The Structure And Properties Research Of ZnO Transparent Conductive Films Prepared By Magnetron Sputtering Method

ZnO transparent conducting oxide (TCO) film is one of important opto-electronic information materials, which has not only high optical transparence in the visible region, high optical reflectance in the infrared region and high electrical conductivity but also much advantages, such as low cost, abound resources(about 1000 multiple of In content ), non-toxicity, high heat stability and chemical stability etc. It has extensive prospects in the fields of solar cell batteries and liquid crystal display devices, and it is hopeful to become the succedaneum of ITO film. In the present work, ZnO nano-sized powders with the purity of 99.9% and Ga₂O₃ powders with the purity of 99.99% are adopted as raw materials. The ZnO/ Ga₂O₃ sputtering target is successfully prepared by pressing at room temperature under isostatic pressure and by sintering at high temperature according to optimum sintering technique. The ZnO/ Ga₂O₃ film is successfully deposited on hard substrate (ordinary glass) and flexible substrate (polymide, PI) by radio frequency magnetron sputtering (RF-MS). With the help of modern analyzers, for examples, XRD, EDS and AFM, the chemical composition, phase constitution and morphology of ZnO/ Ga₂O₃ film are analyzed. The optical, electrical and meehanical properties are studied. The transmission and conduction mechanisms of ZnO/ Ga₂O₃ film are also discussed.The experiment results show that ZnO nano-sized powders with the purity of 99.9% and Ga₂O₃ powders with the purity of 99.99% are adopted as raw materials. Pure ZnO target with shrinkage ratio of 27.72% and ZnO/Ga₂O₃ composite target with shrinkage ratio of 27.65% are successfully prepared by pressing at room temperature under isostatic pressure and by sintering at high temperature according to optimum sintering technique. So these targets possess the high density so as to meet the need for RF sputtering target in the present experiment.The research on ZnO films prepared by RF magnetron sputter indicate that ZnO and ZnO/ Ga₂O₃ films are successfully deposited on ordinary glass substrate and PI substrate in pure Argon gas when self-made ZnO targets with the purity of 99.9% are used as sputtering target. The influence of sputtering technical parameters on the morphology of ZnO films is studied by AFM. The optimal sputtering parameters are as follows, sputtering power 70W, sputtering argon gas pressure 0.1Pa, the distance between the target and the substrate 70mm, sputtering time 60min, sputtering vacuum 6×10^-5Pa.With the help of modern analyzers, XRD, EPMA, EDS and AFM, the chemical composition, phase constitution and morphology of ZnO film are analyzed. Resultsshow that as-deposited ZnO films are hexagonal wurtzite polycrystalline structure. The preferential orientation of (002) is for all deposited films, and Ga ions appear in crystal lattice in form of displacement. No heating on the substrate during the sputtering and no annealing treatment are necessary for as-deposited ZnO films. ZnO films grow in the orientation of c-axis, and have dense structure. The crystal grains of ZnO films become smaller when the distance is closer the substrate.The results of interface bonding strength tested by scarification method show that ZnO films sputtered on glass substrate and PI substrate both have good adhesion properties. Furthermore, there are no formation of cracking and pull-out phenomenon appeared after folding the films deposited on PI substrate for several times.The optical property test and analysis of ZnO/Ga2O3 films show that the ultimate transmittance of ZnO/ Ga2O3 films deposited on glass substrate and PI substrate is 94% and 80% respectively. Moreover, because flexible substrate material has narrow band gap 2.82eV, transmittance curve of ZnO/Ga2O3 films deposited on PI substrate different from that on glass ones. It moves to the direction of long wave. The transparent mechanism of ZnO films lies in that ZnO is direct high-gap semiconductor material. Energy band of valence electron is full, so it can't absorb photon to move freely, while photon energy is too low to make valence electron transit to conduction band. Furthermore, the flaws in ZnO and doping can change its forbidden band width. Therefore, in order to improve the transmittance of ZnO films, its growth conditions should be controlled to get appropriate crystal grain size, good orientation and dense structure.The electrical property test and analysis of ZnO/Ga2O3 films indicate that the minimum electrical resistivity of ZnO/ Ga2O3 films deposited on glass substrate and PI substrate is 2.25><10'3Q.cm and 7.51><10"2D.cm respectively. The resistivity of ZnO/Ga2O3 films deposited on glass substrate is higher than that on PI substrate. It may because of non-activity and worse lattice matching with ZnO films on the surface PI substrate. The resistivity of pure ZnO films is high enough to reach 106Q.cm, but ZnO/Ga2O3( or ZnO: Ga) is much less to 10'3Q.cm. The electricity-conducting mechanism of ZnO films is that ZnO has wurtzite structure, and there are bigger interstices in its tetrahedron and octahedron structure. And the radius of Zn atom is shorter than O atom, so it is easy to enter interstice to form Znj flaw. During doping,Ga3+ take the place of Zn2+ and provide a redundant electron as current carrier, which make current carrier concentration increase with doping proportion.