Study On Microstructure, Optical And Electronical Properties Of ZnO Nanostructure Films

Owing to its excellent opto-electronic properties, ZnO has some important applications in optoelectronic devices, transparent electronics, spintronic devices, piezo electric devices, thermoelectric sensor, etc. It has gradually become a study focus in Ⅱ-Ⅵ semiconductor. Although ZnO has been studied for a long time, some issues of its properties and the regulation of its microstructure and performance are not fully understood. These issues include the preparation of p-type ZnO semiconductor, the identification of the donor and accept defects for the instinctive defects of ZnO, the origins of the emissions in the visible region and the controllable synthesis of ZnO nano-hierarchical structure. The comprehension of these issues will enhance and extend the application of ZnO materials in functional devices and facilitate the practicality of the ZnO-based devices. For this reason, we give attention to ZnO and have carried out many researches about a few of the above issues. The research works and main conclusions are described as follows.(1) The molybdenum doped ZnO (MZO) films were deposited by radio frequency magnetron sputtering. The MZO films are composed of polycrystalline grains with the wurtzite structure. Mo-doping influences the grain size and roughness of the MZO films, and leads to a compressive stress in the MZO films. The grain size increases several times greater after the annealing process. XPS reveals that there is only Mo6+no Mo5+or Mo4+, in the MZO films. This indicates that the valence of Mo6+is not easily changed during the sputtering process. Mo doping introduces two effects on Zn2p3/2XPS spectra. One is the decrease of the binding energy of Zn2p3/2electron, the other is the emergence of a smaller peak at1022.0eV (related to Zni)in Zn2p3/2XPS spectrum. This illustrates that Mo doping can promote the generation of the Zni defects. Mo doping has little effects on the transmittance and the optical bandgap of the MZO films. The resistance of the MZO films firstly increases then decreases with the increase of the Mo doping concentration. The resistivity of MZO films reaches the minimum when the Mo doping concentration is2wt.%. This means that doping suitable amount Mo atoms can reduce the resistance of the MZO films. It’s supposed that the proper Mo doping can stimulate the formation of the Zn; defects and prompt the conductivity of the MZO films. Annealing in air causes a significant increase in the resistivity of the MZO films, which can be attributed to the adsorption of oxygen atoms and the concentration change of the donor defects Zni and its complexes defects in the annealing process. There are one weak UV emission peak at380nm and one stronger blue emission peak in the photoluminescence (PL) spectra of the as-prepared MZO films. The PL spectral intensity of the annealed MZO films is much higher than that of the unannealed film. Such an enhancement should be induced by the increase of the grain size which affects the quantum efficiency of PL significantly. With the annealing temperature rising from600to800℃, the emission peaks located at380and412nm are covered by an emerging emission peak located at400rim, and a new emission peak located at525nm appeared. The spectral intensity of the annealed MZO films is about ten times higher than that of the annealed ZnO film, but the position of the emission peaks (at400and525nm) is almost the same. It’s supposed that in the annealing process Mo-doping benefits the forming of the Vzn and Ozn defects which are the origins of the emissions at400and525nm respectively.(Ⅱ) The Al doped, La doped and Y doped ZnO nanostructure films were prepared by the hydrothermal method. The Al doped ZnO (AZO) nanostructure films are constructed by the nanopyramids with the wurtzite structure. Al doping has. little effect on the surface morphology and the light transmittance of the AZO films. There are two broad emission peaks, a UV-violet emission peak (PI) and a green-red emission peak (P2), in the PL spectra of the AZO films. At room temperature, with the increase of the Al doping concentration, the intensity of the PI peak increased sharply and reached the maximum when the doping concentration is10%; the relative intensity among the components of the green-red emission peak also changed with the increase of the Al doping concentration. The temperature dependent PL spectra show that the P2peak quenched when the temperature rose from10to297K, but the PI peak almost doesn’t change with the increase of sample temperature. It’s supposed that the fluorescence quenching of the green-red emission peak is attributed to the increased probability of the nonradiative exciton recombination caused by the multiphonon emission. The La doped ZnO (LZO) films have the similar microstructure with the AZO films. The diameter of the LZO nanopyramids and the intensity of the (002) XRD diffraction peak of the LZO films increase with the increase of the La doping concentration. The transmittance of the LZO films is much smaller than that of the AZO films. At room temperature the UV emission of the LZO films is very weak, but the emissions in the range of500-800nm is very strong. With the increase of La doping concentration, the intensity of the emissions in the visible region increases and reaches the maximum when La doping concentration is8%. Doping has little effect on the UV emissions. When the sample temperature rising from10to294K, the emissions in the visible region quench quickly, and the UV emission peak appears red shift which should be attributed to the increase of the lattice constant. The Y doped ZnO (YZO) films have similar properties in microstructure and light transmittance. At room temperature the YZO films have strong emissions in the visible region, but Y doping has little effect on the PL spectral intensity and the emissions’peak position. This is different with that of the AZO or LZO films.(Ⅲ) The CdS quantum dots (CdS QDs) were deposited on the surface of the ZnO nanostructure films by chemical bath deposition. The size and the density increase with the increase of the deposition cycles. After four deposition cycles (this sample was marked as CdS-ZnO-4Ts), the CdS QDs have uniform size and large density. The CdS QDs lead to an increase of the absorption coefficient of the ZnO films, but have little effect on the PL spectra of the ZnO films. The sample CdS-ZnO-4Ts has a good photocatalytic performance, its degradation rate of methyl orange solution during120minutes reaches96%, while that of the pure ZnO films is just35%. This suggests that the surface modification of ZnO films by CdS QDs is benefit for its photocatalytic performance.(IV) The ZnO nanostructure films were synthesized by electro-deposition and anodic oxidation. In the electro-deposition process, the volume ratio of alcohol to deionized water in the electrolyte is an important factor in the growth process of ZnO nanostructure. The different ZnO nanostructures, that is, ZnO nanosheets,"*"-like ZnO nanostructures, snowflakes-like ZnO hierarchical structure were synthesized while the ratio is1:3,1:1and3:1, respectively. The higher the ratio is, the more complicated the nanostructures are. The PL spectra of these samples show that when the ratio become higher, the emission in red light region become weaker and the emission in blue light region become stronger. The porous ZnO films were prepared by anodic oxidation. Its surface morphology has certain relation with the anodizing voltage. When the anodizing voltage is20V, the nano-square-tubes were achieved. Its growth mechanism will be studied further.