Nonlinear optical response of GaN layers on sapphire: The impact of fundamental beam interference

GaN layers grown by metalorganic chemical-vapor deposition were characterized by optical second- and third-harmonic generation techniques. The angular dependence of the second-harmonic intensity in transmission showed a c-textured growth of the GaN layers on the sapphire substrates. The measured ratios d33/d15 and d33/d31 are equal to −2.02 and −2.03, respectively, which is indicative of a wurzite structure of the GaN layers. The measured d33 is 33 times that of the d11 of quartz. Fine oscillations were observed in the measured second- and third-harmonic angular dependencies that are explained by taking into account the interference of the fundamental beam in the GaN/sapphire structure.