Vibrational dynamics of bond-center hydrogen in crystalline silicon

The absorption line shape associated with the fundamental transition of the H-related stretch mode of bond-center hydrogen in crystalline silicon is measured as a function of temperature with infrared spectroscopy. In addition to the shift in frequency and increase in linewidth usually observed for local vibrational modes in solids, the absorption line becomes asymmetric at elevated temperatures. A theoretical model is developed, which describes the temperature-dependent line shape in terms of thermal fluctuations in the occupation number of a low-frequency mode coupled anharmonically to the stretch mode. The model successfully describes the shift, broadening, and asymmetry of the absorption line and gives new insight into the nature of the low-frequency mode. The mode has a frequency of 114 cm^-1, is twofold degenerate, and exhibits no isotopic shift when deuterium is substituted for hydrogen. It is assigned to a pseudolocalized, Si-related mode of bond-center hydrogen.