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1. Demonstration of thick phase-pure β-Ga2O3 on a c-plane sapphire substrate using MOCVD

   https://doi.org/10.1117/12.2661097  

   Jewel, Mohi Uddin ; Hasan, Samiul ; Crittenden, Scott R. ; Avrutin, Vitaliy S. ; Özgür, Ümit ; Morkoç, Hadis ; Ahmad, Iftikhar ( March 2023 , Proc. SPIE 12422, Oxide-based Materials and Devices)

   Teherani, Ferechteh H. ; Rogers, David J. (Ed.)

   We demonstrated a metal-organic chemical vapor deposition (MOCVD) of smooth, thick, and monoclinic phase-pure gallium oxide (Ga2O3) on c-plane sapphire using silicon-oxygen bonding (SiOx) as a phase stabilizer. The corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 with a bandgap in the 4.4 – 5.1 eV range, are promising materials for power semiconductor devices and deep ultraviolet (UV) solar-blind photodetectors. The MOCVD systems are extensively used for homoepitaxial growth of β-Ga2O3 on (001), (100), (010), and (¯2 01) β-Ga2O3 substrates. These substrates are rare/expensive and have very low thermal conductivity; thus, are not suitable for high-power semiconductor devices. The c-plane sapphire is typically used as a substrate for high-power devices. The β-Ga2O3 grows in the (¯2 01) direction on sapphire. In this direction, the presence of high-density oxygen dangling bonds, frequent stacking faults, twinning, and other phases and planes impede the heteroepitaxy of thick β-Ga2O3. Previously phase stabilizations with SiOx have been reported for tetragonal and monoclinic hafnia. We were able to grow ~580nm thick β-Ga2O3 on sapphire by MOCVD at 750 oC through phase stabilization using silane. The samples grown with silane have a reduction in the surface roughness and resistivity from 10.7 nm to 4.4 nm and from 371.75 Ω.cm to 135.64 Ω.cm, respectively. These samples show a pure-monoclinic phase determined by x-ray diffraction (XRD); have tensile strain determined by Raman strain mapping. These results show that a thick, phase-pure -Ga2O3 can be grown on c-plane sapphire which can be suitable for creating power devices with better thermal management. 

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2. 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 by 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. 

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3. Al Incorporation up to 99% in Metalorganic Chemical Vapor Deposition‐Grown Monoclinic (Al x Ga 1–x ) 2 O 3 Films Using Trimethylgallium

   https://doi.org/10.1002/pssr.202300224  

   Bhuiyan, A. F. M. Anhar Uddin ; Meng, Lingyu ; Huang, Hsien-Lien ; Chae, Christopher ; Hwang, Jinwoo ; Zhao, Hongping ( August 2023 , physica status solidi (RRL) – Rapid Research Letters)

   Growths of monoclinic (Al_xGa_1−x)₂O₃thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h⁻¹) of β‐(Al_xGa_1−x)₂O₃thin films on (010), (100), and (01) β‐Ga₂O₃substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(Al_xGa_1−x)₂O₃films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (Al_xGa_1−x)₂O₃films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (01) β‐Ga₂O₃substrates, respectively. Beyond 29% of Al incorporation, the (010) (Al_xGa_1−x)₂O₃films exhibit β‐ to γ‐phase segregation. β‐(Al_xGa_1−x)₂O₃films grown on (01) β‐Ga₂O₃show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (Al_xGa_1−x)₂O₃grown on (100) Ga₂O₃are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(Al_xGa_1−x)₂O₃films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices.

    

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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 −1  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. 

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5. Oxygen Incorporation in the Molecular Beam Epitaxy Growth of Sc x Ga 1−x N and Sc x Al 1−x N

   https://doi.org/10.1002/pssb.201900612  

   Casamento, Joseph ; Xing, Huili Grace ; Jena, Debdeep ( January 2020 , physica status solidi (b))

   Secondary‐ion mass spectrometry (SIMS) is used to determine impurity concentrations of carbon and oxygen in two scandium‐containing nitride semiconductor multilayer heterostructures: Sc_xGa_1−xN/GaN and Sc_xAl_1−xN/AlN grown by molecular beam epitaxy (MBE). In the Sc_xGa_1−xN/GaN heterostructure grown in metal‐rich conditions on GaN–SiC template substrates with Sc contents up to 28 at%, the oxygen concentration is found to be below 1 × 10¹⁹ cm⁻³, with an increase directly correlated with the scandium content. In the Sc_xAl_1−xN–AlN heterostructure grown in nitrogen‐rich conditions on AlN–Al₂O₃template substrates with Sc contents up to 26 at%, the oxygen concentration is found to be between 10¹⁹and 10²¹ cm⁻³, again directly correlated with the Sc content. The increase in oxygen and carbon takes place during the deposition of scandium‐alloyed layers.

    

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