Positron Spectroscopy of Defects in Semiconductors

At a vacant lattice cell positron-ion repulsion is reduced leading to positron trapping. This causes observable changes in annihilation characteristics: the positron lifetime increases and the positron-electron momentum distribution narrows. Positron trapping in semiconductors is analogous to carrier capture. Due to the long-range Coulomb interaction, the charge state of a vacancy has a strong effect on positron trapping. The lattice relaxation due to a charge-state transition of a vacancy is also seen in the positron lifetime. Thus ionization levels of vacancy defects can be determined. Applications are shown on native vacancies in GaAs. Positrons reveal As and Ga vacancies in bulk crystals. The As vacancies have negative, neutral and positive charge states in the upper part of the band gap, whereas Ga vacancies are negative. The EL2 defect has a vacancy in its metastable state. A vacancy is also seen in the deep ground state of the DX center in AlGaAs. Positron results thus support the vacancy-interstitial model for the structure of the EL2 and DX centers.