Reflectance difference spectroscopy characterization of Al_xGa_{1 x}N-compound layers

We report reflectance-difference spectroscopy characterization of Al_xGa_1-xN-compound layers grown on sapphire and 6H-SiC. We observe interference features below the bandgap, a derivative feature at the bandgap, and no signal or a broad feature above the bandgap. We present simulations of the data which are based on the assumption that the c-axis is tilted with respect to the surface normal, which results in an apparent bulk anisotropy in the hexagonal layers in normal-incidence reflectance measurements. We discuss implications with respect to a model by Tersoff [1], who suggested the formation of macrosteps during epitaxy.