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$Al_xGa_{1-x}N$ is a critical ultra-wide bandgap material foroptoelectronics, but the deposition of thick, high quality epitaxial layers hasbeen hindered by a lack of lattice-matched substrates. Here we identify the(111) face of transition metal carbides as a suitable class of materials forsubstrates lattice matched to (0001) $Al_xGa_{1-x}N$ and demonstrate the growthof thin film TaC which has an effective hexagonal lattice constant matched to$Al_{0.45}Ga_{0.55}N$. We explore growth conditions for sputtered TaC onsapphire substrates and investigate the effects of sputter power, layerthickness and incident plasma angle on film structure and in- and out-of-planestrain. We then show critical improvements to film quality by annealing filmsin a face-to-face configuration at 1600 $^\circ$C, which significantly reducesfull width at half max (FWHM) of in- and out-of-plane diffraction peaks andresults in a step-and-terrace surface morphology. This work presents a pathtoward electrically conductive, lattice matched, thermally compatiblesubstrates for $Al_xGa_{1-x}N$ heteroepitaxy, a critical step for verticaldevices and other power electronics applications.

Designing low-cost TaC virtual substrates for $Al_xGa_{1-x}N$ epitaxy