Control of Tetragonal Distortion and Ferroelectricity in Bi(Zn1/2Ti1/2)O3-Based Lead-Free Ferroelectric Thin Films
Authors: Yasui Shintaro , Uchida Hiroshi , Funakubo Hiroshi 
Published: February 2020

Lead-free ferroelectric materials with superior property have been demanded, yet a few lead-free ferroelectrics such as alkali-based (K,Na)NbO3 family and bismath-based (Bi,Na)TiO3 family have been reported. Pb(Zr,Ti)O3[PZT] is one of very famous ferroelectric because their ferroelectric and piezoelectric properties can be controlled by their composition and domain structure. And almost of all for application use, PZT having tetragonal and morphotropic phase boundary[MPB] composition have been used. From a view point of material design in PZT case, mother material of tetragonal PbTiO3 is very important for designing MPB. However, it is know that there are only BaTiO3 and (Bi,K)TiO3 in the field of tetragonal perovskite materials. Novel lead-free tetragonal ferroelectric materials are really wanted.

Recently, novel metastable phase materials prepared by high pressure synthesis have been reported. Among them, BiCoO3
1 and Bi(Zn0.5Ti0.5)O3
2 are promising ferroelectric materials due to their high tetragonality, c/a, of over 1.2. Large spontaneous polarization is expected by first principal calculation. BiCoO3 is a one of multiferroic, but unfortunately it is very leaky because of containing Co3+(d
6). On the other hand, Bi(Zn0.5Ti0.5)O3 is constructed by only d
0 and d
10 ions. Unfortunately, nobody reported their ferroelectricity. We can expect that leakage property will be better than that of BiCoO3. Therefore, we have focused on Bi(Zn0.5Ti0.5)O3 and prepare novel ferroelectric materials.

Epitaxial Bi(Zn0.5Ti0.5)O3-BiFeO3 thin films with about 300 nm thickness were grown at 700oC on (100)cSrRuO3//(100)SrTiO3 by pulsed metalorganic chemical vapor deposition using Bi[(CH3)2(2-(CH3)2NCH2C6H4)] (Tosoh Co. Ltd), Zn(C14H25O2)2, Ti(O∙i-C3H7)4, Fe(C2H5C5H4)2 and oxygen gas as the source materials. SrRuO3 films were grown by RF magnetron sputtering method. Film thickness and composition were confirmed by surface profilometry and X-ray fluorescence (XRF) calibrated using standard samples. Crystal structure of the deposited films was characterized by X-ray diffraction (XRD) analysis using a four-axis diffractometer with CuKa radiation. Atomic microstructures of the films were observed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Pt/[Bi(Zn1/2Ti1/2)O3- BiFeO3]/SrRuO3 capacitor structure was used for the electrical measurements after making circular Pt top electrodes of 100 mm in diameter, which were deposited by e-beam evaporation. Piezoresponse mappings and longitudinal piezoelectric coefficient (d
33) was measured using piezoresponse force microscopy.

High resolution Advanced X-ray reciprocal space mapping leads us to know all diffractions from the films, suggesting that crystal structure and domain structure could be investigated easily. This mapping reveals d-spacing information along both the substrate surface normal and off-normal directions. It is observed that all diffraction peaks from the films can be identified to tetragonal P4mm symmetry as same as Bi(Zn1/2Ti1/2)O3, and a single perovskite phase is achieved to fabricate. The out-of-plane (c-axis) and in-plane (a-axis) lattice parameters calculated from diffractions are 0.465 nm and 0.381 nm, respectively, resulting in a giant tetragonal distortion of (c/a)-1 = 22%. This value is 3.5 times larger than the 6.3% of PbTiO3. In addition, this [(c/a)-1] is larger than the reported one for pure Bi(Zn1/2Ti1/2)O3, [(c/a)-1] = 21%. Similar enlargement of the [(c/a)-1] by the substitution with BiFeO3 has been reported in PbTiO3-BiFeO3, even if the substituted BiFeO3 has rhombohedral symmetry.

We have also conducted cross sectional high resolution HAADF-STEM for visualization of the crystal structure directly. The “90o domain boundary” has a 51o/39o angle due to the large tetragonality, which is in good agreement with 50.7o/40.3 o value based on the crystal structure information obtained by XRD measurement. Z-contrast image to perovskite unit cell shows that the B-site ions are also displaced along [001] significantly away from the center. There are in good agreement with structure analysis of the Bi(Zn1/2Ti1/2)O3 prepared by high pressure synthesis. In addition, as XRD results showed no super lattice peaks, B-site cations in perovskite are not ordered. These results clearly indicate that this material is characterized as displacive-type ferroelectric, and unusual angle of the 90o domain is originated from large tetragonal distortion. We will also perform to measure ferroelectricity for discussing relationship between ferroelectricity/piezoelectricity and tetragonality in these materials.

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