Local vibrational modes of hydrogen in GaN: Observation and theory

Gallium nitride (GaN) single crystals were implanted with protons (deuterons) at temperatures below 20 K and studied by infrared absorption spectroscopy at 8 K 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 225 K 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.