The optical absorption edge of single-crystal AlN prepared by a close-spaced vapor process

We have prepared several thicknesses of ∼1 cm by ∼1 cm epitaxial single crystals of AlN with a ’’high temperature (∼1800 °C) close-spaced vapor transport’’ technique. These crystals have been grown both on (011̄2) and (0001) sapphire wafers. The optical absorption edge is measured at 300 K and for the first time at low temperature (5 K). This data is parametrized to include both reflection losses due to scattering off surface imperfections and contributions from impurity absorption bands. This parametrization technique is used to determine the band gap and the nature, direct or indirect, of the absorption edge. A value of 6.28 eV was found to be the best value of Eg(5 K) and the gap was found to be direct.