Local vibrational modes of hydrogen in GaN: Observation and theory

Gallium nitride (GaN) single crystals were implanted with protons (deuterons) at temperatures below 20K and studied by infrared absorption spectroscopy at 8K without any intermediate heating of the samples. A strong absorption line was observed at 1456cm-1 after proton implantation. The annealing of the 1456-cm-1 line at about 225K is correlated with the appearance of two absorption lines at 3027 and 3139cm-1, which were shown previously to represent N-H stretch modes of VGa-H defects in GaN. The observation of the 1456-cm-1 line after low-temperature implantation, together with the observation of a correlated annealing for the 1456-cm-1 line and VGa-H centers, suggests that the line originates from isolated H in GaN. First-principles local-density-functional theory was used to calculate the H-related local mode frequencies of different structures and charge states of isolated H in GaN. The likely candidates for the assignment of the 1456-cm-1 line are discussed on basis of mode frequencies predicted for different structures