Effects of thermal annealing on GaN epilayers deposited on (0001) sapphire

In this work the effects of thermal annealing on the electrical and optical characteristics of GaN layers grown by Hydride-VPE on (0001) sapphire are studied. It is shown that thermal treatments at temperature above 900 °C under flowing nitrogen may have dramatic effects on the electrical properties of GaN epilayers, especially on those grown at low temperature (about 700 °C). In some cases the free electron concentration was reduced by about five orders of magnitude while the Hall mobility did not show consistent changes. The optical properties also varied: the cathodoluminescence (CL) measurements on annealed GaN showed that an additional large band centered around 3.05 eV appeared as a consequence of the thermal annealing while the near-band edge signal was found to be dependent upon the penetration depth of the excitation beam (low signal at the substrate/film interface and enhanced intensity at the film surface).