Direct comparison of photoluminescence lifetime and defect densities in ZnO epilayers studied by time-resolved photoluminescence and slow positron annihilation techniques

The roles of point defects and defect complexes governing nonradiative processes in ZnO epilayers were studied using time-resolved photoluminescence (PL) and slow positron annihilation measurements. The density or size of Zn vacancies (V_Zn) decreased and the nonradiative PL lifetime (τ_nr) increased with higher growth temperature for epilayers grown on a ScAlMgO₄ substrate. Accordingly, the steady-state free excitonic PL intensity increased with increase in τ_nr, at room temperature. The use of a homoepitaxial substrate further decreased the V _Zn concentration. However, no perfect relation between τ_nr and the density or size of V_Zn, or other positron scattering centers was found. The results indicated that nonradiative recombination processes are governed not solely by single point defects, but by certain defect species introduced by the presence of V_Zn such as vacancy complexes.