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1. A Review of Recent Progress in β‐Ga 2 O 3 Epitaxial Growth: Effect of Substrate Orientation and Precursors in Metal–Organic Chemical Vapor Deposition

   https://doi.org/10.1002/pssa.202200616  

   Waseem, Aadil ; Ren, Zhongjie ; Huang, Hsien-Chih ; Nguyen, Kristen ; Wu, Xihang ; Li, Xiuling ( December 2022 , physica status solidi (a))

   Gallium oxide (Ga₂O₃) is a highly promising ultrawide‐bandgap semiconductor for power electronics that emerged about a decade ago. Epitaxial growth Ga₂O₃at the small scale is demonstrated. In order to develop scalable manufacturing of high‐performance epitaxial structures, in‐depth understanding of the fundamental growth processes, control parameters, and mechanism is imperative. This review discusses the recent progress in epitaxial growth of β‐Ga₂O₃films and highlights challenges in obtaining high growth rate, low defects, and high carrier mobilities. Compared with the other epitaxy methods, metal–organic chemical vapor deposition (MOCVD) offers a wider growth window and precursor selection option, to minimize the tradeoff between crystal quality and growth rate. Growth rate is inversely proportional to temperature, within a certain temperature window, because of the unavoidable premature gas‐phase reactions and desorption of the highly volatile gallium suboxide (Ga₂O) at elevated temperatures. Growth defects, background impurity incorporation, and carrier mobilities can be affected by the choice of MOCVD precursors, nucleation, and adsorption/desorption/diffusion of adatoms on substrate surfaces of different orientations, including the effect of growing on cleavage and noncleavage planes. This review summarizes the current status of the epitaxial growth of β‐Ga₂O₃and analyzes the major factors that enhance mobility and reduce background doping concentration. The insights gained help advance the manufacturability of device‐grade epitaxial thin films.

    

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2. 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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3. Probing Charge Transport and Background Doping in Metal‐Organic Chemical Vapor Deposition‐Grown (010) β‐Ga 2 O 3

   https://doi.org/10.1002/pssr.202000145  

   Feng, Zixuan ; Bhuiyan, A. F. M. Anhar Uddin ; Xia, Zhanbo ; Moore, Wyatt ; Chen, Zhaoying ; McGlone, Joe F. ; Daughton, David R. ; Arehart, Aaron R. ; Ringel, Steven A. ; Rajan, Siddharth ; et al ( June 2020 , physica status solidi (RRL) – Rapid Research Letters)

   A new record‐high room‐temperature electron Hall mobility (μ_RT = 194 cm² V⁻¹ s⁻¹atn ≈ 8 × 10¹⁵ cm⁻³) for β‐Ga₂O₃is demonstrated in the unintentionally doped thin film grown on (010) semi‐insulating substrate via metal‐organic chemical vapor deposition (MOCVD). A peak electron mobility of ≈9500 cm² V⁻¹ s⁻¹is achieved at 45 K. Further investigation on the transport properties indicates the existence of sheet charges near the epilayer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epilayer and the interface, originating from both surface contamination and growth environment. The pregrowth hydrofluoric acid cleaning of the substrate leads to an obvious decrease in Si impurity both at the interface and in the epilayer. In addition, the effect of the MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β‐Ga₂O₃film with very low bulk charge concentration, even a reduced sheet charge density plays an important role in the charge transport properties.

    

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4. Si doping in MOCVD grown (010) β-(Al x Ga 1−x ) 2 O 3 thin films

   https://doi.org/10.1063/5.0084062  

   Bhuiyan, A. F. ; Feng, Zixuan ; Meng, Lingyu ; Fiedler, Andreas ; Huang, Hsien-Lien ; Neal, Adam T. ; Steinbrunner, Erich ; Mou, Shin ; Hwang, Jinwoo ; Rajan, Siddharth ; et al ( April 2022 , Journal of Applied Physics)

   In this work, the structural and electrical properties of metalorganic chemical vapor deposited Si-doped β-(Al x Ga 1−x ) 2 O 3 thin films grown on (010) β-Ga 2 O 3 substrates are investigated as a function of Al composition. The room temperature Hall mobility of 101 cm 2 /V s and low temperature peak mobility (T = 65 K) of 1157 cm 2 /V s at carrier concentrations of 6.56 × 10 17 and 2.30 × 10 17  cm −3 are measured from 6% Al composition samples, respectively. The quantitative secondary ion mass spectroscopy (SIMS) characterization reveals a strong dependence of Si and other unintentional impurities, such as C, H, and Cl concentrations in β-(Al x Ga 1−x ) 2 O 3 thin films, with different Al compositions. Higher Al compositions in β-(Al x Ga 1−x ) 2 O 3 result in lower net carrier concentrations due to the reduction of Si incorporation efficiency and the increase of C and H impurity levels that act as compensating acceptors in β-(Al x Ga 1−x ) 2 O 3 films. Lowering the growth chamber pressure reduces Si concentrations in β-(Al x Ga 1−x ) 2 O 3 films due to the increase of Al compositions as evidenced by comprehensive SIMS and Hall characterizations. Due to the increase of lattice mismatch between the epifilm and substrate, higher Al compositions lead to cracking in β-(Al x Ga 1−x ) 2 O 3 films grown on β-Ga 2 O 3 substrates. The (100) cleavage plane is identified as a major cracking plane limiting the growth of high-quality Si-doped (010) β-(Al x Ga 1−x ) 2 O 3 films beyond the critical thicknesses, which leads to highly anisotropic and inhomogeneous behaviors in terms of conductivity. 

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5. 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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