Crack-free ScxAl1−xN(0001) layers grown on Si(111) by plasma-assisted molecular beam epitaxy

We investigate the synthesis of 340-nm-thick ScxAl1−xN layers with 0 ≤ x ≤ 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 (0001̄)-oriented layers, as demonstrated by the (3 × 3) pattern observed in reflection high-energy electron diffraction and confirmed by KOH etching. The subsequent growth of pure wurtzite ScxAl1−xN layers with x ≤ 0.1 is feasible at temperatures ≤740 °C. However, layers with x ≥ 0.2 grown at 740 °C develop cracks due the high thermal mismatch between ScxAl1−xN and Si. Lowering the growth temperature to 500 °C not only prevents cracking but also improves the crystallinity of the layers. For Sc0.3Al0.7N layers grown at 500 °C, additional x-ray reflections due to intermetallic AlSc and Al3Sc inclusions are observed. The formation of these compounds can be inhibited by lowering the temperature further to 300 °C.