High-Quality Single Crystalline Sc 0.37 Al 0.63 N Thin Films Enabled by Precise Tuning of III/N Atomic Flux Ratio during Molecular Beam Epitaxy.

We attained wurtzite Sc _x Al _{1- x} N (0.16 ≤ x ≤ 0.37) thin films by varying the Sc and Al fluxes at a fixed active nitrogen flux during plasma-assisted molecular beam epitaxy. Atomic fluxes of Sc and Al sources via measured Sc percentage in as-grown Sc _x Al _{1- x} N thin films were derived as the feedback for precise determination of the Sc _x Al _{1- x} N growth diagram. We identified an optimal III/N atomic flux ratio of 0.78 for smooth Sc _0.18 Al _0.82 N thin films. Further increasing the III/N ratio led to phase separation under N-rich conditions, validated by the observation of high-Sc-content hillocks with energy-dispersive X-ray spectroscopy mapping. At the fixed III/N ratio of 0.78, we found that phase separation with high-Al-content hillocks occurs for x > 0.37, which is substantially lower than the thermodynamically dictated threshold Sc content of ~0.55 in wurtzite Sc _x Al _{1- x} N. We postulate that these wurtzite-phase purity degradation scenarios are correlated with adatom diffusion and the competitive incorporation process of Sc and Al. Therefore, the Sc _x Al _{1- x} N growth window is severely restricted by the adatom kinetics. We obtained single crystalline Sc _0.37 Al _0.63 N thin films with X-ray diffraction (002)/(102) ω rocking curve full-width at half-maximums of 2156 arcsec and 209 arcsec and surface roughness of 1.70 nm. Piezoelectric force microscopy probing of the Sc _0.37 Al _0.63 N epilayer validates unambiguous polarization flipping by 180°.