Molecular Beam Epitaxy of Nonpolar Cubic Al_xGa_1−xN/GaN Epilayers

Abstract

Cubic AlxGa1-xN films were grown by molecular beam epitaxy on freestanding 3C-SiC (001) substrates with an Al mole fraction of x=0 to 0.74. Using the intensity of a reflected high energy electron beam as a probe we find optimum growth conditions of c-AlGaN when a one-monolayer gallium coverage is formed at the growing surface. Clear reflection high energy electron diffraction oscillations during the initial growth of AlxGa1-xN/GaN layers were observed. The growth rate was about 177 nm/h. We find that the aluminium mole fraction is only determined by the aluminium flux, and that the AlxGa1-xN growth rate is independent on the aluminium content. Atomic force microscopy exhibits smooth surfaces with a RMS roughness of about 5 nm on 5×5 µm2 areas. Cathodoluminescence spectroscopy revealed clear band edge emission up to an aluminium mole fraction of x=0.52, showing a linear relation between the band gap energy and the Al composition.