Fabrication And Properties Of Functional ZnO Thin Films

Znic Oxide (ZnO) is a wide direct band-gap (3.37eV) II-VI semiconductor material with a large exciton binding energy of 60 meV at room temperature, which is much higher than most other semiconductors, such as ZnSe (22 meV), ZnS (40 meV) or GaN (25 meV) . So ZnO is a proper ultraviolet material for short-wavelength photoelectrical device: high efficient light emitting diode (LED), UV detector, and blue LED .etc. Meanwhile, similar with the titanium oxide (TiO₂), after UV light irradiation the surface of ZnO films can produce the electron-hole pairs as reductant-oxidant, which can degrade the organic pollutants in environment. So ZnO film could be as a promising photocatalysis material. As for the solar cell application, ZnO has some advantages: doping with III elements(Al,Ga,In), ZnO-based material could be used as window coats with high transmission ratio in visible light and low resistivity. In this paper, the ZnO films for functional perspective, were successfully prepared by two-step oxidization method, RF sputtering technique, oxidization of Zn₃N₂ films method and in situ oxidization of Zn₃N₂ films, and the different characters of films were investigated by many methods.The ZnO films prepared by two-step oxidization method is shown below: first, Zn films, which prepared by evaporation, were oxidated at oxygen atmosphere below the melt-pointing of metal Zn, and then, the films were continued oxidization at high temperature. The photoactive capability of ZnO films was investigated by degradation of phenol method. The analysis showed that the ZnO films prepared by two-step oxidation show higher photoactive characteristic than that prepared by direct oxidation method. This is ascribed that two-step oxidization method improves the interstitial rate of the films. Excellent adhesion as well as being recyclable and efficient in phenol degradation of ZnO films was found in our experiment, which is in favor of ZnO films used in photocatalysis field.By the co-sputtering method using the metal Zn and Al targets, the ZnO:Al films were deposited on glass substrates. The effects of oxidation temperature and time on structural, electronical and optical properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Hall-effect measurements, UV-Vis transmittance spectra and photoluminescence(PL) spectra. The results indicated highly quality wurtzite ZnO:Al films with (002) orientation were successfully fabricated, which presented high transmittance rate (above 85%) and low resistivity (10^-1～10^-3Ω·cm). The properties of films could be satisfied with the photoelectrical coats in devices fields, especially in solar cells as window coats. Moreover, the films could be produced by this method, which also be extended to other fields.For development of ZnO-based opto-electrical devices, the fabrication of p-type ZnO is of important significance. Low resistance p-type ZnO thin films were prepared by in situ oxidation of Zn₃N₂ films which is deposited by reactive RF magnetron sputtering of zinc in Ar-N₂ mixture atmosphere. The effects of oxidation temperature and time on structural, electronic and optical properties of the samples were investigated by X-ray diffraction, scanning electron microscopy, Hall-effect measurements, UV-Vis transmittance spectra and PL spectra. It was found that the sample which oxidized at 450℃presented Zn₂N₃ structure except for ZnO; the sample at 500℃for 2 hours had better properties: low resistance (0.7Ω·cm), high transmittance rate (above 85%), predominant excitonic ultraviolet emission with narrow full width at half maximum and weak deep levels visible emission. The low resistance p-type ZnO films could be used as short wavelength devices in industrial fields.Except for the ZnO-based films in this paper, considering of the compatibility with Si-based technology, the SiC films, as excellent substrate materials, were also fabricated by RF sputtering and investigated by some measurements.These researches on ZnO materials are valuable for the fabrication of p-type ZnO films. By the amendment and improvement of in situ oxidization, the realization of ZnO-based opto-electrical devices could be developed in near future.