S. Zhong

Dielectric anomaly due to electrostatic coupling in ferroelectric-paraelectric bilayers and multilayers

A thermodynamic model is presented that describes the polarization and the dielectric response of ferroelectric-paraelectric bilayers and multilayers. It is shown that a strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical paraelectric layer thickness. The bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie–Weiss temperature in homogeneous ferroelectrics at this critical thickness. A numerical analysis is carried out for a pseudomorphic (001) BaTiO3∕SrTiO3 heteroepitaxial bilayer on (001) SrTiO3 and a stress-free BaTiO3∕SrTiO3 bilayer. Complete polarization suppression and a dielectric peak are predicted to occur at approximately 66% and 14% of SrTiO3 in these two systems, respectively.

The fabrication and material properties of compositionally multilayered Ba1−xSrxTiO3 thin films for realization of temperature insensitive tunable phase shifter devices

Compositionally layered BaxSr1−xTiO3 (Ba0.60Sr0.40TiO3–Ba0.75Sr0.25TiO3–Ba0.90Sr0.10TiO3) 220nm thin film heterostructures were fabricated on Pt coated high resistivity Si substrates via the metal organic solution deposition technique (MOSD). Optimization of the material design was achieved by evaluating two integration schemes, namely, the single- and multianneal process protocols. Materials characterization demonstrated that both film process protocols resulted in smooth, dense, crack-free films with a single phase perovskite structure. Rutherford backscattering spectroscopy revealed compositionally distinct layers and severe elemental interdiffusion for the films fabricated via the multianneal and single-anneal process protocols, respectively. The retention of the compositional layering subsequent to film crystallization deemed the multianneal processed BaxSr1−xTiO3 (BST) film suitable for evaluation of dielectric properties. The dielectric properties were compared to both paraelectric uniform composit...

Highly tunable and temperature insensitive multilayer barium strontium titanate films

Multilayered Ba1−xSrxTiO3 (BST) films were deposited on Pt coated Si substrates via metalorganic solution deposition. The multilayer heterostructures consisted of three distinct layers of ∼220nm nominal thickness with compositions corresponding to BST 63∕37, BST 78∕22, and BST 88∕12. At room temperature, the heterostructure has a small-signal dielectric permittivity of 360 with a dissipation factor of 0.012 and a dielectric tunability of 65% at 444kV∕cm. These properties exhibited minimal dispersion as a function of temperature ranging from 90to−10°C. These results are explained via a thermodynamic model that incorporates electrical, mechanical, and electromechanical interactions between BST layers.

High dielectric tunability in ferroelectric-paraelectric bilayers and multilayer superlattices

The dielectric tunability of ferroelectric/paraelectric bilayers and multilayer superlattices are examined theoretically. A numerical analysis is carried out for a pseudomorphic (001) BaTiO3∕SrTiO3 heteroepitaxial bilayer on (001) SrTiO3 and a stress-free BaTiO3∕SrTiO3 bilayer. We show that these structures are capable of tunabilities greater than 90% due to electrostatic and electromechanical coupling between layers. Moreover, we develop the methodology for incorporation conventional integrated circuit silicon dielectrics into heteroepitaxial structures that can reduce current leakage while maintaining high tunability, thereby enabling the device designer flexibility toward the optimization of microwave and millimeter wave elements.

Enhanced piezoelectric response from barium strontium titanate multilayer films

Multilayered and homogeneous thin films of BaxSr1−xTiO3 (BST) were grown on Pt-coated Si substrates via metal-organic solution deposition. The multilayer 220nm thick BST heterostructure consisted of Ba0.60Sr0.40TiO3, Ba0.75Sr0.25TiO3, and Ba0.90Sr0.10TiO3. A single composition 220nm thick Ba0.60Sr0.40TiO3 was also grown for comparison. The piezoelectric properties were measured using piezoresponse force microscopy. There is approximately a 50% improvement in the piezoelectric response of the multilayered heterostructure compared to the homogeneous sample, with some spatial inhomogeneity. This enhancement can be attributed to the internal potential that arises from the polarization gradient and the commensurate built-in strain in the multilayer sample.

Effective pyroelectric response of compositionally graded ferroelectric materials

A thermodynamic theory is used to determine the effective pyroelectric coefficients of polarization graded ferroelectrics, with values in excess of 0.1μC∕cm2°C are predicted for various barium strontium titanate thin films. Maximum values closer to 0.4μC∕cm2°C are obtained for a 1μm thick material—compositionally graded between pure barium titanate and one that has barium to strontium in the ratio of 75:25.

Large piezoelectric strains from polarization graded ferroelectrics

The potential applications of polarization graded ferroelectrics as high performance sensors and actuators are theoretically investigated. A static bending can be expected in polarization graded ferroelectric plates, forming a vertical displacement. This is due to the built-in strain gradient that arises from the grading of the composition and concomitantly, the spontaneous self-strain. Numerical results of two compositionally graded ferroelectrics, BaTiO3–Ba1−xSrxTiO3 and PbTiO3–Pb1−xZrxTiO3, show a high dynamic response of the displacement under an external electric field, yielding as much as ∼23% strain at 50kV∕cm in PbTiO3–Pb0.6Zr0.4TiO3, which is comparable to large displacement actuators formed from ceramic/ceramic and ceramic/metal multilayer mesomaterials.

Compositional symmetry breaking in ferroelectric bilayers

Compositional variations across ferroelectric bilayers result in broken spatial inversion symmetry that can lead to asymmetric thermodynamic potentials. For the case of insulating materials, ferroelectric multilayers will self-pole due to the electrostatic coupling between the layers. Polarization-graded ferroelectrics with smooth composition, temperature, or stress gradients are viewed as bilayer structures in the limit of the ever-increasing number of bilayer couples, thus permitting us to conclude that the unconventional hysteresis associated with “up” and “down” polarization graded structures are real phenomena, and not artifacts associated with free charge or asymmetric leakage current.

Interface effects in ferroelectric bilayers and heterostructures

We study the role of interlayer interfaces on the polarization response of ferroelectric-paraelectric bilayers using a nonlinear thermodynamic model. We carry out a numerical analysis for prototypical BaTiO3–SrTiO3 bilayers ranging from 40to800nm total thickness as a function of SrTiO3 fraction. There exists a critical fraction of SrTiO3 at which the polarization is suppressed due to the depolarization field arising from the interlayer coupling and a large dielectric response is predicted. It is shown that this critical fraction decreases with decreasing total bilayer thickness, indicating that the interfacial effects are more pronounced in thinner bilayers.

Effect of strain on tunability in Ba0.60Sr0.40TiO3 thin films on Pt-Si substrates

Ba0.6Sr0.4TiO3 films with a thickness of 200nm were deposited on Pt–Si substrates at 400 and 700°C. Room-temperature tunability was measured and found to improve with deposition temperature, but losses also increased. The dielectric constant, tunability, and loss tangent are found to be 350, 52%, and 0.07 at 300kV∕cm for the 700°C deposition. The film grown at 700°C has a larger grain size, leading to approximately 5% higher tunability compared to the film deposited at 400°C. Supporting theoretical calculations were carried out using a modified Landau-Devonshire thermodynamic formalism that takes into account the internal stresses that arise from the differences of coefficients of thermal expansion between the film and the substrate.