Designing TaC Virtual Substrates for Vertical Al_{x}Ga_{1−x}N Power Electronics Devices

Abstract

Power electronics are critical for a sustainable energy future, playing a key role in electrification and integration of renewable energy sources into the grid. Advances in ultrawide band gap materials are needed to handle higher powers in smaller form factors while reducing electrical and thermal losses. High Al content Al_{x}Ga_{1−x}N is theoretically capable of meeting these demands, but its impact in power electronics has been severely restricted by a lack of substrates that can satisfy conductivity, lattice matching, and/or thermal expansion requirements. We demonstrate that electrically conductive TaC can be used as a virtual substrate for Al_{x}Ga_{1−x}N heteroepitaxy. Scaleably sputtered TaC grown on Al_{2}O_{3}, followed by high-temperature face-to-face annealing, produces a thin film TaC template with an effective hexagonal lattice constant matched to Al_{0.70}Ga_{0.30}N. Annealing of the TaC promotes recrystallization, significantly improving crystallinity and reducing crystalline defects from as-deposited columnar grains to a step-and-terrace surface morphology, enabling the subsequent growth of high-quality Al_{0.70}Ga_{0.30}N by molecular beam epitaxy. X-ray diffraction and scanning transmission electron microscopy confirm that the Al_{x}Ga_{1−x}N layer is heteroepitaxially aligned, strain-free, and lattice-matched, transitioning abruptly from TaC to Al_{x}Ga_{1−x}N without intermediate phases. These results demonstrate TaC virtual substrates as electrically conductive, lattice-matched, and thermally compatible templates for vertical Al_{x}Ga_{1−x}N devices that can meet the growing power needs of a sustainable energy future.