Self-induced Crystallinity in RF Magnetron Sputtered ZnO Thin Films

ZnO films were coated on the order of micrometer thickness on various substrates using RF magnetron sputtering. Glass, mica and Si were used as amorphous and crystalline substrates to study film growth. X-ray diffraction measurements revealed a self-induced, (002) oriented texture on all substrates. Effects of residual stresses on growth behavior and possible mechanisms of textured crystallization on crystalline and amorphous substrates are discussed. Introduction ZnO is a large band-gap (3.20-3.43 eV [1]) semiconductor with the added advantage of being a low cost electronic ceramic with good optical quality, excellent piezoelectric properties and several others. Thin films of ZnO have already found many electronic and opto-electronic applications such as piezoelectric devices, optical modulators, and acoustic wave filters. Various coating methods had been successfully applied for good quality textured layers, not only on crystalline but also on amorphous substrates. Textured growth of ZnO on amorphous substrates deposited by RF magnetron sputtering [1,2], CVD [3], DC magnetron sputtering [4], and laser ablation [5] had been reported. Morphological uniformity, phase purity and crystallinity are important parameters that strongly affect the electrical, mechanical, and optical properties of a material. For ZnO, such examples are the electromechanical coupling for the piezoelectric devices [6], the elastic constant [7], and optical indices of thin films in the UV-VIS wavelength range [1,8]. Thus, understanding the influence of substrate structure and the coating parameters on film structure is critical. Experiment ZnO films were coated on various amorphous and crystalline substrates by a radio frequency magnetron sputtering equipment (Rescue-E, Teknoplazma, Ankara, Turkey). Microscope slides (IsoLab, Wertheim, Germany) with dimensions of 20x20x0.16 mm were used as glass substrates. High quality mica slides (Ted Pella Inc., CA, USA) and phosphor doped, (100) oriented silicon wafer pieces were used as crystalline substrates. Si wafer pieces were of various sizes all with a thickness of 508 ± 20 μm. Pure (99.999 %) ZnO sputtering targets were provided by Kurt J. Lesker (Clairton, PA, USA). Target diameter and thickness were 50.8 and 3.175 mm, respectively. Experimental parameters such as temperature, pressure and specimen rotation were controlled during deposition. Substrate-to-target distance was kept at 14 cm. Before coating the chamber was evacuated to 10 mbar and then the substrate was subjected to argon-ion plasma cleaning. Coating was held under pure argon atmosphere at 0.02 mbar. Two different initial temperatures were 50 and 200 °C. The system was heated to these Key Engineering Materials Vols. 264-268 (2004) pp. 1225-1228 online at http://www.scientific.net © 2004 Trans Tech Publications, Switzerland