| Zinc oxide has been a great deal of interest in zinc oxide (ZnO) semiconductor materials since many decades, as seen from a surge of relevant number of publications. ZnO has wide and direct bandgap (3.37eV), high exciton energy (60meV) at room temperature and excellent properties of piezoelectricity, gas-sensitivity, pressure-sensitivity etc. In addition, inexpensive and abundant raw materials, nontoxicity, good chemical and thermal stability, and strong radioresistance are also the advantages of ZnO. As a result, various aspects of ZnO have become hot research topics. Therein, thin film has significant research and application value as a main morphostructure of ZnO.In last few years, with the development and improvement of ZnO thin film fabrication methods and growth mechanism, research has begun to focus on the industrial application. ZnO is one of the most promising candidates for many fields, such as a viable alternative to polysilicon used for solar cell, GaN thin films used for light-emitting diodes, ITO for transparent conductive glass, as well as surface acoustic wave (SAW), ultraviolet laser, and special polarity reversing switches fields. However, the existence of photoelectric anisotropy in the application can seriously influence the device performance. As a result, more and more research work has been focused on to control the growth orientation of ZnO thin films thereby optimizing the photoelectric properties.At present, with the development and improvement of thin film fabrication methods, almost all methods can be used for the fabrication of ZnO thin films. Because wurtzite structure ZnO has the intrinsic dipole moment and spontaneous polarity, almost all the methods can fabricate ZnO films with the c-axis preferred growth orientation easily. However, preparation of non-polar preferred growth orientated ZnO films is very difficult. Most commonly used mature methods for preparing non-polar preferred growth oriented ZnO thin films such as magnetron sputtering, molecular beam epitaxy (MBE), chemical vapor deposition (CVD), atomic layer deposition (ALD) contain nonequilibrium state and all these methods use the special substrates such as m-or r-plane sapphire, y-LiA102, Si and Mg (100), etc., which have certain mismatch with the non-polar crystal plane of ZnO. ZnO thin films prepared by these nonequilibrium state methods contain residual stress caused by the mismatch of thin film and substrate, which greatly limits the industrial applications. So, to control the film growth orientation proves to be the bottleneck, which restraints the optoelectronic field application of non-polar ZnO thin films.In order to avoid the generation of residual stress, we look for the way out from equilibrium state through the chemical methods. After trying of several chemical methods such as sol-gel, reverse micelle microemulsion, etc., the chemical bath deposition which can control the film growth orientation was found. In this paper, we optimize the experiment and technological parameters in order to control the growth orientation of ZnO thin films steadily.The experiment results indicated that high quality non-polar (110) oriented ZnO thin films can be prepared when the SG-ZnO(sol-gel derived zinc oxide) thin films were chosen as the substrate, the chemical bath deposition was carried out at80℃for4h, the solvent was composited with the isopropanol (90%) and the deionized water (10%). Through changing the solvent type and amount growth orientation of the film was controlled, keeping other conditions unchanged. Using this technique, various (002),(101),(112),(110) and (100) oriented ZnO thin films were fabricated successfully.In addition, the structure, surface chemistry and optical properties were tested and analysed in order to discuss the film growth orientation control mechanism specifically. The polarity matching between the solvent and ZnO crystal plane is very important, the specific growth oriented ZnO thin films can steadily obtained as long as the solvent polarity is appropriate. |
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