Full Length ArticleTransparent thin films of indium tin oxide: Morphology–optical investigations, inter dependence analyzes
Introduction
Indium tin oxide (ITO) have been widely studied in the last decades by many prestigious research groups, mainly because of the broad spectrum of applicability of these thin films. However, the optical and electrical properties of these thin films depend heavily on the influence of oxygen pressure correlated with thickness of the films, the preparation method and the growth parameters (like deposition rate or temperature).
The choosing a suitable transparent conductive oxide (TCO) may contribute to the advancement of research in photovoltaics [1], [2] and optoelectronic devices applications such as light emitting diodes, transparent coatings for solar energy, photo transistors, lasers or flat-panel displays [3], [4], [5]. ITO is one of the most appropriate TCO candidate for such applications [6], [7] being the most used TCO material due to its excellent structural, electrical and optical properties (low resistivity and high transmissivity) [3], [8], [9]. In addition, it may be obtained on large surfaces. These are the main reasons, it has been used as thin layer in different devices such as transparent conductors [10] or solar cells [11], where the performance is always influenced by the size and morphology.
As part of optoelectronic devices, it is involved in the control of transmission [8] and electrical conduction [8], [9]. The quality of the oxide thin films is highly influenced by the deposition technique [3], [7], [12], [13], [14]. Radio frequency magnetron sputtering (rfMS) [15], [16], diode (DC) magnetron sputtering [17] reactive ion beam sputtering [18], spray pyrolysis method [19], sol gel process [20] and pyrosol method [9] are among the methods used for deposition of ITO thin films. The rfMS deposition method [12], [15], [16] used to obtain ITO thin films with very good qualities, has several advantages as compared to other techniques [13], [14], [16], [17]. For instance, it allows the deposition of uniform films on large areas with a good adherence to the substrate and low residual tensions. Variations of deposition parameters lead to structural changes [3] and development of physical processes in these oxide films. ITO is an ideal material as a contact electrode in CuInGaSe2 (CIGS) solar cells [11].
The topic of this work is interesting as a contribution to the development of new optoelectronic devices. Changing the optical and electrical properties by tuning the coating process is an important field of investigation.
In this paper, we report a comparative analysis of ITO thin films obtained by rfMS with different thickness. To track changes in the performance of structural and optical properties of oxide thin films, we varied the thickness of the layers. In this study, different approaches were used to measure the optical parameters of transparent oxide films with different thicknesses. Estimating the thickness and transmission of ITO thin films from Swanepoel [21] and Drude methods [22], [23], implied determinations of several parameters and investigation of the optical properties. Refractive index dispersion, absorption spectrum, the real and the imaginary parts of dielectric constants, optical conductivity and mobility are some of the important characteristics that could be determined by using these valuable methods. Calculation of plasma frequency is also possible using the Drude method [22], [23], [24].
In this paper, we have shown that quality of the obtained transparent oxide thin films, improves with increasing thickness of the deposited layer. This way, the increase in thickness determined an increase of the electrical resistivity at room temperature, for the ITO films (i.e. 7.1 × 10 − 4 Ωcm for ITO 05, 5.5 × 10 − 4 Ωcm for ITO 06 and 4.6 × 10 − 4 Ωcm for ITO 07).
Section snippets
Experimental
A solid strategy (regarding experimental, theoretical and applied research) to obtain efficient transparent conductive films using the rfMS deposition technique, is developed in this paper. In this respect, the ITO thin films were deposited by rfMS method [15], [25], [26], at room temperature onto MENZEL-GLASER optical glass substrates with dimensions of 2 cm × 2 cm × 0.6 cm. The substrates were cleaned ultrasonically in a chromic acid and distilled water bath. Deposition was performed using a VARIAN
Results and discussion
AFM analysis was used to investigate the microstructure and morphology of the oxide thin films. Fig. 1(a)–(c) shows two-dimensional (5 μm × 5 μm) AFM images of the oxide thin films. The roughness (root mean square) is estimated from the topographic images between 2.9 nm and 9.5 nm. Following these scans, the presence of nanoparticles (Table 1) was noticed. It was also found that nanoparticle size increases as the thin film thickness increases, from about 138 nm (for ITO 05 sample), 154 nm (for ITO 06
Conclusions
Polycrystalline ITO thin films with different thickness were prepared using the rf magnetron sputtering technique. We obtained highly transparent and conductive ITO films with smooth surface (Rrms < 10 nm). The results indicate that surface nature of the thin films (with a uniform surface morphology) leads to samples with properties that may be integrated into solar cells structures. The crystallinity as well as optical and electrical properties of the samples are influenced by variation of the
Acknowledgements
This research is supported by the Romanian Ministry of Education, Research, Youth and Sport under grant number PN-II-RU-ID 52/2012. We also acknowledge the STAR 65/2013 project of the Romanian Space Agency.
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