Structural and optical properties of epitaxial Sc_xAl_1−xN coherently grown on GaN bulk substrates by sputtering method

Three scandium aluminum nitride (ScAlN) thin films with different Sc compositions of 6%, 10%, and 14% were heteroepitaxially grown on n-type GaN bulk substrates by a low-temperature sputtering method. Atomically flat and smooth surfaces were observed by atomic force microscopy. The ScAlN films were coherently grown on GaN, and the c-axis lattice constants increased with increase in the Sc composition, confirmed by x-ray diffraction. The refractive index and the extinction coefficient of ScAlN were extracted by variable angle spectroscopic ellipsometry. The refractive index slightly increased and the extinction coefficient showed red shift with increase in the Sc composition. The optical bandgap of the ScAlN films was also extracted, which slightly shrunk with increase in the Sc composition.