Study On Deposited ZnO Thin Film By Mist-CVD Method And Its Characteristics

Zinc oxide(ZnO) is a II-VI direct band gap semiconductor. Due to its wide band gap, large exciton binding energy(60 me V) at room temperature, non-toxic, raw materials readily available from nature, low cost, strong anti-radiation and favorable electromechanical coupling properties, ZnO has been investigated as a functional material for some electronic and optoelectronic devices such as solar cell windows, surface acoustic wave devices, liquid crystal display, gas sensors, and pressure-sensitive devices and so on. In this study, ZnO thin films were grown on(10-10) plane sapphire substrate by mist CVD under atmospheric pressure. Zinc chloride solution was used as Source solution and carrier gas was Nitrogen. The effects of the growth temperature and the carrier gas flow rate on the thickness, crystallinity, surface morphology and the photoluminescence of the ZnO thin films were investigated. Mist CVD is a new thin film preparation technology with Many advantages such as energy saving, safe, simple equipment, inexpensive, and easy to operate, etc.Firstly, we investigated the thickness, crystallinity, surface morphology and the photoluminescence of zinc oxide thin films prepared under different carrier gas flow rate by using profilometer, X-ray diffraction(XRD), scanning electron microscopy(SEM), and photoluminescence(PL) measurements. we concluded that while carrier gas flow rate varied from 3 to 10L/min, Thickness of the ZnO thin film increase generally along the direction of the gas flowing. With the increase of the gas flow rate, the thickness of the film at each fixed point decreases. This showed that lower gas flow rate is good for the growth of the film, but the film surface is uneven. From the XRD pattern, we know that the ZnO thin films grown at low carrier gas flow rate have no(10-10) preferred orientation growth characteristics, but are in(10-10),(0002),(10-11) and other crystal orientation random growth. In contrast, the ZnO films grown at the carrier gas flow rate of 8L/min only have single diffraction peak, and grow along the(10-10) preferred orientation. This showed that the ZnO films grown at the(10-10) sapphire substrates with the carrier gas flow rate of 8L/min are of good crystallinity. The reason for this result may be related to the relationship between the supply of raw materials and the oxygen content in the reaction furnace and the growth mechanism of the ZnO thin films. It is also considered that the result is related to the mismatch between the(10-10)plane ZnO and the(10-10)plane sapphire. The results of SEM and XRD were in agreement. When the carrier gas flow rate is 3 or 5L/min, the result of SEM shows that the surface of the film is disordered and the grain shape is different. When the carrier gas flow rate is 8L/min, the grain shapes are roughly the same, and the grains are closely connected. But when the carrier gas flow rate is 10L/min, the surface morphology of the films decrease. When the carrier gas flow rate varied from 3 ~ 10 L, The photoluminescence of the samples is mainly green emission near 500 nm, and ultraviolet emission about 380 nm is very weak. The change of the carrier gas flow rate has little effect on the emission intensity.Secondly, we studied the changes of the basic properties of ZincOxide films prepared at different growth temperatures. When the growth temperature changes from 500 ? to 700 ?, the thickness of the film increase As the temperature increases. This is because as the temperature increases, the reaction activation energy of the reaction is increased, and the reaction efficiency is improved. Interestingly, the thickness of the film increases along the carrier gas flow when the growth temperature changes from 500? to 575?,but when the temperature changes from 600? to 700?, the film thickness decreases with the gas flow direction. The effect of the change of growth temperature on the X- ray diffraction of the sample was small. The samples deposited at various growth temperatures were essentially only(10-10) diffraction peak. With the increase of the growth temperature, the surface state of ZnO thin films changed significantly. When the growth temperature is increased from 500, the surface grains gradually become larger, and the grains changed from isolation to interaction with each other. Uniform and dense films were obtained when the growth temperature rose to 625 ? and 650?. However, when the temperature increased further, the quality of the film is reduced. This is related to the anti-evaporation and particle deposition of the deposited thin film atoms at high temperature. When the growth temperature changed from 500? to 700?,The photoluminescence of the samples is mainly green emission near 500 nm as well, but ultraviolet emission about 380 nm is almost undetectable. The change of the carrier gas flow rate has little effect on the emission intensity.