Microstructure And Optical Properties Of ZnO/PS Nanocomposite Films

ZnO is an important II-VI semiconductor oxide material with a wide direct bandgap of3.37eV. The most significant feature of ZnO is very large exciton bindingenergy of60meV at room temperature, twice as GaN (25meV) and much larger thanthermal ionization energy(26mev) at room temperature, which results in excitonstimulated emission at room temperature with high stability. There has been anincreasing interest in ZnO due to its electrical, optical and piezoelectric propertiesmaking suitable for many applications such as light emitting diodes，liquid crystaldisplay, solar cell, photodetectors, surface acoustic waves device and so on.In our paper, porous silicon (PS) templates were formed by electrochemicalanodization on p-type (100) silicon wafer and ZnO films were deposited on PSsubstrates using radio frequency (RF) reactive magnetron sputtering technique indifferent oxygen partial pressures, annealing temperature and annealing ambience.The effects of various preparation parameters on the crystalline and optical propertiesof ZnO thin films were investigated by the X-ray diffraction (XRD), scanningelectron microscope (SEM), UV spectrophotometer and fluorescencespectrophotometer. In this way some experimental data and the theoretical basis areprovided for the application of ZnO films. The results are summarized as follows:1. XRD shows that all films exhibit a dominant peak corresponding to the (002)direction of ZnO and other weak peaks corresponding to (100) and (101). XRD resultsreveal that all ZnO films are polycrystalline in nature with a hexagonal wurtzitestructure and a preferential orientation along the C-axis to the PS substrate.Meanwhile, a broad and weak peak at about2θ=56.6corresponding to PS is observed.In addition, it was shown that the preparation parameters have significant effects.2. Optical transmission and absorption spectra show that all samples have hightransmission, the result means that samples have better crystal quality. In addition,with the change of the preparation parameters, the optical band gaps of films haveslight movement, which can be attributed to quantum confinement effect. Moreover,the optical band gaps calculated based on the quantum confinement model are in goodagreement with the experimental values.3. The PL spectra measured in room temperature show that the ZnO/PS nanocomposite systems form a broad PL band in the visible region including the twoblue peaks located at about440nm (2.81eV) and480nm (2.58eV) and one greenpeak located at about520nm (2.37eV) from ZnO and a weak red-orange peaklocated at about600nm(2.07eV) from PS. Combining the blue and green emissionfrom ZnO with red-orange emission from PS, white light can be obtained. In addition,the red-orange emission band of PS located around670nm has a big blue shift afterZnO film was deposited. The mechanism of PL emission of ZnO/PS nanocompositeshas been understood by an oxygen-bonding model in PS and a native defects model inZnO.