Preparation Of Lithium Tantalate Thin Film And Study On Its Electrical And Optical Properties

Lithium tantalate?LiTaO₃?is an important ferroelectric material because of its excellent piezoelectric,pyroelectric and photorefractive properties.Lithium tantalate crystals have been successfully used in many devices,especially in electro-optic devices,infrared detector and surface acoustic wave?SAW?devices.At present,the focus of the microelectronics industry has shifted from the use of bulk lithium crystal lithium tantalate ceramics to the deposition of lithium tantalate thin films with low optical losses,and incorporates them into current semiconductor technologies.Therefore,it is of great value to carry out research on the preparation of pyroelectric lithium tantalate film and its related physical properties.In this dissertation,Lithium tantalate thin films have been prepared by sol-gel method and radio frequency magnetron sputtering.The influence of the key experimental conditions such as substrate material,annealing temperature,annealing atmosphere and argon-oxygen ratio in sputtering atmosphere,on the composition,microstructure,crystallinity,optical and electrical properties of the deposited LiTaO₃ films,has been systematically investigated by means of various instruments and methods.Based on the experimental results,the growth mechanism of microstructures and defects in lithium tantalate thin films has been analyzed and discussed.The main contents can be summarized as follows:1.LiTaO₃ thin films have been prepared on Pt/Ti/SiO₂/Si?100?substrate and FTO conductive glass substrate by sol-gel method.The influence of lattice mismatch on the growth process and properties of lithium tantalate thin films has been investigated.The experimental results show that the lithium vacancy defects caused by the interfacial lattice mismatch between lithium tantalate thin films and substrates are the main factors affecting the properties of LiTaO₃ thin film.The larger the degree of lattice mismatch,the higher the diffraction peak of lithium-deficient phase in LiTa₃O₈,the more lithium vacancy defects in the lithium tantalate film,the deteriorated crystallinity of the film,and the surface morphology crack of the film.In the XPS analysis,the atomic percentage concentration of+4 valence Ta corresponds to the lithium vacancy defect in the film,the atomic percentage of Ta⁴⁺increases,and the maximum pyroelectric coefficient of the film decreases significantly.2.Lithium tantalate thin films have been prepared by magnetron sputtering.The influence of annealing temperature on the growth mechanism and properties of the film has been systematically studied.The crystal growth kinetics of the second phase LiTa₃O₈formed during thermal annealing of LiTaO₃ film has also been studied.The results show that growth and recrystallization of LiTaO₃ grains lead to the growth of the film to the lithium-deficient LiTa₃O₈ structure during the annealing process,which is the main reason for the deteriorates of the crystallinity of the film.The results show that when LiTaO₃ film is annealed above 600^oC,Li₂O will desorb and volatilize from the surface of the film and the grain boundary,which will cause the LiTa₃O₈ phase to precipitate and crystallize on the surface of LiTaO₃ film.Loss of Li₂O creates vacancies,and lithium vacancies in LiTaO₃ film reduce the barrier to atomic migration.The phase transition of LiTaO₃ is achieved by atomic rearrangement,which promotes the nucleation of LiTa₃O₈crystallites.A further increase of the annealing temperature will enhance the volatilization loss of Li₂O from the film,thereby forming more LiTa₃O₈ phase structure.When the annealing temperature is increased,the atoms in the film are mutually diffused,the film particles are grown,the particle size is increased,and LiTaO₃ particles of different sizes in the form of"hills"appear on the surface of the film.When the annealing temperature is further increased,the interdiffusion of the particle in the film is further enhanced,and the particles are swallowed and agglomerated to form larger particles,and the roughness of the film is also increased.3.The influence of annealing atmosphere on the growth mechanism and properties of lithium tantalate films has been investigated.The higher the annealing temperature,the larger the atomic mobility of the film and the more intense the recrystallization process of the crystal.The desorption and volatilization of Li₂O are more severe.The results show that the annealing of lithium tantalate film in anoxic atmosphere will increase the concentration of oxygen vacancy defects in the film,and the film will tend to the growth of LiTa₃O₈.The increase of oxygen vacancy defects in the film is the main cause of film performance deteriorates.If the oxygen atoms are insufficient during the film annealing and recrystallization process,the film tends to the growth of lithium-deficient phase LiTa₃O₈,inhibiting the growth of the LiTaO₃ phase structure,and the intensity of the diffraction peak of LiTa₃O₈ in the film is enhanced.When the film is annealed in an O₂-rich atmosphere,oxygen atoms are sufficient during the annealing and recrystallization process,and oxygen molecules permeating into the film hinder atom migration and slow the crystallization of the LiTa₃O₈ phase structure.The optical band gap of the film is closely related to the crystal state,defects and structure.The LiTa₃O₈ phase structure leads to the appearance of defect energy levels in the energy band,resulting in a decrease in the optical band gap and pyroelectric coefficient.4.The influence of argon-oxygen ratio on the growth and properties of lithium tantalate films has been investigated.The experimental results show that the oxidation of Li and Ta atoms in the film during the sputtering process to form Li₂O and Ta₂O₅ is the main reason for the film performance deteriorates.A lithium tantalate film with an argon-oxygen ratio of 1:1 in a sputtering atmosphere has a high partial pressure of oxygen.Excess oxygen reacts with the Li atoms sputtered from the target during sputtering.Leading to the oxidation of Li atoms to produce volatile Li₂O,The Li atom is desorbed from the surface and grain boundary of the film,and the LiTa₃O₈ crystallite is more easily deposited on the surface of LiTaO₃ to form a LiTa₃O₈ phase crystal seed layer,which forms LiTa₃O₈ crystal in the film.For a lithium tantalate film with an argon-oxygen ratio of 2:1 in a sputtering atmosphere,the argon partial pressure content is increased.Thereby,the probability of the Ar ion bombarding the target is increased,so that the sputtering rate of the Li atom and the Ta atom is increased.At the same time,since the partial pressure of oxygen is sufficiently large,the supply of oxygen atoms is sufficient.Li atoms are oxidized to form volatile Li₂O,and Li vacancies are formed on the surface of the film.Part of Ta atoms and the remaining Li atoms form a lithium-deficient LiTa₃O₈ crystal.The other part of the Ta atom is oxidized sufficiently with oxygen to form Ta₂O₅ before being deposited on the substrate,so that a Ta₂O₅ diffraction peak appears in the LiTaO₃ film.