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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 ofmore »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.« 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. MOCVD growth and band offsets of κ-phase Ga ₂ O ₃ on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) YSZ substrates

   https://doi.org/10.1116/6.0002106  

   Bhuiyan, A F ; Feng, Zixuan ; Huang, Hsien-Lien ; Meng, Lingyu ; Hwang, Jinwoo ; Zhao, Hongping ( December 2022 , Journal of Vacuum Science & Technology A)

   Epitaxial growth of κ-phase Ga 2 O 3 thin films is investigated on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) oriented yttria stabilized zirconia (YSZ) substrates via metalorganic chemical vapor deposition. The structural and surface morphological properties are investigated by comprehensive material characterization. Phase pure κ-Ga 2 O 3 films are successfully grown on GaN-, AlN-on-sapphire, and YSZ substrates through a systematical tuning of growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, whereas the growth on c-sapphire substrates leads to a mixture of β- and κ-polymorphs of Ga 2 O 3 under the investigated growth conditions. The influence of the crystalline structure, surface morphology, and roughness of κ-Ga 2 O 3 films grown on different substrates are investigated as a function of precursor flow rate. High-resolution scanning transmission electron microscopy imaging of κ-Ga 2 O 3 films reveals abrupt interfaces between the epitaxial film and the sapphire, GaN, and YSZ substrates. The growth of single crystal orthorhombic κ-Ga 2 O 3 films is confirmed by analyzing the scanning transmission electron microscopy nanodiffraction pattern. The chemical composition, surface stoichiometry, and bandgap energies of κ-Ga 2 O 3 thin films grown on different substrates are studied bymore »high-resolution x-ray photoelectron spectroscopy (XPS) measurements. The type-II (staggered) band alignments at three interfaces between κ-Ga 2 O 3 and c-sapphire, AlN, and YSZ substrates are determined by XPS, with an exception of κ-Ga 2 O 3 /GaN interface, which shows type-I (straddling) band alignment.« less
4. 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 cmmore »−3 and E c −1 eV with concentration ∼10 16 cm −3 .« less
5. Structural and mechanical properties of nanocrystalline Ga ₂ O ₃ films made by pulsed laser deposition onto transparent quartz substrates

   https://doi.org/10.1088/2632-959X/abf3d3  

   Zade, Vishal ; Makeswaran, Nanthakishore ; Boyce, Brad L. ; Paraguay-Delgado, Francisco ; Ramana, C. V. ( April 2021 , Nano Express)

   This work reports on the correlation between structure, surface/interface morphology and mechanical properties of pulsed laser deposited (PLD)β-Ga₂O₃films on transparent quartz substrates. By varying the deposition temperature in the range of 25 °C–700 °C, ∼200 nm thick Ga₂O₃films with variable microstructure and amorphous-to-nanocrystalline nature were produced onto quartz substrates by PLD. The Ga₂O₃films deposited at room temperature were amorphous; nanocrystalline Ga₂O₃films were realized at 700 °C. The interface microstructure is characterized with a typical nano-columnar morphology while the surface exhibits the uniform granular morphology. Corroborating with structure and surface/interface morphology, and with increasing deposition temperature, tunable mechanical properties were seen in PLD Ga₂O₃films. At 700 °C, for nanocrystalline Ga₂O₃films, the dense grain packing reduces the elastic modulus E_rwhile improving the hardness. The improved crystallinity at elevated temperatures coupled with nanocrystallinity, theβ-phase stabilization is accounted for the observed enhancement in the mechanical properties of PLD Ga₂O₃films. The structure-morphology-mechanical property correlation in nanocrystalline PLDβ-Ga₂O₃films deposited on quartz substrates is discussed in detail.