Studies Of The Photoelectric Properties Of Zinc Oxide Ultrafine Particles And Zinc Oxide Thin Films

As a new generation of direct wide band gap semiconductor material, ZnO band gap is 3.37 eV, with the exciton binding energy of 60 meV, and has very excellent optoelectronic properties. It will greatly reduce the cost of device fabrication to apply ZnO to the light emitting device because of its rich sourse and low cost.This work is aim at studing the photoelectric properties of ZnO ultrafine particles and ZnO thin films. The microstructures and photoelectric properties of ZnO ultrafine particles with different grain sizes were characherized by XRD, SEM, PL and EPR, respectively. The ZnO, ZnMgO and NiO ceramic target materials were prepared by conventional solid-state reaction method. The influence of processing techniques on the density and shrinkage of the target were studied. The ZnO thin films were deposited on the glass substrate by a magnetron sputtering system. The microstructures, the transmittance and the optical band gap of the ZnO thin films were tested by XRD, SEM, and UV spectrophotometer, respectively. The effects of the substrate temperatures, the sputtering powers and the thicknesses on the microstructures and the optical properities of the ZnO films were investigated. The p-Si/n-ZnO and p-Si/SiO2/n-ZnO heterojunctions were were prepared by a magnetron sputtering system. The electrical properties of the p-Si/n-ZnO and p-Si/SiO2/n-ZnO heteroj unctions were measured by a Resistance and Hall Coefficient Test System.The main results are as follows:(1) SEM images of the ZnO ultrafine particles show that increasing heating temperature can result in expanding the particle size and fusion. Increasing the particle size was accompanied by major changes in the photoluminescence properties of the ZnO, and the decrease in peak intensity ratios of UV:visible emissions with increasing temperature can be attributed to a decrease in the number of defects on the surfaces.(2) The EPR results of the ZnO ultrafine particles indicate that there is a resonance peak with g= 1.957 in the EPR spectrum of the unheated ZnO nano-particles, and it can be dominated by a resonance from Znj. On heating above 1000℃, results in a progressive increase in a component with g= 2.146, which probably due to an enhancement in the effects of oxygen since the ZnO particles expanded and fusioned.(3) There are several types of defect centres associated with isomorphous substitutions, vacancies at Zn and O sites, and interstitial atoms in the ZnO structure that can contribute to its semiconductor and luminescence properties.(4) During the sintering process of the ZnO target, most of the water was volatilized at 220 ℃, the organic matter (PVA) decomposed and released at 640℃, and the organizations of the ZnO target dissolved at 1225℃ where the liquid phase was formed. When the ZnO target annealing at 1225℃ for a period of time, the grains packed together closely and the pores of the target get smaller, and glass phase formed on the grain boundaries, which reduced the porosity of target and increased the target density.(5) As the substrate temperature is 300℃ and sputtering power is 150 W, the deposited ZnO film has a good crystallization properties. If the substrate temperature is higher or lower than 300 ℃, the sputtering power is higher or lower than 150 W, the quality of the ZnO film will be deterioration. The thickness of film had little effect on its crystallinity. The transmittances of ZnO films prepared in different sputtering process are higher than 86%.(6) The optical band gap Eg of the ZnO films prepared at different substrate temperatures are in the range of 3.24-3.27 eV, and reached a maximum value at 300℃, it will decrease for the ZnO films prepared at higher or lower substrate temperatures. The optical band gap Eg of the ZnO films prepared under different sputtering powers are in the range of 3.22-3.28 eV, and reached a maximum value at 150 W, it will decrease for the ZnO films prepared at higher or lower sputtering powers. The thickness of the film had no significant effects on the optical band gap of the ZnO film. The change in the optical band gap may be caused by the lattice defects.(7) ZnO thin films growth preferentially along the C axis on Si substrate, however there are large lattice mismatch in these films, result in bad crystalline quality. The crystalline properties of ZnO film can be improved by embedding a layer of SiO2 film between the ZnO film and the Si substrate. In comparision with the p-Si/n-ZnO heterojunction, the p-Si/SiO2/n-ZnO heteroj unction has relatively good rectifying properties. The p-Si/SiO2/n-ZnO heterojunction has a smaller threshold voltage and a good rectification. Thus, the SiO2 layer can improve the rectifying characteristics of heterojunction.(8) The rectifying properties of p-NiO/n-ZnO heterojunction is not very good, however if the MgZnO layer for electron blocking was inserted between p-NiO and n-ZnO layer, the threshold voltage of the p-NiO/MgZnO/n-ZnO heterojunction is reduced to about 0.5V, and the ratio of the rectification is increased, thus the rectifying characteristic of the device can be improved, and weak light luminescence can be observed in the device.