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.