Study Of The ZnO Thin Film Growth Based On Polar And Nonpolar GaN/AIN Templates

The development and breakthrough of new semiconductor materials have brought the new technology revolution and the development of emerging industry, as well as promoted the rapid development of information technology. Because of the potential huge market of optoelectronic devices, wide bandgap seconductor materials have become the focus of researches. ZnO thin film is a transparent optical thin film with wide bandgap and high exciton binding energy, which can realize the ultraviolet stimulated emission at room temperature. These excellent properties make it very large applications in optoelectronic devices such as solar cells, light emitting diodes, gas sensors, and so on. Furthermore, ZnO material is easy to get, cheap, low toxicity, and one of the most potential thin film materials for the development of optoelectronic.ZnO thin film is generally obtained by heteroepitaxial, whose crystal quality still needs to be improved. Because the group III-V AlN, GaN, and the group II-VI ZnO are hexagonal close-packed structure, the lattice constants are similar. Therefore, AlN and GaN can be used as the buffer layers for the growth of ZnO material. Through reducing the lattice mismatch with the substrate material, the crystal quality of ZnO material can be improved. Therefore, how to grow high-quality AlN, GaN buffer layers and how to take use of those buffer layers to improve the quality of ZnO thin film have been the hotspot of international research.The researches of ZnO-based materials and devices have been carried out mostly in the polar plane (c-plane) material. However, at room temperature, ZnO materials have a stable wurtzite structure without the center of inversion symmetry, resulting in the strong spontaneous polarizations in the c direction in ZnO and its heterojunctions. This can lead to the decrease of the ZnO light emission efficiency and red shift of the peak wavelength. In order to eliminate the influence of polarization effect on the luminescence efficiency and emission wavelength, the most fundamental method is growth of non-polar plane ZnO thin film and its heterostructures on non-polar plane AlN or GaN buffer layers. Currently, growths of high quality non-polar plane GaN and ZnO materials are becoming one of the international research focuses in the field of the wide bandgap semiconductor. Several research groups from the United States, Japan, and Europe have been engaged in the researches of this field.In this paper, high quality ZnO thin films have been grown on the polar c-plane and nonpolar a-plane by using pulsed laser deposition (PLD) and magnetron sputtering method with sapphire used as substrate, AIN and GaN as the buffer layers. Growth process and conditions have been studied, and the material surface morphology and defects have been analyzed. Contents are divided into the following three parts:(1) Through inserting of buffer layer and optimizing the growth of nucleation layer, high quality c-plane and a-plane AIN and GaN films have been grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD).(2) High quality non-polar plane ZnO thin films have been grown on non-polar α-plane GaN templates by magnetron sputtering. Through changing the main growth parameters and using many methods to characterize the non-polar plane ZnO, the optimization condition of non-polar a-plane ZnO thin films have been achieved.(3) Polar c-plane ZnO thin films have been grown on high quality c-palne AIN template by pulsed laser deposition (PLD) and the optimized growth conditions have been researched. Furthermore, we explored the method to use nickel plated sapphire as the buffer layer to growth ZnO thin films, which can make the process simple. Finally, the optimization conditions of non-polar plane ZnO thin films grown on non-polar plane GaN buffer layer have been studied.In this paper, we obtained the following significant and innovative results:(1) High quality AIN thin films have been grown by mix using of pulsed atomic layer epitaxy (PALE) and high temperature continuous growth on low temperature nucleation layer by MOCVD. This method can not only decrease the dislocation density by tuning the nucleation density at low temperature, but also realize high quality AIN thin films with a high growth rate. At the same time, adjusting the stress and inhibition of dislocation can be realized by optimizing the MOCVD growth conditions, which can lead to AIN thin films with smooth surface. The RMS is1.4nm. The XRC results show that the FWHMs of (002) and (102) are82arcsec and575arcsec, respectively.(2) The better lattice matched a-GaN/r-Al2O3template has been proposed as the substrate for the growth of nonpolar a-ZnO thin film by magnetron sputtering. The results show that it is easy to obtain a-plane oriented ZnO thin films on a-GaN/r-Al2O3template than on r-Al2O3. It is also shown that when the growth temperature is300℃, the crystallization of ZnO thin film is best, and the high resolution X-ray diffiactometer FWHMis0.51°(3) The influences of thickness of A1N buffer layer on the polar plane ZnO have been studied by PLD. The results show A1N buffer layer can improve the crystal nucleation density when ZnO thin films are grown on the c-plane sapphire substrate. Furthermore, when the thickness of A1N is150nm, the crystal quality of ZnO is the best with the FWHM of (002) plane XRC is0.09°.In addition, the process of non-polar plane ZnO growth by PLD has been optimized. After optimization, the XRC of ZnO is0.28°, better than the films grown by magnetron sputtering.