Enhancement of thermal donor formation in germanium by preliminary irradiation: infrared absorption measurements

Infrared absorption at 10 K of oxygen-rich Ge crystals after irradiation wi th fast electrons (E = 4 MeV, T approximate to 100 degreesC) and subsequent annealing at 220-420 degreesC has been studied. It is found that upon heat -treatments of the irradiated samples at 220-280 degreesC the simultaneous formation of four bistable thermal donors (TD1-TD4) occurs. This process is not accompanied by detectable changes in intensities of absorption bands a t 780.4 and 818.0 cm(-1), which are related to the oxygen dimer. Upon follo wing anneals of the samples at 300-420 degreesC, the strongly enhanced gene ration of the higher order TDs is observed with a simultaneous decrease in intensities of the bands originated from the oxygen dimer, TD1 and TD2. It is inferred that direct transformations of some radiation-induced defects i nto the first TD species occur in the irradiated samples. Radiation-induced complexes giving rise to local vibrational modes in the region 570-577 cm( -1) and/or at 729, 733.5 and 745 cm(-1) are suggested as possible precursor s to the bistable TDs in Ge. (C) 2001 Published by Elsevier Science B.V.