MBE Growth and Properties of Monoclinic (Al_xGa_1-x-yIn_y)₂O₃ Alloys

Gallium oxide (Ga₂O₃) is an emerging ultra-wide bandgap semiconductor material that has attracted attention for its potential to outperform existing SiC and GaN based devices operating at high breakdown voltages and high temperature. The thermodynamically stable phase at room temperature and pressure is the monoclinic β-phase with symmetry <i>C2/m</i>. β-Ga₂O₃ has a direct bandgap energy of 4.76 eV and critical field as high as 8 MV/cm. Isovalent alloying of In and Al in β-Ga₂O₃ provides the ability to engineer bandgap energy and strain of the material, however the monoclinic structure is not the ground state for Al₂O₃ or In₂O₃. First-principles calculations indicate a range of bandgap energies from 7.2-7.5 eV for monoclinic θ-Al₂O₃ to 2.7 eV for monoclinic In₂O₃. The corresponding lattice mismatch to β-Ga₂O₃ ranges from about 4% for θ-Al₂O₃ to 10% for monoclinic In₂O₃. In principle the quaternary (Al_xGa_1-x-yIn_y)₂O₃ can span this bandgap energy and strain range, providing device designers with additional degrees of freedom to tune band offsets and electronic properties.<br/><br/>However, efforts to synthesize isovalent alloys are complicated by their tendency to phase separate. Strain balanced alloys of (Al_xGa_1-x)₂O₃and (In_xGa_1-x)₂O₃ may be able to overcome the tendency for phase separation and stabilize the monoclinic crystal, however literature reports of the quaternary (Al_xGa_1-x-yIn_y)₂O₃are limited to &lt;1% unintentional indium incorporation in In-catalyzed (Al_xGa_1-x)₂O₃.¹ The primary limitation to quaternary formation is the limited incorporation of indium at elevated growth temperatures. This limited incorporation is due to both the volatility of indium oxide and Al and Ga cation exchange reactions which replace indium in In₂O₃.²<br/><br/>We report on the first successful synthesis of phase pure monoclinic (Al_xGa_1-x-yIn_y)₂O₃ by molecular beam epitaxy. Plasma-assisted molecular beam epitaxy (MBE) is used to grow (Al_xGa_1-x-yIn_y)₂O₃ on (010) oriented Fe doped β-Ga₂O₃ substrates at 750 °C and Ga and In beam equivalent pressures (BEP) of 5×10^-8 and 2×10^-7 torr, respectively. The Al BEP varies from 1×10^-9 to 1.5×10^-8 torr. X-ray diffraction measurements of the (020) plane indicate a strained film consistent with the structural incorporation of both Al and In, and this incorporation is chemically corroborated by x-ray fluorescence (XRF) and X-ray photoemission spectroscopy (XPS). Wide-angle ω-2θ survey scans confirm the absence of additional XRD peaks, indicating the films are monoclinic with no phase separation. Spectroscopic ellipsometry measurements show a monotonic shift in the extinction coefficient edge from approximately 4.7 to 5.2 eV with increasing Al BEP. Additional structural, chemical, optical, and electrical characterization measurements including Rutherford back-scattering spectrometry, scanning tunneling microscopy will be presented, concluding with an outlook on the applicability of these quaternary films to gallium oxide device design.<br/><br/>[1] P. Vogt, A. Mauze, F. Wu, B. Bonef, and J. S. Speck, "Metal-oxide catalyzed epitaxy (MOCATAXY): the example of the O plasma-assisted molecular beam epitaxy of β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ heterostructures", Appl. Phys. Express <b>11</b>, 115503 (2018).<br/>[2] P. Vogt, O. Brandt, H. Riechert, J. Lähnemann, and O. Bierwagen, "Metal-Exchange Catalysis in the Growth of Sesquioxides: Towards Heterostructures of Transparent Oxide Semiconductors", Phys. Rev. Lett. <b>119</b>, 196001 (2017).