Study On Al³⁺-doped ZnO Thin Films Prepared By Magnetron Sputtering And Their Transparent Conductive Properties

In order to find an inexpensive, environmentally friendly and performance transparent conducting films, Al3+-doped ZnO (AZO) transparent conductive films were prepared by DC reactive magnetron sputtering on ordinary glass substrates using Zn-Al alloy (Znwt%:Alwt% 98.5:1.5) as the target. This paper focuses on the impacts of magnetron sputtering process parameters (oxygen-argon ratio, working pressure, sputtering power) and different annealing process (temperature and atmosphere) on the microstructures and their transparent conductive properties of AZO films. The microstructures of AZO thin films were characterizated by XRD and SEM, and then, the electrical properties and transmittance of AZO thin films were tested with HMS-3000 Hall test systems and UV spectrophotometer. Optimization of process parameters of AZO thin films prepared by magnetron sputtering was achieved by analyzing the influences of sputtering process.The results show that AZO thin films with good light transmittance and conductivity characteristics have been successfully made by the DC Reactive Magnetron Sputtering. XRD results show that the crystal structure of AZO thin films is corresponding to the (002) crystal face diffraction of ZnO, having a high preferred orientation, and Al3+-doped into ZnO does not change the original crystal structure of ZnO, in which the structure for AZO samples still remains the hexagonal wurtzite structure of ZnO crystal structure. SEM observation shows that there is a flat surface with denser grain for AZO thin films. Also the tested results of electrical properties and transmittance show that the light transmission rate is more than 90%, and the lowest resistivity is 3.665×10-4Ω.cm for the AZO thin films.The results further show that there are great effects of the magnetron sputtering process parameters, i.e., oxygen-argon ratio (9.33%-19.33%), working pressure (0.0Pa-2.5Pa), and sputtering power (70W-170W) on the organizational structure, optical and electrical properties of the AZO thin films. When an oxygen-argon ratio is 9.33%, because the flow of oxygen is insufficient to fully integrated with the zinc to form zinc oxide, the film still mainly shows as a Zn metal film, opaque. While when the oxygen-argon ratio is in the ranges of 11.33%-19.33%, the metal film is converted into the oxide film, that is, AZO film. The resistivity of AZO thin films increases gradually with the increase of the oxygen-argon ratio and working pressure, but decreases with raise of the sputtering power. Under optimum conditions in this study (i.e. oxygen-argon ratio of 11.33%, working pressure of 0.5Pa, and sputtering power of 90W), the minimum square resistance is 6.6Ω, and the lowest resistivity is 3.665× 10-4Ω.cm, and the light transmissionrate within the range of visible spectrum is 91% for the AZO thin films.After annealing under an atmosphere of vacuum, or Ar gas, the electrical resistivity of AZO films reduces, while the resistivity increases after annealing under an atmosphere of O2, or N2, or air, etc. When annealing in the ranges of 25℃-500℃ under Ar atmosphere, with the increase of annealing temperature, the grain size of samples grows up, and the transmittance of the films is slightly increased, and the resistivity of the films decreases first and then increases. Especially, the resistivity of the films is minimized when the annealing temperature of 400℃.