Role of shallow donor defects in inducing broad visible photoluminescence in ZnO thin film

Abstract

In the last three decades, ZnO and its different variants have received deeper attention among the scientific community due to growing research interest and the emergence of their diverse applications. Comprehensive investigations of ZnO thin films have been carried out by many focusing on their optical properties. The broad visible photoluminescence (PL) spectrum of ZnO has been attributed to the defects and imperfections in the thin films. In present work, undoped ZnO thin films are fabricated on quartz substrates via sol-gel spin coating approach followed by post-annealing at three different temperatures to vary the defect concentration. All films exhibit a hexagonal wurtzite phase and the dominance of pyramidal-tipped ZnO grains without the hexagonal cylinder, as confirmed by X-ray diffraction plots. The analysis of X-ray diffraction plots reveals that the interstitial site defects are dominant in our samples. To determine the elemental composition and chemical status of defects, X-ray photoelectron spectroscopy is performed. The characteristic ZnL₃M_4.5M_4.5 and O 1s spectra reveal the dominance of Zinc interstitial defects over the Oxygen vacancies in our thin films and their concentration to be sensitive to post-annealing temperature. The electron paramagnetic resonance measurements indicate the presence of zinc vacancies, the concentration of which is also monitored using the same strategy. The features of visible region PL spectra are analysed and are observed to be controlled by post-processing temperatures and hence by the defect concentration. The features of PL spectra are described using a schematic density of states diagram for the involved energetic electron relaxation mechanism and by invoking the key role of Zinc interstitial defects leading to a broad visible PL spectrum.