Structural optimization of lattice-matched Sc0.14Al0.86N/GaN superlattices for photonic applications

Abstract

ScxAl1−xN is an emerging III-nitride material known for its high piezoelectric coefficient and ferroelectric properties. Integration of wide-bandgap ScxAl1−xN with GaN is particularly attractive for quantum photonic devices. Achieving low defect complex multilayers incorporating ScxAl1−xN, though, requires precise lattice-matching and carefully optimized growth parameters. This study systematically investigates the molecular-beam epitaxy of short-period ScxAl1−xN/GaN superlattices with total thicknesses of up to 600 nm on GaN templates. X-ray diffraction reciprocal space mapping confirmed lattice-matching at x = 0.14 ± 0.01 Sc composition regardless of the thickness of GaN interlayers, as evidenced by symmetric superlattice satellites aligned in-plane with the underlying substrate peak. Superlattices with Sc compositions deviating from this lattice-matching condition exhibited strain-induced defects ranging from crack formation to partial relaxation. Scanning transmission electron microscopy investigation of the ScxAl1−xN/GaN interfaces identified temperature-dependent intermixing as a major factor in setting the nitride composition variation and implicitly band structure profile along the growth direction. Energy-dispersive x-ray spectroscopy also revealed that Sc incorporation exhibits delays relative to Al at both onset and termination. Optimal growth conditions for superlattices with 6 nm ScxAl1−xN layers were observed at approximately 600 and 550 °C for structures with thick GaN layers (6 nm) and ultra-thin GaN layers (≤2 nm), respectively.