TaC Virtual Substrates for AlGaN Epitaxy

High quality ultra-wide bandgap materials and devices require high quality substrates. Homoepitaxial substrates for ultra-wide band gap semiconductors like AlGaN are prohibitively expensive and alternative heteroepitaxial schemes to date have been complex and resulted in poorly performing devices. We have identified TaC as a high temperature stable material as capable of serving as a low cost and high-quality lattice-matched heteroepitaxial template for AlGaN. The (111) plane of rock-salt TaC has a lattice parameter perfectly lattice matched to the hexagonal (001) plane of AlGaN. Cost-effective substrates for high quality AlGaN growth are a critical component to the development of vertical devices needed for next generation high power electronics.<br/>In this work we grow (111)-oriented epitaxial TaC on Al₂O₃ substrates by RF sputtering. We explore structure and crystallinity as a function of sputter gun power, plasma incidence angle, chamber pressure, and film thickness. We find temperatures above 800 C and pressures of 5 mTorr result in pure-phase (111) oriented TaC. Rocking curves confirm the presence of a (113) peak; these plane angles geometrically differentiate the TaC layer from a competing Ta₂C phase. X-ray diffraction confirms registry to the oxygen sublattice of the underlying sapphire substrate. Transmission electron micrographs also show epitaxial registry of the grown layer to the substrate.<br/>We also investigate crystallinity improvements resulting from annealing TaC films at. We find the optimal annealing geometry is a face-to-face arrangement, which appears to restrict oxidation and evaporation of the thin film layers. Precedent with AlN thin films suggests that face to face annealing promotes recrystallization while maintaining a local partial pressure near the surface of the film. [1] XRD of TaC films before and after annealing above 1500 C shows sharper peaks with reduced full width half max for the (111) plane while still maintaining phase purity. We also assess surface roughness and dislocation density before and after annealing. Our results indicate TaC virtual substrates may be a commercially viable path forward for growth of AlGaN devices.<br/>[1] Miyake, H., Lin, C.-H., Tokoro, K. & Hiramatsu, K. Preparation of high-quality AlN on sapphire by high-temperature face-to-face annealing. <i>Journal of Crystal Growth</i> <b>456</b>, 155–159 (2016).