Optical properties of high-Al-content crack free AlxGa1−xN (x<0.67) films grown on Si(111) by molecular-beam epitaxy

We report on the optical properties of high-Al-content crack free AlxGa1−xN (x<0.67) films grown by molecular-beam epitaxy on Si(111) substrates using ammonia as nitrogen source. The energetic position of the A free exciton as a function of the Al content is determined from photoluminescence and reflectivity measurements at low temperature. A bowing parameter of b=1 eV is deduced from these measurements. The excitonic linewidth increases as a function of Al concentration. The observed variation agrees very well with the one calculated using a model in which the broadening effect is assumed to be due to alloy compositional disordering.