Preparation And Growth Characteristics Of Piezoelectric Films On Diamond Substrates

Recently,study of Chemical Vapor Depostion(CVD) diamond films is going maturity. CVD diamond films have excellent characteristics nearly the same as natural diamond while with much lower cost.Therefore,CVD diamond films have a very bright future in their application and the market.In this dissertation,Freestanding Thick Diamond Films(FTDF) prepared by CVD method are applied as substrates of piezoelectric materials and the piezoelectric films are investigated systematically.Piezoelectric films/FTDF can be used as substrates of Surface Acoustic Wave(SAW) devices,which can make full use of the highest SAW velocity and heat conductivity of diamond.So,piezoelectric films/FTDF structure SAW devices can be expected to be more excellent than conventional devices.In the deposition technique,the use of smooth nucleation side of FTDF avoids the tedious task of diamond polishing.With this simple method, piezoelectric films of high quality on FTDF were prepared,which can meet the requirement of application and would lead to the elaboration of high quality and high power durability SAW devices in the future.In this work,ZnO and GaN films were deposited on FTDF by three different methods and how to prepare piezoelectric films of high quality on the special substrates has been studied systematically.The results mainly in three parts are summarized as follows:1 ZnO films were deposited on FTDF by the reactive radio-frequency magnetron sputtering method.(1) The results demonstrate that the crystalline quality of ZnO films on FTDF can be improved with substrate temperature increasing,which is slightly different from the tendency on silicon.The surface morphology tendency of ZnO films on FTDF is accorded with the tendency on silicon.Higher substrate temperature can lead to stronger Ultraviolet(UV) emission of ZnO films.Substrate temperature plays a key role on the quality of ZnO films on FTDF while FTDF has little effect at the same time.(2) Other process conditions,such as radio-frequency power,working pressure and O₂/Ar ratio,have little influence on the crystalline quality,surface morphology and optical property of ZnO films on FTDF,while the defects and grain boundary of FTDF are closely related with the quality of ZnO films deposited on FTDF. (3) All the samples are of high resistivity,which can meet the requirement of SAW devices.(4) Multi-orientation texture and cluster development mechanism of ZnO films on FTDF indicate that characteristics of ZnO films are closely related with surface of FTDF.2 ZnO films were deposited on FTDF by plasma enhanced MOCVD system.(1) Highly c-axis-oriented undoped ZnO films on FTDF are obtained at 38 Pa and 600℃with a strong UV emission.The ZnO grains are of uniform density with clear boundaries and a clean surface.(2) With substrate temperature increasing from 500℃to 700℃,the morphological, crystalline and optical qualities of N-doped ZnO films tend to decline,which maily result from the increase of structural defects induced by high temperature.From variable temperature PL spectra,it can be proved by the increase of emission of exciton bound related with structural defects.The sample at 500℃has a cleaner surface,narrower FWHM,higher resistivity and stronger PL intensity with a relatively higher Zn/O atomic ratio and relatively more Zn-O bond structure.(3) Both chemical reaction and surface atom diffusion render an appropriate substrate temperature for ZnO films on FTDF.The grain size of N doped ZnO films decreases with substate temperature increasing,which may be due to the increase of chemical active O^2-. induced by N₂O.3 GaN films were deposited on FTDF by Electron Cyclotron Resonance MOCVD (ECR-MOCVD) system.N₂ affluent atmosphere may lead to GaN film growth in a preferred orientation and a very smooth surface with larger clusters.GaN films with different N₂ flowrates have a very smooth surface with a probability of the fabrication of SAW device with a lower insertion and transmission loss.