Transparent conductive oxides (TCO) are electrically conductive materials that absorb a small amount of light in the visible range of electromagnetic spectrum and are consequently transparent. They are used in various optoelectronic devices such as solar cells, thermochromic glasses, touch displays and sensors. Thin films based on indium oxide with the addition of tin (ITO) are mostly used as material for electrically conductive components in these devices. Physical methods, such as laser deposition and magnetron sputtering, are used for the preparation of ITO thin films, which require deposition in vacuum and use of expensive equipment. In addition, In2O3 is expensive, therefore alternative materials, that are cheaper and more accessible than In2O3, are being researched as well as methods for preparing thin films, that do not require expensive equipment and take place under normal conditions. One of the promising materials is zinc oxide (ZnO) with a wide band gap, good mechanical/chemical/thermal stability, and in the form of a thin film, it is also highly transparent. In addition, the structural, electrical and optical properties of ZnO thin films can be influenced by changing the chemical composition (doping) and changing the conditions during thin film preparation process.
As part of my master`s thesis, I prepared thin films IZO with the atomic ratio Zn/(In+Zn) = 0,36 on glass and silicon, using the spin-coating method. IZO solutions, with concentrations 0,3 mol/l and 0,147 mol/l, were prepared by dissolving the starting raw materials in appropriate solvents. Zinc acetate hydrate was dissolved in ethylene glycol, and indium nitrate hydrate in ethanol with the addition of acetic acid. Mixing the starting solutions, I obtained a homogeneous and clear IZO solution.
With thermal analysis and infrared spectroscopy with Fourier transformation of solutions and powders, obtained with drying of solutions, I determined the drying conditions of individual coatings and annealing of thin films. IZO thin films were characterized by field-emission scanning electron microscopy, atomic force microscopy and X-ray powder diffraction. The electrical resistivity of the prepared IZO thin films was measured by the four-point probe method.
When drying the film at 350 0C, I obtained IZO thin film with a layered microstructure, and by lowering the drying temperature to 200 0C, I managed to obtain a film with a homogeneous microstructure. I also compared the influence of annealing temperature on the microstructure of the IZO thin films on glass. I found that the most suitable annealing temperature is 600 0C. The thickness of the IZO thin films with the layered and homogeneous microstructure is around 80 nm with grain size of up to 20 nm. The surface of all analysed IZO thin films is flat and their structure does not have long-range order. IZO film with a layered microstructure had the lowest resistivity of 0,12 Ω*cm. By additionally annealing IZO thin films in an Ar/H2 atmosphere, the resistivity was reduced by approximately two orders of magnitude to 4*10-3 Ω*cm. This is the result of the formation of oxygen vacancies in the microstructure of IZO thin films. I also found that these thin films transmit between 80% and 92% of light in the visible part of electromagnetic spectrum and the IZO thin film with a layered microstructure shows better transparency.
|
