Study On Preparation And Dielectric Properties Of CaCu₃Ti₄O₁₂Thin Films

With the development of microelectronics industry,higher requirements for devices,such as low power consumption,high performance,miniaturization and integration,have been put forward.Recently,a new kind of dielectric material,calcium copper titanate(CaCu3Ti4O12,CCTO),has attracted wide interest due to its unique dielectric behavior and high dielectric constant(?104).CCTO is expected to become an ideal material for supercapacitors,variable resistors,microwave devices,gas sensors,and energy storage devices.However,the origin of the huge dielectric constant of CCTO materials remains unclear,which limits its further optimization.Besides,although CCTO materials possess giant permittivity,it has high dielectric loss,which is not conducive to its applications.With respect to the above problem,this thesis mainly studies on CCTO thin films prepared by magnetron sputtering method..The effects of different processing conditions on crystal structure,microstructure and dielectric properties of CCTO films were investigated,so as to obtain the optimum process parameters.In addition,preliminary research about the basic technique of Nb doped CCTO thin films(Nb-CCTO)by Co-sputtering method had been done in order to reduce the dielectric loss of the films.The main conclusions are drawn as follows:(1)CCTO thin films at different sputtering pressures(0.3 Pa,0.45Pa 0.6 Pa,0.8 Pa,and 0.9 Pa)were prepared.The influences of sputtering pressure on crystal structure and dielectric properties of as-prepared CCTO thin films were investigated.It was found that when the sputtering pressure was 0.45 Pa,the characteristic peaks of CCTO thin films were obvious,and the peaks of the secondary phases were weak.The films had the largest dielectric constant at this time.(2)The sample prepared at a sputtering pressure of 0.45 Pa was annealed at different temperatures(200 ?,400 ?,600 ?,800 ?,and 900 ?),and the effects of annealing temperature on crystal structure and dielectric properties were investigated.The crystallization and dielectric properties of CCTO films can be improved by annealing.As the annealing temperature is 800 ?,the crystallization of CCTO films is the best with obvious characteristic peaks and less impurity phases.In addition,the higher the annealing temperature is,the bigger the dielectric constant of the films is.When the annealing temperature is 800?,the dielectric constant of the CCTO film is the largest of 256 at 1kHz.(3)Based on the above process parameters,CCTO samples annealed in air and oxygen atmospheres were compared with as-sputtered samples.As a result,CCTO thin films were pure cubic phase with(422)preferential orientation,and further grain growth can be observed after annealing treatment at high temperature.The films after annealing showed a distinctive increasement of grain boundary resistance.The annealed films exhibited different dielectric behaviors from those as-sputtered ones:the dielectric relaxation originated from the secondary ionization of oxygen vacancies at the intermediate frequency of as-sputtered films disappeared after annealing;The permittivity of the sample annealed in oxygen atmosphere reached 104 at 1 Hz,which was an order of magnitude higher than that of the as-sputtered samples.The oxygen vacancy in grain boundaries of films is filled during annealing in oxygen,and then the crystallization of films were enhanced,therefore,the dielectric behaviors of the films were promoted as a result.(4)Nb-doped CCTO thin films(Nb-CCTO)were fabricated by co-sputtering method at various sputtering powers.The relationship of dielectric properties and structure of CCTO films is explored.The X-ray diffraction pattern indicated that Nb-CCTO films contained a small amount of secondary phases such as CaTiO3,TiO2 and CuO,and the CCTO phase exhibited a preferential(422)orientation.It was found that the particle size of Nb-CCTO films became smaller and the arrangement of the particles became denser as sputtering power increased.The dielectric constant decreased as increasing sputtering power,while the dielectric loss increased at the same time.The inhibited grain growth by the introduction of Nb elements should be probably responsible for the variation of the dielectric behaviors.Additionally,the local conductance caused by small amount of Nb particles remained in CCTO films might also contribute to this phenomenon.When the direct sputtering power of Nb target was 30 W,the film acquired a relatively high permittivity and low dielectric loss.