| In recent years, traditional materials could not adapt to the development of modern science and technology. New materials have been under intense investigation, and thin film is one of them. As a new type of semiconductor thin films, transparent conducting oxides (TCO), are widely applied optical field due to their low resistivity and high optical transmittance in the visible region. Such as surface acoustic wave devices, gas sensors, solar cell technology and transparent conducting electrodes.Nowadays, the energy issue has caused large attention around the world, the problem that people need to solve quickly is how to use renewable sources of energy. Solar energy is the most abundant and clean energy, how to improve the efficiency of solar energy utilizing has become a research priority. At present, research of solar energy technology has made a certain progress, further more, some solar energy products appear in the market and got people's praise, solar energy enterprise has been gradually industrializing. Solar energy devices are made from silicon, but all the properties of silicon indicate that it is not the best solar energy material, therefore, it is great significant to find substitution of silicon.Recently the applications in solar cells and optoelectronic field of cadmium oxide (CdO) received more attention. CdO is an n-type semiconductor, there are a lot of native defects which provide a large number of free electrons makes CdO have high carrier concentration and low resistivity (10-4 ?·cm), and it exhibits excellent electrical properties. CdO shows a high optical transmission in the visible region which may come up to 80%. The band gap of CdO is 2.3 eV corresponding to the absorption edge of 530 nm, near the region which has strongest energy in the solar spectrum. Different methods could be used to fabricate CdO films because of their excellent crystalline properties. CdO is a cheap material and has less pollution. All of these advantages make CdO to be a suitable candidate for solar cell application. Hence, the purpose of researching CdO films is how to further enhance their Optical and electrical properties. It has been reported that other ions doped CdO films show an increase in their properties. Indium (In) has been used as a suitable dopant for transparent conducting films to increase the carrier concentration and improve the electrical conductivity, but there were few reports about In-doped CdO (In-CdO) films.There are several methods could be used to prepare CdO thin films, pulsed laser deposition (PLD) is a physical vacuum deposition technique which rapidly developed in recent years, laser is monochromatic light and has high energy, the laser beam irradiate the target and plasma is formed of high temperature in the surface of target in a short time, the plasma expand along target normals and deposited on the substrate finally. PLD technique can exactly control the stoichiometric proportion and maintain target composition in the deposited thin films. The properties of films can be easily controlled by changing the processing parameters such as laser energy density, pressure of atmosphere gas in vacuum chamber, substrate temperature, etc. Films are deposited in vacuum environment to avoid introduction of impurities. PLD technique could solve the problem of refractory materials deposition because of high energy of laser, and could be applied for the fabrication of precision thin films.According to the above contents, the In-doped CdO transparent conducting films were deposited by pulse laser deposition technique and their structure, optical and electrical properties were studied in the present work. Pusled Nd-YAG laser with a wavelength of 1064 nm was used in ablation of Cd-In alloy target in oxygen reactive atmosphere, the films were deposited on quartz glass substrate. The effect of In concentration and substrate temperature on structure and optical and electrical properties of In-CdO films were studied in detail. It included the following: In-CdO films with different In concentration were deposited on quartz glass substrates by PLD, and other processing parameters were fixed. CdO film without doping was also fabricated for comparison to study the effect of In on properties of In-CdO films. The possible mechanism of In doping effect on the properties of films was discussed, and the best In concentration was found. Furthermore, In-CdO films were deposited by PLD from ablating Cd-In metallic target with the best In concentration at different substrate temperatures. The effect of substrate temperature on structure and optical and electrical properties of In-CdO films were studied. The main results are shown as follows:1. The actual In concentration in the films is lower than the nominal composition of the targets. That may be attributed to the melting temperature of In (156.6°C) is much lower than that of Cd (321.18°C), the metal In evaporates more easily than Cd does under the substrate temperature, the In in the targets is not deposited on substrates completely.2. The films show NaCl cubic crystalline structure. In doping weakens (200) preferential growth and the lattice parameter shows slight variation due to substitution of cadmium by indium atoms. The doping of indium induces an obvious increase of grain sizes and grains become more uniform, that means crystalline quality is improved. As In00q concentration increases to 5.6 wt%, In2O3 crystals appear which means that the maximum solid solution of indium element in CdO is less than 5.6 wt%. 3.9 wt% indium doping CdO film shows a very low electric resistivity of 5.95×10-5Ω·cm, which is attributed to the increase of carrier concentration due to the substitution of Cd2+ ions by In3+ ions doped in CdO structure. The films show high optical transmission. The doping of indium induces a blue shift of absorption edge of In-CdO films, and the increase of optical band gap energy from 2.41 eV to 2.97 eV. To sum up, the In-CdO thin film with In concentration of 3.9 wt% has the best properties.3. The metal Cd in the target was not be oxidized completely, and the films become a composite of In-CdO and Cd that deposited at substrate temperature of 100°C and 200°C. It is found that the intensity of (200) orientation of the films decreases with the increase of substrate temperature, stress in the thin films induce a decrease of lattice constant. The lowest surface roughness and finest uniform grains are obtained by the film grown under 300°C. Because the combined action of crystallinity and rough surface morphology of the films as well as point defects and interstitial impurities in crystals, low resistivity, high carrier concentration and high mobility are observed when the film deposited at substrate temperature of 300°C. All films show high transmittance in the visible region, especially around the wavelength of 500 nm which has the strongest energy in the solar spectrum. The band gap of In-CdO films is widened with the substrate temperature increases from 300°C to 500°C, however, combine with the decrease of carrier concentration, the band gap of In-CdO films increases with substrate temperature have reverse change according the Burstein-Moss effect. It attribute to the combined action of the band shrinkage effect and the Burstein-Moss effect at high carrier concentrations in the order of 1020 cm-3.4. To sum up, the In-CdO films with low resistivity, high carrier concentration and high mobility were obtained. The band gap was widened to 2.97 eV and the absorption edge was moved to about 400nm, Films show the highest transmittance in the region which has the strongest energy in the solar spectrum, hence the light absorption of solar spectrum was increased about 30% in comparison with that of pure CdO film. The blue shift of absorption edge, combining with the obvious decrease of electric resistivity, makes In-CdO film to be a potential candidate for window layer in solar cells and other possible optoelectronic applications. |
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