Characteristics of ZnO–Cu–ZnO multilayer films on copper layer properties D.R. Sahu * , Jow-Lay Huang Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 26 September 2005; received in revised form 11 January 2006; accepted 11 January 2006 Available online 24 February 2006 Abstract ZnO/Cu/ZnO multilayers on glass with different copper layer thickness were prepared by simultaneous RF magnetron sputtering of ZnO and dc magnetron sputtering of Cu. Different optimization procedure were used for good transparent conductive film. Several analytical tools such as spectrophotometer, scanning electron microscope (SEM), four point probes were used to explore the causes of the changes in electrical and optical properties. The sheet resistance of the structure was severely influenced by the deposition condition of both top ZnO and intermediate Cu layer. Effect of substrate temperature and annealing treatment on ZnO and Cu layer was analyzed. A sheet resistance of 10 V/sq and transmittance over 85% at 580 nm wavelength was achieved and could be reproduced by controlling the preparation process parameter. The results of an optimization condition of both oxide layers and metallic Cu layers are illustrated. # 2006 Elsevier B.V. All rights reserved. Keywords: ZnO; Cu; Multilayers; Optical properties and electrical properties 1. Introduction There have been extensive studies on thin films exhibiting high electrical conductivity as well as high transparency in the visible solar spectrum and high reflectivity in the infrared region [1–3]. Transparent metal electrodes of gold, silver and copper in very thin form (approximately up to 15 nm thick) were used in early optoelectronic devices [4]. Practical applications of transparent conducting coatings increase significantly after the introduction of wide band gap semi- conducting films. Different semi-conducting oxides such as tin oxide, indium oxide and zinc oxides having transparent properties are of practical significance [5]. ZnO thin film coatings developed swiftly into commercial products due to their easy manufacturing and excellent properties [6]. The high stability, melting point and excitation energy make it a promising ultraviolet and blue optoelectronic material [7]. In addition ZnO thin film offer a variety of application in integrated optic, piezoelectric and gas sensitive devices [8–10]. ZnO also attracted interest as transparent conductive coating materials, because the materials consists of cheap and abundant elements, can be readily produced for large scale coatings, allow tailoring of the ultraviolet absorption, have high stability in a hydrogen plasma and have low growth temperature [11]. Besides, it is non-toxic and easy to fabricate [12]. The electrical resistivity of ZnO thin film is also readily modified by the addition of dopants or post deposition annealing [13]. However, high conductivity and optical transparency are mutually exclusive properties since photons are strongly absorbed by the high density of charge carriers. Although there are materials that are far more conductive or transparent, the transparent conductors exhibit a useful compromise of both desirable properties. Broadly speaking, transparent conducting films consist of either of very thin metals or semi-conducting oxides. Simultaneous optimization of conductivity and transparency presents a considerable challenge in film deposition. At one extreme are discontinuous islands of considerable transparency but high resistivity; at the other are films that coalesce early and are continuous, possessing high conductivity but low transpar- ency. For these reason semi-conducting oxides and more common their alloys are used. Recently, combination of semi- conducting materials and metal multilayers are used to achieve high transparent conducting oxides [14]. Extension of multi- layer system, by suitable variations in design, it is possible to obtain improved electrical and optical properties over a broader spectral range. Systems with a vast variety of properties can be achieved usually with the use of many film layers through proper optimization of processing parameters. www.elsevier.com/locate/apsusc Applied Surface Science 253 (2006) 827–832 * Corresponding author. Tel.: +886 6 2754410; fax: +886 6 2754410. E-mail address: sahu@mail.ncku.edu.tw (D.R. Sahu). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.023