Research On Transparent Conducting ITO Films Doped By High-Valence Metal Element

Since 1990s, the rapid development of panel displays promotes the progress of optical-electrical films. Due to high carrier concentration and wide optical band gap, transparent conductive oxide films exhibit outstanding optical and electrical properties such as low resistivity and high transmittance in the visible range. ITO (Sn doped In2O3) films possess the better optical and electrical properties, which make them have wide applications as the transparent electrodes. But some critical factors, such as chemical and thermal instability and lower surface energy, limit the wider application of ITO films, and optical-electrical properties also should be further improved. The improvement of over-all properties of ITO films is one of the key points. In this paper, In2O3-based multi-component oxides films were deposited by magnetron sputtering.The first-principles calculations regarding electronic band structure are performed. The characteristics of ITO films strongly depend on its oxidation state and the content of impurities. Carrier concentration can be modified by the dopant activation state, which is due to a donor atom to substitute the lattice site and produce some free electrons to increase carrier concentration. In this study, zirconium and tantalum are regarded as the donor, which replaces indium in the In2O3 matrix. As a result, some free electrons are released to contribute the electrical conductivity. The ionic radius of Zr4+ or Ta5+ is lower than that of In3+, which implies that a limited solid solution can be formed. In order to realize the effective doping of high valence metal elements, the dual sputtering sources with two targets, including a DC power and a RF power, were used for co-sputtering with an ITO target and a Zr target or Ta2O5 target.ITO, ITO:Zr and ITO:Ta films were deposited on glass substrates by magnetron sputtering. Electrical and optical properties of the films at different experiment parameters were contrastively studied. The results show that the doping of high-valence metal element favors a better crystalline structure for ITO films and leads to the formation of the (400) plane preferred orientateon. ITO:Zr and ITO:Ta films show better electrical and optical properties at low substrate temperature than ITO films. The dopings of Zr and Ta improve the figure of merit of ITO films deposited at room temperature from 0.003×10-3Ω-1 to 0.15×10-3Ω-1 and 0.88×10-3Ω-1. The increase in substrate temperature remarkably improves the electrical and optical properties. The excessive oxygen can worsen the optical properties of the films. Better optical-electrical properties of the films can be achieved after the proper annealing treatment, but the atmosphere annealing treatment with overhigh temperature can worsen the electrical property. Obvious"Burstin-Moss"effect can be revealed by transmittance spectra with different parameters, and the direct transition models show the change of optical band gap of the films. ITO:Zr and ITO:Ta films prepared by co-sputtering reveal better optical-electrical properties and higher optical band gap than that of ITO films. Under the same parameters, ITO:Ta films show lower conductivity but higher transmission in the visual region than that of ITO:Zr films.The first-principles calculations were performed for In2O3-based transparent conductive oxides. The characters of the highest valence states and lowest conduction band are the key characteristics of the band structure responsible for its optical-electrical properties. When Sn, Zr or Ta is introduced into the lattice, it prefers to occupy the less distorted In1 site of In2O3. It is found that the tops of the valence states of those materials are mainly formed by O 2p states and In 4d states. The bottoms of the conduction bands are due to In 5s states hybridized with Sn 5s states (and Zr 4d or Ta 5d). The introduction of Zr or Ta into ITO stimulates the shifts of the Fermi level, conduction band bottom and valence band top, which is related with the improvement in eigen electrical and optical properties observed in the high-valence metal element doped materials.According to the result of the relative resistance change of ITO films in the special medium environments and the analysis of the film surface, the doping films show better chemical and thermal stability than that of ITO films. Besides the influence of crystal structure, the better stability of zirconium oxide and tantalum oxide can improve the chemical and thermal stability of ITO films. Based on the measurements of the contact angles, the surface energy and surface polarity of the films were calculated. The experimental data and calculated results show that the doping decreases contact angle, increases surface energy and enhances surface polarity. The surface condition and the present of active high-valence atom with d-orbital states close to Fermi lever are the main contributions to improve the surface energy.