| In recent years, bismuth layer-structured ferroelectrics (BLSFs), such as SrBi2Ta209(SBT), Bi4-xLaxTi3O12 and SrBi4Ti4O15 (SBTi) thin films, have been investigated for the applications in ferroelectric random access memories, high temperature piezoelectric devices, and high-density capacitors due to its excellent endurance properties against polarization switching, high Curie temperature, and low loss tangent. These compounds belong to the Aurivillius family with a general chemical formula of (B2O2)2+(Am-1BmO3m+1)2-, where A is mono-, di-or trivalent elements with dodecahedral coordination, B is a transition cation suited to octahedral, and m is integer which presents the number of perovskite layers. Their structure can be regarded as a regular intergrowth of (B2O2)2+ layers and (Am-1BmO3m+1)2- pervoskite-like slabs.Among them SBTi has larger remnant polarization and fatigue-free behavior, but the low Curi Temperature (Tc=520℃) handicaps its practical use. Recently, higher Curie temperature materials are needed for higher temperature application. While CBTi has high Curie temperature (Tc=790℃), with the same structure of SBTi. So we did a series of experiments on replacing Sr2+ with Ca2+ of SBTi film and found that Cao.4Sr0.6Bi4Ti4O15 (C0.4S0.6BTi) thin film exhibits good ferroelectric properties。The high anisotropic structure of BLSFs results in the ferroelectric properties to be strongly dependent on the crystallographic orientation. The polarization vector of four-layer perovskite BLSFs is only along the a-axis due to the presence of a glide plane and mirror plane along the b and c axes, respectively. To improve the film polarization, it is thus of great interest to design a process that can adjust the crystallization behavior of BLSFs to stimulate the growth of a-axis-oriented crystals. In this paper, (100)-oriented C0.4S0.6BTi thin films were prepared by controlling the heat treatment, introducing the TiO2 buffer layer between C0.4S0.6BTi thin film and Pt bottom and Sm3+ doping.The polycrystalline (100)-oriented C0.4S0.6BTi thin films were successfully prepared by sol-gel method on Pt/Ti/SiO2/Si substrates at various conditions. The phase structure and morphology of films were characterized by X-ray diffraction (XRD) and a scanning electron microscope (SEM). The effects of different annealed temperature, pre-annealed temperature and spinning speed on the crystallization behavior of C0.4S0.6BTi films have been investigated. It is found that the films made by the spin coating at 4000r/m, decomposed at 400℃and then annealed at 800℃preferred a-axis orientation, Pr and Ec was 16.1μC/cm2 and 85kV/cm, respectively. No evident fatigue can be observed after 109 switching cycles.A (100)-oriented C0.4S0.6BTi ferroelectric thin film with high volume fraction ofα(100)= 81% was obtained using the solution-gelatin method on Pt/Ti/SiO2/Si substrate coated by TiO2 rutile buffer layer. The X-ray diffraction patterns and SEM photographs exhibited that the (100)-oriented C0.4S0.6BTi thin film are homogeneous, crack-free and compact, with a-axis-orientation global crystals. Besides, the epitaxial growth of C0.4S0.6BTi film on TiO2 was investigated. It is found that the epitaxial relationship is that: (100)[C0.4.S0.6BTi]//(101)[010]TiO2. The enhancement of the growth of a-axis-oriented crystals for C0.4S0.6BTi film demonstrated that TiO2 is potential for the integrated devices.Samarium doped calcium strontium bismuth titanium Ca0.4Sr0.6SmxBi4-xTi4O15 ferroelectric films were prepared on Pt/Ti/SiO2/Si substrates by sol-gel method. The effects of Sm content on microstructure, orientation, ferroelectric property of films were investigated. The results show that Sm can inhibit the pining effect result of oxygen vacancies and hinder the grain growth. The ferroelectric property of Ca0.4Sr0.6Bi4Ti4O15 can be improved by adding proper amount of Sm. The Pr reaches a maximum value of 10.2μC/cm2, and the I(200)/I(119)reaches 0.869,when x=0.05.
|
PDF Full Text Request