Lattice parameters of Sc_xAl_1-xN layers grown on GaN(0001) by plasma-assisted molecular beam epitaxy

An accurate knowledge of lattice parameters of ScxAl1-xN is essential for understanding the elastic and piezoelectric properties of this compound as well as for the ability to engineer its strain state in heterostructures. Using high-resolution x-ray diffractometry, we determine the lattice parameters of 100-nm-thick undoped ScxAl1-xN layers grown on GaN(0001) templates by plasma-assisted molecular beam epitaxy. The Sc content x of the layers is measured independently by both x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy, and it ranges from 0 to 0.25. The in-plane lattice parameter of the layers linearly increases with increasing x, while their out-of-plane lattice parameter remains constant. Layers with x approximate to 0.09 are found to be lattice matched to GaN, resulting in a smooth surface and a structural perfection equivalent to that of the GaN underlayer. In addition, a two-dimensional electron gas is induced at the ScxAl1-xN/GaN heterointerface, with the highest sheet electron density and mobility observed for lattice-matched conditions.