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Many of the FTIR results are discussed in detail in the text so will not be covered here. There has been a recent major breakthrough in FTIR analysis of thermal donors, as a series of vibrational absorptions in the region 975-1015 cm-1 have been identified as corresponding to the thermal donors [239]. In addition each of these centres also has a corresponding lower frequency mode at around 700-800 cm-1. These lie very close to the dimer vibration at 1012 cm-1 (see Chapter 6), and the earlier TD1/2 modes are also observed in as-grown material. The annealing characteristics of these lines has been observed at 370, 420 and 450$^\circ$C over long time scales, as well as their behaviour when subjected to high temperature dispersion treatments.

In addition, IR has been used to identify the electronic transitions associated with the thermal donors [240,241], where the (0/+) and (+/++) occupancy levels were shown to lie at around Ec - 0.07 and 0.15 eV respectively, with excited states in agreement with effective mass theory predictions.

Chris Ewels