Metal-modulated epitaxy of ScAlN thin films and lattice-matched ScAlN/GaN superlattices

Scandium aluminum nitride (ScxAl1−xN) is attractive for novel quantum photonic applications due to the prospect of growing arbitrarily thick low-defect ScxAl1−xN/GaN heterostructures on c-plane GaN. While high-quality GaN growth by molecular-beam epitaxy (MBE) is performed under metal-rich conditions at substrate temperatures >700 °C, ScxAl1−xN undergoes phase segregation and roughening under these conditions. For this reason, MBE of Sc-containing nitrides has been typically done under nitrogen-rich conditions at relatively low substrate temperatures (<600 °C). We demonstrate a metal-modulated epitaxy method for ScxAl1−xN growth under metal-rich conditions that produces smooth ScxAl1−xN layers and ScxAl1−xN/GaN superlattices (SLs) with enhanced structural quality. In our approach, metal and nitrogen shutters are precisely timed to maintain approximately one monolayer (ML) of metal adlayer during the majority of ScxAl1−xN growth while preventing excessive metal buildup on the surface in each ∼1 nm cycle. This approach is especially beneficial for ScxAl1−xN/GaN SLs with ultrathin GaN quantum wells that require near-ML control of interface abruptness to limit intermixing and roughness that undermine optical or electronic properties. Using this metal-modulated approach at 600 °C, interface root mean square roughness in a 6 nm Sc0.14Al0.86N/2 nm GaN SL is kept below 0.1 nm, and interface widths are improved substantially relative to SLs grown under nitrogen-rich conditions at the same temperature.