Search

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

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.

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.

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.

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.

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.

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.

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.

$\beta$ -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.

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.

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.

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.

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.

We developed a coherent Raman microspectroscopy that realizes a three-dimensional visualization of the spatial distribution of optical phonon modes in wide-bandgap semiconductors. This microspectroscopy will be a powerful tool for measuring the carrier concentration in semiconductors.

β-Ga 2 O 3 is being actively pursued for power devices owing to its wide bandgap of 4.5 eV and the availability of melt-grown native substrates for high quality epitaxy. Depletion and enhancement mode Ga 2 O 3 metal-oxide-semiconductor field-effect transistors (MOSFETS) reported to date have been implemented as lateral devices. For high voltage and high power ratings, vertical topologies are preferred since chip area utilization is more efficient and device operation is insensitive to surface effects. This paper presents the first experimental demonstration of a vertical Ga 2 O 3 MOSFET, wherein the source was electrically isolated from the drain by a current blocking layer (CBL) except at an aperture opening through which drain current (I DS ) was conducted [1]. Modulation of I DS was effected by gating a channel above the CBL. Similar to Si and SiC technologies, this planar device structure was fabricated with no regrowth steps. The buried CBL, which acted as a back barrier for the channel, was formed by Mg-ion (Mg++) implantation doping in light of the anticipated deep acceptor nature of Mg in β-Ga 2 O 3 . Despite large source-drain leakage due to an unoptimized CBL, successful transistor action was realized.

AlN layers were grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) at 1100 °C with a source gas supply sequence of (1) NH3 preflow or (2) AlCl3 preflow. An Al-polarity AlN layer without inclusion of a N-polarity region was grown when AlCl3 was preflown to the sapphire surface prior to AlN growth, while N- and Al-polarity regions were both present in the same AlN layer when NH3 was preflown, since growth was performed on a nitrided sapphire surface. Compared with the AlN layers grown with NH3 preflow, the Al-polarity AlN layers grown with AlCl3 preflow had improved crystalline structural quality, a low concentration of oxygen impurity, and a photoabsorption edge energy of 6.08 eV, which is close to an ideal value. Therefore, the source gas supply sequence has a significant influence on the growth of AlN layers on (0001) sapphire substrates. Thus, preflow of AlCl3 gas to a sapphire surface prior to AlN growth is a key process for high crystalline quality AlN layer growth with uniform Al-polarity on (0001) sapphire substrates by HVPE.

We succeeded in fabricating HVPE-grown Ga2O3 FP-SBDs with a record Vbr/sub> of over 1 kV. This is an important step in the research and development of Ga2O3 power devices toward practical applications and future commercialization. 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).

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and farinfrared active modes. We apply our model approach to monoclinic $\beta$-Ga$_2$O$_3$ single crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and ($\bar{2}$01), are investigated by generalized spectroscopic ellipsometry within infrared and farinfrared spectral regions. We determine the frequency dependence of 4 independent $\beta$-Ga$_2$O$_3$ Cartesian dielectric function tensor elements by matching large sets of experimental data using a point by point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infared and farinfrared active transverse optic phonon modes with $A_u$ and $B_u$ symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in $\beta$-Ga$_2$O$_3$. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in $\beta$-Ga$_2$O$_3$ and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and farinfrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for $\beta$-Ga$_2$O$_3$., Comment: 18 pages, 12 figures, 105 references; Presented at IWGO 2015 Nov. 4th, Kyoto, Japan

The morphology, structural and optical properties of void-assisted freestanding HVPE-AlN films were investigated by a combination of non-destructive microscopic and spectroscopic techniques. The freestanding approximately 80 μm thick clear film has a wurtzite crystalline structure with remarkable properties around the central film region. The E 2 (high)-phonon frequency coincides with reported stress-free film phonon frequency. The low temperature luminescence study of the growth and interface sides of the film is consistent with the incorporation of a high concentration of oxygen impurities. These results are promising as the growth method amenable to the production of freestanding stress-free large area substrates for epitaxial growth.

A vertical β-Ga2O3 metal–oxide–semiconductor field-effect transistor featuring a planar-gate architecture is presented. The device was fabricated by an all-ion-implanted process without requiring trench etching or epitaxial regrowth. A Mg-ion-implanted current blocking layer (CBL) provided electrical isolation between the source and the drain except at an aperture opening through which drain current was conducted. Successful transistor action was realized by gating a Si-ion-implanted channel above the CBL. Thermal diffusion of Mg induced a large source–drain leakage current through the CBL, which resulted in compromised off-state device characteristics as well as a reduced peak extrinsic transconductance compared with the results of simulations.

Thick freestanding GaN films were grown by hydride vapor-phase epitaxial method on both pattern-masked and unmasked GaAs substrates. Both approaches resulted on films characterized by a large excess of free carriers at room temperature. The use of a GaAs back surface NiTi substrate protective layer increased the concentration of incorporated iron impurities and yield semi-insulating films. The morphology and crystalline quality of these films show a strong dependence on nucleation layer growth approach and Fe doping concentration. Near infrared photoluminescence and Raman scattering verified the incorporation and activation of the Fe impurities, and its effect on the free carrier concentration.

Gallium oxide (Ga2O3) possesses excellent material properties especially for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured by using simple methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this report, we describe the recent progress in development on fundamental technologies for Ga2O3 devices, covering wafer production from melt-grown bulk single crystals, homoepitaxial thin-film growth by halide vapor phase epitaxy, as well as device processing and characterization of metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes.

The new wide-bandgap oxide semiconductor, gallium oxide (Ga 2 O 3 ), has gained attraction as a promising candidate for power device applications because of its excellent material properties and suitability for mass production. The Baliga's figure of merit of Ga 2 O 3 is expected to be much larger than those of SiC and GaN due primarily to Ga 2 O 3 's extremely large bandgap of 4.5∼4.9 eV, which will enable Ga 2 O 3 power devices with higher breakdown voltage (V br ) and efficiency than SiC and GaN devices [1]. The other important advantage of Ga 2 O 3 is that large, high-quality bulk single crystals can be grown by using melt growth methods. Recently, we developed a homoepitaxial growth technique for high-purity Ga 2 O 3 thin films on single-crystal Ga 2 O 3 substrates by halide vapor phase epitaxy (HVPE) [2, 3]. This is the first report on Ga 2 O 3 Schottky barrier diodes (SBDs) with epitaxial Si-doped n−-Ga 2 O 3 drift layers grown by HVPE.

We studied experimentally the crystallization of a low-temperature (LT)-GaN buffer layer and its effect on the subsequent crystalline quality of the GaN epitaxial layer. The degree of crystallization was sensitive to the thickness and the temperature ramp rate of the GaN buffer layer. When the LT-GaN buffer layer was more than 100 nm thick, there was relatively little crystallization of the buffer layer and subsequently poor crystalline quality of the GaN epitaxial layer. The optimum crystallization condition for the GaN buffer layer just prior to high-temperature growth was found when the single-crystal regions in the GaN buffer layer occupied a volume 40% as large as the randomly aligned matrix. We could reduce the threading dislocation density in the GaN epitaxial layer by inserting the LT-GaN buffer layers in periodically. To explain this result, we propose a mechanism by which the periodic layer reduces the density of threading dislocations.

We carried out Nd:YAG pulse laser-assisted metalorganic vapor-phase epitaxy (LMOVPE) of InGaN at low temperatures in order to obtain films with high indium content. The results suggest that the reaction rate between group-III source gas and ammonia is enhanced by the pulse laser irradiation. Moreover, it is found that pulse laser may enhance the surface migration of the elements, and crystalline quality becomes good. These results imply that LMOVPE using Nd:YAG pulse laser is useful for the low-temperature growth of InGaN.

In 0.53 Ga 0.47 N film was grown at 600 °C by Nd:YAG pulse laser assisted MOVPE. The optical and structural properties of the film were compared with that grown without laser assistance at the same condition. The results of XRD measurements showed that the crystallinity of the film grown with laser was better than that of the one grown without laser. The surface morphology and cross-sectional SEM image of the film grown with laser revealed that there were no In droplets on the film. The band-edge emission of the film grown with laser at room temperature and 77 K was observed at 840 nm. The results of micro-Raman measurement showed that the film grown with laser had better crystalline structure than that of the film grown without laser and the radiative recombination which contributed to photoluminescence mainly occurred at In 0.53 Ga 0.47 N region. Those results imply that pulse laser enhances the surface migration and reaction of elements in spite of low-growth temperature. We suggest that pulse laser assisted technique is effective for low-temperature growth of InGaN with high indium content.

The correlation between crystallization of low-temperature (LT) GaN buffer layer and crystalline quality of subsequently grown thick GaN layer at high-temperature on GaAs (1 1 1)A substrate has been investigated. During heating of LT-GaN buffer layer to 1000°C, the degree of crystallization highly depended on the thickness and the temperature ramping rate of GaN buffer layers. When the LT-GaN buffer layer was less than 40-nm-thick or temperature ramping rate was low, excess crystallization and/or agglomeration of the buffer layer occurred, which resulted in a poor crystalline quality of the GaN epitaxial layer subsequently grown on it. On the other hand, when the thickness of LT-GaN buffer layer exceeded 100 nm, there was relatively little agglomeration of buffer layer, which also resulted in poor crystalline quality of the subsequently grown GaN. To grow high-quality GaN layer at high-temperature, optimum crystallization condition for GaN buffer layer just prior to high-temperature growth was found to be as follows: single crystallites region in the GaN buffer layer occupied a volume 40% as large as the randomly aligned matrix. The trade-off between thickness and temperature ramping rate of GaN buffer layer clearly indicates that the degree of crystallization of the GaN buffer layer is an important parameter for achieving high-quality GaN growth at high-temperature.

Thermodynamic analyses incorporating the contribution of lattice constraint from the substrates were carried out to understand the relationship between input In mole ratio and solid composition of stress-free InGaN, InGaN on GaN and InGaN on InN during metalorganic vapour phase epitaxy. The calculated results suggest that a compositionally unstable region is found at the InN-rich region for InGaN on GaN at high temperatures, while that for InGaN on InN can be seen at the GaN-rich region due to the lattice constraint from the substrate. These results imply that incorporating the contribution of lattice constraint is indispensable to predict the thermodynamic properties of such materials.

Thick and high quality GaN layer growth using periodic insertion of low-temperature (LT)-grown GaN buffer layers was investigated by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). Both morphological and optical properties of GaN epitaxial layers were drastically improved by inserting a second LT-GaN buffer layer. Also, the thickness of the second buffer layer was found to affect the quality of subsequently grown epitaxial layers. The full-width at half maximum (FWHM) value of X-ray diffraction for () plane (φ scan) of the GaN layer with double buffer layer structure decreased to 608 arcsec whereas that with single buffer structure was 3600 arcsec. These results indicate that the free-standing GaN substrate with low dislocation density can be possible by reiterating the growth sequence of buffer layer and epitaxial layer. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermodynamic analyses were carried out to understand influence of lattice constraint from GaN and InN substrates on relationship between solid composition x of In x Ga 1− x N films and input mole ratio R In (= P In 0 /( P In 0 + P Ga 0 ) where P i 0 is the input partial pressure of element i ) during molecular beam epitaxy. The calculated results suggest that compositional unstable region is found at small R In region for InGaN on InN while that for InGaN on GaN can be seen at large R In region at higher temperatures. This implies that InN-rich thin films are possible to form on InN substrate though it is difficult to form on GaN substrate.

High temperature growth of thick GaN layers on GaAs (111)A substrates was performed using a two-step growth process by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). Temperature ramping rate of the low-temperature (LT)-grown GaN buffer layer was found to be a key factor controlling the crystalline quality of subsequently grown GaN layer at high temperature. When a 50 nm thick LT-GaN buffer layer grown at 550 °C is heated to 1000 °C, it is essential to apply a temperature ramping rate of 15 °C/min or more in order to grow a high-quality GaN layer with a specular surface on a GaAs (111)A substrate.

The growth of thick, high-quality GaN layers on GaAs (1 1 1)A substrates was investigated using a two-step growth process by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). The thickness and temperature ramping rate of the low-temperature (LT)-grown GaN buffer layer as well as the growth temperature of the subsequent thick GaN layer at high temperatures were found to be the key factors controlling the crystalline quality of the GaN layer on GaAs (1 1 1)A substrate. Growth of a 50-nm-thick LT-GaN buffer layer at 550°C and subsequent heating to 1000°C at a temperature ramping rate of 16.1°C/min afforded a 100-μm-thick high-quality GaN layer with a specular surface. The results indicate that a thick GaN layer can be successfully grown on a GaAs (1 1 1)A substrate at high temperatures and represent a promising technique for the preparation of freestanding GaN substrates.

A freestanding GaN substrate of over 2-in. size with low dislocation density was prepared by hydride vapor phase epitaxy (HVPE) using GaAs (111)A as a starting substrate. A SiO 2 mask pattern with round openings was formed directly onto the GaAs (111)A substrate. Then, a thick GaN layer was grown with numerous large hexagonal inverse-pyramidal pits constructed mainly by {11–22} facets maintained on the surface. After removing the GaAs substrate and subsequent lapping and polishing, a freestanding GaN about 500 μm in thickness was obtained. Etch pit observation reveals that etch pit groups with etch pit density 2×10 8 cm −2 at the center exist in the matrix area with etch pit density as low as 5×10 5 cm −2 . This distribution is due to the effect of large hexagonal pits on collecting dislocations at the bottom of the hexagonal pit. Dislocations propagate into the bottom of the pit mainly in the 〈11–20〉 or 〈1–100〉 direction parallel to (0001).

A freestanding GaN substrate of over 2 inch size with low dislocation density was prepared by hydride vapor phase epitaxy (HVPE) using GaAs (1 1 1)A as a starting substrate. An SiO2 mask pattern with round openings was formed directly onto the GaAs (1 1 1)A substrate. Then, a thick GaN layer was grown with numerous large hexagonal inverse-pyramidal pits constructed mainly by {1 1 2 2} facets maintained on the surface. After removing the GaAs substrate and subsequent lapping and polishing, a freestanding GaN substrate about 500 μm in thickness was obtained. The distribution of dislocations was examined by transmission electron microscope (TEM), cathodoluminescence (CL) and etch-pit density (EPD) measurement. They show that the restricted area with high dislocation density up to 1×109 cm2 exists in the area with low dislocation density as low as 2×105 cm2. This dislocation distribution is caused by {1 1 2 2} facet growth in hexagonal pits. The dislocations are concentrated to the center of the hexagonal pit, and therefore large area with low dislocation density is formed. A new reduction mechanism of dislocation density is proposed.

Ga2O3 field-plated Schottky barrier diodes (FP-SBDs) were fabricated on a Si-doped n−-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n+-Ga2O3 (001) substrate. The specific on-resistance of the Ga2O3 FP-SBD was estimated to be 5.1 mΩ·cm2. Successful field-plate engineering resulted in a high breakdown voltage of 1076 V. A larger-than-expected effective barrier height of 1.46 eV, which was extracted from the temperature-dependent current–voltage characteristics, could be caused by the effect of fluorine atoms delivered in a hydrofluoric acid solution process.

Control of the in-plane epitaxial relationship of a c -plane AlN layer grown at 1450 °C by hydride vapor phase epitaxy on an a -plane sapphire substrate is achieved. In addition, two in-plane epitaxial relationships are compared. It was possible to grow a c -plane AlN layer on an a -plane sapphire substrate with selectable in-plane matching: AlN [100]//sapphire [0001] could be obtained by growing an intermediate AlN layer at 1065 °C, whereas AlN [110] // sapphire [0001] could be grown on the intermediate AlN layer at a low temperature of 700 °C. The full-width at half-maximums of X-ray rocking curves for the (0002) and (100) planes were respectively 313.2 and 543.6 arcsec from the AlN layer with the AlN [100]//sapphire [0001] relationship and 428.4 and 788.0 arcsec from the AlN layer with the AlN [110] // sapphire [0001] relationship. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of low-temperature (LT-) GaN buffer layers grown on GaAs {111} surfaces has been performed. LT-GaN buffer layers of 0-160 nm thickness were grown simultaneously both on GaAs (111)A and (111)B surfaces by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE) at 550 °C. On GaAs (111)A surface, the LT-GaN buffer layers showed smooth surface and covered the GaAs homogeneously. On GaAs (111)B surface, however, the LT-GaN buffer layers showed rough surface with some pits, and surface roughness increased with increasing thickness of the LT-GaN buffer layer. Due to the poor morphology of the LT-GaN buffer layers on GaAs (111)B, the GaAs was eroded during subsequent heating up to 1000°C in NH 3 ambient, whereas no deterioration was observed on GaAs (111)A surfaces covered with the LT-GaN buffer layers.

Gallium oxide (Ga2O3) is an emerging wide-bandgap semiconductor for high-power, low-loss transistors and diodes by virtue of its excellent material properties and suitability for mass production. In this paper, we begin by discussing the material properties of Ga2O3 that make it an attractive alternative to not only Si but also other wide-bandgap materials such as SiC and GaN. State-of-the-art Ga2O3-based devices that have been fabricated to date demonstrate the performance potential for power electronics applications.

We investigated the temperature-dependent electrical properties of Pt/Ga2O3 Schottky barrier diodes (SBDs) fabricated on n–-Ga2O3 drift layers grown on single-crystal n+-Ga2O3 (001) substrates by halide vapor phase epitaxy. In an operating temperature range from 21 °C to 200 °C, the Pt/Ga2O3 (001) Schottky contact exhibited a zero-bias barrier height of 1.09–1.15 eV with a constant near-unity ideality factor. The current–voltage characteristics of the SBDs were well-modeled by thermionic emission in the forward regime and thermionic field emission in the reverse regime over the entire temperature range.

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

The effects of temperature ramping rate on GaAs (111)A substrate covered with a thin GaN buffer layer grown at 550°C are investigated in order to determine the ideal parameters for protecting the substrate from thermal decomposition during the growth of a thick GaN layer at 1000°C by metalorganic hydrogen chloride vapor-phase epitaxy (MOHVPE). Crystallization and agglomeration of the GaN buffer layer during heating is found to increase markedly as the temperature ramping rate is decreased, and thus high ramping rates are found to be applicable to the high temperature growth of GaN on GaAs substrate. When a temperature ramping rate of 20.9°C/min is used, a thick GaN layer exhibiting no voids or cracks is obtained due to the protection of the GaAs substrate.

III-nitrides can be grown by employing several different techniques, such as molecular-beam epitaxy (MBE), metalorganic vapor-phase epitaxy (MOVPE), halide vapor-phase epitaxy (HVPE), high-pressure solution growth, and sputtering. Each of these are suited for a particular application; the specific property of HVPE is a much larger growth rate, which makes this technique the natural choice for growth of very thick layers that can be used as high-quality native substrates for subsequent growth of device structures using other techniques. Such substrates will be needed for certain devices with high current density or high voltage load, where the high defect density caused by growth on foreign substrates (heteroepitaxy) cannot be tolerated. The HVPE technology is still under development, and below we present the present situation with emphasis on GaN. The thermodynamic limitations of HVPE growth are discussed first, including the high-temperature chemistry in both the source zone and growth zone of a growth reactor. Examples of the design of growth systems are given; in particular, issues such as flow patterns, parasitic growth, and growth rates are discussed. Methods to reduce the defect density for growth on foreign substrates are discussed, as well as various lift-off techniques to prepare free-standing GaN wafers. Common characterization techniques are mentioned, and important physical properties of high-quality GaN wafers are given. The ongoing developments of HVPE growth for AlN and InN are also briefly summarized.

A novel technique for misorientation determination based on energy dispersive X-ray diffraction is introduced. The energy dispersive diffractometer equips with a silicon drift detector as a energy dispersive detector and a micro focus air cooled X-ray tube as a source. The diffractometer has advantages in speed of measurement and alignment of sample compared with conventional angular dispersive diffractometer. Dependence of misorientation of AlN layers on misorientation angle and direction of (111) Si starting substrates were investigated by this method. It is found that the AlN has (0001) orientation and the (0001) plane is tilted when the film thickness is thin. Moreover, it is shown that the tilt becomes smaller with increasing the film thickness. The atomic force microscopy (AFM) showed clear difference in surface morphology of obtained AlN layer depending on misorientation direction of Si substrate. The AlN film grown on (111) Si with 1 degree misorientation angle toward [] showed the best step and terrace structure compared with those grown on that having larger than 1 degree misorientation angle. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 1014 cm−3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ~8. The reverse breakdown voltage, defined as the leakage current level of 10−3 A/cm2, ranged from 550 to 770 V.