Characterization of the absorption edges of epitaxial AlGaN grown by plasma-induced molecular beam epitaxy

The dependence of the absorption edge of wurtzite Al _XGa _1-XN on the AlN mole fraction was studied. The Al mole fraction was varied from 0 to 1. The absorption coefficient at room temperature was determined by transmission and photothermal deflection spectroscopy. Photothermal deflection spectroscopy can be applied to determine the low absorbance values. With those results, the effective bandgaps of the Al _XGa _1-XN alloys were defined as the photon energy E _4.8 at an absorption coefficient of 10 _4.8 cm ^-1. From the energy position of the absorption edge versus AlN mole fraction, a bowing parameter of 1.3 eV could be determined. The bowing parameter agreed quite well with the measured effective bandgaps of AlGaN alloys.