Dependences of the Al thickness and annealing temperature on the structural, optical and electrical properties in ZnO/Al multilayers Y.M. Hu a, * , C.W. Lin b , J.C.A. Huang a,b a Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan b Department of Physics, National Cheng-Kung University, Tainan, Taiwan Received 18 December 2004; received in revised form 22 September 2005; accepted 21 October 2005 Available online 28 November 2005 Abstract High-quality (0001) oriented ZnO (300 A ˚ ) film and [ZnO(100 A ˚ )/Al(t Al )] 3 (t Al = 0.6, 1.7, 2.8 A ˚ ) multilayers have been established at room temperature on Al 2 O 3 (0001) substrates by ion-beam sputtering. The structural, optical and electrical properties of multilayers as functions of both the Al thickness and annealing temperature are reported. We have verified that Al thickness and annealing temperature are the key factors to optimize transparency-conducting property in ZnO/Al multilayers. The optimum Al thickness and annealing temperature for ZnO/Al multilayer of 300 A ˚ thin is 1.7 A ˚ (about one Al atomic layer) and 400 -C, respectively, leading to the relatively lower resistivity (2.8  10  3 V cm) and higher Hall mobility (10 cm 2 /VIs) without suppression of the visible transmittance (above 85%). D 2005 Elsevier B.V. All rights reserved. PACS: 73.66.Ga; 78.66.Hf Keywords: Multilayers; Sputtering; Resistivity; Zinc oxide 1. Introduction Transparent conducting oxide (TCO) films are currently of great commercial and scientific importance for applications in flat panel displays, electrochromic windows, electro-optical devices, gas sensors and solar cells. Nowadays, tin-doped indium oxide (In 2 O 3 /Sn, ITO) has been the most commercially selected TCO material due to its excellent characteristics of low DC electrical resistivity (¨ 10  4 V cm) and high visible transmittance (> 80%) [1]. Owing to the scarcity of its rare indium element, however, the cost of ITO films is expensive. In the last decade, zinc oxide (ZnO), an alternative TCO material, has received considerable attention by taking the advantages of abundance in nature, non-toxicity and good stability in hydrogen plasma processes. The electrical conductivity of ZnO film is readily modified by creation of intrinsic defects (oxygen vacancies or zinc interstitials) or introduction of extrinsic dopants [2–4] such as B, Al, In, Ga, Si, Sn, F and Cl. In most ZnO studies for TCO application, metallic dopants are used because of the increase of carrier mobility and also the thermal and oxidation stabilities. Among these impurity doped ZnO films, Al-doped ZnO (AZO) films exhibit both the low resistivity (down to ¨ 2  10  4 V cm) and high visible transmittance comparable with ITO films. AZO films have been grown by various deposition techniques such as thermal evaporation [5], chemical vapor deposition [6], pulsed laser deposition [7], spray pyrolysis [8], sol–gel [9], magnetron sputtering [10], etc. Much effort has been made on decreasing the resistivity of AZO film by increasing either the carrier concentration or the carrier mobility. However, the further increase of carrier concentration trades with the decrease of transparency and vice versa, due to the effect of ‘‘electron gas’’ reflecting/absorbing light [11]. A possible route to improve the electrical conduction and maintain the transmittance is to prepare AZO films with better crystal structure. In comparison with the other sputtering techniques, there are several advantages of ion-beam sputtering (IBS) deposition [12,13]. Particularly, IBS deposition is executed in a high-vacuum environment and with tunable ion-beam energy that enables the preparation of high-purity films. Besides, epitaxial ZnO films on sapphire substrates have 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.10.058 * Corresponding author. Tel.: +886 7 5919468; fax: +886 7 5919357. E-mail address: ymhu@nuk.edu.tw (Y.M. Hu). Thin Solid Films 497 (2006) 130 – 134 www.elsevier.com/locate/tsf