Stress-dependent band gap shift and quenching of defects in Al-doped ZnO films

Abstract

Al-doped ZnO (AZO) films were deposited on quartz substrates by the ultrasonically assisted chemical vapour deposition technique. The undoped ZnO film was found to be subjected to a stress which increases initially up to 3% Al doping, and then a slight decrease was observed for 5% Al doping. The band gap of AZO shows a blue shift up to 3% of Al doping as compared with the undoped ZnO. The blue shift in the band gap of the AZO films cannot be understood in the framework of Burstein–Moss shift and has been attributed to an increase in the stress present in the film. The photoluminescence spectrum of the undoped ZnO film shows a wide peak in the visible region which is suppressed with a small red shift after Al doping in the ZnO film. A detailed analysis of photoluminescence of ZnO and AZO films indicates suppression of zinc interstitials (Zni) and oxygen vacancies (VO) and creation of oxygen interstitial (Oi) defects after Al doping in ZnO films. X-ray photoelectron spectroscopy study also reveals suppression of oxygen vacancy related defects after Al doping in the ZnO film. The presence of Al in the ZnO matrix seems to change the defect equilibria leading to a suppression of Zni and VO and enhancement of Oi defects. The suppression of Zni defects is correlated with the increase in stress in Al-doped ZnO films.