Raman spectroscopy of nanocrystalline and amorphous GaN

We report Raman measurements on thin films of strongly disordered GaN and GaN:O prepared by ion-assisted deposition. The incident photon energies used in the experiments ranged from 1.95 to 3.8eV, spanning the interband edge. Under subgap excitation the signal resembles the crystalline GaN vibrational density-of-modes, with significant broadening as expected for disordered material. There is a strong resonant behavior at the interband edge of the same mode for which a strong resonance is found in crystalline GaN, with a width suggesting that the entire vibrational branch contributes to the signal. Even nanocrystalline material is found to display Raman spectra characteristic of very short-range (<1n) translational symmetry, in agreement with x-ray diffraction evidence for the random stacking nature of the 3nm diameter crystallites. The presence of oxygen at even 25at.% has only a subtle effect on Raman spectra at the network vibrational frequencies below 800cm−1, but its presence is signaled by the appearance of an oxygen mode at 1000cm−1. An N2 line at 2360cm−1 correlates with a nitrogen excess introduced during growth.