Positron annihilation spectroscopic studies of Zn-doped material revealed the presence of a defect, with a positron lifetime of about 330ps, in samples in which the carrier concentration had saturated. This lifetime was attributed to a complex that involved vacancies and Zn atoms. A model was proposed according to which this complex had a (-/0) level which was near to the bottom of the band gap and underwent a large inward lattice relaxation during transition to the neutral charge state. This caused a decrease in the positron lifetime to about 281ps. The model explained positron annihilation results for annealed samples, and at low temperatures, and was supported by Hall effect measurements. The concentration of these complexes was less than 1017/cm3. Therefore, they could not entirely account for the observed discrepancy between the carrier concentration and the Zn concentration in very heavily Zn-doped material.

J.Mahony, P.Mascher, W.Puff: Journal of Applied Physics, 1996, 80[5], 2712-9