Correlation between the free carrier lifetime and total amount of deep centers in ZnO single crystals

Abstract

We report on optical study of charge carrier dynamics in vapor phase grown ZnO single crystals with different content of deep defects. Deep defects were controlled by applying various heat treatment regimes to each crystal. A time-resolved picosecond transient grating technique was used to determine the carrier lifetime and diffusion coefficient, while the light absorption spectra of transparency region together with the numerical simulations provided information on deep level content, their spectral positions and concentrations. A linear dependence was obtained between the carrier recombination rate and the integral absorption coefficient in 2.0–3.1 eV spectral region. This proves the role of the deep defects as the main recombination centers of excess carriers in ZnO. Numerical simulations of absorption spectra revealed five different species of intrinsic deep defects. The most recombination-active defects were attributed to oxygen vacancy and zinc–oxygen vacancy complex.