Study On Preparation And Properties Of Ga-Doped ZnO Transparent Conductive Oxide Films

ZnO film is a II-VI semiconductor material with a wide direct band gap. Due to its various advantages, such as abundance, non-toxicity, good stability in plasma etc, it is considered as the most promising substitute transparent conductive oxide(TCO) film for widely used, scarce and expensive ITO. ZnO thin film can be widely applied in opto-electronic fields such as solar cell, liquid crystal display, light-emitting devices, gas sensor etc, so it has gained great attention in recent years. In order to improve the poor conductivity and stability of intrinsic ZnO, Group III elements (Al, In and Ga) have usually been choosed as n-type dopants, among which Al doping has been most investigated. Compared to ZnO:Al thin films, ZnO:Ga thin films are more resistive to oxidation and have smaller deformation of lattice. But at present, there are a relative few papers related to ZnO:Ga films, so it is necessary to research ZnO:Ga films systematically to establish a basis for its application in opto-electronic devices etc.In this paper, the ZnO:Ga films were deposited on glass and quartz substrates by Radio frequency(RF) magnetron sputtering. The films were characterized and analyzed by XRD, SEM, XPS, Hall measurement and ultraviolet-visible-infrared spectrophotometer. The structure, morphology, composition, conductivity, transmission, thermoelectrical and NO₂ gas sensing properties were studied, the optical constants were calculated, and the photoluminescence property was also observed. The main results are as following:?The obtained ZnO:Ga thin films exhibit a polycrystalline wurzite structure with a preferred c-axis orientation. The surface is flat and dense. The crystallinity of thin films was affected by the sputtering parameters. The increase of substrate temperature, film thickness, sputtering power, the decrease of sputtering pressure and post annealing in Air or N₂ all contribute to the improvement of crystallinity. The best crystallinity is found to occur in the 3at.% ZnO:Ga films.?Zn, Ga, O and C elements are observed in the films. The Ga content and the concentration of oxygen vacancies in thin films increase with substrate temperature, leading to the increase of carrier concentration and the improvement of conductivity.?ZnO:Ga thin film is n-type conductive, its optical and electrical properties are closely related to prepartion parameters. The higher substrate temperature, the lower sputtering pressure , the thicker film thickness and larger sputtering power, the higher the conductivity, while after annealing in Air or N2, the conductivity decreases. The average transmittaces of ZnO:Ga thin films in visible range are generally over 80%. Increasing sputtering pressure, sputtering power, and annealing process are beneficial to the improvement of transmittance, but substrate temperature and Ga doping concentration have no apparent influence on the transmittance. Based on a trade-off consideration of transmittance and conductivity, the optimum deposition parameter is obtained: substrate temperature 300?,Ga doping concentration 3at.%, sputtering pressure 2.0Pa, sputtering power 150W, sputtering time 2h, distance between target and substrate 7.0cm. The resistivity of the ZnO:Ga thin film deposited under the above parameter is 1.41×10^-3?·cm, the transmittance is over 85%, so the film meets the demand of TCO film.?Seebeck effects were apparently found in ZnO:Ga films. The Seebeck coefficients are negative, further illustrating the n-type conduction of the films. Typically, the Seebeck coefficient of 3at.% Ga doped thin film depositied at 200?is -54.31?V/K, the power factor(7.530×10^-5W/K²m). Under magnetic field, magnetoresistive and magnetothermoelectric effect are obviously observed in ZnO:Ga films. The magnetoresistance and magneticthermoelectric effect increases with magnetic field intensity.?The optical band gap of ZnO:Ga thin film is related to deposition parameters. The increase of substrate temperature, sputtering power, and the decrease of sputtering pressure lead to the increase of optical band gap. With the increase of Ga doping concentration and film thickness, the optical band gap increases first and decrease aferward. Annealing in air and N2 will result in the decrease of optical band gap.?The photoluminescence spectra of ZnO:Ga thin film is composed of one near band edge emission and several deep level emissions(DLE) which are dominated by a blue emission (at 467nm). After annealing in air or N₂, the enhancemant of these DLEs is ascribed to the increase of radiative recombination of photon-excited non-equilibrium carriers.?The results calculated by Swanepoel and unconstrained optimal method have been compared. It is found that the thickness and transmission spectrum calculated by the latter method is better agreement with the measured data, implying this method is better than Swanepoel method.?The optical constants of ZnO:Ga thin films with various thickness(including the thinner film with the thickness less than 100nm) were calculated by using unconstrained optimal method. The result indicates that the refractive index decreases sharply with the increase of wavelength in the ultraviolet region, while the refractive index in the visible region change a bit between 1.73 and 2.15. In the wavelength range of 435-600nm, the refractive index decreases as the thickness increases. The extinction coefficient decreases sharply with the increase of wavelength in the ultraviolet region, while it changes little and near to 0 in the visible region.?ZnO:Ga thin films are sensitivie to NO₂ gas. The best sensing propertity can be gotten in the 1at.% ZnO:Ga thin film operated at 250?.The sensitivity to 5ppm and 200ppm is 2.34 and 135.90, respectively.