Temperature-dependent widths of infrared and far-infrared absorption lines of oxygen in silicon

For temperatures below 70 K, we measured the widths of the absorption lines in the 1100- and 30-cm(-1) bands of oxygen in silicon. Strong temperature dependences of the line widths observed for both bands were ascribed to the one-phonon transitions in the energy-level ladders of the 30-cm(-1) excitation belonging to the ground and first excited states of the 1100-cm(-1) mode. A formulation was given to the interaction between the two-dimensional low-energy anharmonic excitation of oxygen and the phonons. This clarified the selection rule for the phonon transitions, and suggested the essential phonon transitions for the observed behavior of the line widths. Origin for the extreme narrowness of the low-temperature 29.3-cm(-1) line was also clarified.