BiFeO₃/BaTiO₃ Bilayer Ferroelectric Thin Films And Their Integration On Silicon

Ferroelectric thin films are one class of important electronic functional ceramic materials,exhibiting excellent ferroelectricity,piezoelectricity,dielectric and pyroelectric properties.Especially,BaTiO₃ and BiFeO₃ ferroelectric thin films have attracted intensive research interestes in this field.BaTiO₃ is a kind of dielectric material with high dielectric constant,low dielectric loss and good ferroelectricity,which is considered as one of the ideal materials for energy storage capacitors.However,the small spontaneous polarization of BaTiO₃ has to be modified to improve its energy storage capacity.Meanwhile,BiFeO₃ has a large spontaneous polarization,which is one of the rare single-phase multiferroic materials at room temperature,but its dielectric constant and electrical resistivity are too low to limit its development.In our work,BaTiO₃/BiFeO₃ bilayer films are prepared in order to combine the advantages of both BaTiO₃ and BiFeO₃.The theoretical storage density and dielectric properties of BaTiO₃/BiFeO₃ bilayer films can be improved by the polarization coupling formed between the two layers and a space charge layer formed in the BaTiO₃ layer near the bottom electrode.Generally,the formation of BaTiO₃ and BiFeO₃ films with a good performance needs high temperature（higher than 500℃）or thermal annealing treatments,which is not compatible with CMOS integrated processes.Therefore,it is important to prepare BaTiO₃/BiFeO₃ bilayer films in situ under low substrate temperature（below 500℃）on the silicon substrates.In this paper,firstly,on the basis of properties of materials and preliminary work of our research group,BiFeO₃/BaTiO₃/SrRuO₃/SrTiO₃ heterostructures were fabricated by magnetron sputtering.By adjusting the thickness ratio of each layer and the total thickness of the bilayer films,we obtained films with excellent electrical properties and storage characteristics.Based on the above experiments,to compatible with the CMOS process,we replaced the previous substrate with silicon and prepared the ferroelectric films at low temperature（500℃ 450℃,400℃,375℃,350℃）.In this paper,the main research aspects are as follows:1、Electrical and energy storage properties of BiFeO₃/BaTiO₃/SrRuO₃/SrTiO₃ heterostructures（1）Fixed the total thickness of the films on 300 nm,investigating the effect of thickness ratio of each layer on electrical properties of the films.The thickness ratio of each layer has a significant effect on the shape of the hysteresis loop of the films.The shape of hysteresis loop becomes more slender as the thickness of the BaTiO₃ layer become more lower,indicating the increase of the energy storage density with the decrease of thickness of BaTiO₃ layer.For the film with thickness ratio of 50/250nm,its breakdown electric field can be up to 3750kV/cm,besides,saturation polarization is～79.45μC/cm2,the effective energy density is～102J/cm3,and efficiency is～74.5%.（2）Keeping the thickness ratio of each layer at 1:1,the total thicknesses of the films were set to 300nm,1300nm and 2700nm;respectively.As the thickness increases,the polarization of the film increases and the leakage current decreases.films with thickness of 300nm exhibits the best dielectric properties.When the thickness ratio of BiFeO₃/BaTiO₃ is 9:1,the hysteresis loop appears a characteristic of"double loop",which corresponds to the current loop.（3）BiFeO₃/BaTiO₃ films（150/150nm）were prepared on（100）,（110）and（111）oriented SrTiO₃ substrates,respectively.The film grown on SrTiO₃（100）substrate has the best dielectric properties.2、Medium and low temperature preparation of BiFeO₃/BaTiO₃ films on Si substrate（1）BiFeO₃/BaTiO₃ bi-layer thick films（～1 μm）were deposited on Pt/Ti/SiO2/Si（100）substrates with LaNiO₃ buffer layer at 500 ℃,which show outstanding ferroelectric and dielectric properties.On the basis of this,the preparation temperature was reduced,and the films with good crystallinity and good ferroelectric and dielectric properties were prepared at 450 ℃,400 ℃,375℃ and 350 ℃respectively.（2）Preparing BiFeO₃/BaTiO₃ double layered thin films on（100）and（111）oriented SiO2/Si substrates with LaNiO₃ buffer layer,and study its electrical properties.The thin films prepared on Si（100）substrates show better shape of hysteresis loops and higher voltage-resistance,as well as more excellent dielectric properties.