High-Quality Single Crystalline Sc0.37Al0.63N Thin Films Enabled by Precise Tuning of III/N Atomic Flux Ratio during Molecular Beam Epitaxy

We attained wurtzite Sc<i>_x</i>Al_1-<i>x</i>N (0.16 ≤ <i>x</i> ≤ 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<i>_x</i>Al_1-<i>x</i>N thin films were derived as the feedback for precise determination of the Sc<i>_x</i>Al_1-<i>x</i>N growth diagram. We identified an optimal III/N atomic flux ratio of 0.78 for smooth Sc_0.18Al_0.82N 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 <i>x</i> > 0.37, which is substantially lower than the thermodynamically dictated threshold Sc content of ~0.55 in wurtzite Sc<i>_x</i>Al_1-<i>x</i>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<i>_x</i>Al_1-<i>x</i>N growth window is severely restricted by the adatom kinetics. We obtained single crystalline Sc_0.37Al_0.63N 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_0.37Al_0.63N epilayer validates unambiguous polarization flipping by 180°.