Structural and optical properties of sputtered ZnO thin films
Introduction
Zinc oxide is a promising transparent conductive oxide (TCO) and there is an unceasing interest on its luminescence and electrical properties during the last decades. Transparent conductive oxides (TCOs) are unusual materials that are both electrically conductive and visual transparent, so they are widely used in high- and low-tech applications such as antistatic coatings, touch display panels, flat panel displays, heaters, defrosters, and optical coatings [1]. TCOs such as Sn-doped In2O3 (indium tin oxide, ITO), Sb-doped SnO2 and N and Al-doped ZnO have applications as transparent electrodes for liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and solar cells. ZnO films are widely utilized as a front contact in thin film solar cells, and are a material of interest for silicon thin film solar cell applications. In general thin film technologies are less costly compared to crystalline–silicon technologies for the fabrication of photovoltaic-energy systems. Sputtered ZnO films emerged as a possible competitor to other TCOs films having a higher conductivity and transparency than SnO2 and lower cost than ITO. In additional ZnO is a material of great natural abundance and environment friendly [2], [3].
Zinc oxide is a direct band gap semiconductor (band gap of 3.37 eV at room temperature), with large exciton binding energy of 60 meV, and high refractory (melting point of 1975 °C) and chemical stability [4]. Un-doped zinc oxide exhibits intrinsic n-type conductivity like other TCOs.
Aluminium-doped ZnO (ZnO:Al) films have low resistivity and good optical characteristics. They exhibit a sharp UV cut-off, a high refractive index in the IR range, and are transparent in the visible light. Their optical band gap can be modified by Al doping-level [5]. ZnO:Al films have good thermal stability and are stable in hydrogen plasma atmosphere.
ZnO thin films can be deposited by various deposition techniques such as chemical vapour deposition, reactive and ion-assisted evaporation, spray pyrolysis, sol–gel processing, and sputtering.
In our research we utilized the advantages of RF diode sputtering to obtain ZnO and ZnO:Al thin films with optimal parameters for solar cell applications. The method is favourable for fabrication of large-scale coatings of good adhesion to substrate and flexible concerning the processing parameters. The processing parameters of major influence on the structure and properties of the sputtered ZnO films are the substrate temperature, sputtering pressure, discharge power, and substrate bias. The optimisation of these parameters can result in required surface texture and electrical and optical parameters. The influence of sputtering pressure and substrate temperature on ZnO film properties has been the subject of numerous studies [6], [7]. To the best to our knowledge there are only few studies of influence of bias voltage on ZnO films [8]. RF diode sputtering itself is associated with a considerable electron and ion bombardment on the growing film, which provide additional energy to the growing ZnO film on the substrate, resulting in significant variation in film structure and electrical properties and effectively cleans the film of adsorbed gases from the sputtering ambient. The negative bias applied to the substrate intensified the ion bombardment and its effects. With the aim to find a correlation between sputtering conditions and film's properties we investigated ZnO and ZnO:Al thin films prepared by RF diode sputtering at varying negative bias voltage and RF power. We report our observations on the effect of RF power and negative bias on deposition rate, surface morphology, structure and optical properties of ZnO and ZnO:Al thin films prepared by RF diode sputtering on glass substrate.
Section snippets
Experimental procedure
The ZnO and ZnO:Al films were deposited on Corning glass substrates in a planar RF sputtering diode system Perkin Elmer 2400/8L with a base pressure of 10−4 Pa. For thin film preparation two targets were used. One of high purity ZnO (99.9995%) and the other sintered ceramic target with a mixture of ZnO:Al2O3 (purity 99.99%). The percentage of Al2O3 in the ZnO target was 2 wt.%. The Ar gas pressure of 1.33 Pa was maintained constant during the sputtering process. Table 1 showed the deposition rate
Deposition rate
The deposition rate at Ar gas pressure of 1.33 Pa and substrate room temperature increases linearly with RF power, which means that the number of atoms sputtered from the target is nearly proportional to the RF power (Fig. 1).
Structural properties
XRD analysis indicated that all investigated ZnO films are polycrystalline with a fine-grained hexagonal structure (Fig. 2, Fig. 3). Furthermore, XRD patterns show (Fig. 2, Fig. 3, Fig. 4, Fig. 5) that there is a very strong preferred orientation of crystallites (texture)
Conclusion
The deposition rate of un-doped ZnO and Al-doped ZnO films increases linearly with RF power and higher RF power causes changes of the preferred orientation of crystallites. Stronger effect has the negative substrate bias voltage: increasing of it leads to decreasing of the intensity of (0 0 2) diffraction line and give rise to (1 0 0), (1 0 1) and (1 1 0) diffraction lines (texture is more random).
The integral transmittance of about 90% in the range of 400–1000 nm was not significantly influenced either
Acknowledgements
Presented work was supported by the MSMT CZ grant project 1M06031 and in part by projects VEGA of the Slovak Grant Agency.
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