3, by both ozone and plasma-assisted MBE were performed. In either case, under O-rich conditions, the driving force for III2O3 (III = Al, Ga, In) growth ( Δ P III 2 O 3 ) increased with increasing input partial pressure of the group-III metal ( P III o ), without generation of metal droplets. Conversely, under group-III-metal-rich conditions, Δ P III 2 O 3 decreased with increasing P III o and/or decreasing input partial pressure of O3 or O. This decrease was caused by the formation of Ga2O or In2O during growth of β-Ga2O3 and c-In2O3. The decrease of Δ P Al 2 O 3 was smaller because the equilibrium constant of α-Al2O3 formation reaction was very large. Ga and In droplets formed at low temperatures (2O3 < β-Ga2O3 << α-Al2O3.

The heteroepitaxial growth of In2O3 on a (0001) α-Al2O3 substrate was carried out using the mist chemical vapor deposition method. The concentrations of In2O3 powder in the source precursor solution varied between 0.025 and 0.250 mol l−1. An increase in the growth rate was observed with the increasing In2O3 powder concentration, and the highest growth rate of 4.2 μm h−1 was obtained at a powder concentration of 0.150 mol l−1. An improvement in the electrical properties was eventually observed, i.e. the lowest carrier concentration of 3.1 × 1017 cm−3 and the highest Hall mobility of 241 cm2 V−1 s−1 at room temperature were obtained at a powder concentration of 0.150 mol l−1. The temperature-dependent Hall effect measurement for the sample showed pronounced carrier freeze-out in a temperature range of 300–30 K, indicating that the α-In2O3 film had near-non-degenerate conductivity.

In metalorganic vapor phase epitaxy of β-Ga2O3 using triethylgallium (TEGa) and O2 as precursors and Ar as the carrier gas, the gases directly above the substrate were sampled and analyzed by time-of-flight mass spectrometry. TEGa was found to decompose at 400 °C–600 °C via β-hydrogen elimination reaction to generate gaseous Ga, hydrocarbons (C2H4, C2H2, C2H6), and H2. When β-Ga2O3 was grown at temperatures greater than 1000 °C and with input VI/III ratios greater than 100, the hydrocarbons and H2 were combusted and CO2 and H2O were generated. The C and H impurity concentrations measured by secondary-ion mass spectrometry in the β-Ga2O3(010) homoepitaxial layer grown under these conditions were less than their respective background levels. Thus, to grow β-Ga2O3 without C and H contamination, conditions that favor the complete combustion of hydrocarbons and H2 generated by the decomposition of TEGa should be used.

β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (0 1 ¯ 0)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

The electrical properties of beta-gallium oxide (β-Ga2O3) and gallium arsenide semiconductors were characterized using the emerging terahertz time-domain ellipsometry (THz-TDE) technique. The dielectric and conductivity properties were obtained from the complex ratio of the measured p- and s-polarized THz pulses reflected from the samples. The carrier concentration and mobility were then deduced using the Drude model, and the results showed good accuracy. This work demonstrates THz-TDE as a promising tool for characterizing semiconductors, especially those with high carrier concentrations and significant absorption in the THz region.

Enhancement-mode (E-mode) vertical $\beta $ -Ga2O3 metal–oxide–semiconductor (MOS) field-effect transistors featuring a current aperture were developed on a single-crystal $\beta $ -Ga2O3 (001) substrate. Nitrogen ions were implanted into a drift layer grown by halide vapor phase epitaxy to form current blocking layers (CBLs) for vertical source–drain isolation, while Si ions were implanted to form degenerately doped source contact regions and a top-gated lateral channel that was fully depleted at 0-V gate bias. The devices delivered a high output current on/off ratio of $2\times {10}^{{7}}$ despite a nonideal MOS interface that limited the maximum drain current density to

Thermodynamic analyses of β-Ga2O3 growth by both ozone and plasma-assisted molecular beam epitaxy (MBE) were performed. In either case, the growth mechanism was found to differ depending on whether the input VI/III ratio was above or below 1.5. Under O-rich conditions (VI/III > 1.5), the driving force for β-Ga2O3 growth ( Δ P Ga 2 O 3 ) was determined to increase linearly with increasing Ga input partial pressure ( P Ga o ) because almost all the supplied Ga was used for the growth of the β-Ga2O3. In contrast, Ga-rich conditions (VI/III Δ P Ga 2 O 3 to decrease. Etching of the β-Ga2O3 occurred with increasing P Ga o due to the formation of volatile Ga2O. This work also demonstrated that the use of ozone allowed growth at higher temperatures than the use of O radicals. The calculated results were in good agreement with experimental values, indicating that β-Ga2O3 growth by MBE can be explained by thermodynamics.

The influence of growth rate on the homoepitaxial growth of AlN at 1450 °C by hydride vapor phase epitaxy on bulk AlN(0001) substrates was studied. X-ray diffraction and Raman spectroscopy revealed that high structural quality comparable to that of the initial substrate can be achieved even when the growth rate is increased to over 150 μm h−1. Although the concentration of Si impurities increased with increasing growth rate, a freestanding AlN substrate prepared from a homoepitaxial layer grown at 155.6 μm h−1 showed a steep optical absorption edge at 207 nm and high optical transmittance at longer wavelengths.

We investigated the monoclinic β-Ga 2 O 3 ultrawide-bandgap semiconductor using transmission terahertz time-domain spectroscopy (THz-TDS). To characterize the anisotropic properties, the complex refractive index of a semi-insulating bulk and of an n-type Si-doped homoepitaxial film were obtained along the [100] and [010] directions. The material response can be well-described by the Drude-Lorentz model, from which the static permittivity and the electrical properties were determined in good agreement with electrical contact measurements.

Depletion-mode vertical Ga2O3 metal-oxide-semiconductor field-effect transistors featuring a current aperture were developed on a halide vapor phase epitaxial drift layer grown on a bulk $\beta $ -Ga2O3 (001) substrate. Three ion implantation steps were employed to fabricate the ${n}^{++}$ source regions, lateral ${n}$ channel, and ${p}$ current blocking layers, where Si and N were selected as the donor and deep acceptor dopant species, respectively. The transistors delivered a drain current density of 0.42 kA/cm2, a specific on-resistance of 31.5 $\text{m}\Omega \cdot \text {cm}^{2}$ , and an output current on/off ratio of over 108. High-voltage performance of the present devices was hampered by a large gate oxide field in the off-state causing high gate leakage, a limitation that can be readily overcome through optimized doping schemes and an improved gate dielectric. The demonstration of a planar-gate vertical Ga2O3 transistor based on a highly manufacturable all-ion-implanted process greatly enhances the prospects for Ga2O3-based power electronics.

This study presents vertical Ga2O3 Schottky barrier diodes (SBDs) with a staircase field plate on a deep trench filled with SiO2. It was clarified from device simulation that at high reverse voltage operation, the staircase field plate and the deep trench can effectively alleviate electric field concentration in the Ga2O3 drift layer and the SiO2 layer, respectively. The Ga2O3 SBDs successfully demonstrated superior device characteristics typified by an on-resistance of 7.6 mΩ cm2 and an off-state breakdown voltage of 1.66 kV. These results offer the availability of the trench staircase field plate as an edge termination structure for the development of Ga2O3 SBDs.

In this paper, we investigated the effects of the growth temperature and chlorine gas on the growth of thick GaN on an N-polar GaN substrate via tri-halide vapor phase epitaxy. The results revealed that free Cl2 is necessary for high-speed growth. When the growth temperature increases, the crystal quality improves, and a high growth rate and high crystalline quality can be simultaneously achieved.

High-temperature AlN homoepitaxial growth up to 1600℃ by HVPE was investigated. High-purity AlN with excellent crystallinity was reported where HVPE-AlN was homoepitaxially grown on the PVT-AlN substrate. However, quartz-derived impurities incorporation was slightly problematic in the HVPE-AlN. In this study, a new reactor was introduced that is a quartz glass reactor including a high-temperature growth zone constructed with heat-resistant materials. O and Si impurity incorporation was reduced with high growth rates of around 150 μm/h by the newly introduced high-temperature growth system. This work was partially supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan.

The development of high-power and reliable deep-UV optical devices requires low dislocation density AlN substrates. In this study, thick AlN layers were homoepitaxially grown on PVT-AlN(0001) substrates by HVPE method at a growth rate above 100 μm/h. The grown layer showed low dislocation densities of less than 1E4 cm^-2 and a high deep-UV optical transparency. Recent expansions in both diameter of the substrate and size of the growth reactor have enabled mass-production of 2-inch-diameter HVPE-AlN substrates for deep-UV optical devices. This work was partially supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan.

Gallium oxide (Ga 2 O 3 ) bulk substrate and homoepitaxial layer were investigated using terahertz time-domain spectroscopy in the frequency region from 0.2 to 3.0 THz and with polarization along the $a$ crystal axis. From the transmittance measurement, the refractive index spectra of the bulk substrate and the epilayer were obtained. The carrier density, electron mobility, and resistivity of the epilayer were then extracted by employing the Drude-Lorentz model.

The influence of substrate orientation on homoepitaxial growth of beta-gallium oxide by halide vapor phase epitaxy was investigated. Substrates were cut at various angles Δb from the (001) plane ( Δb = 0°) to the (010) plane ( Δb = 90°) of bulk crystals grown by the edge-defined film-fed growth method. The growth rate increased with increasing absolute value of Δb near the (001). However, from the (001) to the (010), as Δb increased, the growth rate decreased sharply, and streaky grooves observed in the grown layer on the (001) substrate became triangular pits. The length of the pits decreased with increasing Δb, and a pit-free homoepitaxial layer grew at Δb ≈ 60°. The valley line of the pits was parallel to the [010] direction; therefore, the length of the pits decreased with increasing Δb. In addition, transmission electron microscopy observations of the deepest part of a pit revealed that the pits originate from dislocations propagating in the substrate at an angle of 60° with respect to the (001) plane. Therefore, pits are not formed on the grown layer surface when the Δb of the substrate is ∼60°, because its surface is substantially parallel to the dislocations. The homoepitaxial growth of a pit-free layer on the (011) substrate ( Δb = 61.7°) was demonstrated, and void defects and dislocations in the substrate were confirmed by the etch-pit method to not be inherited by the homoepitaxial layer.

Gallium oxide (Ga 2 O 3 ) is an attractive material for next-generation power device applications due to its large breakdown electric field $(> \mathbf{7\ MV}/\mathbf{cm})$ . This paper highlights current-aperture vertical Ga 2 O 3 metal-oxide-semiconductor field-effect transistors fabricated using silicon- and nitrogen-ion implantation processes.

Homoepitaxial growth of β-Ga2O3 on β-Ga2O3 substrates by halide vapor-phase epitaxy (HVPE) using GaCl and O2 was investigated by both thermodynamic analysis and growth experiments. The thermodynamic analysis clarified that growth of Ga2O3 is expected at high temperatures around 1000 °C using an inert carrier gas. The experimental results revealed that homoepitaxial growth of unintentionally doped (UID) layers with a low effective donor concentration (Nd − Na) of less than 1013 cm−3 is possible at 1000 °C on β-Ga2O3 (001) substrates with a high growth rate of up to 28 μm/h. Furthermore, HVPE growth of intentionally Si-doped β-Ga2O3 layers was investigated by supplying SiCl4, which revealed that n-type carrier density almost equal to the Si-doping concentration can be controlled in the range of 1015–1018 cm−3. The carrier mobility decreased with increasing Si impurity concentration and was about 150 cm2/V·s at room temperature for a layer with a carrier density of 3.2 × 1015 cm−3. Thus, the intentionally Si-doped homoepitaxial layers grown on β-Ga2O3 substrates can be applicable for the production of β-Ga2O3-based power devices.

Gallium oxide (Ga2O3) is an emerging material for power electronics. The final penetration in the market is limited by several issues, including a stable and effective isolation between different devices and between different regions of the same device. In this work, we analyze lateral and vertical isolation structures, obtained by Mg implantation and annealing at 1000°C in Halide Vapor Phase Epitaxy β-Ga2O3. By means of repeated current-voltage characterization, it is possible to detect a severe current collapse, which can be completely recovered by white light illumination. When a constant bias is applied, the current collapse increases in magnitude at higher bias, showing a stronger filling of the deep levels. The transients closely follow the stretched-exponential model, an indication that the charge trapping is originated by extended defects, mini-bands or surface states. From the recovery transients carried out at various temperatures, it is possible to extrapolate a dominant thermal activation energy of 0.34 eV. The results of the recovery transients under monochromatic illumination show gradual variation in a broad energy range, consistent with the presence of extended defects. Temperature-dependent current-voltage characterization highlights the good performance of the bulk isolation and the presence of a significant surface leakage. Long-term stability tests show that the lateral structure is able to withstand a higher voltage level before catastrophic failure, but is less stable and is affected by a time-dependent degradation process. Charge trapping at the surface may act as a field-limiting element and partially explain the experimental findings.

Gallium oxide $(\mathbf{Ga}_{2}\mathbf{O}_{3})$ is an emerging ultra-wide-bandgap semiconductor especially suitable for high-power and/or high-voltage switching device applications. In this paper, device fabrication process and characteristics of depletion-mode and enhancement-mode vertical $\mathbf{Ga}_{2}\mathbf{O}_{3}$ transistors fabricated by using a multiple ion-implantation doping process are discussed.

Homoepitaxial growth of β-Ga2O3 layers by halide vapor phase epitaxy (HVPE) using O2 or H2O as an oxygen source was investigated by thermodynamic analysis, and compared with measured properties after growth. The thermodynamic analysis revealed that Ga2O3 growth is expected even at 1000 °C using both oxygen sources due to positive driving forces for Ga2O3 deposition. The experimental results for homoepitaxial growth on (0 0 1) β-Ga2O3 substrates showed that the surfaces of the layers grown with H2O were smoother than those grown with O2, although the growth rate with H2O was approximately half that with O2. However, in the homoepitaxial layer grown using H2O, incorporation of Si impurities with a concentration almost equal to the effective donor concentration (2 × 1016 cm−3) was confirmed, which was caused by decomposition of the quartz glass reactor due to the presence of hydrogen in the system.

The growth of twin-free single-crystal cubic-indium oxide (c-In2O3) layers was investigated by halide vapor phase epitaxy on c-plane sapphire substrates with various off-axis angles. The growth rate of the c-In2O3 layer increased and twin formation was suppressed as the off-axis angle of the substrate was increased. A single-crystal c-In2O3(111) layer grown on a sapphire substrate with a 5° off-axis angle showed a room temperature carrier density and mobility of 1.4 × 1016 cm−3 and 232 cm2 V−1 s−1, respectively. Temperature-dependent Hall measurements of the layer revealed that the mobility is dominated by optical phonon scattering.

A hydrogen-environment anisotropic thermal etching (HEATE) method, which is a top-down etching process with the advantages of high controllability, large-area adaptability, and low damage to the etched surface, was applied to c-In2O3 layer. The etching characteristics under the HEATE conditions using a H2 flow at various temperatures and various low total pressures were investigated using (1 1 1) c-In2O3 layers grown by halide vapor phase epitaxy on (0 0 0 1) sapphire substrates. The onset temperature of etching at a low pressure of 5.7 × 10−4 atm and the onset total pressure of etching at 800 °C were approximately 600 °C and 2.0 × 10−5 atm, respectively. Etching occurred without the formation of In droplets on the surfaces under the investigated conditions. The etching rate for c-In2O3 increased with increasing temperature and pressure and could be controlled with nanometer-scale order in the range from 1 to 75 nm min−1. The etching rate estimated from the thermodynamic analysis was in good agreement with the experimental results, indicating that the experimental results follow thermodynamics.

Silicon doped homoepitaxial films were grown on beta‑gallium oxide (001) substrates by halide vapor phase epitaxy using gallium monochloride, oxygen and silicon tetrachloride gases as precursors. It was confirmed that the n-type carrier density at room temperature was almost equal to the doped silicon concentration, which was controlled in the range of 1015 to 1018 cm−3. In the doped film with the carrier density of 1 × 1016 cm−3, the activation energy and the mobility at room temperature were 45.6 meV and 145 cm2/V⋅s, respectively. The carrier scattering mechanism in the low carrier density film was dominated by optical phonon scattering with the phonon energy of 33 meV. These results suggest that the doped homoepitaxial film grown by halide vapor phase epitaxy is a high quality film with good crystallinity comparable to bulk crystals.

The effect of the growth conditions on halide vapor phase epitaxy of In2O3 on sapphire (0 0 0 1) substrates was investigated. Only the most thermally stable phase c-In2O3 grows at growth temperatures of 400 to 1000 °C. The growth rate increased as the growth temperature increased up to 700 °C, and layers with rough surfaces and a preferred (1 0 0) orientation were grown. Above 700 °C, the growth rate became constant, the preferential orientation changed to (1 1 1), and layers with smooth surfaces were grown. At 1000 °C, the volume fraction of the (1 1 1)-oriented domains in the grown layer reached 99.0%, although there were in-plane twins rotated by 180°. The growth rate also increased as the input partial pressure of the InCl or O2 source gas was increased, and a high growth rate exceeding 10 μm/h was found. The layer grown at 1000 °C was of high purity and incorporated no impurities other than Cl. Optical transmission measurements of this layer showed high optical transmittance at energies below the optical gap of 3.47 eV.

The growth of thick InGaN layers on free-standing GaN (0001¯) substrates was studied using tri-halide vapor phase epitaxy. It was found that high-indium-content InGaN can be grown under higher InCl 3 input partial pressure at higher growth temperature, which allows the fabrication of a high crystalline quality InGaN layer with a smooth surface morphology. Using the growth conditions of high InCl 3 input partial pressure and high growth temperature, crack- and droplet-free InGaN layers with a thickness of over 10 µm and with an indium fraction of 0.05 were successfully grown. Although the surface showed many hillocks, the number of hillocks was reduced upon growth of thicker InGaN layers. Photoluminescence measurements confirm that thick InGaN layers could be successfully grown without degradation of the crystalline quality.

Tri-halide vapor phase epitaxy (THVPE) of thick GaN using GaCl3 was investigated for fabricating low-cost, high-crystalline-quality GaN substrates instead of the conventional manufacturing method of GaCl-based hydride vapor phase epitaxy (HVPE). The growth rate and upper growth temperature limit of GaN using THVPE were found to be much higher than those obtained using conventional HVPE under the same growth conditions. Drastic reduction in the number of dark spots measured by cathodoluminescence at room temperature was observed for the high-temperature-grown GaN layer on the (000-1) GaN/sapphire template due to the enhancement of precursor migration on the growing surface. It was found that the incorporation of impurities such as O, C, and Cl can be reduced even on the N-polarity GaN by increasing the growth temperature. The possibility of enlargement of the crystal diameter by growing the N-polarity GaN layer using THVPE was also proposed.

Ga 2 O 3 is attractive for power electronics owing to its wide bandgap of 4.5 eV and the availability of economical melt-grown native substrates. Normally-off vertical power switches are highly sought-after since they allow for superior field termination and current drive at the device level while ensuring fail-safe operation and simplified designs at the system level. Capitalizing on ion-implantation technologies for donor (Si) [1] and deep acceptor (N) [2] doping of Ga 2 O 3 , we have demonstrated depletion-mode (D-mode) vertical Ga 2 O 3 MOSFETs [3] in which Si-ion (Si+) implanted top $n^{++}$ source contacts are electrically isolated from the bottom drain contact by a N-ion (N++) implanted current blocking layer (CBL) except at an aperture bounded by CBLs through which drain current $(I_{\mathrm{D}})$ is conducted. The CBL also serves as a back-barrier for a top-gated lateral channel defined by another Si+ implantation step. Based on this manufacturable all-ion-implanted structure, this paper presents accumulation-mode normally-off vertical Ga 2 O 3 MOSFETs by appropriately designing the channel doping to control the threshold voltage $(V_{\mathrm{T}})$ without requiring fundamental process modifications.

High speed growth of cubic-In 2 O 3 by halide vapor phase epitaxy was investigated on c-plane sapphire substrates with various off-axis angles. The growth rate was found to increase with increase of the off-axis angle. In addition, it was found that (111) oriented single crystal layers could be grown when off-axis angle of the substrate was at and above 5°, while polycrystalline layers with mixed domains of {100} and {111} were grown when off-axis angle was less than 5°. A smooth layer showing n-type electron concentration and mobility of $5.4\times 10^{16}\mathbf{cm}^{-3}$ and 177 cm2V−1/s−1 at room temperature was obtained on the sapphire substrate with off-axis angle of 5°.

$\beta$ -Ga 2 O 3 has captured attention in recent years for power electronics. Opportunities also exist for $\beta$ -Ga 2 O 3 transistors to operate as radio-frequency amplifiers. For highly-scaled devices, strong confinement of the channel is critical for mitigating short-channel effects. In this work, a depletion-mode $n$ -channel $\beta$ -Ga 2 O 3 MOSFET with a back-barrier formed by implantation of deep nitrogen acceptors was demonstrated. Despite being fabricated on a conductive base material, the device delivered a drain current density of 103 mA/mm commensurate with the channel charge density and a large output current on/off ratio of $4\times 10^{9}$ , both of which attested to the efficacy of back-barrier isolation. Pulsed current-voltage measurements revealed the presence of electron trapping under the gate manifested as a threshold voltage shift. The dominant charge trapping effect was mitigated instead of exacerbated by applying a large reverse gate bias - a behavior consistent with field-assisted trap emission. These results indicate a need for further process optimization to improve the speed and reliability of the device.

Thermodynamic analysis and experimental demonstration of β-Ga2O3 growth by metalorganic vapor phase epitaxy using triethylgallium (TEG) and oxygen (O2) precursors were performed. Thermodynamic analysis revealed that the O2 supplied is preferentially used for the combustion of hydrocarbons and H2 derived from TEG. Therefore, the use of high growth temperatures and high input VI/III ratios is essential for the complete combustion of hydrocarbons and H2, and β-Ga2O3 growth. The use of an inert gas as the carrier gas was also determined as necessary to grow β-Ga2O3 at high temperatures. Based on these results, a ( 2 ¯ 01) oriented smooth β-Ga2O3 layer could be grown on a c-plane sapphire substrate at 900 °C with a growth rate of 1.4 μm h−1 at an input VI/III ratio of 100. The grown layer showed a clear optical bandgap of 4.84 eV, and impurity concentrations of hydrogen and carbon were below the background levels of the measurement system.

We report a positron annihilation study using state-of-the-art experimental and theoretical methods in n-type and semi-insulating β - Ga 2 O 3. We utilize the recently discovered unusually strong Doppler broadening signal anisotropy of β - Ga 2 O 3 in orientation-dependent Doppler broadening measurements, complemented by temperature-dependent positron lifetime experiments and first principles calculations of positron-electron annihilation signals. We find that split Ga vacancies dominate the positron trapping in β - Ga 2 O 3 single crystals irrespective of the type of dopant or conductivity, implying concentrations of at least 1 × 1 0 18 c m - 3.

Homoepitaxial film and semi-insulating bulk β-Ga2O3 with (001) orientation were studied using terahertz time-domain spectroscopy (THz-TDS) in the frequency region from 0.2 to 3.0 THz parallel to the [100] and [010] directions. The static permittivity of the bulk was determined to be 10.0 and 10.4 along the a-axis and b-axis, respectively, and the refractive index values at 0.2 THz are 3.17 and 3.23 for each axis. The electrical resistivity of the epilayer was extracted with good accuracy by employing the Drude–Lorentz model and without the use of electrical contacts. This noninvasive and contact-free material evaluation through THz-TDS proves to be a powerful tool for probing and obtaining various types of information about β-Ga2O3 materials such as bulk and thin films for the development of β-Ga2O3-based device applications.

An anomalous diode-like turn-on behavior was observed in the drain characteristics of current aperture vertical β-Ga2O3 transistors. This phenomenon was attributable to an electron barrier created by negative fixed charges in the aperture opening, through which electrons were funneled from the gated channel to the drift layer. Electrostatic analysis for deriving the turn-on voltage yielded effective sheet charge densities on the order of 1011–1012 cm−2. The charged species was conjectured to be acceptor-like point defects diffusing from nitrogen-implanted current blocking layers with an activation energy consistent with migration of gallium vacancies. These results alluded to a possible role of point-defect diffusion in the performance and reliability of ion-implanted Ga2O3 devices.

The advent of acceptor-type doping of β-Ga2O3 through ion-implantation of nitrogen has opened a new design space for junction-type devices with estimated breakdown voltages in excess of a few kVs. However, the presence of deep states due to intrinsic defects in β-Ga2O3 and implantation damage could be detrimental to the performance and reliability of such devices. We give a phenomenological description and experimental demonstration of the effects of nitrogen implantation in a buried blocking layer on the performance of transistors. The partial activation of acceptor-like states in the buried implanted region has been revealed and estimated to be ∼20% through a junction spectroscopic technique involving substrate-bias and sub-bandgap illumination, which remains elusive to standard characterization techniques. The characterization technique, along with a space-charge model of the channel and band model of the buried implanted layer, has revealed the presence of photosensitive mid-bandgap (∼2.47 eV below the conduction band) and tail states near the valence band edge of nitrogen-implanted β-Ga2O3.

The influence of growth temperature on Ga2O3 growth by atmospheric-pressure halide vapor phase epitaxy was investigated on sapphire and β-Ga2O3 substrates. In the growth-temperature range of 700–1000 °C, the growth rate of Ga2O3 was in agreement with that estimated by thermodynamic analysis under the assumption of growth under thermal equilibrium. However, when the growth temperature was lower than 700 °C, the growth rate, which decreased as the growth temperature decreased, deviated from that estimated by thermodynamic analysis, reflecting growth behavior under nonthermal equilibrium. X-ray diffraction and optical absorption measurements of the grown layers revealed that the Ga2O3 growth under nonthermal equilibrium was constrained by the crystal structure of the substrate, i.e., the metastable phase α-Ga2O3(0001) grew on the sapphire (0001) substrate, whereas the stable phase β-Ga2O3 grew homoepitaxially on a β-Ga2O3(001) substrate. However, under thermal equilibrium, the growth of the stable phase β-Ga2O3 occurred irrespective of the substrate and the constraint from the crystal structure of the substrate was no longer observed. We also observed that in the β-Ga2O3 homoepitaxial layer grown under nonthermal equilibrium, crystal twinning occurred in the homoepitaxial layer, presumably due to an insufficient growth temperature.

Deep-level luminescence at 3.3 eV related to the presence of Al vacancies (V Al ) was observed in room temperature photoluminescence (RT-PL) spectra of homoepitaxial AlN layers grown at 1450 °C by hydride vapor-phase epitaxy (HVPE) and cooled to RT in a mixture of H 2 and N 2 with added NH 3 . However, this luminescence disappeared after removing the near surface layer of AlN by polishing. In addition, the deep-level luminescence was not observed when the post-growth cooling of AlN was conducted without NH 3 . Secondary ion mass spectrometry (SIMS) studies revealed that although the point defect density of the interior of the AlN layers remained low, the near surface layer cooled in the presence of NH 3 was contaminated by Si impurities due to both suppression of the surface decomposition by the added NH 3 and volatilization of Si by decomposition of the quartz reactor walls at high temperatures. The deep-level luminescence reappeared after the polished AlN wafers were heated in presence of NH 3 at temperatures above 1400 °C. The surface contamination by Si is thought to generate V Al near the surface by lowering their formation energy due to the Fermi level effect, resulting in deep-level luminescence at 3.3 eV caused by the shallow donor (Si) to V Al transition.

AlGaN alloys are the building blocks for deep UV optoelectronics and high-power devices. It has been demonstrated that the highest crystalline quality AlGaN films with high Al content are obtained on AlN single crystal substrates. The resulting mismatch between AlGaN and AlN causes a compressive strain within the AlGaN layers, which varies with composition. Nevertheless, pseudomorphic AlGaN films with Al content higher than 50%, and dislocation densities lower than 104 cm-2 have been achieved, sustaining compressive stresses with thicknesses exceeding 3 µm. Such results demonstrate the advantages of using AlN substrates for this technology, and at such some have been realized on several deep-UV optoelectronics applications. Kinoshita et al. have demonstrated UV LEDs emitting at 265 nm with output powers exceeding 80 mW. UV LEDs grown on these native substrates have higher reliabilities and higher output powers. In addition, optically pumped lasers emitting at wavelengths between 230 nm and 280 nm that display cavity modes and single polarized-state emission with low lasing thresholds have been developed. Nevertheless, there are several limitations related to the performance and further improvement of the active regions as related to their quantum efficiencies and the polarization of their emission is desired. These limitations will be classified in two main categories: (1) identification and control of point defects, and (2) efficient doping. One of the main limitations based on point defects is the one related to the UV absorption band present at 265 nm in AlN. This relatively broad absorption band limits the use of the substrate within the deep UV range and several complicated fabrication procedures were devised to overcome this limitation. In this work, the origin of this absorption band will be discussed, along with other related optical features and the use of co-doping as a way of removing this undesired absorption. Experimental and theoretical results that explore the optical properties of native and impurity point defects as well as point-defect complexes in AlN with respect to the 265 nm absorption band will be presented. Theoretical results, based on DFT, are compared to photoluminescence and absorption measurements of AlN samples grown by physical vapor transport, metal-organic chemical vapor deposition, and hydride vapor-phase epitaxy . Our results demonstrate that there are multiple sources of a deep-UV absorption between 4.5 and 5.0 eV. The presence of the isolated carbon substitutional CN - produces an absorption band centered at 4.7 eV and emission at 3.9 eV, thus its reduction leads to the reduction of the absorption band. In addition, one mechanism that contributes to the apparent UV transparency when the material is co-doped with Si is discussed. These schemes produced transparent AlN wafers suitable for deep UV LED production. The other limitation relates to obtaining technologically useful n-type conductivities in high Al-content AlGaN within the low and high doping regimes. Besides dislocation density reduction, identification and elimination of compensating defects are necessary to achieve this goal. AlGaN films with high Al mole fractions (0.650.8 was primarily caused by an increase in activation energy. This is due to the formation of a Si DX-center, thus inspiring the search for alternative dopants. Preliminary data on Ge doping of AlGaN will be presented to assess its effectiveness as a donor throughout the whole composition regime. In addition, for samples grown with different Si doping levels it is observed that up to a critical Si concentration, the carrier concentration was always proportional to the Si source flow. Any further increase led to a free carrier concentration reduction. PL spectra exhibited deep defect-related peaks corresponding to transitions between Si and deep acceptor states. For Si concentrations less than an Al-content dependent critical concentration, a higher energy peak dominates the spectra. This peak was related to a (VAl-complex)2- based on DFT calculations. For Si concentrations greater than the critical concentration, a (VAl)3- related lower energy peak dominates the spectra. The energy levels associated with these defects increased with Al composition where the energy increase was the same as the increase in band gap. Acknowledgement: Partial financial support from NSF (DMR-1312582 and ECCS-1508854), DARPA (W911QX-10-C-0027), ARPA-E (DE-AR0000299), ARO (W911NF14C0008) is greatly appreciated. All PVT AlN wafers used for homoepitaxial growth were supplied by HexaTech, Inc., Morrisville, NC.

Growth of Si-doped AlN layers by hydride vapor phase epitaxy on AlN/sapphire templates prepared by metalorganic vapor phase epitaxy and bulk AlN substrates prepared by physical vapor transport was investigated using silicon tetrachloride (SiCl4) as a doping gas. On the AlN/sapphire templates, when the SiCl4 supply was low, the incorporation ratio of Si decreased due to the influence of O impurities resulting from the decomposition of the sapphire. In addition, pits were formed on the surfaces, and the pit density increased significantly at Si concentrations exceeding 2 × 1019 cm-3. In contrast, on the bulk AlN substrates, the Si concentration increased linearly with increasing SiCl4 supply, and a pit- and stress-free layer could be grown with a Si concentration as high as 6.5 × 1019 cm-3. Hall effect measurements revealed that this layer exhibited n-type conductivity with a donor activation energy of 253 meV. However, the carrier density at room temperature (RT) was as low as 3.6 × 1013 cm-3 because of the high compensation ratio due to the presence of acceptors. A broad peak centered at 3.3 eV was observed in the RT photoluminescence spectra of the Si-doped AlN layers grown on the bulk AlN substrates, indicating that Al vacancies formed by the Fermi level effect due to Si doping acted as acceptors that compensated for carriers.

Unintentionally doped n-type beta -Ga2O3 becomes highly resistive after annealing at high temperatures in oxygen ambient. The annealing process also induces an electron paramagnetic resonance (EPR) center, labeled IR1, with an electron spin of S=1/2 and principal g-values of g(xx)=2.0160, g(yy)=2.0386, and g(zz)=2.0029 with the principal axis of g(zz) being 60 degrees from the [001](*) direction and g(yy) along the b-axis. A hyperfine (hf) structure due to the hf interaction between the electron spin and nuclear spins of two equivalent Ga atoms with a hf splitting of similar to 29G (for Ga-69) has been observed. The center can also be created by electron irradiation. Comparing the Ga hf constants determined by EPR with corresponding values calculated for different Ga vacancy-related defects, the IR1 defect is assigned to the double negative charge state of either the isolated Ga vacancy at the tetrahedral site (V-Ga(I)(2-)) or the V-Ga(I)-Ga-ib-V-Ga(I) complex. Funding Agencies|DFGGerman Research Foundation (DFG) [FR2833/63-1]; HLRN [hbc00027]; Institute of Global Innovation Research, Tokyo; University of Agriculture and Technology, Japan

Ga2O3 has exploded onto the semiconductor landscape for next-generation power electronics because its enticing material properties, most notably a large critical field strength stemming from its ultra-wide bandgap of 4.5 eV, promise miniaturized circuits and systems with high conversion efficiency. High-quality native substrates produced by melt-growth methods offer an economical platform to this technology. Ga2O3 transistors reported to date have predominantly taken on a lateral geometry, yet vertical switching devices are desirable for applications that demand high voltage ratings and high power levels since they allow for large current drives, simplified thermal management, and superior field termination. Capitalizing on cost-effective ion implantation technologies for shallow donor (silicon) and deep acceptor (nitrogen) doping of Ga2O3 with efficient dopant activation at a low thermal budget, we developed both depletion-mode (D-mode) [1] and enhancement-mode (E-mode) [2] current aperture vertical Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs). The vertical Ga2O3 MOSFETs were fabricated on low-doped (~1016 cm-3) halide vapor phase epitaxial drift layers grown on single-crystal β-Ga2O3 (001) substrates [3]. In these devices, silicon ion-implanted top source contacts were electrically isolated from the bottom drain contact by a nitrogen ion-implanted current blocking layer (CBL) [4] except at an aperture bounded by CBLs through which drain current was conducted. The CBL simultaneously served as a back-barrier for a top-gated lateral channel defined by another silicon ion implantation step. D-mode devices delivered a drain current density of 0.42 kA/cm2, a specific on-resistance of 31.5 mΩ·cm2, and an output current on/off ratio of over 108. E-mode devices, whose positive threshold voltage was attained by appropriately designing the channel doping concentration such that the channel was fully depleted at 0-V gate bias, showed a high output current on/off ratio of 2×107 despite a nonideal MOS interface that limited the maximum drain current density to 2. Both D-mode and E-mode devices did not exhibit current collapse in pulsed measurements when subjected to off-state voltage stress; however, their hard breakdown occurred prematurely owing to leakage through the CBLs, a deficiency expected to be readily overcome with an optimized nitrogen implantation process. Further details regarding fabrication, performance, and physics of the devices, as well as potential directions for future development, will be discussed in the talk. The demonstration of planar-gate vertical Ga2O3 transistors based on a highly manufacturable all-ion-implanted process greatly enhances the prospects for Ga2O3-based power electronics applications. [1] M. H. Wong, K. Goto, H. Murakami, Y. Kumagai, and M. Higashiwaki, IEEE Electron Device Lett. 40, 431 (2019). [2] M. H. Wong, H. Murakami, Y. Kumagai, and M. Higashiwaki, Proc. 77th Device Research Conference (2019). [3] H. Murakami, K. Nomura, K. Goto, K. Sasaki, K. Kawara, Q. T. Thieu, R. Togashi, Y. Kumagai, M. Higashiwaki, A. Kuramata, S. Yamakoshi, B. Monemar, and A. Koukitu, Appl. Phys. Express 8, 015503 (2015). [4] M. H. Wong, C.-H. Lin, A. Kuramata, S. Yamakoshi, H. Murakami, Y. Kumagai, and M. Higashiwaki, Appl. Phys. Lett. 113, 102103 (2018).

Historically, the exploration of III-V compound semiconductors has begun with small bandgap materials and proceeded to large bandgap ones in recent years, that is, from GaAs-based compounds to GaN-based ones. We consider that gallium oxide (Ga 2 O 3 ) is no exception in following this history and is poised to become the next mainstream of compound semiconductor research due to its attractive material properties based on an extremely large bandgap of about 4.5 eV [1]. This bandgap energy is not only much larger than those of representative wide bandgap semiconductors such as GaN and SiC but also unique among single-crystal semiconductors. Furthermore, Ga 2 O 3 has another important advantage for commercialization over the other wide bandgap materials in that large-size, high-quality bulk single crystals can be synthesized by melt growth methods, thus allowing native substrates to be produced at a relatively low cost [2]. Recently, these two features have drawn much attention to Ga 2 O 3 , resulting in a rapid expansion of the Ga 2 O 3 community.