Green luminescence and donor acceptor pair emission associated with Zn-vacancy related defect in ZnO grown by pulsed laser deposition

Abstract

Undoped ZnO (002) films are grown on sapphire by pulsed laser deposition at different substrate temperautres and oxygen pressures. Low temperature (10K) photoluminescence (pL) study shows that the green luminescence (GL) without fine structure peaked at 2.45 eV and the donor-acceptor-pair (DAP) at 3.23 eV are introudced after the 900°C annealing in all the sarnples irrespcctive of the initial growth condition. The accptor level of the DAp emission is found to be at E v +0.18 eV. The level (0/-) of Zn-vacancy calculated by LDA+U [Janotti and Van de Walle, Phys. Rev. B vo1.76, 165202 (2007)] agrees well with the acceptor level of the DAP. The energy corresponding to the transition betwecn the conduction band (or shallow donor) and the (-/2-) of the Zn-vacancy also coincides with the GL photon energy. The samples were also studied by positron annihilation spectroscopy (PAS), which is selectively sensitive to Zn-vacancy related defects. A Zn-vacancy related defect is induced at the annealing temperature of 900°C, which correlates with thc introductions of the GL and the DAp. This thus provides strong evidence for associating the GL and the DAP observed in the present study to the Zn-vacancy defect.