Fabrication And Doping Of ZnO Based Transparent Conductive Films

ZnO is a direct wide band-gap compound semiconductor. Due to abundant material and low cost, ZnO is considered as a promising material instead of GaN for short-wavelength optoelectronic devices. If the doping engineering and the bandgap engineering are meanwhile realized, namely p-type ZnMgO is prepared, luminous efficiency of light-emitting diodes (LEDs) can be improved and the working waveband can be modulated. In this paper, p-type ZnMgO thin films were realized by In-N codoping method.The transmittance in the visible range for ZnO thin films is over 90 %. Undoped ZnO is n-type conductive. And the conductivity of ZnO films can be improved by doping IIIA group elements such as Ga, Al and so on. Compared with Sn doped In2O3 (ITO), ZnO has several merits, such as innoxious, cheap, stable in H. So ZnO is considered as a promising candidate for substituting ITO. In this paper, we investigated the properties of Al-and Ga-doped ZnO transparent conductive films. Nowadays, many electronic devices become flexible, which claims that transparent conductive oxide (TCO) films must be flexible. Therefore, it is practical to prepare flexible TCO films with good optical and electrical properties. So we also investigated the properties of Ga-doped ZnO (GZO) transparent conductive films on polycarbonate (PC) substrates. The work included:1. In-N codoped ZnMgO films have been prepared on glass substrates by direct current reactive magnetron sputtering. We investigated the effect of N2O partial pressures and Mg contents in the targets on the properties of ZnMgO thin films. The p-type conductivity could be obtained in ZnMgO films by adjusting the N2O partial pressures. The presence of In-N and Zn-N bonds was identified by x-ray photoelectron spectroscopy (XPS), which may enhance the nitrogen incorporation and respond for the realization of good p-type behavior in In-N codoped ZnMgO films. According to the analysis results of XPS and x-ray diffraction (XRD), we found that Mg substituted Zn in the crystal lattice. The ZnMgO-based p-n homojunction was fabricated by deposition of an In-doped n-type ZnMgO layer on an In-N codoped p-type ZnMgO. The p-n homostructural diode exhibits electrical rectification behavior of a typical p-n junction.2. Al-doped ZnO (AZO) thin films have been prepared on glass substrates by pulsed laser deposition. The properties of films were strongly dependent on the growth temperatures. When the temperature was 350℃, we obtained tansparent. conductive and near infrared (IR) reflective thin films. The good IR-reflective properties of AZO films show that they are promising for near-IR reflecting mirrors and heat reflectors.3. GZO thin films have been prepared on PC substrates by radio frequency magnetron sputtering. The dependence of the properties of films on the sputtering pressures and the oxygen partial pressures was investigated. The lowest resistivity of 7.8x10-4 Qcm was obtained. The average transmittance in the visible light range of all the films was over 80%.4. In order to conquer the disadvantages of polymer substrates, a ZnO buffer layer is necessary to be used before that GZO films are deposited on polymer substrates, which will make the polymer surface smoother and reduce diffusion of vapor and oxygen. In order to optimize the design of ZnO buffer layer deposition process, the Taguchi experimental design was used. We investigated the effect of sputtering power, sputtering pressure, sputtering time and the O2 partial pressure ratio on the optical properties in the visible light range and the resistivity for the GZO thin films deposited on PC substrates with undoped ZnO layers. The optimal parameters of the ZnO buffer layer could be obtained according to the range analysis method. Compared with the single layer GZO films, the electrical resistivity of GZO/ZnO films decreased from 7.85×10-4Ωcm to 5.21xlO-4 Qcm. Due to the thickness effect, the transmittance of the GZO film with the ZnO buffer layer is lower than that of the GZO film without the ZnO buffer layer, which is high and acceptable for applications.5. We also investigated the properties of multilayer structure TCO films. According to the single layer ZnO TCO films, multilayers are thinner and more conductive. According to the single layer metal films, multilayers are more transparent. We mainly investigated the effect of Cu layer thickness and GZO layer thickness on the properties of GZO/Cu and GZO/Cu/GZO multilayers. The highest figure of meritφTC is 4.66x10-3Ω-1 for the GZO(30 nm)/Cu(12 nm) multilayer. The highest value of figure of merit (φTC is 2.68x10-3Ω-1 for the GZO(10 nm)/Cu(10 nm)/GZO(10 nm) multilayer. The highest average near infrared reflectivity in the wavelength range 1000-2500 nm is as high as 70 % for the GZO(10 nm)/Cu(10 nm)/GZO(10 nm) multilayer. The good IR-reflective properties of GZO/Cu/GZO multilayers show that they are promising for near-IR reflecting mirrors and heat reflectors.