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1. 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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2. Transport and trap states in proton irradiated ultra-thick κ-Ga2O3

   https://doi.org/10.1116/6.0002673  

   Polyakov, A. Y.; Nikolaev, V. I.; Pechnikov, A. I.; Yakimov, E. B.; Lagov, P. B.; Shchemerov, I. V.; Vasilev, A. A.; Kochkova, A. I.; Chernykh, A. V.; Lee, In-Hwan; et al (May 2023, Journal of Vacuum Science & Technology A)

   Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change. 

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3. (Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge ₂ Sb ₂ Te ₅

   https://doi.org/10.1149/10801.0029ecst  

   Kashem, Md Tashfiq; Muneer, Sadid; Adnane, Lhacene; Dirisaglik, Faruk; Gokirmak, Ali; Silva, Helena (May 2022, ECS Transactions)

   We calculate critical electronic conduction parameters of the amorphous phase of Ge 2 Sb 2 Te 5 (GST), a common material used in phase change memory. We estimate the room temperature bandgap of metastable amorphous GST to be E g (300K) = 1.84 eV based on a temperature dependent energy band model. We estimate the free carrier concentration at the melting temperature utilizing the latent heat of fusion to be 1.47 x 10 22 cm -3 . Using the thin film melt resistivity, we calculate the carrier mobility at melting point as 0.187 cm 2 /V-s. Assuming that metastable amorphous GST is a supercooled liquid with bipolar conduction, we compute the total carrier concentration as a function of temperature and estimate the room temperature free carrier concentration as p(300K) ≈ n(300K) = 1.69×10 17 cm -3 . Free electrons and holes are expected to recombine over time and the stable (drifted) amorphous GST is estimated to have p-type conduction with p(300K) ≈ 6×10 16 cm -3 . 

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4. Structural and electrical properties of thick κ -Ga2O3 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-Ga2O3 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-Ga2O3 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-Ga2O3 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 ∼103 cm−2) approximately circular (diameter ∼50–100 µm) uncoalesced regions, containing κ-Ga2O3 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 Ec − 0.7 eV deep centers and a deeper region (∼2 µm from the surface) dominated by shallow donors with concentrations of ≤1016 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-Ga2O3 on GaN/sapphire templates. 

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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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