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1. Structural and electrical properties of thick κ -Ga ₂ O ₃ grown on GaN/sapphire templates

   https://doi.org/10.1063/5.0091653  

   Polyakov, A. Y.; Nikolaev, V. I.; Pechnikov, A. I.; Stepanov, S. I.; Yakimov, E. B.; Scheglov, M. P.; Shchemerov, I. V.; Vasilev, A. A.; Kochkova, A. A.; Chernykh, A. V.; et al (June 2022, APL Materials)

   Thick (23 µm) films of κ-Ga 2 O 3 were grown by Halide Vapor Phase Epitaxy (HVPE) on GaN/sapphire templates at 630 °C. X-ray analysis confirmed the formation of single-phase κ-Ga 2 O 3 with half-widths of the high-resolution x-ray diffraction (004), (006), and (008) symmetric reflections of 4.5 arc min and asymmetric (027) reflection of 14 arc min. Orthorhombic κ-Ga 2 O 3 polymorph formation was confirmed from analysis of the Kikuchi diffraction pattern in electron backscattering diffraction. Secondary electron imaging indicated a reasonably flat surface morphology with a few (area density ∼10 3  cm −2 ) approximately circular (diameter ∼50–100 µm) uncoalesced regions, containing κ-Ga 2 O 3 columns with in-plane dimensions and a height of about 10 µm. Micro-cathodoluminescence (MCL) spectra showed a wide 2–3.5 eV band that could be deconvoluted into narrower bands peaked at 2.59, 2.66, 2.86, and 3.12 eV. Ni Schottky diodes prepared on the films showed good rectification but a high series resistance. The films had a thin near-surface region dominated by E c − 0.7 eV deep centers and a deeper region (∼2 µm from the surface) dominated by shallow donors with concentrations of ≤10 16  cm −3 . Photocurrent and photocapacitance spectra showed the presence of deep compensating acceptors with optical ionization energies of ∼1.35 and 2.3 eV, the latter being close to the energy of one of the MCL bands. Deep level transient spectroscopy revealed deep traps with energies near 0.3, 0.6, 0.7, 0.8, and 1 eV from the conduction band edge. The results show the potential of HVPE to grow very thick κ-Ga 2 O 3 on GaN/sapphire templates. 

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2. Point defect creation by proton and carbon irradiation of α-Ga ₂ O ₃

   https://doi.org/10.1063/5.0100359  

   Polyakov, Alexander Y.; Nikolaev, Vladimir I.; Meshkov, Igor N.; Siemek, Krzysztof; Lagov, Petr B.; Yakimov, Eugene B.; Pechnikov, Alexei I.; Orlov, Oleg S.; Sidorin, Alexey A.; Stepanov, Sergey I.; et al (July 2022, Journal of Applied Physics)

   Films of α-Ga 2 O 3 grown by Halide Vapor Phase Epitaxy (HVPE) were irradiated with protons at energies of 330, 400, and 460 keV with fluences 6 × 10 15  cm −2 and with 7 MeV C 4+ ions with a fluence of 1.3 × 10 13  cm −2 and characterized by a suite of measurements, including Photoinduced Transient Current Spectroscopy (PICTS), Thermally Stimulated Current (TSC), Microcathodoluminescence (MCL), Capacitance–frequency (C–f), photocapacitance and Admittance Spectroscopy (AS), as well as by Positron Annihilation Spectroscopy (PAS). Proton irradiation creates a conducting layer near the peak of the ion distribution and vacancy defects distribution and introduces deep traps at E c -0.25, 0.8, and 1.4 eV associated with Ga interstitials, gallium–oxygen divacancies V Ga –V O , and oxygen vacancies V O . Similar defects were observed in C implanted samples. The PAS results can also be interpreted by assuming that the observed changes are due to the introduction of V Ga and V Ga –V O . 

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3. Electrical and Structural Properties of Two-Inch Diameter (0001) α-Ga ₂ O ₃ Films Doped with Sn and Grown by Halide Epitaxy

   https://doi.org/10.1149/2162-8777/ac9edb  

   Nikolaev, V. I.; Polyakov, A. Y.; Stepanov, S. I.; Pechnikov, A. I.; Yakimov, E. B.; Chernykh, A. V.; Vasilev, A. A.; Shchemerov, I. V.; Kochkova, A. I.; Guzilova, L.; et al (November 2022, ECS Journal of Solid State Science and Technology)

   Two-inch diameter α -Ga 2 O 3 films with thickness ∼4 μ m were grown on basal plane sapphire by Halide Vapor Phase Epitaxy (HVPE) and doped with Sn in the top ∼1 μ m from the surface. These films were characterized with High-Resolution X-ray Diffraction (HRXRD), Scanning Electron Microscope (SEM) imaging in the Secondary Electron (SE) and Micro-cathodoluminescence (MCL) modes, contactless sheet resistivity mapping, capacitance-voltage, current-voltage, admittance spectra, and Deep Level Transient Spectroscopy (DLTS) measurements. The edge and screw dislocations densities estimated from HRXRD data were respectively 7.4 × 10 9 cm −2 and 1.5 × 10 7 cm −2 , while the films had a smooth surface with a low density (∼10 3 cm −2 ) of circular openings with diameters between 10 and 100 μ m. The sheet resistivity of the films varied over the entire 2-inch diameter from 200 to 500 Ω square −1 . The net donor concentration was ∼10 18 cm −3 near the surface and increased to ∼4 × 10 18 cm −3 deeper inside the sample. The deep traps observed in admittance and DLTS spectra had levels at E c −0.25 eV and E c −0.35 eV, with concentration ∼10 15 cm −3 and E c −1 eV with concentration ∼10 16 cm −3 . 

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4. 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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5. Mechanism for Long Photocurrent Time Constants in α-Ga ₂ O ₃ UV Photodetectors

   https://doi.org/10.1149/2162-8777/acc900  

   Polyakov, A. Y.; Almaev, A. V.; Nikolaev, V. I.; Pechnikov, A. I.; Shchemerov, V. I.; Vasilev, A. A.; Yakimov, E. B.; Kochkova, A. I.; Kopyev, V. V.; Kushnarev, B. O; et al (April 2023, ECS Journal of Solid State Science and Technology)

   Deep centers and their influence on photocurrent spectra and transients were studied for interdigitated photoresistors on α -Ga 2 O 3 undoped semi-insulating films grown by Halide Vapor Phase Epitaxy (HVPE) on sapphire. Characterization involving current-voltage measurements in the dark and with monochromatic illumination with photons with energies from 1.35 eV to 4.9 eV, Thermally Stimulated Current (TSC), Photoinduced Current Transients Spectroscopy (PICTS) showed the Fermi level in the dark was pinned at E c −0.8 eV, with other prominent centers being deep acceptors with optical thresholds near 2.3 eV and 4.9 eV and deep traps with levels at E c −0.5 eV, E c −0.6 eV. Measurements of photocurrent transients produced by illumination with photon energies 2.3 eV and 4.9 eV and Electron Beam Induced Current (EBIC) imaging point to the high sensitivity and external quantum efficiency values being due to hole trapping enhancing the lifetime of electrons and inherently linked with the long photocurrent transients. The photocurrent transients are stretched exponents, indicating the strong contribution of the presence of centers with barriers for electron capture and/or of potential fluctuations. 

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