Crack-free Sc${}_x$Al${}_{1-x}$N(000$\bar{1}$) layers grown on Si(111) by plasma-assisted molecular beam epitaxy

Abstract

We investigate the synthesis of 340-nm-thick Sc${}_x$Al${}_{1-x}$N layers with $0\le x\le 0.35$ on AlN-buffered Si(111) by plasma-assisted molecular beam epitaxy. We employ an AlN nucleation layer under conditions giving rise to single-domain N-polar [(000$\bar{1}$)-oriented] layers, as demonstrated by the ($3\times 3$) pattern observed in reflection high-energy electron diffraction and confirmed by KOH etching. The subsequent growth of pure wurtzite Sc${}_x$Al${}_{1-x}$N layers with $x\le 0.1$ is feasible at temperatures $\le$ 740°C. However, layers with $x\ge 0.2$ grown at 740°C develop cracks due the high thermal mismatch between Sc${}_x$Al${}_{1-x}$N and Si. Lowering the growth temperature to 500°C not only prevents cracking but also improves the crystallinity of the layers. For Sc${}_{0.3}$Al${}_{0.7}$N layers grown at 500°C, additional x-ray reflections due to intermetallic AlSc and Al${}_3$Sc inclusions are observed. The formation of these compounds can be inhibited by lowering the temperature further to 300°C.