Abstract
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°.