We report the observation of infrared (IR) vibrational bands related to ind
ividual thermal double donors (TDs) in Ge crystals enriched with either O-1
6 or O-18 isotopes of oxygen. The TDs were generated by heating the oxygen-
doped Ge at 300 degrees C and 350 degrees C. IR absorption spectra were mea
sured at room temperature (RT) and 10 It (LT). In the RT spectra two broad
bands at about 600 and 780 cm(-1) are found to develop upon the TD generati
on. In the LT spectra a splitting of the bands into series of rather sharp
bands (up to 9 resolved lines) related to a double-donor (DD) configuration
of individual TDs (TD1-TD9) is observed. A manifestation of bistability of
the first four TD species (TD1-TD4) is observed in absorption spectra meas
ured at 10 K after different cooling conditions. The pairs of lines due to
bistable TDs in the DD configuration are detected after cooling under the b
and-gap illumination. They are found to transform to the sets of three line
s after cooling in the dark. These triplets are assigned to the local vibra
tional modes (LVMs) of a neutral (X) configuration of bistable TDs. Oxygen
isotopic shifts of LVMs due to TDs are determined and compared with those f
or vibrational modes of interstitial oxygen in Ge. (C) 1999 Elsevier Scienc
e B.V. All rights reserved.