Preparation And Properties Of ZnO Materials

Zinc oxide, similar to GaN, is a novel direct and wide band-gap (3.37eV,300K)Ⅱ-Ⅳcompound semiconductor material with wurtzite crystal structure. The most significant feature of ZnO is very large exciton binding energy of 60meV at room temperature, twice as GaN (25meV) and much larger than thermal ionization energy (26meV) at room temperature, which leads to exciton stimulated emission at room temperature with high stability. It ensures an ultraviolet (UV) exciton emitting with low activation energy at room temperature. In addition, ZnO has been recognized as one of the most important and promising semiconductor materials in optoelectronic devices fields such as ultraviolet detectors, lasers and blue-violet light-emitting diodes (LEDs) due to its high electrical conductivity, thermal conductivity, chemical stability and good UV absorption properties.Compared with the bulk materials, one-dimensional ZnO nanomaterials have a wide range of applications in the field of nano-devices due to its unique physical and chemical properties. UV stimulated emission is easier to achieve at room temperature because of the radial quantum confinement effect in one-dimensional ZnO nano materials. One-dimensional ZnO nano-materials show good field emission properties, as same as carbon nanotubes, due to the high surface-to-volume ratio and small tip radius of curvature. In the future, ZnO nanostructure will be an ideal substitute for carbon nanotubes in field emission display because of their better chemical stability in the environment. ZnO nano-materials have been greatly applied in optoelectronic devices fields because of their low-threshold ultraviolet stimulated emission at room temperature.In recent years, many research works were reported about preparation and growth mechanism of ZnO nano-structures. Many different morphologies of ZnO nano-structure, such as nanowires, nanorods, nanobelts, nano-ring, were prepared in the past research work. Chemical methods or Metal organic chemical vapor deposition (MOCVD) were popular used in these studies, however few papers were reported about magnetron sputtering in ZnO nano-materials growth in the past. The important problem was the control of structure, morphology and scale in the micro-structure research of ZnO:So an effective approach to realize blue-violet light emission is to realize the controlled growth of one-dimensional ZnO nano-materials with high quality, so the research about it was received broad attention.In this paper, ZnO nanorods with aligned single-crystal blocks were synthesized by annealing and magnetron sputtering technology on the Au-template substrates, realizing the controlled growth of ZnO materials and the rare earth (Eu) doped ZnO materials. The structure, morphology, compositions and optical properties of the products under different conditions were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM & HRTEM), energy-dispersive X-ray spectroscopy (EDS), flourier transform infrared spectrum (FTIR), x-ray photoelectron .spectroscopy (XPS) and photoluminescence (PL). The main contents are as follows:1. The synthesis of ZnO nanorods with aligned single-crystal blocksAt first, simple Au-dot templates were prepared by annealing Au films at 900℃which were deposited on the Si (111) substrates with direct current (DC) sputtering. A large number of Au nanoparticles with uniform size are found on the substrate through SEM results. Compared with sputtering for 10 seconds, smaller diameter and greater density of Au dots were found in the template with sputtering for 5 seconds, and it is more suitable for the growth of ZnO materials.The ZnO nanostructures with aligned single-crystal blocks were synthesized by annealing ZnO film at 1000℃which was deposited on the Au-template substrate with radio frequency magnetron sputtering. The structure, morphology and optical properties of the products were characterized by XRD, SEM, HRTEM, FTIR, XPS and PL. The influence of different annealing conditions and different templates on ZnO nanorods was discussed in detail in the text.The results show that the annealing temperature has a great influence on the morphology and the crystal quality of ZnO nanorods. As the annealing temperature increases, the diameters of nanorods become smaller, then thicker. ZnO nanorods with the minimum diameter were obtained at 1000℃. The crystal quality of nanostructures improved firstly and then fell. We believe that this transformation is caused by the different atomic mobility at different temperature. Because of the typical polar structure of ZnO, the atomic mobility increased with the temperature increase, then more atoms arranged in [0001] direction. So the smaller diameter and better crystalline quality were found. But when the temperature rises to a certain value, the influence of the atomic mobility is greater than that of the polar structure of ZnO, and leading to the increase of lateral movement of atoms. So the diameter of nanorods becomes bigger with the declined crystalline quality.The annealing time also has important influence on the growth of ZnO nanorods. As the annealing time increased, nanorods grow gradually from clutter to order, then from order to bond with each other. A short time is not enough for atoms to move from initial state to an ordered state controlled by Au dots. But too long time will cause the increase of nanorods growth along the diameter direction, thus rods will bond with each other with the bigger diameter. The influence of ambience on ZnO growth is related to the oxygen content. In the oxygen-rich ambience, more ZnO grains appear and the re-crystallization of ZnO with high crystal quality will be complete. However, no significant effect of the flow on the ZnO growth was found in our experiments. The optimum annealing condition was found by detail analysis, that is:annealing for 20 min in O2 ambience at 1000℃.The ZnO nanorods with aligned single-crystal blocks were re-synthesized in the optimum condition in our research. The structure, morphology and optical properties were studied in detail by many testing methods. The results show that the ZnO nanorods were composed of aligned single-crystal blocks along the [0001] direction, which were hexagonal wurtzite single-crystal blocks. The nanorods arranged compactly with high density ordered by the template to some extent. PL spectrum shows that the nanorods possess good optical properties with a sharp UV emission at 380nm caused by the ZnO band edge emission and a broad defect-related green emission at 500nm. The Au-dot substrates play the role of control in the ZnO nanorods growth to some extent. So it can be used as a simple template in ZnO nano-materials growth in the future.2. The preparation of one dimension ZnO worm-like structure doped with EuThe film with alternate layers of Eu and ZnO was synthesized by magnetron sputtering technique on the Au-dot template. Rare earth Eu targets and ZnO sintered targets are used in the alternating sputtering technology. The ZnO worm-like structure doped with Eu was prepared by annealing the multiplayer film in the quartz tube furnace in O2 ambience. The morphology, composition and influence of doping on optics properties are studied by various testing technology.SEM results show that the worm-like curved structures with diameter of 80～100nm and length of the micron order of magnitude were found in the samples. When temperature increased from 900℃to 1000℃, no significant change in morphology was found. XPS and EDS results showed that a small amount of rare earth Eu was doped in ZnO with atomic ratios of only about 0.42%. The bigger spacing caused by Eu doping was found in the crystal structure from XRD and HRTEM results. And also two crystal structures, structure with aligned single-crystal blocks at 1000℃and cumulate crystal grains structure at 900℃, were found in the experiment. This result also shows that the growth of ZnO is a process of change from stacking of small crystal grains to aligned single crystal slowly. All the testing results show that Eu-doping ZnO material was synthesized in our research. PL results show that the UV emission peak at 380nm become weak but the green emission at 500nm increase because of Eu doping in ZnO material. The doping of Eu in ZnO material will induce electron traps or recombination centers, increasing the internal defects. And also it can increase the density of the zinc interstitial atoms (Zn;) and oxygen vacancy (Vo). All the defects will contribute to the increase of green emission to some extent. According to the PL spectra, the emission at 615.9nm is due to the electrons transition at 4f shell of Eu from 5D0 to 7F2, which further illustrate the doping of rare earth Eu.3. The growth of ZnO material under magnetic fieldBased on the theory of the movement of charged particles in the electromagnetic field, an external magnetic field was applied under the substrate to change the spatial distribution of the magnetic field in the magnetron sputtering system. It can change the deposition parameters and affect the material growth. Firstly, ZnO films were prepared on Si (111) substrates by RF magnetron sputtering. Significant difference was found in the samples because of the external magnetic field and the film thickness increased significantly. Then the samples after sputtering were annealed in the tube furnace. XRD, SEM and microscopy are applied to study the influence of magnetic field on the ZnO material growth. The testing results show that the external magnetic field can greatly improve the sputtering rate and the film crystallization. The ZnO film after annealing in O2 ambience is wurtzite single crystal with highly crystallized. Optical microscopy results show that the external magnetic field has changed dramatically the ZnO surface morphology distribution due to the polarity of ZnO itself.4. The discuss about the possible growth mechanism of ZnO with aligned single-crystal blocksCombined with the analyses in the process of ZnO material preparation, a preliminary growth mechanism of ZnO with aligned single-crystal blocks was discussed in this paper. There is no Au particle present at the tip of the nanorod, which demonstrates that the growth mechanism is not VLS growth mechanism. From the SEM results, we can found that the Au dot template play a key role in the formation of ZnO nanorods. After comprehensive analysis, we think the growth process is divided into two steps, one is the formation of ZnO single crystal grains and the other is the arrangement of ZnO single crystal grainsAt the beginning of ZnO growth, Au molten metal droplets change the distribution of surface energy, resulting in a large number of defects which are the best nucleation sites for ZnO growth. Many small single crystals with polarity formed because of the unique polar properties of ZnO crystals. To reduce the surface energy of polarization, one possible approach is to interface the positively charged Zn-(0001) plane with the negatively charged O-(0001) plane under the long range electrostatic interaction, resulting in neutralization of the local polar charges, thus forming the structure with aligned single-crystal blocks ordered by [0001] direction. However, further detailed investigations are thought necessary to identify the growth mechanism of these ZnO nanorods.