Preparation And Dielectric Relaxation Properties Of Co-sputtering Niobium Doped CaCu₃Ti₄O₁₂ Thin Films

With the development of miniaturization,integration,high performance and precision in the field of integrated circuits and microelectronics,higher demands have been put forward for the performances of dielectric materials.Copper calcium titanate(CaCu₃Ti₄O₁₂,CCTO),a new potential dielectric material,has extremely high dielectric constant and considerable stability of its dielectric constant.Therefore,CCTO material has prospects in supercapacitors,memories,sensors and microwave devices.However,the application of CCTO is hampered by its high dielectric loss.It has been reported that the dielectric loss of the Nb doped CCTO ceramics can be effectively suppressed,while the performance optimization of the thin films by Nb doping has not yet been reported.Therefore,we adopted the method of RF/DC(radio frequency/direct frequency)co-sputtering to introducing Nb element into CCTO thin films by controlling the papering parameters,so as to reduce the dielectric loss of CCTO thin films.The main contents are as follows:(1)The dielectric constant of CCTO films prepared in the atmosphere of oxygen-argon mixture is greatly increased due to the filling of oxygen vacancy,but the dielectric loss is also too large,the grain growth is poor,and the peak value of XRD main phase is low.(2)Preparation and phase structure of pure argon Nb-CCTO thin films:Nb was introduced into CCTO thin films by controlling the DC sputtering power from 0W to 7W to achieve pure cubic perovskite CCTO phase.The results show that perovskite-like CCTO thin films with basic pure cubic phase can be obtained within DC power of 6W,and the films displayed a(422)preferential orientation.(3)Micromorphology of pure argon Nb-CCTO thin films:SEM surface morphology shows that the surface of undoped CCTO thin films was flat and smooth;However,small particle distribution can be found on the surface of the doped Nb-CCTO thin films.The growth of surface grains can be inhibited by increasing the DC sputtering power,that is,the larger the DC power,the smaller the surface grain size.Additionally,it can be seen from the cross-section morphology of CCTO thin films that the thickness of the film becomes larger with increasing the DC power that implies that the film growth speed has increased.According to EDS analysis,the content of Nb element of CCTO thin films also increases with the increase of DC sputtering power.(4)Dielectric relaxation behaviors of pure argon Nb-CCTO thin films:The dielectric constant spectra illustrates that the dielectric constant of CCTO films almost remains unchangeable in the frequency range of 20Hz?10⁵Hz at room temperature.Above 10⁵Hz,the dielectric constant decreases sharply as increasing the frequency.Amongst all the dielectric constant of Nb-CCTO films with DC sputtering power of 7W is the largest and the most stable with frequency as well.The loss spectra shows that the dielectric loss of the films has been effectively suppressed by Nb doping.As it can be seen that the dielectric loss value of the film drops from 0.060 without doping to 0.027 of DC power of 6W at 10³Hz.(5)Relaxation mechanisms of pure argon Nb-CCTO thin films:Two parts can be seen in the complex impedance spectra.A semi-arc at the middle-low frequency region is contributed by the grain boundary.The longer the diameter of the semi-arc is,the greater the resistance at the grain boundary is.CCTO films at DC power of 7W has the largest grain boundary resistance of 490M.A non-zero intercept at the end of the high frequency range is due to the contribution of CCTO grain.The smaller the non-zero intercept is,the smaller the grain resistance is.The smallest the grain resistance happened at DC power of 7W.The ?-? curve of CCTO films presents nonlinear characteristics,and the nonlinear coefficient of CCTO films can be improved from 0.35 to 0.83 by Nb doping.