Local vibrational mode spectroscopy of thermal donors in germanium

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.