Growth, Characterization And Application Of Potassium Tantalate(KTaO₃) Crystal

With the rapid development of communication and information,diversified photoelectronic devices develop at a tremendous speed and come to the trend of advanced performance.Therefore,the quality for the key part of photo-electronic devices-the materials of substrates and the degree of processing nicety-has to face newer and stricter standard.That is to say,substrates single crystal material must have excellent qualities of piezo-electricity,photo-electricity,thermo-electricity;the degree of processing nicety of device substrates must amount to the level of nanometer;the crystal lattices must have super-smooth surfaces without distortion or damage.Because any tiny flaw in surface of substrates will destroy the surface performance of crystal materials and even arouse to the change of the structure of crystal lattices,witch will affect the processing nicety and reliability of devices.Furthermore,high rate of finished products also be required.However,the substrate of photo-electronic devices,as typical ultra-thin part,such as the thickness of the substrate of the surface wave filter of cell-phone just amount to 350 um,has bad stiffness and cannot meet the requirement of processing nicety and quality.As a result,how to meet the requirement of the high nicety of modern photo-electronic devices and surface quality and how to gain the substrate with super-smooth surface are the very processing methods that people in this field have explored for many years.Potassium Tantalate single crystal(KTaO₃)which has no phase transition from room temperature to melting is a topically isostructural perovskite.This crystal is good thermal and mechanical properties,can be grown for high quality by using the Czochralski method.There is small crystal lattice mismatching and structure matching for extending kinds of perovskite oxide film.KTaO₃ crystal has such performances as good chemical stability,heat-conduction and insulation under the high temperature, very low dielectric constant and dissipation in high frequency.The substrate surface quality,such as surface roughness,micro-structure directly influences the quality of High Temperature Superconductor thin film grown on it.So,it is an important problem how to get KTaO₃ substrate with high surface quality.Now,there are some disadvantages in mechanical polishing of single crystal KTaO₃ substrates,such as low material removal rate(MRR),poor surface quality and high percent off ragment waste. Moreover,to the author's best knowledge,systematic study on ultra-precision polishing of KTaO₃ substrates has not been reported.Thereby,in this dissertation,the growth, structure,defects,properties and ultra-precision polishing of KTaO₃ crystal are systematiclly studied and discussed.The outline of this dissertation is as follows:1.Crystal GrowthKTaO₃ polycrystalline material was synthesized by means of a solid-state reaction with 4N K₂CO₃ and Ta₂O₅.Free-cracked KTaO₃ single crystals with high optical quality and large dimensions were successfully grown by using Czochralski method.Single crystal growth is a complex process,the factors that affect quality of crystals during were analyzed,according to the theories of thermodynamics and dynamics.High quality polycrystalline material and a suitable filed of temperature are the precondition to grow an excellent crystal;the optimal technics is the key for crystal growth;using a good crystal seed and eliminating outside influence can ensure the quality of crystals.We also presented the resolvents according to the characteristics of these factors.2.Crystal Components,Structure and DefectsThe concentrations of K,Ta and O elements in the different parts of as-grown KTaO₃ single crystal were measured by the JXA-8800 electron probe analysis method. The measured results indicate that as-grown KTaO₃ in rich K melt is non stoichimetric KTaO₃ and has high homogeneousness in components,yet exist obvious O vacancy in crystals.The structure of as-grown KTaO₃ single crystal was studied by X-ray powder diffraction combined with conoscopic diagram method.The X-ray diffraction confirmed that as-grown KTaO₃ single crystal belongs to the cubic crystal system,and calculated unit cell dimensions a=3.9884(?)by Dicvol 91 software.Defects in the as-grown KTaO₃ single crystal have been studied by using the chemical etching method and high-resolution X-ray diffraction.The results of observations and discussion of growth defects and formation mechanisms in as-grown KTaO₃ single crystal were presented in this dissertation.It is important for us,since the investigation can guide the growth of single crystal with high perfection.The main defects in the as-grown KTaO₃ single crystal are dislocations and sub-grain boundaries. These defects seriously affected the perfection of crystals.In order to avoid or decrease these defects,we take some corresponding steps according to the above results.3.Basic PropertiesThe density,hardness,linear optical properties and thermal properties of the as-grown KTaO₃ single crystal were measured,and influences of these properties on crystal growth and applications were also discussed.The density of KTaO₃ crystal was measured by using the buoyancy method at room temperature and its value is 7.0146 g/cm³.The Mohs hardness of KTaO₃ was about 6 by using Mohs sclerometer.The refractive indices of KTaO₃ single crystal was measured by using V-prism method at Hg lamp e line(λ=633nm andλ=1539nm)and its value is 2.2256 and 2.1520.The transmission spectra were measured with Hitachi U-3500 FIIR spectrometer at room temperature and the results show that KTaO₃ crystal has a broad transparent wavelength band.The cut-off wavelength of the crystals is about 340 nm.From 340 nm to 3 500 nm,only an absorption band between 2 847 nm and 2 910 nm appears,and the peak is located at about 2 875 nm.The thermal properties of KTaO₃ single crystal were systemically studied.The specific heat were measured by the method of differential scanning calorimetry,which values is 0.371～0.385 jg^-1K^-1in the temperature range of 303.15 K～843.15 K.The thermal expansion of KTaO₃ single crystal was measured by using a thermal dialatometer and the average linear thermal expansion coefficient were calculated in the temperature from 298.15 to 773.15K,which values are ac=6.44×10^-6/℃,a_a=a_b= 6.63×10^-6/℃.The thermal diffusion coefficient of KTaO₃ single crystal was measured by the laser flash method in the temperature range from 298.15 K to 773.15 K and its value decreases with increasing temperature.At room temperature,the thermal diffusion coefficients are 3.259 mm²s^-1,the calculated thermal conductivities of KTaO₃ single crystal are 8.551 Wm^-1K^-1.The influences of these thermal properties on crystal growth and applications were also analyzed.4.Crystal Ultra-precision PolishingThe chemical mechanical polishing(CMP)technology of KTaO₃ substrate is studied in order to efficiently obtain the ultra smooth and free damage surface of KTaO₃ substrates by use of the chemical and mechanical interaction.A series of CMP experimental studies on different technics parameter on the surface quality of KTaO₃ substrate were carried out.According to analysis result of different parameter,we have compared to different polish mechanism and polish effect,summed up optimal CMP technics parameter for KTaO₃ crystal.Finally,the suitable CMP parameters for single crystal KTaO₃ are obtained by using the orthogonal experiment method.It is proved that the surface roughness of KTaO₃ substrate can reach Ram=0.752 nm and the MRR of KTaO₃ substrate is improved.A series of CMP experimental studies on the influences of the main factors(polishing pad,polishing slurry,polishing load,table rotational speed) on the surface quality of substrate were carried out.