We show that the Doppler broadening of positron annihilation radiation can be used in the identification of vacancy defects in compound semiconductors. Annihilation of trapped positrons with surrounding core electrons reveals chemical information that becomes visible when the experimental background is reduced by the coincidence technique. We also present a simple calculational scheme to predict the high-momentum part of the annihilation line. The utility of the method is demonstrated by providing results for vacancies in InP. In electron irradiated InP the isolated In and P vacancies are distinguished from each other by the magnitude of the core electron annihilation. In heavily Zn-doped InP we detect a native vacancy defect and identify it to a P vacancy decorated by Zn atoms.

After electron irradiation two positron lifetimes 250+or-1 and 268+or-3 ps, are observed in phosphorus-doped Si ((P)=3*1016 cm-3) depending on the temperature and the electron fluence. The lifetimes are assigned to the negative and neutral charge states of the phosphorus-vacancy pair (P-V). The longer lifetime at the neutral charge state indicates a larger open volume of the vacancy and implies an outward relaxation of the phosphorus-vacancy pair in the charge-state transition (P-V)- to (P-V)0. A comparison with the theoretical lifetimes yields a breathing-mode relaxation of the nearest-neighbour atoms Delta r/r0 approximately 5.1% (r0 is the Si-Si bond length), or Delta r approximately 0.12 AA. The positron trapping coefficients 2*1016 s-1 at 20 K and 2*1015 s-1 at room temperature at the negatively charged state (P-V)- are estimated. A weakly bound Rydberg-like precursor state is invoked to explain the temperature dependence of positron trapping at the negatively charge centres (P-V)- and V- in Si. The binding energy at the precursor state is 15-30 meV.

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