A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively. ͓S0163-1829͑96͒04327-5͔

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.

No abstract

Doppler-broadening of the 511 keV positron annihilation line was used for defect identification in CdTe materials. In electrically compensated lightly n-type CdTe(In) and lightly p-type or semi-insulating CdTe(Cl) crystals positron lifetime measurements show vacancy defects with characteristic positron lifetimes of 323 ps and 370 ps, respectively. The shapes of the highmomentum parts of the measured electron-momentum distributions indicate that both defects contain a cadmium vacancy V Cd . The defects are assigned to vacancy-donor complexes V Cd -In and V Cd -Cl, respectively. A vacancy in MBE-grown CdTe(I) layers observed with a low-energy positron beam is also identified as a cadmium vacancy V Cd which is most likely complexed with I-donors.

The optical ionization of the silicon divacancy in 2-MeV electron-irradiated Si was studied by using positron-lifetime and positron-electron momentum distribution measurements under illumination with monochromatic light. Upon irradiation at room temperature, negative and neutral divacancies are detected in both float zone and Czochralski Si by positron-annihilation measurements in darkness. The positron-annihilation characteristics of the divacancy are determined as d ϭ300(5) psϭ1.35(2)ϫ b , S d ϭ1.055(3)ϫS b , and W d ϭ0.75(2)ϫW b. Illumination at 15 K with monochromatic 0.70-1.30 eV light has a strong effect on the positron trapping rate to the divacancies. The results can be understood in terms of optical electron and hole emission from the electron levels V 2 Ϫl0 and V 2 2ϪlϪ of the divacancy. The changes in the positron trapping rate are due to the different sensitivities of the positron to the charge states V 2 0 , V 2 Ϫ , and V 2 2Ϫ. The spectral shape of the positron trapping rate under illumination reveals an electron level at E v ϩ0.75 eV, which is attributed to the ionization level V 2 2Ϫ/Ϫ of the divacancy. ͓S0163-1829͑98͒10819-6͔