Two groups of infra-red localized vibrational bands, at 975 to 1015 and 724 to 748/cm, were correlated with well-known infra-red electronic bands that were due to thermal double donors, and with the thermal donor concentration that was deduced from resistivity measurements. The 2 groups were suggested to be due to 2 different vibrational modes of O atoms in a thermal donor core. Vibrational bands at 975, 988, 999 and 1006/cm were correlated with TD1, TD2, TD3 and thermal donors numbered TD4 and above, respectively. A band at 1012/cm was correlated with the NL10 center. A calibration coefficient was determined, for donor-related vibrational bands, that could be used to estimate the sizes of donor-related centers; assuming that the calibration coefficient for interstitial O was applicable to the O atoms of the centers. This indicated that all of the donor-related bands originated from centers with one to two O atoms; thus suggesting that these bands were due to vibrations of O atoms in a thermal donor core. The various positions of the donor-related bands were attributed to differently strained environments that were produced by different O clusters. It was suggested that these clusters would develop into larger O precipitates which, at the end of thermal donor formation, appeared in the spectrum; with a broad infra-red band at about 1060/cm. The early stages of thermal donor formation at temperatures below about 450C were closely related to the transformation of pre-existing clusters that were related to the 1012/cm band. This explained the formation of the early thermal donors at low temperatures, when the interstitial O concentration was nearly constant.

T.Hallberg, J.L.Lindström: Journal of Applied Physics, 1996, 79[10], 7570-81