Electrically air-stable ZnO produced by reactive RF magnetron sputtering for thin-film transistor (TFT) applications.
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Electrically air-stable ZnO produced by reactive RF magnetron sputtering for thin-film transistor (TFT) applications. D. K. Ngwashi and R. B. M. Cross and S. Paul Emerging Technology Research Centre, De Montfort University, Leicester LE1 9BH, UK ABSTRACT The influence of native point defects on the electrical and optical stability of zinc oxide (ZnO) layers in air produced by reactive RF magnetron sputtering is investigated. ZnO thin films are strongly affected by oxygen (O2) molecules in ambient atmosphere. For instance, surface defects such as oxygen vacancies act as adsorption sites of O2 molecules, and the chemisorption of O2 molecules depletes the surface electronic states and reduces channel conductivity. Thin films of ZnO produced have electrical resistivities between 8.6 x 103 and 8.3 x 108 Ω-cm, and were found to be electrically-stable in air. TFTs fabricated using these films exhibited effective mobilities of ~3 cm2V-1s-1 and the threshold voltage shifts by < 5 V under gate bias stress of 1 MV/cm for up to 104 s. INTRODUCTION Wide bandgap semiconducting II-VI oxides are an important class of materials in optoelectronics devices. Zinc oxide (ZnO) is cheap, abundant and when produced at room temperature, is compatible with cheap flexible and plastic substrates. The physical properties of sputtered ZnO are strongly dependent on the deposition parameters and on the post-deposition treatment such as post-annealing. Furthermore, a reduction in the concentration of O2 vacancies in thin films has been found to reduce O2 chemisorption in an ambient atmosphere, [[1-7]] which could be a major cause of electrical degradation in ZnO-based applications (such as thin-film transistors (TFTs)). TFTs using ZnO as a channel material were first demonstrated by Hoffman et al. [8] and have since then been the focus of a substantial amount of research. However, the stability of these TFTs remains one of the major issues delaying its commercial application [9]. The stability of TFTs under gate bias stress is also of great importance for switching applications where prolonged turn-on is needed such as in static images (e.g. a logo). Threshold voltage instability as a result of bias stressing in TFTs is often attributed to defect creation in the channel material, charge trapping in the gate dielectric material or at the dielectric/channel interface [10,11]. In this work, we investigate the influence of deposition parameters on the stability of sputtered ZnO layers. EXPERIMENT ZnO layers deposited by reactive radio frequency (RF) magnetron sputtering at room temperature in an argon (Ar)/O2 atmosphere have been investigated. The different ZnO thin films obtained are characterized by x-ray diffraction (XRD) using Siemens/Bruker D5000 diffractometer equipped with a 0.1540562 nm (Cu kα) line and UV-Vis spectroscopy using UNICAM UV/Vis spectrometer (UV 2), together with electrical resistivity measurements to investigate the influence of oxygen within the sputtering chamber on the film properties. The
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