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

The development and breakthrough of new semiconductor materials have brought thenew technology revolution and the development of emerging industry, as well as promotedthe rapid development of information technology. Because of the potential huge market ofoptoelectronic devices, wide bandgap seconductor materials have become the focus ofresearches. ZnO thin film is a transparent optical thin film with wide bandgap and highexciton binding energy, which can realize the ultraviolet stimulated emission at roomtemperature. These excellent properties make it very large applications in optoelectronicdevices 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 filmmaterials for the development of optoelectronic.ZnO thin film is generally obtained by heteroepitaxial, whose crystal quality stillneeds to be improved. Because the group III-V AlN, GaN, and the group II-VI ZnO arehexagonal close-packed structure, the lattice constants are similar. Therefore, AlN andGaN can be used as the buffer layers for the growth of ZnO material. Through reducingthe lattice mismatch with the substrate material, the crystal quality of ZnO material can beimproved. Therefore, how to grow high-quality AlN, GaN buffer layers and how to takeuse of those buffer layers to improve the quality of ZnO thin film have been the hotspot ofinternational research.The researches of ZnO-based materials and devices have been carried out mostly inthe polar plane （c-plane） material. However, at room temperature, ZnO materials have astable wurtzite structure without the center of inversion symmetry, resulting in the strongspontaneous polarizations in the c direction in ZnO and its heterojunctions. This can leadto 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 efficiencyand emission wavelength, the most fundamental method is growth of non-polar plane ZnOthin 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 theinternational research focuses in the field of the wide bandgap semiconductor. Several research groups from the United States, Japan, and Europe have been engaged in theresearches of this field.In this paper, high quality ZnO thin films have been grown on the polar c-plane andnonpolar a-plane by using pulsed laser deposition （PLD） and magnetron sputteringmethod with sapphire used as substrate, AlN and GaN as the buffer layers. Growth processand conditions have been studied, and the material surface morphology and defects havebeen 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 AlN and GaN films have been grown on the sapphiresubstrates by metalorganic chemical vapor deposition （MOCVD）.（2） High quality non-polar plane ZnO thin films have been grown on non-polara-plane GaN templates by magnetron sputtering. Through changing the main growthparameters and using many methods to characterize the non-polar plane ZnO, theoptimization 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 AlNtemplate by pulsed laser deposition （PLD） and the optimized growth conditions have beenresearched. Furthermore, we explored the method to use nickel plated sapphire as thebuffer layer to growth ZnO thin films, which can make the process simple. Finally, theoptimization conditions of non-polar plane ZnO thin films grown on non-polar plane GaNbuffer layer have been studied.In this paper, we obtained the following significant and innovative results:（1） High quality AlN thin films have been grown by mix using of pulsed atomic layerepitaxy （PALE） and high temperature continuous growth on low temperature nucleationlayer by MOCVD. This method can not only decrease the dislocation density by tuningthe nucleation density at low temperature, but also realize high quality AlN thin films witha high growth rate. At the same time, adjusting the stress and inhibition of dislocation canbe realized by optimizing the MOCVD growth conditions, which can lead to AlN thinfilms 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 thesubstrate 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-Al₂O₃templatethan on r-Al₂O₃. It is also shown that when the growth temperature is300℃, thecrystallization of ZnO thin film is best, and the high resolution X-ray diffractometerFWHM is0.51°（1836arcsec）.（3） The influences of thickness of AlN buffer layer on the polar plane ZnO have beenstudied by PLD. The results show AlN buffer layer can improve the crystal nucleationdensity when ZnO thin films are grown on the c-plane sapphire substrate. Furthermore,when the thickness of AlN is150nm, the crystal quality of ZnO is the best with theFWHM of （002） plane XRC is0.09°（324arcsec）.In addition, the process of non-polarplane ZnO growth by PLD has been optimized. After optimization, the XRC of ZnO is0.28°（1008arcsec）, better than the films grown by magnetron sputtering.