Effect of the annealing environment on the optical properties of ZnO/GaAs grown by MOCVD

The optical properties of ZnO grown on (1 0 0) GaAs substrate using metalorganic chemical vapor deposition are investigated by photoluminescence (FL) spectroscopy. Postgrowth annealing in nitrogen and oxygen was performed for different times and temperatures in order to incorporate As from the substrate into the ZnO thin films. The PL spectra of the samples annealed in different ambients reveal that the effect of As diffusion into the ZnO thin films is more pronounced when the annealing is performed in oxygen at 550 degrees C. The 11 K PL spectra show the appearance of a transition at similar to 3.35 eV after annealing in oxygen at 550 degrees C for 1 h. A further increase in the annealing temperature leads to the disappearance of this line, while for annealing times longer than 2 h at 550 degrees C, it is no longer prominent. The increase in intensity of this new transition is also accompanied by the enhancement of radiative centers related to structural defects, such as the stacking fault-related transition at similar to 3.31 eV and the Y-line. Temperature dependent PL illustrates the excitonic nature of the new transition at similar to 3.35 eV, which is therefore assigned to (A(0), X) transition, where the acceptor is possibly the 2V(Zn)-As-Zn complex, with an activation energy E-A in the range of 160-240 meV. Furthermore, the enhancement of the radiative centers related to structural defects is regarded as evidence that As atoms tend to segregate in the vicinity of structural defects to relieve local strain. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.