Zinc oxide (ZnO) thin films were grown by nonreactive RF sputtering at room temperature under varying argon pressures ( ). Their optical band gap was found to increase from 3.58 to 4.34？eV when the argon pressure increases from 2.67 to 10.66？Pa. After annealing at 200°C and 500°C, optical band gaps decrease considerably. The observed widening of the band gap with increasing can be understood as being a consequence of the poorer crystallinity of films grown at higher pressures. Measurements of morphological and electrical properties of these films correlate well with this picture. Our main aim is to understand the effects of on several physical properties of the films, and most importantly on its optical band gap. 1. Introduction ZnO is a wide-band-gap semiconductor with a high transparency in the whole visible range and electrical properties that can be tailored from insulating to semimetallic by doping. This has attracted strong interest in this material because of its potential for applications, which include transparent conducting electrodes, gas sensors, light emitting devices, laser diodes, and optical waveguides [1, 2]. ZnO thin films have been obtained by techniques such as sol-gel [3, 4], metal organic chemical vapor deposition [5, 6], pulsed laser deposition [7, 8], and sputtering [9, 10]. As compared to other deposition methods, sputtering has several advantages. Uniformity of film thickness over large areas, a high degree of film adhesion, and relatively simple scalability properties are some of the most important advantages. Several recent works [11–14] discuss the mechanisms involved in ZnO thin film growth by magnetron sputtering in a mixture of argon and oxygen, that is, by reactive sputtering, at varying Ar/O2 ratios. This technique produces films with a fixed optical band gap, in the absence of doping. Most applications of ZnO as a semiconductor, on the other hand, require the tailoring of the band gap. Band gap modulation of ZnO is usually obtained by doping with Cd and Mg ions [15]. A significant widening of the band gap, reaching values as high as 5.23？eV, has been reported for ZnMgO films [16, 17]. However, doping may sometimes be undesirable, for example, because this may compromise the crystallographic properties of the film [17]. Therefore, there is a strong interest in exploring different approaches to control the band gap in ZnO films. An approach to control band gap that has not been widely explored up to now consists in depositing ZnO from a ceramic target, without the addition of O2, under varying Ar pressures. This
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