THE EFFECT OF TEMPERATURE ON THE GROWTH OF HIGH QUALITY AL-DOPED ZINC OXIDE THIN FILMS BY RF MAGNETRON SPUTTERING FOR THE 28th EU PVSEC 2013 Mohammed Mannir Aliyu 1,2 , M. Sajedur Rahman 3 , Towhid H. Chowdhury 3 , Fei-Lu Siaw 4 , Kok-Keong Chong 4 , Kamaruzzaman Sopian 3 , Nowshad Amin 1,3 1 Dept of Electrical, Electronic and Systems Engineering, The National University of Malaysia, 43600 Bangi, Selangor Malaysia. 2 Dept of Electrical and Electronics Engineering, College of Engineering, Kaduna Polytechnic, PMB 2021, Kaduna, Nigeria. 3 Solar Energy Research Institute (SERI), The National University of Malaysia, 43600 Bangi, Selangor, Malaysia. 4 Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kuala Lumpur 53300, Malaysia ABSTRACT: Al-doped zinc oxide is a popular transparent conducting oxide material commonly used in thin film solar cells as front contact or buffer layer. In this work, we attempt to investigate and assess the impact and significance of deposition temperature on the material properties of AZO by RF sputtering technique. The structural, optical and electrical behaviors of the films were studied by varying the substrate temperature from room temperature to 500 o C. Results show that the morphology, crystallinity and electrical indices all improve with increasing temperature. The optical characteristics of the films include high transmittance (>90%) and bandgap energy in the range of 3.5 – 3.8 eV. All in all, these films are found suitable to be used as buffer layer in CdTe and CIGS thin film solar cells, while at higher film thicknesses, they can equally be used as front contact TCO layer in these cells. Keywords: AZO, Bandgap, Figure of Merit, Sputtering, Resistivity, Solar Cell 1 INTRODUCTION Aluminum doped zinc oxide (AZO) is an important semiconducting material with several attractive properties, making it a viable material in several opto- electronic applications. Its lower cost, non-toxicity and availability, among other material properties, make it a suitable choice to replace other TCOs such as indium tin oxide (ITO). However, the fabrication of this thin film layer depends on the deposition conditions, chief amongst which is temperature. Thus, in order to obtain best material properties, it is necessary to optimize the deposition variables. Chief amongst its unique characteristics are good electrical conductivity and high transparency in the optical and near infra-red region (NIR), making it a good transparent conducting oxide (TCO) in devices such as solar cells, flat panel displays, electro chromic displays, coatings in thermal collectors and mirrors, liquid crystal displays, thin film transistors (TFT), transparent heating elements, anti-electrostatic coatings, laser diodes, ultraviolet photo-detectors and energy-efficient window systems [1-3]. Although AZO films have been deposited by several methods such as vacuum evaporation, chemical vapor deposition (CVD), spray pyrolysis and pulsed laser deposition (PLD), but sputtering is the most widely used technique [4-6]. The structural, optical, and electrical properties of AZO films are strongly influenced by deposition parameters, including substrate temperature [6-7]. It is therefore important that optimal film properties can only be achieved if the deposition temperature is optimized. Thus, this work presents our attempt to achieve good quality AZO film by optimization of the substrate temperature using RF sputtering technique. 2 EXPERIMENTAL PROCEDURE Clear soda lime glass with dimension of 30x30x1mm, was cleaned for 15 minutes in ultrasonic bath, using 3- solvent sequence of acetone-ethanol-DI water and dried with pure nitrogen gas. AZO target (98% ZnO, 2% Al) was then sputtered by non-reactive RF magnetron sputtering in an argon gas ambient only. In order to remove oxides and surface impurities, the target was pre- sputtered for 20 – 30 minutes, during the first activation of plasma. Deposition was carried out at constant values of 40 W RF power, 6-8 mTorr of argon pressure and uniform substrate speed rotation of 4 rpm, while the substrate temperature was varied from room temperature RT up to 500 o C. Next, the films were characterized in order to gain insight into their material characteristics. The characterization tools used include FESEM (SEISS SUPRA 55VP and CARL ZEISS EVO models), XRD (Bruker aXS-D8 Advance X-Ray diffractometer with Cu Kά, λ=1.54050), AFM, UV-UVis (Perkin Elmer Instruments Lambda 35), EDX (Oxford Instruments INCAPenta FETx3), Four-point probe (Agilent 34401A model) and BIORAD Hall System.. 3 RESULTS AND DISCUSSION The results of the characterization are hereby presented and analyzed. The surface morphology of the deposited films is studied from the SEM images presented in Figure 1. It is observed that all the films are compact, uniform and have good coverage. Also, the grain sizes increase with deposition temperature, until 400 o C, after which it appears to reduce. These are corroborated by the results of the AFM. The SEM images show that all the films have the honey comb structure described by Zhang, et al [8]. At low temperatures the grains are small, however, with temperature rise, the sizes increase due to the coalescence of smaller grains into larger ones, making the structure to change from columnar to granular. But as the temperature increases further (beyond 400 o C in this case), the film changes to columnar structure without grain boundaries, where they agglomerated according to Zhang et al [8] and Oh et al [9]. Here, it is seen from Figure 1 that at 500 o C, the grains are columnar and without grain boundary, as described above. The XRD patterns, as presented by Figure 2 reveal that the deposited films are wurtzwite hexagonal having the c- 28th European Photovoltaic Solar Energy Conference and Exhibition 200