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1. Electrical properties of α-Ga ₂ O ₃ films grown by halide vapor phase epitaxy on sapphire with α-Cr ₂ O ₃ buffers

   https://doi.org/10.1063/5.0090832  

   Polyakov, Alexander; Nikolaev, Vladimir; Stepanov, Sergey; Almaev, Alexei; Pechnikov, Alexei; Yakimov, Eugene; Kushnarev, Bogdan O.; Shchemerov, Ivan; Scheglov, Mikhail; Chernykh, Alexey; et al (June 2022, Journal of Applied Physics)

   We report on growth and electrical properties of α-Ga 2 O 3 films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr 2 O 3 buffers predeposited on sapphire by magnetron sputtering. The α-Cr 2 O 3 buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr 2 O 3 bandgap and a sharp MCL line near 700 nm due to the Cr + intracenter transition. Ohmic contacts to Cr 2 O 3 were made with both Ti/Au or Ni, producing linear current–voltage ( I– V) characteristics over a wide temperature range with an activation energy of conductivity of ∼75 meV. The sign of thermoelectric power indicated p-type conductivity of the buffers. Sn-doped, 2- μm-thick α-Ga 2 O 3 films prepared on this buffer by HVPE showed donor ionization energies of 0.2–0.25 eV, while undoped films were resistive with the Fermi level pinned at E C of 0.3 eV. The I– V and capacitance–voltage ( C– V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr 2 O 3 buffer indicated the presence of two face-to-face junctions, one between n-Ga 2 O 3 and p-Cr 2 O 3 , the other due to the Ni Schottky diode with n-Ga 2 O 3 . The spectral dependence of the photocurrent measured on the structure showed the presence of three major deep traps with optical ionization thresholds near 1.3, 2, and 2.8 eV. Photoinduced current transient spectroscopy spectra of the structures were dominated by deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga 2 O 3 system. 

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2. Silicon-doped β -Ga ₂ O ₃ films grown at 1 µ m/h by suboxide molecular-beam epitaxy

   https://doi.org/10.1063/5.0139622  

   Azizie, Kathy; Hensling, Felix V.; Gorsak, Cameron A.; Kim, Yunjo; Pieczulewski, Naomi A.; Dryden, Daniel M.; Senevirathna, M. K.; Coye, Selena; Shang, Shun-Li; Steele, Jacob; et al (April 2023, APL Materials)

   We report the use of suboxide molecular-beam epitaxy ( S-MBE) to grow β-Ga 2 O 3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 10 16 to 10 19  cm −3 . In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga 2 O, i.e., gallium suboxide, is supplied. Directly supplying Ga 2 O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga 2 O 3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature ( T sub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO 2 ) producing an SiO suboxide molecular beam are used to dope the β-Ga 2 O 3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 10 17  cm −3 reveal a room-temperature mobility of 124 cm 2  V −1  s −1 that increases to 627 cm 2  V −1  s −1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga 2 O 3 films grown by S-MBE at growth rates of ∼1 µm/h. 

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3. Growth and characterization of α-Ga ₂ O ₃ on sapphire and nanocrystalline β-Ga ₂ O ₃ on diamond substrates by halide vapor phase epitaxy

   https://doi.org/10.1116/6.0002115  

   Modak, Sushrut; Lundh, James Spencer; Al-Mamun, Nahid Sultan; Chernyak, Leonid; Haque, Aman; Tu, Thieu Quang; Kuramata, Akito; Tadjer, Marko J.; Pearton, Stephen J. (December 2022, Journal of Vacuum Science & Technology A)

   Halide vapor phase epitaxial (HVPE) Ga 2 O 3 films were grown on c-plane sapphire and diamond substrates at temperatures up to 550 °C without the use of a barrier dielectric layer to protect the diamond surface. Corundum phase α-Ga 2 O 3 was grown on the sapphire substrates, whereas the growth on diamond resulted in regions of nanocrystalline β-Ga 2 O 3 (nc-β-Ga 2 O 3 ) when oxygen was present in the HVPE reactor only during film growth. X-ray diffraction confirmed the growth of α-Ga 2 O 3 on sapphire but failed to detect any β-Ga 2 O 3 reflections from the films grown on diamond. These films were further characterized via Raman spectroscopy, which revealed the β-Ga 2 O 3 phase of these films. Transmission electron microscopy demonstrated the nanocrystalline character of these films. From cathodoluminescence spectra, three emission bands, UVL′, UVL, and BL, were observed for both the α-Ga 2 O 3 /sapphire and nc-Ga 2 O 3 /diamond, and these bands were centered at approximately 3.7, 3.2, and 2.7 eV. 

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4. Growth and characterization of (Sc ₂ O ₃ ) _x (Ga ₂ O ₃ ) _1−x by molecular beam epitaxy

   https://doi.org/10.1116/6.0001805  

   Hlad, Mark S.; Gila, Brent P.; Abernathy, Cammy R.; Ren, Fan; Pearton, S. J. (July 2022, Journal of Vacuum Science & Technology A)

   (Sc 2 O 3 ) x (Ga 2 O 3 ) 1−x was grown by molecular beam epitaxy at low temperatures (100 °C) using a variety of growth sequences to avoid surface segregation of Ga. Continuous and digital growth techniques always produced Ga segregation. This surface segregation was attributed to the stronger bond between the Sc and O compared to the Ga and O. A digital growth technique (alternate opening of Sc and Ga shutters with the O shutter open continuously during the growth) was unsuccessful in eliminating this effect. The segregation was eliminated using a growth technique in which the Ga shutter was closed for a set amount of time toward the end of the growth while the O and Sc shutters remained open. Characterization with reflection high energy electron diffraction, x-ray diffraction, and transmission electron microscopy revealed the growth of a fine-grained polycrystalline film under these conditions. A third growth technique was used that involved closing the Ga shutter for a set amount of time toward the end of the growth while the O and Sc shutters were open continuously. This technique was successful in depositing a uniform film. However, the breakdown field was only 1.40 MV/cm (at 1 mA/cm 2 ). The addition of Ga to Sc 2 O 3 diminished the insulating properties of the film. These initial experiments indicate that phase segregation is likely to be a major issue with most growth techniques and that alloying Ga 2 O 3 with elements other than Sc, such as Gd or Al, might be a more successful approach. 

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5. H trapping at the metastable cation vacancy in α -Ga ₂ O ₃ and α -Al ₂ O ₃

   https://doi.org/10.1063/5.0094707  

   Venzie, Andrew; Portoff, Amanda; Stavola, Michael; Fowler, W. Beall; Kim, Jihyun; Jeon, Dae-Woo; Park, Ji-Hyeon; Pearton, Stephen J. (May 2022, Applied Physics Letters)

   α-Ga 2 O 3 has the corundum structure analogous to that of α-Al 2 O 3 . The bandgap energy of α-Ga 2 O 3 is 5.3 eV and is greater than that of β-Ga 2 O 3 , making the α-phase attractive for devices that benefit from its wider bandgap. The O–H and O–D centers produced by the implantation of H + and D + into α-Ga 2 O 3 have been studied by infrared spectroscopy and complementary theory. An O–H line at 3269 cm −1 is assigned to H complexed with a Ga vacancy (V Ga ), similar to the case of H trapped by an Al vacancy (V Al ) in α-Al 2 O 3 . The isolated V Ga and V Al defects in α-Ga 2 O 3 and α-Al 2 O 3 are found by theory to have a “shifted” vacancy-interstitial-vacancy equilibrium configuration, similar to V Ga in β-Ga 2 O 3 , which also has shifted structures. However, the addition of H causes the complex with H trapped at an unshifted vacancy to have the lowest energy in both α-Ga 2 O 3 and α-Al 2 O 3 . 

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