Yu, J.; Luo, S.; Splith, D.; Selle, S.; Thieme, K.; Gierth, S.; Schultz, T.; Schlupp, P.; Sturm, C.; von Wenckstern, H.; Lorenz, M.; Koch, N.; Höche, T.; Grundmann, M.: Ultrawide bandgap spinel γ-(Ga0.8Ge0.2)2O3 alloy semiconductor epitaxial thin films. Journal of Applied Physics 137 (2025), p. 175302/1-12
10.1063/5.0255699
Open Accesn Version
Abstract:
Epitaxial growth of phase-pure and high-quality spinel ?-Ga2O3-based semiconductor thin films has been a big challenge for fundamental research on metastable defective inverse spinel ?-Ga2O3 semiconductors in view of potential device application. We report experimental results on epitaxial growth, microstructural, and electrical transport properties of (001)-oriented nominal ?-(Ga0.8Ge0.2)2O3 alloy semiconductor single crystal thin films with a coherent interface on cubic spinel (001) MgAl2O4 substrates by pulsed laser deposition using a Ge-rich target. Pristine films are found to be composed of about 2?nm thick insulating Ge-rich surface layers and the high-quality epitaxial n-type semiconductor film layers consisting of partially subvalent Ge2+ and Ga1+ cations as well as major components of normal Ge4+ and Ga3+ cations. Epitaxial films exhibit a direct bandgap of about 5.2?±?0.1?eV and a valence band maximum of about 3.3?±?0.1?eV below the Fermi level at room temperature. We further report a demonstration of ?-(Ga0.8Ge0.2)2O3 thin film-based metal-semiconductor field-effect transistor (MESFET) with the PtOx/Pt Schottky gate contact realized upon the surface pretreatment by Ar/O2 plasma etching. The MESFET device exhibits a clear field-effect with drain current modulation of about 105 orders of magnitude. This work not only significantly advances the fundamental and application-oriented research on epitaxial spinel ?-Ga2O3-based semiconductor films for practical device application but also offers new insight into microstructural characteristics of ultrawide bandgap spinel oxide semiconductor epitaxial thin films.