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1. Ga ⁺ -focused ion beam damage in n-type Ga ₂ O ₃

   https://doi.org/10.1116/5.0099892  

   Xia, Xinyi ; Al-Mamun, Nahid Sultan ; Warywoba, Daudi ; Ren, Fan ; Haque, Aman ; Pearton, S. J. ( September 2022 , Journal of Vacuum Science & Technology A)

   Focused Ga + ion milling of lightly Si-doped, n-type Ga 2 O 3 was performed with 2–30 kV ions at normal incidence and beam currents that were a function of beam voltage, 65 nA for 30 kV, 26 nA for 10 kV, 13 nA for 5 kV, and 7.1 nA for 2 kV, to keep the milling depth constant at 100 nm. Approximate milling rates were 15, 6, 2.75, and 1.5  μm 3 /s for 30, 10, 5, and 2 kV, respectively. The electrical effects of the ion damage were characterized by Schottky barrier height and diode ideality factor on vertical rectifier structures comprising 10  μm epitaxial n-Ga 2 O 3 on n + Ga 2 O 3 substrates, while the structural damage was imaged by transmission electron microscopy. The reverse bias leakage was largely unaffected even by milling at 30 kV beam energy, while the forward current-voltage characteristics showed significant deterioration at 5 kV, with an increase in the ideality factor from 1.25 to 2.25. The I–V characteristics no longer showed rectification for the 30 kV condition. Subsequent annealing up to 400 °C produced substantial recovery of the I–V characteristics for all beam energies and was sufficient to restore the initial ideality factor completely for beam energies up to 5 kV. Even the 30 kV-exposedmore »rectifiers showed a recovery of the ideality factor to 1.8. The surface morphology of the ion-milled Ga 2 O 3 was smooth even at 30 kV ion energy, with no evidence for preferential sputtering of the oxygen. The surface region was not amorphized by extended ion milling (35 min) at 5 kV with the samples held at 25 °C, as determined by electron diffraction patterns, and significant recovery of the lattice order was observed after annealing at 400 °C.« less
2. 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-Ga₂O₃films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr₂O₃buffers predeposited on sapphire by magnetron sputtering. The α-Cr₂O₃buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr₂O₃bandgap and a sharp MCL line near 700 nm due to the Cr⁺intracenter transition. Ohmic contacts to Cr₂O₃were 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-Ga₂O₃films 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_Cof 0.3 eV. The I– V and capacitance–voltage ( C– V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr₂O₃buffer indicated the presence of two face-to-face junctions, one between n-Ga₂O₃and p-Cr₂O₃, the other due to the Ni Schottky diode with n-Ga₂O₃. 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 bymore »deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga₂O₃system.

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3. High conductivity β-Ga ₂ O ₃ formed by hot Si ion implantation

   https://doi.org/10.1063/5.0127457  

   Sardar, Arka ; Isaacs-Smith, Tamara ; Lawson, Jacob ; Asel, Thaddeus ; Comes, Ryan B. ; Merrett, Joseph N. ; Dhar, Sarit ( December 2022 , Applied Physics Letters)

   This work demonstrates the advantage of carrying out silicon ion (Si⁺) implantation at high temperatures for forming controlled heavily doped regions in gallium oxide. Room temperature (RT, 25 °C) and high temperature (HT, 600 °C) Si implants were carried out into MBE grown (010) β-Ga₂O₃films to form ∼350 nm deep Si-doped layers with average concentrations up to ∼1.2 × 10²⁰cm⁻³. For such high concentrations, the RT sample was too resistive for measurement, but the HT samples had 82.1% Si dopant activation efficiency with a high sheet electron concentration of 3.3 × 10¹⁵ cm⁻²and an excellent mobility of 92.8 cm²/V·s at room temperature. X-ray diffraction measurements indicate that HT implantation prevents the formation of other Ga₂O₃phases and results in reduced structural defects and lattice damage. These results are highly encouraging for achieving ultra-low resistance heavily doped Ga₂O₃layers using ion implantation.
4. 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 −1more » 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.« less
5. Impact of radiation and electron trapping on minority carrier transport in p -Ga ₂ O ₃

   https://doi.org/10.1063/5.0096950  

   Modak, Sushrut ; Schulte, Alfons ; Sartel, Corinne ; Sallet, Vincent ; Dumont, Yves ; Chikoidze, Ekaterine ; Xia, Xinyi ; Ren, Fan ; Pearton, Stephen J. ; Ruzin, Arie ; et al ( June 2022 , Applied Physics Letters)

   Highly resistive undoped p-type gallium oxide samples were subjected to cumulative proton irradiation with energies ranging from 25 to 70 keV and doses in the 1.6 × 10¹⁴–3.6 × 10¹⁴cm⁻²range. Proton irradiation resulted in up to a factor of 2 reduction of minority electron diffusion length in the samples for temperatures between ∼ 300 and 400 K. Electron injection into the samples under test using a scanning electron microscope beam leads to pronounced elongation of diffusion length beyond the pre-irradiation values, thus demonstrating stable (days after injection) recovery of adverse radiation impact on minority carrier transport. The activation energy of 91 meV estimated from the temperature dependent diffusion length vs electron injection duration experiments is likely related to the local potential barrier height for native defects associated with the phenomenon of interest.