Study On Preparation And Properties Of Sn-doped ZnO Transparent Conductive Oxide Thin Films

ZnO is an important Ⅱ-Ⅵ semiconductor oxide material with a wide direct bandgap of3.37eV at room temperature. It belongs to hexagonal structure. Recently, ZnO hascaused diamond fever at home and abroad due to its good conductivity, high transmittance,abundance, non-toxicity and more preparation metheods etc. Studies have shown that ZnOis considered as the most promising substitute transparent conductive oxide for nanoindium tin oxide （ITO） films which is widely used, rare and expensive. ZnO thin film canbe widely used in many fields, such as flat panel liquid crystal displays, solar cells, gassensor, ultraviolet detectors, surface acoustic wave devices and so on. In order to improvethe poor conductivity and stability of intrinsic ZnO, typical elements such as Ⅲ elements（Ba、Al、Ga、In） and Ⅳ elements （Si、Sn） have usually been choosed as n-type dopants,among which Sn is an important dopant element. When ZnO is doped with Sn, Sn4+substitutes Zn²⁺site in the ZnO crystal structure resulting in two more free electrons,compared to the Ⅲ group elements with the same dopant concentration, ZnO can obtainmore carrier concentration. Furthermore, Zn can be easily substituted by Sn and does notresult in a large lattice distortion due to their almost equal radii (rZn²⁺=0.074nm and rSn4+=0.069nm). But at present, there are a relative few papers related to ZnO:Sn films, so it isnecessary to research ZnO:Sn films systematically to prepare ZnO:Sn films with highquality and establish a basis for its application.In this paper, the ZnO:Sn thin films were deposited on quartz substrates by Radiofrequency（RF） magnetron sputtering using high purity ZnO and SnO2powders. The filmswere characterized and analyzed by XRD, SEM, XPS, Hall measurement andultraviolet-visible-infrared light spectrophotometer. The structure, morphology,composition, conductivity, electrical and optical properties of the films were also studied.The influence of Sn content, substrate temperature, film thickness on the film propertieshave been studied systematically. The main results obtained are as follows:○1All the ZnO:Sn films were polycrystalline hexagonal wurtzite structure with （002）orientation perpendicular to the substrate. No tin oxide peaks are observed in the films. Snexists in the crystal lattice by replacing or in the grain boundaries by interstitial atoms. Thesuface of the films are flat and dense.○2The XPS results show that Zn、Sn、O and C elements are observed in all the ZnO:Sn films. Sn is successfully doped into the ZnO in the ZnO:Sn films. In the ZnO:Snfilms, Zn and Sn elements existed in Zn²⁺and Sn⁴⁺form, respectively.○3The Hall measurements indicate that ZnO: Sn films are n-type conductive; itselectrical properties are closely related to preparation parameters. The resistivity of thefilms shows increase and then decrease with increasing Sn doping and film thickness. Butwith the increase of the substrate temperature, the resistivity decreases. In our experiment,the lowest resistivity of the ZnO:Sn film is2.063×10^-2.cm.○4Sn has a great influence on the optical properties of ZnO thin films. When thedopant concentration is less, the average transmittance of the ZnO: Sn film in visible rangeis higher. The optical band gap value of the films shows increase and then decrease withincreasing Sn doping. The transmittances of the films with higher substrate temperature arehigher than that of lower substrate temperature.With the increase of the substratetemperature, the optical band gap value of the films shows increase and then decrease, atlast increase. With the increase of film thickness, the transmittance in the visible lightrange decreases, and the optical band gap value has shown a tendency to increase. In ourexperiment, the highest transmittance is76.93%in the visible light range.○5Based on a trade-off consideration of transmittance and conductivity, the optimumdeposition parameter is obtained: doping concentration of3at.%, the substrate temperatureof300℃, film thickness of440nm, working pressure of2.5Pa, power of120W, Ar flow of40sccm, target distance of7cm. Under the above parameters, the resistivity of the ZnO:Snthin film is4.182×10^-2Ω·cm, the transmittance is59.81%.