Role of oxygen vacancy in optical and gas sensing characteristics of ZnO thin films

Highly (002) textured, ultra-thin (100-160 nm), nano-crystalline zinc oxide thin films are grown by metal organic chemical vapor deposition. These films are characterized in terms of structure, microstructure, optical and carbon monoxide gas sensing characteristics. The room-temperature photoluminescence (PL) spectra of these films are characterized by two peaks: the first one at 387 nm is due to the near band edge UV emission and the other one similar to 496 nm is green emission from defect levels. The PL intensity ratio of the defect level and near band edge emission (I-DL/I-NBE) is argued to be an indicator of oxygen vacancy concentration of these films. The response (%) of carbon monoxide gas sensing is found to increase with the film thickness and maximized in film similar to 130 nm thick. With further increase of film thickness the response % is found to be reduced. Good correlation is observed between the variation of the PL intensity ratio (I-DL/I-NBE) and response% as a function of film thickness. It has been argued that the concentration of oxygen vacancies in turn controls the gas sensing characteristics of the synthesized films. Finally, we have elucidated the interrelation between the oxygen vacancy concentration, film microstructure and the gas sensing characteristics of the synthesized films. (C) 2012 Elsevier B. V. All rights reserved.