Investigations On The Microstructure Of ZnO Thin Films Derived By Chemical Method

Zinc oxide (ZnO) is a compound semiconductor with wide direct band gap of 3.37eV and a large exciton binding energy of 60meV. Theoretically, it can realize stimulated ultraviolet (UV) emission at room temperature. Therefore, as a wide direct-gap semiconductor, ZnO attracts as much attention as GaN in optoelectronics research field. Because there are more advantages in structure, energy gap, optoelectronic field and more methods of fabricating ZnO film, which suitable for different demands, ZnO materials possess a lot of potential applications in device fabrication. Therefore, ZnO thin films can be used to fabricate transparency electrode, piezoresistor, cell battery window, surface acoustic wave device, gas sensor and light-emitting diode etc. In short wave region, ZnO can be used to fabricate UV light emitting and UV laser devices, which is very important for improvement of memory density and optical information access speed.It is very important that higher quality films and nanostructures are needed for the application of ZnO. Because liquid chemical method has many advantages such as low cost, easy to preparation for a large scale and so on, synthesis of nano materials has received much more attentions in recent years. However, there are always lots of problems in many aspects including of improving the quality of the films, growing the special nanostructures in a controlled mode and doping technology. The mechanism of film growth is unclear until now, which needs much more experimental investigations by using all kinds of methods to solve these existing problems.In this paper, the ZnO films were prepareded by sol-gel and chemical bath deposition methods. We investigated the microstructures of the two kinds of ZnO films by using X-ray (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) so as to reveal their growth mechanisms of ZnO thin films combining with the process of the film preparation.The hexagonal wurtzite structure of ZnO:Al thin films deposited on the glass prepared by sol-gel method were identified. The X-ray results indicate that with increase of the Al-doping content, the peak moves to the right in the X-ray curve and the peak reaches to the maximum when Al content rises up to 3 at%. However, when the Al-doped content exceeds 3 at%, the peak moves to the left in the X-ray curve. The film structure becomes worse when the Al-doping reaches to 6 at%, the lattice no longer changes. AFM results reveal that the wrinkle exists on the ZnO thin film surface, which is due to the stress change during the heat treatment in the process of the gel formation.All thin film with different Al-doping has the c-axis preferred orientation. When the doping content reaches to 3 at%, the strongest c-axis preferred orientation was found. The preferred orientation decreases evidently when the Al-doping exceeds 3 at%. TEM results reveal that the ZnO thin film is composed of the granular or ellipsoid particles and characterized by the layer structure obviously. ZnO thin film was spin-coated through five cycles, and crystals with larger size form in the coarse-grained layer which located at between the two cycle layers. There are fine-grained layers in the middle of coarse-grained layers which consist in plenty of small crystallines. Compared with the higher Al-doping films, the films with lower Al-doping (1 at%-3 at%) contain larger coarse particles, these larger particles grow along in-plane direction of the coarse-grain layers. The analysis results show that the mechanisms of crystal nuclearation, growth and coarsing in each stage are complex. In final, the growth mechanism of ZnO thin film may be a comprehensive effect due to self-assembly, self-texture and ostwald ripening processes.The experimental results show that the ZnO crystallines refine obviously with increase of Al-doping. In addition, the redundant Al segregates near the grain boundary in the form of amorphous Al2O3 to inhibit the growth of grains, which results in the decrease of the grain size simultaneously.The microstuctures of ZnO film on the ZnO seed layer prepared by chemical bath deposition has been investigated. The film is characterized by a columnar grain structure of polycrystalline. It is shown that the film epitaxily grows on the single seed crystal and becomes bulk columnar one. Therefore, the growth orientation of the columnar crystal depends on the one of the seed crystal. Meanwhile, TEM analysis shows that the thin film grows based on the oriented attachment mechanism. The thin film has plenty of defects which result in a poor stablility and the compact film will break down into the numoureus particles when the film surffers electron beam irradiation for a long time.