Theory of local-phonon-coupled low-energy anharmonic excitation of the interstitial oxygen in silicon

The previous model for the low-energy anharmonic excitation of the interstitial oxygen in silicon [D. R. Bosomworth, W. Hayes, A. R. L. Spray, and G. D. Watkins, Proc. R. Soc. London, Ser. A 317, 133 (1970)] is expanded so that the coupling to the local phonon is included. The result of the calculation explains the previously observed absorption peaks of ¹⁶O and ¹⁸O in the 30-, 1100-, and 1200-cm^-1 bands, confirming the energy-level scheme and transition assignment of Bosomworth et al. The coupling significantly reduces the level separations of the low-energy anharmonic excitation, and plays an important role in explaining the ¹⁸O isotope peak shifts. A physical interpretation is given to the calculated negative coupling constant. The origin of the 517- and 1700-cm^-1 bands is also discussed.