TEMPERATURE-DEPENDENCE OF THE ENERGY-GAP IN GAN BULK SINGLE-CRYSTALS AND EPITAXIAL LAYER

We performed optical-absorption studies of the energy gap in various GaN samples in the temperature range from 10 up to 600 K. We investigated both bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate. The observed positions of the absorption edge vary for different samples of GaN (from 3.45 to 3.6 eV at T=20 K). We attribute this effect to different free-electron concentrations (Burstein-Moss effect) characterizing the employed samples. For the sample for which the Burstein shift is zero (low free-electron concentration) we could deduce the value of the energy gap as equal to 3.427 eV at 20 K. Samples with a different free-electron concentration exhibit differences in the temperature dependence of the absorption edge. We explain the origin of these differences by the temperature dependence of the Burstein-Moss effect.