Study On The Preparation And Properties Of BMT/PZT Heterogeneous Thin Films

Thin films with only dielectric or ferroelectric properties can not meet the need of micromation,multi-function and integration of device.The advanced properties and extended application can be realized by multilayered films,which is fabricated by combining dielectric and ferroelectric materials.In this paper,the research is focused on the coupling effect between the dielectric and ferroelectric.The effects of stacking order,interface number and dielectric of interface on the properties of the BMT/PZT thin films were investigated.The mechanism of enhanced properties of BMT/PZT thin films was also studied.In this paper,PZT thin films were prepared by sol-gel method and BMT thin films were prepared by aqueous solution-gel method.The effects of thickness on the structure of PZT thin film and the effects of annealing temperature on the structure of BMT thin film were studied.The residual stress in PZT and BMT thin films was determined by Raman spectra and nanoindentation technique.There is no significant difference in grain size for different thickness of PZT thin films and the grain size of BMT thin films increases with the increase in the annealing temperature.As the thickness of PZT thin films increases,the residual stress reduces and the dielectric constant,the remnant polarization,the effective piezoelectric coefficient of PZT thin films increase.As the annealing temperature increases,the dielectric constant of BMT thin films firstly increases rapidly,and then increases slowly.PZT thin films with poor temperature stability and anti-fatigue properties are difficult to be used for many devices design.We study the properties of PZT thin films by using BMT thin film as a buffer layer in this work.In order to analyze the effects of polarization mechanism of BMT thin films on the PZT thin films,the complex impedance was studied.As the thickness of BMT buffer layer increases,the dielectric constant for PZT/BMT thin films increases first,and then decreases.The leakage current density of PZT/BMT thin films reduces from 1.64×10^-5 A/cm² to4.37×10^-6 A/cm².For PZT/BMT thin films,the temperature coefficients of the dielectric constant decrease with the increase thickness of BMT buffer layer.After1.04×10⁸ cycles,the remnant polarization value of buffered PZT thin films decreases12.1%,while it decreases 58.2%in PZT thin films.The space charge polarization in the PZT and PZT/BMT thin films dominates the dielectric response at low frequency.As the frequency increases to 1 MHz,the dipole polarization in the PZT and PZT/BMT thin films dominates the dielectric response and the contribution of the dipole polarization in the PZT thin films is greater than that in the PZT/BMT thin films since there is no dipole polarization in the BMT thin films.PZT thin films were used to reduce the hole in the BMT thin films and the effects of PZT thin films on the properties of BMT thin films were investigated.The dielectric constant and dielectric loss of BMT/PZT thin films increase as PZT thin films thickness increases.Compared with BMT thin films,BMT/PZT thin films show strong voltage dependence.BMT and BMT/PZT thin films show ohmic conduction mechanism at low electric field.As the electric field increases,BMT thin films show ohmic conduction mechanism and space charge limited conduction?SCLC?mechanism,while BMT/PZT thin films show SCLC mechanism.The temperature coefficients of the dielectric constant decrease with the increase thickness of PZT thin films.The space charge polarization in the BMT and BMT/PZT thin films dominates the dielectric response at low frequency.As the frequency increases to 1 MHz,the dipole polarization in the BMT/PZT thin films dominates the dielectric response.The auger electron spectroscopy?AES?and complex impedance were used to analyze the structure and polarization of interface.The effects of stacking order,interface number and dielectric of interface on the properties of the BMT/PZT thin films were investigated.The thickness of BMT-substrate interface and PZT-BMT interface are about 40 nm and 90 nm,respectively.While the thickness of PZT-substrate interface and BMT-PZT interface are about 110 nm and 8.6 nm,respectively.The results reveal that the increase number of interface can improve the dielectric properties,temperature stability and anti-fatigue of BMT/PZT thin films.After 1.04×10⁸ cycles,the remnant polarization value of PPBB thin films,PBPB thin films,BBPP thin films and BPBP thin films decrease 11.2%,9%,44%and 38.2%,respectively.Interface polarization model was set up to explain the effects of the number and dielectric constant of interface on the properties of BMT/PZT thin films and the total dielectric constant increases linearly with the interface number and dielectric constant of interface.As the interface number increases,the relaxation activation energy of interfaces increase and the space charges at the interface are not easy to stimulate,which is beneficial to improve the dielectric constant of interface and temperature stability of interface.