1. Introduction
The demand for transparent conducting oxides (TCOs) is increasing rapidly in the field of optoelectronic technology on account of their excellent optical and electrical properties. Tin-doped indium oxide (ITO) has long been regarded as a TCO material, which led to the diverse applications because of its optimal performance. On the other hand, the rarity and the high cost of its prime material indium highlights to explore an excellent new inexpensive material. In this scenario, by doping of appropriate element such as aluminum (Al) in ZnO host lattice, better optical transparency can be achieved in the visible region. Thus, Al-doped ZnO (AZO) behaves like a best alternative TCO candidate for ITO. Considering the relevant capabilities, such as coexistence of high conductivity and excellent optical transparency in the visible region for the AZO material, it is expected to be the most favorable for the fabrication of many optoelectronic devices at moderate doping level of Al with a comparable yield. Owing to these key features, AZO films have taken many applications in diverse fields, such as thin film solar cells [1], photoluminescence [2], thin film transistors [3] and transparent electrodes for organic photovoltaics [4]. Thin films of AZO can be produced by using numerous deposition techniques, including RF sputtering [5], DC sputtering [6], spray plasma technique [7], pulsed laser deposition [8], atomic layer deposition [9], metalorganic chemical vapor deposition [10] and sol-gel spin coating [11] and several studies have been performed to control the crystalline nature and surface morphology. Among these various techniques, RF magnetron sputtering has attracted much special interest at room temperature deposition because of it provides large scale uniformity for industrial production, good adhesion and for preventing the target from poisoning. On the other hand, the film properties mainly depends on the chemical composition, microstructure and surface morphology. The unbalanced RF magnetron sputtering technique has been developed to produce advanced films, i.e., the films with a better microstructure and morphology because of it is most versatile and useful technique for the deposition of high-quality films at room temperatures than so-called conventional RF magnetron sputtering through the extended plasma region by applying of external magnetic field. The applied magnetic field superimposes with the conventional planar RF magnetron configuration during the glow discharge process. The major advantage of unbalanced RF magnetron sputtering is that it could confine the plasma in the vicinity of the substrate. In this context, unbalanced RF magnetron sputtering contributes to the significant changes during film growth on the nature of a film through the variations in the extended plasma characteristics towards the substrate and it also forces the electron movement along the helical path around the extended magnetic field lines with strengthened induced ionization. The induced ionization substantially helps to improve the mechanism of film growth in comparison with the conventional RF magnetron sputtering. Limited attempts have so far been made and reported for ITO [12], AlN [13] and AZO [14,15] thin films with improved structural, electrical and optical properties using the unbalanced magnetron sputtering technique.
1. บทนำ The demand for transparent conducting oxides (TCOs) is increasing rapidly in the field of optoelectronic technology on account of their excellent optical and electrical properties. Tin-doped indium oxide (ITO) has long been regarded as a TCO material, which led to the diverse applications because of its optimal performance. On the other hand, the rarity and the high cost of its prime material indium highlights to explore an excellent new inexpensive material. In this scenario, by doping of appropriate element such as aluminum (Al) in ZnO host lattice, better optical transparency can be achieved in the visible region. Thus, Al-doped ZnO (AZO) behaves like a best alternative TCO candidate for ITO. Considering the relevant capabilities, such as coexistence of high conductivity and excellent optical transparency in the visible region for the AZO material, it is expected to be the most favorable for the fabrication of many optoelectronic devices at moderate doping level of Al with a comparable yield. Owing to these key features, AZO films have taken many applications in diverse fields, such as thin film solar cells [1], photoluminescence [2], thin film transistors [3] and transparent electrodes for organic photovoltaics [4]. Thin films of AZO can be produced by using numerous deposition techniques, including RF sputtering [5], DC sputtering [6], spray plasma technique [7], pulsed laser deposition [8], atomic layer deposition [9], metalorganic chemical vapor deposition [10] and sol-gel spin coating [11] and several studies have been performed to control the crystalline nature and surface morphology. Among these various techniques, RF magnetron sputtering has attracted much special interest at room temperature deposition because of it provides large scale uniformity for industrial production, good adhesion and for preventing the target from poisoning. On the other hand, the film properties mainly depends on the chemical composition, microstructure and surface morphology. The unbalanced RF magnetron sputtering technique has been developed to produce advanced films, i.e., the films with a better microstructure and morphology because of it is most versatile and useful technique for the deposition of high-quality films at room temperatures than so-called conventional RF magnetron sputtering through the extended plasma region by applying of external magnetic field. The applied magnetic field superimposes with the conventional planar RF magnetron configuration during the glow discharge process. The major advantage of unbalanced RF magnetron sputtering is that it could confine the plasma in the vicinity of the substrate. In this context, unbalanced RF magnetron sputtering contributes to the significant changes during film growth on the nature of a film through the variations in the extended plasma characteristics towards the substrate and it also forces the electron movement along the helical path around the extended magnetic field lines with strengthened induced ionization. The induced ionization substantially helps to improve the mechanism of film growth in comparison with the conventional RF magnetron sputtering. Limited attempts have so far been made and reported for ITO [12], AlN [13] and AZO [14,15] thin films with improved structural, electrical and optical properties using the unbalanced magnetron sputtering technique.
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