Designing Ta⁢C Virtual Substrates for Vertical Al𝑥⁢Ga1−𝑥⁢N Power Electronics Devices

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 Al𝑥⁢Ga1−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 Ta⁢C can be used as a virtual substrate for Al𝑥⁢Ga1−𝑥⁢N heteroepitaxy. Scaleably sputtered Ta⁢C grown on Al2⁢O3, followed by high-temperature face-to-face annealing, produces a thin film Ta⁢C template with an effective hexagonal lattice constant matched to Al0.70⁢Ga0.30⁢N. Annealing of the Ta⁢C 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 Al0.70⁢Ga0.30⁢N by molecular beam epitaxy. X-ray diffraction and scanning transmission electron microscopy confirm that the Al𝑥⁢Ga1−𝑥⁢N layer is heteroepitaxially aligned, strain-free, and lattice-matched, transitioning abruptly from Ta⁢C to Al𝑥⁢Ga1−𝑥⁢N without intermediate phases. These results demonstrate Ta⁢C virtual substrates as electrically conductive, lattice-matched, and thermally compatible templates for vertical Al𝑥⁢Ga1−𝑥⁢N devices that can meet the growing power needs of a sustainable energy future.