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1. H trapping at the metastable cation vacancy in α-Ga2O3 and α-Al2O3

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

   α-Ga2O3 has the corundum structure analogous to that of α-Al2O3. The bandgap energy of α-Ga2O3 is 5.3 eV and is greater than that of β-Ga2O3, 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-Ga2O3 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 (VGa), similar to the case of H trapped by an Al vacancy (VAl) in α-Al2O3. The isolated VGa and VAl defects in α-Ga2O3 and α-Al2O3 are found by theory to have a “shifted” vacancy-interstitial-vacancy equlibrium configuration, similar to VGa in β-Ga2O3 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-Ga2O3 and α-Al2O3. 

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2. Hydrogen centers as a probe of V _Ga(2) defects in β -Ga ₂ O ₃

   https://doi.org/10.1063/5.0138904  

   Portoff, Amanda; Stavola, Michael; Fowler, W. Beall; Pearton, Stephen J.; Glaser, Evan R. (February 2023, Applied Physics Letters)

   While a number of O-H and O-D vibrational lines have been observed for hydrogen and deuterium in β-Ga 2 O 3 , it has been commonly reported that there is no absorption with a component of the polarization E parallel to the [010], or b, axis. This experimental result has led to O-H defect structures that involve shifted configurations of a vacancy at the tetrahedrally coordinated Ga(1) site [V Ga(1) ] and have ruled out structures that involve a vacancy at the octahedrally coordinated Ga(2) site [V Ga(2) ], because these structures are predicted to show absorption for E//[010]. In this Letter, weak O-D lines at 2475 and 2493 cm −1 with a component of their polarization with E//[010] are reported for β-Ga 2 O 3 that had been annealed in a D 2 ambient. O-D defect structures involving an unshifted V Ga(2) are proposed for these centers. An estimate is made that the concentration of V Ga(2) in a Czochralski-grown sample is 2–3 orders of magnitude lower than that of V Ga(1) from the intensities of the IR absorption lines. 

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3. Impact of process anneals on high-k/β-Ga ₂ O ₃ interfaces and capacitance

   https://doi.org/10.1116/6.0002264  

   Hawkins, Roberta; Wang, Xinglu; Moumen, Naim; Wallace, Robert M.; Young, Chadwin D. (March 2023, Journal of Vacuum Science & Technology A)

   Gallium oxide (β-Ga 2 O 3 ) is becoming a popular material for high power electronic devices due to its wide bandgap and ease of processing. In this work, β-Ga 2 O 3 substrates received various annealing treatments before atomic layer deposition of HfO 2 and subsequent fabrication of metal–oxide–semiconductor (MOS) capacitors. Annealing of β-Ga 2 O 3 with forming gas or nitrogen produced degraded capacitance–voltage (C–V) behavior compared to a β-Ga 2 O 3 control sample with no annealing. A sample annealed with pure oxygen had improved C–V characteristics relative to the control sample, with a higher maximum capacitance and smaller flat-band voltage shift, indicating that oxygen annealing improved the C–V behavior. X-ray photoelectron spectroscopy also suggested a reduction in the oxygen vacancy concentration after O 2 annealing at 450 °C, which supports the improved C–V characteristics and indicates that O 2 annealing of β-Ga 2 O 3 may lead to better MOS device performance. 

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4. Type II band alignment of NiO/α-Ga ₂ O ₃ for annealing temperatures up to 600 °C

   https://doi.org/10.1116/6.0002257  

   Xia, Xinyi; Li, Jian-Sian; Chiang, Chao-Ching; Yoo, Timothy Jinsoo; Hershkovitz, Eitan; Ren, Fan; Kim, Honggyu; Kim, Jihyun; Jeon, Dae-Woo; Park, Ji-Hyeon; et al (December 2022, Journal of Vacuum Science & Technology A)

   There is increasing interest in the alpha polytype of Ga 2 O 3 because of its even larger bandgap than the more studied beta polytype, but in common with the latter, there is no viable p-type doping technology. One option is to use p-type oxides to realize heterojunctions and NiO is one of the candidate oxides. The band alignment of sputtered NiO on α-Ga 2 O 3 remains type II, staggered gap for annealing temperatures up to 600 °C, showing that this is a viable approach for hole injection in power electronic devices based on the alpha polytype of Ga 2 O 3 . The magnitude of both the conduction and valence band offsets increases with temperature up to 500 °C, but then is stable to 600 °C. For the as-deposited NiO/α-Ga 2 O 3 heterojunction, ΔE V  = −2.8 and ΔE C  = 1.6 eV, while after 600 °C annealing the corresponding values are ΔE V  = −4.4 and ΔE C  = 3.02 eV. These values are 1−2 eV larger than for the NiO/β-Ga 2 O 3 heterojunction. 

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5. 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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