Substrate Temperature Effects of the ZnO:AlF 3 Transparent Conductive Oxide
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Substrate Temperature Effects of the ZnO:AlF3 Transparent Conductive Oxide Tien-Chai Lin1, Wen-Chang Huang2*, Chin-Hung Liu1 and Shang-Chou Chang 1 Department of Electrical Engineering, Kun Shan University, No. 949, Da Wan Road, YungKang District, Tainan, 710, Taiwan, ROC 2 Department of Electro-Optical Engineering, Kun Shan University, No. 949, Da-Wan Road, Yung-Kang District, Tainan, 710, Taiwan, ROC *Corresponding author: email: [email protected] 1
ABSTRACT Thermal effects on the crystal structure, electrical and optical characteristics of the Al and F co-doped ZnO films (ZnO:AlF3) are discussed in the paper. The ZnO:AlF3 thin films are prepared by RF sputtering with a constant power (ZnO/AlF3=100W/75W) toward the ZnO and AlF3 targets. The substrate temperature varied from room temperature to 250 °C with a step of 50 °C during thin film deposition. The crystalline quality of the ZnO:AlF3 film improved as the substrate temperature increased, with a corresponding increase in grain size. The improvement of the film quality leads to a higher electron mobility, with electron mobility of 0.85 cm2/V-s for the film deposited at the substrate temperature of 250 °C. The doping effect of fluorine in ZnO, and hence carrier concentration, was reduced at high temperature due to the vaporization of fluorine. This led to a reduction of carrier concentration with increase of temperature from 25 to 200°C. The corresponding resistivity increased from 3.60×10-2 to 6.0×10-2 ȍ-cm. While for a further increase in substrate temperature, the doping of Al to the ZnO film was increased and resulted in an increase in carrier concentration. INTRODUCTION The transparent conducting film, Indium tin oxide (ITO) has been widely used because of its good electrical and optical properties. However, it shows the disadvantages of high cost, toxicity and low stability to H2 plasma. On the other hand, zinc oxide (ZnO) films have the advantages of low cost, non-toxicity and high stability in H2 plasma atmosphere with good electrical and optical properties [1]. Furthermore, the electrical properties of ZnO film can be modified by appropriate doping process [2]. Al-doped ZnO thin films (AZO) have been obtained by several techniques [3]. The resistivity values of these films are in the range of 10-4–10-2 -cm depending on the preparation technique used. One approach to further increase conductivity of AZO is the addition of another dopant element to the thin film. Fluorine is an adequate anion doping candidate, due to its similar ionic radius relative to oxygen. F-doped ZnO thin films (FZO) have also been deposited by many methods [4,5]. Lately, ZnO thin films doped with Al and F have been deposited by radio frequency (RF) magnetron co-sputtering of a ZnO target containing Al2O3 and a ZnO target containing ZnF2 [5],with notable effect on the resistivity and carrier mobility. It was found that F dopants improved crystallization of ZnO films, and with additional post-deposition vacuum
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annealing desorption of oxygen at the grain boundaries led to
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