Correlation of spectral features of photoluminescence with residual native defects of ZnO thin films annealed at different temperatures

Abstract

We investigated the effects of post-growth annealing in the temperature range of 873 to 1273 K on the spectral features of photoluminescence (PL) vis-à-vis the crystalline and compositional native defects of ZnO thin films grown at 773 K by pulsed laser deposition (PLD) on sapphire substrates. It is found in the PL spectra at 10 K that the deep level emission (DLE) shifted from red-orange spectral region of ∼1.8–2.4 eV to yellow–green region of ∼2.4–2.9 eV with the increasing temperature of annealing. We propose that the PL in red-orange region originating from the singly ionized oxygen vacancies diminished due to increased replenishment of oxygen with increasing annealing temperature and that in the yellow–green region originating from the oxygen interstitials and/or zinc vacancies increased due to enhanced concentration of these point defects. As the annealing temperature was increased, the overall intensity of PL in the DLE region increased slightly up to 973 K but beyond that it increased steeply and made a quantum leap at 1073 K. In contrast to that, intensity of PL due to the near band-edge emission (NBE) in UV region of ∼3.15 to 3.45 eV increased very steeply up to the annealing temperature of 973 K, which is found to be due to improvement in the crystalline and compositional qualities of the films and beyond that it dropped drastically due to deteriorations of these qualities. The high resolution PL spectra at 10 K in the NBE region mainly consisted of peaks due to the recombinations of neutral donor bound excitons’ complexes (D0X) at ∼3.36 eV, free excitons (FXA) at ∼3.38 eV with their conspicuous LO phonon replicas and some other features such as exciton complexes bound to surface states or transitions of conduction band electrons to acceptor levels located in stacking faults and recombination of neutral acceptor bound excitons. The relative intensities of these individual features were strongly dependent on the annealing temperature of the films and the ensuing crystalline and compositional qualities. The 10 K PL spectra from the interfacial region of the annealed ZnO films and the sapphire substrates observed from the backside of the samples showed that the annealing temperature affected the crystalline and compositional qualities at the interface in a complex manner. Particularly, the features corresponding to the interface deteriorations resulting from the diffusion of Al into the ZnO films and the crystalline defects at the interface caused by the sputtering due to the PLD plume were prominently present in these PL spectra. These studies provide deeper insight into the fundamental PL processes in ZnO thin films annealed at different temperatures.