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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.

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 $(\mathbf{Ga}_{2}\mathbf{O}_{3})$ 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.

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.

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.

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.

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).

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.