BaTiO₃ based materials for piezoelectric and electro-optic applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003.Includes bibliographical references (p. 93-95).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Ferroelectric materials are key to many modem technologies, in particular piezoelectric actuators and electro-optic modulators. BaTiO₃ is one of the most extensively studied ferroelectric materials. The use of BaTiO₃ for piezoelectric applications is, however, limited due to the small piezoelectric coefficient of the room temperature-stable tetragonal phase. Furthermore, research on BaTiO₃ for integrated optics applications remains sparse. In this work Zr-, Hf-, and KNb- doped BaTiO₃ materials were prepared in a composition range that stabilizes the rhombohedral phase. These materials were prepared as bulk polycrystals using a standard solid-state reaction technique in order to test the piezoelectric and dielectric properties. Some compositions were then chosen for thin film deposition. The films were deposited using pulsed laser deposition on MgO and SOI substrates. Growth orientation, remnant strain and optical properties were then measured. X-ray diffraction was used to confirm the existence of a stable rhombohedral phase. Dielectric measurements confirmed the expected phase transition temperatures. A piezoelectric coefficient of d₃₃=290-470pc/N was measured for Zr- and Hf- doped BaTiO₃, compared with d₃₃=75pC/N for pure BaTiO₃. The electrostrictive coefficient of the KNb-doped material, was measured as Q33=0.37m⁴/C², compared with Q33=0.11m⁴/C² for pure BaTiO₃. The maximum strain measured for the doped samples was 5-10 times higher then that of pure BaTiO₃. The effect of growth conditions on the orientation and strain of BaTiO₃ thin films was studied. As the substrate temperature and laser fluency were increased the film orientation varied from (111) to (110), then to (100). Zr- and Hf- doping helped lower the forming temperature for the(cont.) orientations. The index of refraction for the thin films was measured and a model based on the Clausius-Mossotti relation was used to explain the data. The refractive index for BaTiO₃ films was extracted from the model, giving n=2.334 and n,=2.163.by Ytshak Avrahami.Ph.D

Study of orientation effect on nanoscale polarization in BaTiO3 thin films using piezoresponse force microscopy

We have investigated the effect of texture on in-plane (IPP) and out- of plane (OPP) polarizations of pulsed-laser-deposited BaTiO3 thin films grown on Pt and La0.5Sr0.5CoO3 (LSCO) buffered Pt electrodes. The OPP and IPP polarizations were observed by piezoresponse force microscopy (PFM) for three-dimensional polarization analyses in conjunction with conventional diffraction methods using x-ray diffraction and reflection high energy electron diffraction measurements. BaTiO3 films grown on Pt electrodes exhibited highly (101) preferred orientation with higher IPP component whereas BaTiO3 film grown on LSCO/Pt electrodes showed (001) and (101) orientations with higher OPP component. Measured effective d(33) values of BaTiO3 films deposited on Pt and LSCO/ Pt electrodes were 14.3 and 54.0 pm/ V, respectively. Local piezoelectric strain loops obtained by OPP and IPP-PFM showed that piezoelectric properties were strongly related to film orientation

Ridge waveguide using highly oriented BaTiO3 thin films for electro-optic application

In this work, 750 nm-thick BaTiO3 thin films with highly (0 0 1) preferred orientation were grown on single crystal MgO substrates by RF-sputtering. Hydrogen silsesquioxane (HSQ) resist material based ridge waveguides, which were fabricated on BaTiO3 thin films, were formed by e-beam lithography. Au electrodes were deposited on top of the BaTiO3 films beside the waveguide. Propagation losses of the BaTiO3 ridge waveguide were 3–5 dB/cm in transverse electric polarization. The measured electrooptic coefficient value (r51) was 110 pm/V, which is three times larger than the electrooptic coefficient (r33 = 30.8 pm/V) of single crystal LiNbO3. SiO2 strip waveguide formed by HSQ exhibited light propagation with loss lower than 5 dB/cm. This result demonstrates potential possibility of creating highly oriented and/or epitaxially grown BaTiO3 waveguides and optical components on oxide substrates