Tin-vacancy acceptor levels in electron-irradiated n-type silicon

Si crystals (n-type, fz) with doping levels between 1.5×10¹⁴ and 2×10¹⁶cm^-3 containing in addition ~10¹⁸Sn/cm³ were irradiated with 2-MeV electrons to different doses and subsequently studied by deep level transient spectroscopy, M\"ossbauer spectroscopy, and positron annihilation. Two tin-vacancy (Sn-V) levels at Ec-0.214eV and Ec-0.501eV have been identified (Ec denotes the conduction band edge). Based on investigations of the temperature dependence of the electron-capture cross sections, the electric-field dependence of the electron emissivity, the anneal temperature, and the defect-introduction rate, it is concluded that these levels are the double and single acceptor levels, respectively, of the Sn-V pair. These conclusions are in agreement with electronic structure calculations carried out using a local spin-density functional theory, incorporating pseudopotentials to eliminate the core electrons, and applied to large H-terminated clusters. Thus, the Sn-V pair in Si has five different charge states corresponding to four levels in the band gap.