Structural and optical analysis of epitaxial GaN on sapphire

We investigate epitaxial layers of GaN on c-plane sapphire by photoluminescence, optical density and x-ray diffraction. Besides the well known luminescence from hexagonal GaN we have identified two emission bands from cubic GaN. We observe the emission of the donor bound exciton in cubic GaN at 3.279 eV. The luminescence at 3.15-3.21 eV is explained as the cubic donor-acceptor recombination. The corresponding acceptor binding energy is found to be low as E-A less than or equal to 100 meV. For layers of 400 nm thickness, the optical density yield values for the average contents of cubic GaN between 10 and 25%. Proper growth conditions minimize the cubic contents in the upper regions of such layers.