Optical and electrical properties of aluminum zinc oxide (AZO) nanostructured thin film deposited on polycarbonate substrate

Abstract

Transparent polymer materials, due to their unique properties, such as light weight, optical transparency, and electrical and mechanical properties, have become very attractive as a replacement for inorganic glass substrates in a wide range of optoelectronic applications. In this research, aluminum zinc oxide nanostructured thin film was deposited on polycarbonate polymer substrates using a magnetron sputtering technique. The structure, morphology, and surface composition of the thin film were investigated by X-ray diffraction and field emission scanning electron microscopy. The optical and electrical properties of the thin film were investigated by UV–VIS-NIR spectrophotometer, ellipsometer, and four point probe method. The X-ray diffraction pattern showed that the aluminum zinc oxide thin film had a polycrystalline structure. The optical and electrical results indicated that the refractive index, band gap, and sheet resistance of the aluminum zinc oxide thin film were 1.8, 3.2 eV, and 265 Ω/sq, respectively.

Introduction

Transparent conductive oxide (TCO) thin films have been used extensively as a transparent electrode in optoelectronic devices such as flat panel displays, light-emitting diodes, and thin-film solar cells [1], [2]. In addition, TCO thin films have been used in aircraft windshields in order to prevent surface icing [3]. Most of TCO thin films are based on indium oxide (In₂O₃), zinc oxide (ZnO), tin oxide (SnO₂), and their mixed compounds such as indium tin oxide (ITO), aluminum zinc oxide (AZO), etc. Among these materials, ITO is the most favorable TCO due to its electro-optical properties. However, ITO use is limited because of its high price and toxicity. ZnO has attracted much attention as a replacement for ITO because of desirable properties such as high transmittance, non-toxicity, and lower cost. ZnO is a natural n-type semiconductor material. Due to its wide direct band gap (3.3 eV), its transmittance in the visible region is extremely high [4], [5], [6]. ZnO thin films are also highly resistive. In contrast, doped ZnO such as AZO can increase the conductivity by several orders of magnitude [7], [8], [9], [10], [11], [12].

AZO thin film immobilized on rigid glass substrates has been widely studied because of its excellent electro-optical properties. Nowadays, there is increasing interest in replacing conventional glass substrates with flexible substrates such as plastic polymers in optoelectronic devices. As it is well known, glass is very heavy, brittle, and easily deformed, especially when used for certain purposes, such as smart cards, electronic maps, and flat panel displays, where flexibility and light weight are needed. Therefore, transparent conducting films deposited on flexible polymer substrates could overcome these problems. In addition, transparent conducting films must be deposited on polymer substrates at low temperature because of the weak thermal stability of polymer materials [13], [14]. Many techniques have been developed for the deposition of AZO thin films such as electron beam evaporation, chemical vapor deposition (CVD), spray pyrolysis, the sol–gel process, metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), and magnetron sputtering [15], [16]. Sputtering is a desirable technology for immobilizing films on polymer substrates since it permits deposition at low temperatures, leading to smooth films with good surface uniformity [15], [16]. In this work, transparent conducting AZO thin film (200 nm thickness) was deposited on polycarbonate polymer substrates by magnetron sputtering. The structural and electro-optical property of the AZO thin film was investigated.