Effects of ain buffer layer on crystallographic structure and on electrical and optical properties of GaN and Ga1−AlN (0 < x ≦ 0.4) films grown on sapphire substrate by MOVPE

GaN and Ga1−xAlxN (0 < x ≦ 0.4) films grown by MOVPE on (0001) sapphire substrate are found to consist of many mosaic crystallites with various orientations. By preceding deposition of a thin AIN buffer layer, the microscopic fluctuation in crystallite orientation can be considerably reduced and the crystalline quality of the film is remarkably improved. Both the thickness and the deposition temperature of the AIN layer are found to be optimal as a buffer layer to convey the information of the substrate such as the crystallographic orientation and to relax the strain in this heteroepitaxial growth. The essential role of the AIN buffer layer is thought to be the supply of nucleation centers having the same orientation as the substrate and the promotion of lateral growth of the film due to the decrease in interfacial free energy between the film and the substrate.