RF-Sputtered ZnO Thin Films: The Tailoring of Structural, Electrical and Optical Properties
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RF-Sputtered ZnO Thin Films: The Tailoring of Structural, Electrical and Optical Properties Cristina Besleaga1, George E. Stan2, Lucian Ion1 and Stefan Antohe1 1 MDEO, Faculty of Physics, University of Bucharest, P.O. Box MG-11, Bucharest-Magurele, 077125, Romania. 2 National Institute of Materials Physics, P.O. Box MG-7, Bucharest-Magurele, 077125, Romania. ABSTRACT We report on the influence of radio-frequency magnetron sputtering variables (working pressure and deposition atmosphere) and post-deposition thermal treatment processing upon the structural, optical and electrical properties of c-axis highly textured ZnO thin films. The films’ crystallinity increased progressively with argon pressure for the inert atmosphere experiments and with the oxygen dilution in the working atmosphere (up to 10%) for the reactive atmosphere experiments. The post-deposition annealing treatment at 450°C/1h in air reduced the strain in the ZnO films and enhanced their crystallinity and texturing. The ZnO films had an average transmittance of ~90% in visible range and an average band-gap of ~3.4 eV, regardless of the sputtering variables used. The samples prepared at the higher argon pressure (0.45 Pa) had a resistivity with one order of magnitude smaller than the samples prepared at the lower pressures. INTRODUCTION Zinc oxide (ZnO) constitutes a highly promising material for electronic and optoelectronic applications. A current and challenging task for the researchers is finding viable deposition algorithms for producing adherent and reproducible nano-sized ZnO films with low resistivity and a high transparency value. Due its properties such as large band gap value (~3.3 eV at room temperature), high exciton binding energy (60 eV), high transmittance (>90% in visible spectrum), high electro-chemical stability [1], ZnO is considered presently as a viable alternative for the material that have its own era in electronics, gallium nitride. During the last years various techniques, including pulsed laser deposition [2,3], sol-gel [4], pulsed electron beam deposition [5], or magnetron sputtering [6,7], have been proposed to prepare high quality ZnO thin films. Among them, Radio Frequency – Magnetron Sputtering (RF-MS) is a flexible technique that allows high deposition rates, efficient sputtering of virtually any compound, high-purity adherent films, good quality coverage of steps or small features and automation [8]. Highly oriented nanostructured ZnO thin films were synthesized onto silicon and glass substrates by RF-MS, aiming at electronic and optoelectronic applications. The possibility of tailoring the ZnO films’ structural, optical and electrical properties by varying the sputtering parameters is investigated.
EXPERIMENTAL Silicon wafers (100) and UV grade glass slides (Fisher Scientific Microscope Slides) were used as substrates. The substrates were cleaned successively in acetone and ethanol for 10 minutes in an ultrasonic bath, then dried in argon flow and fixed on the substrates holders in the RF-MS reactor chamber. Mild-pre
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