| Transparent electronics are nowadays an emerging technology for the next generation of optoelectronic devices. The most commercially important application for transparent electronics seems to be as a transparent thin film transistor (TFT) for active-matrix liquid crystal display (AMLCD) and organic light emitting display (OLED). TFT based on Si technology (especially the amorphous silicon) actually present some limitations like: light sensitivity, low field effect mobility and opacity. One possible way to overcome such problem is the utilization of transparent oxide semiconductor based transistors, which have recently been proposed using as active channel intrinsic zinc oxied (ZnO). ZnO is a wide direct band gap (3.37eV at room temperature) semiconductor with a hexagonal wurtzite structure. It is transparent in the visible region of the spectra. The main advantage of using ZnO deals with the fact that it is possible to grow at relative low temperature high quality polycrystalline ZnO films by magnetron sputtering, which is a particular advantage for electronic drivers, where the response speed is of major importance. Aluminum nitride (A1N) is an important wide band gap III- V compound semiconductor. The wide band gap of 6.2eV, high resistivity as well as the large dielectric strength of A1N films make them more suitable as insulating layers in TFT structures. In addition, A1N has the same structure as ZnO, which makes it as the buffer layer to grow high-quality ZnO thin film and improve the interface structure.In this paper, A1N and ZnO thin films with high quality have been deposited on various substrates by radio frequency (RF) reactive magnetron sputtering. The microstructural and optical properties of the deposited films were characterized by x-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), fourier transform infrared spectroscopy (FTIR), raman spectroscopy and spectrophotometer. The results indicated that the preferred orientation of A1N films was influenced by many depositon parameters such as P_N2/P_Ar, sputtering pressure P_s, substrate temperature T_s and RF power P. At the condition of P_N2/P_Ar=3/1, P_s =0.5Pa, T_S =300℃ and P =300W, the A1N films showed an excellent preferred orientation along the c axis. With the introduction of low-temperature A1N buffer layers, the microstructure of A1N films deposited on ITO glass substrates was significantly improved with enhanced c-axis preferred orientation, increased grain size and smoothened morphology. Additionally, according to theenvelope method, the refractive index and the extinction coefficient were calculated by several extrema in the transimission spectrum of AIN film deposited on K9 glass substrate, which were respectively 2.0187 and 0.0077 at the wavelength of 554 nm.It was found that the crystalline quality of ZnO films was affected greatly by the deposition parameters. The (002) diffraction peak of ZnO films was obviously enhanced with the elevation of substrate temperature and the increase in RF power, which also resulted in largened grains. The best fabrication condition of ZnO films was:P=200W, Ps=1.0Pa, Ts=300°C, PO2/PAr=l/2. It can be seen from theAFM images that the ZnO films presented a densely-packed, pebble or cell-like surface appearance with columnar growth crystals perpendicular to the substrate surface. In addition, the transmission curve was fitted with the dispersive model proposed by Forouhi and Bloomer to determine the refractive index of ZnO film. The results indicated that the refractive index at the wavelength of 633nm was about 1.989, which was in good agreement with that previously reported in the literature. |
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