Fabrication and vacuum annealing of transparent conductive AZO thin films prepared by DC magnetron sputtering

Abstract

Using highly conductive Al-doped ZnO (AZO) ceramic target, (0 0 2)-oriented transparent conductive AZO thin films are prepared by DC planar magnetron sputtering deposition on glass sheet substrate. Structural, electrical and optical properties of the films deposited at different temperatures and subsequently annealed at 400°C for 2 h under 10⁻³ Pa are characterized with various techniques. Experimental results show that the electrical resistivity of AZO thin films deposited at 320°C can be as low as 1.5×10⁻⁴ Ω cm with post-deposition annealing. The annealing process leads to improvement of (0 0 2) orientation, wider band gap, increased carrier concentration and blue shift of absorption edge in the transmission spectra of AZO thin films.

Introduction

ZnO is a II–VI group semiconductor material with wide direct bandgap and wurtzite structure. The high stability, melting point and excitation energy make it a promising ultraviolet (UV) and blue optoelectronic material. In addition, ZnO thin films offer a variety of applications in integrated optic, piezoelectronic, and gas sensitive devices.

A transparent conducting electrode is a necessary component of most types of flat panel displays (FPDs). The commercially most important material for a transparent conducting film nowadays is Sn-doped In₂O₃ (ITO), owing to its unique characteristics of high visible transmittance (90%), low DC resistivity, high infra-red reflectance and absorbance in the microwave region. The high quality of ITO films deposited by sputtering of oxide targets has already been successfully achieved on a commercialized production scale. On the other hand, ZnO films have attracted interest as a transparent conductive coating material, because the materials (1) consist of cheap and abundant element, (2) are readily produced for large-scale coating, (3) allow tailing of ultraviolet absorption, (4) have a high stability in hydrogen plasma, and (5) have low growth temperature. Besides, it is nontoxic and easy to fabricate. The electrical resistivity of ZnO thin film is readily modified by the addition of impurity or post-deposition annealing [1], [2], [3]. Aluminum-doped zinc oxide (AZO) thin films have been prepared by thermal evaporation [4], [5], chemical vapour deposition (CVD) [6], plasma enhanced chemical vapour deposition (PECVD) [7], sol–gel [8], [9], spray pyrolysis [10], [11], [12], pulsed laser deposition (PLD) [13] and magnetron sputtering [14], [15], [16], [17], [18], [19], etc. Magnetron reactive sputtering, using a Zn–Al alloy target, has advantages in target fabrication and film deposition rate [18]. Its drawback is that the chemical composition of AZO thin films is easily modified by the Zn oxidation on the target surface and is hard to control. A relative high-density (94%) AZO conductive ceramic target has been used for AZO film fabrication in this work. The results of fabrication, post-deposition annealing and their effects on the optical and electronic properties are reported in this paper. Influence of substrate temperature, sputtering power, sputtering pressure (argon pressure), target to substrate distance, and post-deposition annealing in air, argon or reducing gas on AZO film quality has been reported [11], [12], [18], [19], [20], [21]. The structural and physical properties of AZO films synthesized by thermal evaporation, sol–gel, spray pyrolysis, and RF sputtering following by annealing in vacuum have been studied [4], [9], [10], [20]. However, post-deposition annealing of DC sputtered AZO thin film in vacuum has been less studied. This area is our main concern in this work.