DEFECT MODES AND DISORDER-INDUCED RAMAN-SCATTERING IN GAN

We present results of Raman-scattering experiments on GaN layers with different ratios of cubic and hexagonal phase grown on GaAs and sapphi re. In the low-energy region (60-250 cm(-1)) we found a series of shar p modes which increase exponentially in intensity with decreasing temp erature. The modes disappear when reaching room temperature. Wavelengt h-dependent measurements reveal that they are strongest under excitati on at around 568 nm. No magnetic-field dependence could be observed. F rom their exclusive presence in GaN layers grown on GaAs, we conclude that they are likely to be caused by As-defects in GaN or N-impurities in GaAs. Apart from the host phonon modes in the high-energy region w e found a broad band ranging from the cubic TO to the LO phonon freque ncies, which is caused by disorder-induced scattering.