IWGO 2026 Session IWGO-WeM2: Epitaxial Growth and Doping Control II

α-Ga₂O₃ is a promising power semiconductor material; however, due to its metastable nature, high-quality bulk substrates are not available. Consequently, epitaxial growth of α-Ga₂O₃ has mainly been performed on sapphire substrates with large lattice mismatch, resulting in high dislocation densities. Recently, high-quality α-Cr₂O₃ templates, which exhibit a small in-plane lattice mismatch of approximately −0.45% with c-plane α-Ga₂O₃, have attracted significant attention. Epitaxial growth of α-Ga₂O₃ thin films (<~1 μm) on α-Cr₂O₃ has been demonstrated using mist CVD [1] and HVPE [2], showing dislocation densities on the order of 10⁷ cm^-2, about three orders of magnitude lower than those on sapphire substrates [1].

In this study, α-Ga₂O₃ films were grown on high-quality α-Cr₂O₃ templates to thicknesses far exceeding previously reported values, and their crystalline quality and electrical properties were investigated [3]. The films were grown by HVPE at 520 °C with thicknesses ranging from 0.24 to 21 μm and a growth rate of approximately 14 μm/h. X-ray diffraction measurements confirmed phase-pure, single-crystalline α-Ga₂O₃. The tilt angle estimated from X-ray rocking curves was close to that of the underlying α-Cr₂O₃ layer and did not increase with film thickness, while the twist angle increased for films thicker than 10 μm. The dislocation density estimated from etch pit density (EPD) was 5.6×10⁷ cm^-2 for the thinnest film. Although it increased to 3.9×10⁸ cm^-2 for the 21 μm-thick film, this value remains much lower than that of α-Ga₂O₃ layers of similar thickness grown directly on sapphire (~3×10⁹ cm^-2).

Furthermore, Si-doped α-Ga₂O₃ films with a thickness of 2 μm and EPD of ~(1–3)×10⁸ cm^-2 were fabricated and their electrical properties were evaluated by Hall measurements at room temperature. The mobility increased with decreasing carrier concentration, reaching 144 cm²/Vs at a carrier concentration of 1.8×10¹⁸ cm^-3. This value significantly exceeds previously reported highest mobilities of approximately 52 cm²/Vs for c-plane [4] and 99 cm²/Vs for m-plane α-Ga₂O₃ [5].

[1] K. Yamada et al, the 72nd JSAP Spring Meet. 14-17 Mar, 2025, Noda, Japan, 16p-Y1311-7 (2025).

[2] K. Takeda et al, 44th Electron. Mater. Symp. (EMS-44), Nara, Japan, Oct 15-17, Fr1-12 (2025).

[3] Y. Oshima et al, J. Appl. Phys. 139, 075302 (2026).

[4] H. Son et al, ECS J. Solid State Sci. Technol. 9, 055005 (2020).

[5] T. Wakamatsu et al, Appl. Phys. Lett. 128, 012105 (2026).

Parasitic conduction from accumulated Si between β-Ga₂O₃ films and substrate remains a persistent challenge for β-Ga₂O₃-based devices, particularly in lateral structures. Efforts to remove this interfacial Si via various etching methods have been mostly successful, however, in oxide molecular beam epitaxy (MBE), such efforts appear unsuccessful due to re-accumulation of Si from within the MBE including the Si dopant source. Instead of Si removal, we demonstrate how a thin layer of Fe-doped β-Ga₂O₃ on the substrate surface can compensate interfacial Si and eliminate the double-channel effect from lateral devices. Secondary ion mass spectrometry (SIMS) and capacitance-voltage (C-V) measurements confirm the Fe confinement to the substrate-film interface and interfacial charge compensation, respectively. Devices fabricated from epitaxial material using this interfacial Fe compensation show no double channel effect in ~97% of structures, significantly improved from device yield of <50% without Fe compensation. These results demonstrate a potential method to mitigate parasitic conduction channels due to interfacial Si in β-Ga₂O₃. While Fe seems initially promising, we are doing further studies on the stability of the interfacial Fe-doped layers to both processing conditions and electrical cycling during device operation. Alternative compensating acceptors including N or Mg need to be explored given the observation of capacitance transients in Fe-doped structures. Overall mitigating this parasitic interface will help improve yield and performance uniformity in fabricated devices.

α-(Al,Ga)₂O₃ possesses the largest bandgap of 5.4-8.8 eV among (Al,Ga)₂O₃ polymorphs, and electrical conductivity is theoretically expected when the Al composition x is lower than 0.7 [1].Sapphire substrates are commonly used for the growth of α-(Al,Ga)₂O₃; however, for x<0.7, the critical thickness is less than 50 nm, leading to high dislocation densities and consequently high resistivity [2]. α-Cr₂O₃, which is lattice matched with (Al,Ga)₂O₃, is promising as a substrate for Ga-rich α-(Al,Ga)₂O₃ growth. We previously reported the lattice-matched growth of α-(Al,Ga)₂O₃ using commercial α-Cr₂O₃ substrates, but the α-(Al,Ga)₂O₃ layers inherited the low crystallinity of the substrate [3]. High-quality α-Cr₂O₃/sapphire templates, whose dislocation density is lower than 10⁸ cm^-2, have been realized by a group at NGK Insulators, Ltd. Dislocation density in α-Ga₂O₃ layers grown on the template were lower than 10⁸ cm^-2, which was three orders of magnitude smaller compared to α–Ga₂O₃ films grown on sapphire substrates [4,5]. In this study, we report coherent growth of α-(Al,Ga)₂O₃ on the α-Cr₂O₃/sapphire templates.

(Al,Ga)₂O₃ films were grown on c-plane α–Cr₂O₃/sapphire templates manufactured by NGK Insulators, Ltd using a hot-wall-type mist-CVD system. The Al composition x in the grown films varied from 0.05 and 0.5. A symmetric x-ray diffraction 2θ/ω scan profile for the samples showed that the successful growth of single-phase α–(Al,Ga)₂O₃ on the α–Cr₂O₃ templates. Reciprocal space mapping revealed that the α–(Al,Ga)₂O₃ films with a film thickness of ca. 50 nm were coherently grown on the α–Cr₂O₃ templates when x was lower than 0.33. Surface atomic force microscopy images of the coherently grown samples revealed smooth surface roughness with root mean square roughnesses smaller than 0.12 nm. When x = 0.09 (almost lattice matched to α–Cr₂O₃), the flim was coherently grown on the template when the film thickness was ca. 900 nm. These findings suggest that the α–Cr₂O₃ template holds promise for the growth of high quality α–(Al,Ga)₂O₃ in the Ga-rich region, thus paving the way for the development of advanced α-(Al,Ga)₂O₃-based heterostructure devices.

This work was partly supported by Grants-in-Aid for Scientific Research (25K17958) and Nippon Sheet Glass Foundation for Materials Science and Engineering.

[1] D. Wickramaratne, et al., Appl. Phys. Lett. 121,042110P. (2022).

[2] H. Okumura and J, B. Varley, Jpn. J. Appl. Phys. 63 075502.

[3] R. Jinno, et al., IWGO-4, WeP-9 (2024).

[4] M. Watanabe, et al., JSAS Autumn Meet., 20p-A302-9 (2023).

[5] Y. Oshima, et al., J. Appl. Phys. 139, 075302 (2026).

⁺Author for correspondence:jinno@iis.u-tokyo.ac.jp

Gallium oxide (Ga₂O₃) is widely recognized as a premier next-generation, ultra-wide-bandgap (UWBG) semiconductor for energy, power and deep-UV optoelectronics, offering a compelling pathway beyond the limits of Si, SiC, and GaN. However, to fully exploit the benefits of Ga₂O₃ in high-power applications, achieving p-type conductivity and, consequently, an effective p-n junction is mandatory. This remains as one of the major challenges of the technology.

Ion implantation technique has attracted attention to form stable p-type conduction when applied into epitaxial layer in homojunction systems. Very recently, vertical p-n junctions using Phosphorous (P) have been reported. This is an unexpected result as, in general, P implantation in β-Ga₂O₃ is regarded as amphoteric and only a deep acceptor. However, when the P doping dose is sufficiently high in the implanted regions, a delocalization-localization regime has been observed typically associated with disordered systems in Anderson-localization physics, i.e., a metal-insulation transition (MIT) at low-T that enables hole transport.

In this work, a homoepitaxial PiN diode was engineered and presents unambiguous high-injection bipolar operation. We apply the recently reported methodology of acceptor extended wavefunctions in disordered system (related to Anderson disorder) via a P implantation route performed on epitaxial highly resistive p-type Ga₂O₃ grown via MOCVD on commercial highly conductive Sn-doped (001) substrates. Here, we demonstrate that using a native lightly doped p-type intrinsic epi reduces the residual donor and increase bipolar conductivity modulation which results in an easier minority lifetime control. The consistent device demonstration of 8-6x10¹⁸ cm^-3 free hole depletion and electron-hole plasma adds further evidence that bipolar devices are possible in Ga₂O₃ and confirm that P implantation is a route for implementing homoepitaxial p-n junctions.

In summary, our results show that the introduction of native p-type conductivity compensates residual donor defects, enhances bipolar conductivity modulation, and enables improved control of carrier lifetime. The reproducible observation of free-hole depletion and electron-hole plasma in the range of ~1–8× 10¹⁸ cm⁻³ at room T provides further compelling evidence for the feasibility of bipolar device operation in Ga₂O₃. In unterminated devices, it is yet possible to achieve minority carrier lifetimes of 0.1-0.2 ms, current densities above 6000 A/cm², breakdown voltages larger than 500V and on-off ratios of 10⁷. These results open new research avenues toward advanced Ga₂O₃ power electronic devices based on disordered extended waveform concepts.

β-Ga₂O₃ is a promising candidate material for next-generation power devices. The growth of thick homoepitaxial layers with controlled conductivity is essential for vertical device fabrication. Recently, our group demonstrated high-speed growth of Si-doped n-type homoepitaxial layers with a carrier concentration of 2 × 10¹⁶ cm^-3 on β-Ga₂O₃(010) small-sized substrates (10 × 15 mm²) by low-pressure hot-wall metalorganic vapor phase epitaxy (MOVPE) [1]. In this study, homoepitaxial growth was carried out on 2-inch β-Ga₂O₃(010) substrates, and the crystallinity and uniformity of the grown layers were investigated.

A horizontal low-pressure hot-wall MOVPE reactor (TAIYO NIPPON SANSO, FR2000-OX) was used. Si-doped homoepitaxial layers were grown on a 2-inch-diameter Sn-doped β-Ga₂O₃(010) substrate for 150 min at a reactor pressure of 3.4 kPa and a growth temperature of 1000ºC using trimethylgallium (TMGa), O₂, and tetramethylsilane (TMSi) as the Ga, oxygen, and Si dopant sources, respectively, using Ar as the carrier gas.

An approximately 11 µm-thick homoepitaxial layer with a smooth surface was obtained over the entire substrate. The X-ray rocking curves of β-Ga₂O₃(020) measured at various positions on the 2-inch substrate all exhibited full width at half maximum (FWHM) values in the range of 27–38 arcsec, comparable to those of the original substrate. Furthermore, secondary-ion mass spectrometry measurements revealed a uniform Si doping concentration of approximately 1 × 10¹⁶ cm^-3 at all measured positions, suggesting that the prepared homoepitaxial substrate is suitable for device fabrication.

This work was supported by MIC under a grant entitled “R&D of ICT Priority Technology (JPMI00316): Next-Generation Energy-Efficient Semiconductor Development and Demonstration Project (second period) (in collaboration with MOEJ).”

[1] J. Yoshinaga et al., Appl. Phys. Express 18, 055503 (2025).