The thickness of the epitaxial layers was measured using Fourier transform infrared (FTIR) reflectance and the surface morphology of the epitaxial layers was studied by an optical microscope with  Nomarski differential interference contrast, and by atomic force microscopy (AFM) in tapping mode on 20 × 20 μm 2  areas at the center of the substrate andat two different spots about 2 mm from the sample edge.