Vacancy defect reactions associated with oxygen and bismuth in irradiated germanium

Defect reactions occurring in Ge:Bi, O crystals upon electron and gamma irradiations at room temperature and the following isochronal annealing in the temperature range 80-350 °C have been studied by means of deep level spectroscopy (DLTS) and high-resolution Laplace DLTS. It is shown that bismuth and oxygen impurity atoms are the most effective traps for mobile vacancies created by irradiation in the crystals. Both the vacancy-oxygen (VO or A center) and vacancy-bismuth (Bi-V or E center) defects in Ge have three charge states and introduce two energy levels into the gap. The ratio of concentrations of the VO and Bi-V centers in as-irradiated samples depends on the relative concentrations of interstitial oxygen (Oi) and substitutional bismuth (Bis) atoms. Annealing studies demonstrate that in the temperature range 120-150 °C the concentration of the Bi-V center increases. This has been associated with the dissociation of VO that occurs in the range 100-150 °C. In consequence the proposed mechanism is the release of vacancies from the A center which are subsequently trapped by the Bi impurity atoms to form the Bi-V E center. It is suggested that the enhanced disappearance of the Bi-V centers in electron-irradiated oxygen-rich Ge samples is associated with the interaction of the E centers with mobile Ge self-interstitials released from oxygen-related traps at T≥140°C.