Positron annihilation spectroscopy of vacancy complexes in SiGe

The aim of this contribution is to present a review of the results related to vacancy complexes in SiGe obtained in the positron group at Helsinki University of Technology. We have studied proton irradiated undoped and P doped SiGe layers with Ge concentration from 4% to 30%. The dominating defect after irradiation in the n-type layers is found to be the E-center, i.e. the vacancy phosphorus complex, no preferred association with either Ge or Si is found for the vacancy after irradiation. The E-center is observed to be mobile in the temperature interval 150-200 °C and to migrate until it encounters a Ge atom and forms the V-P-Ge complex. This complex anneals out in temperatures above 200 °C. We estimate that the binding energy increases by approximately 0.1-0.2 eV when a Ge atom neighbours an E-center. For the undoped irradiated samples, we find no indication of vacancies surrounded by one or several Ge atoms, i.e. the presence of Ge around a vacancy is not enough to make the defect stable at room temperature. The dominating defect in undoped irradiated samples is most likely the divacancy.