Volume 91, Issue 11, 01 June 2002
Index of content:
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
91(2002); http://dx.doi.org/10.1063/1.1476078View Description Hide Description
The electromechanical coupling factor for the thickness-extensional mode, in crystals has been predicted to be as high as 69% for the 49.5° rotated X-cut about the Y-axis, which is the highest among known piezoelectrics. This paper presents the experimental confirmation of the high coupling factor and elucidates the relationship between the domain structure and the piezoelectric properties in the pseudocubic cut, which is close to the maximum cut and has a piezoelectric strain constant about 2.9 times that of the Z-cut. Etching of crystals is found to have an outstanding effect on the piezoelectric characteristics. That is, in the case of “as-cut” crystals, the strain versus electric field curves have a large hysteresis and instability due to the occurrence of 60° domains in a high electric field region, whereas etched crystals remain the single domain state and exhibit linear piezoelectric properties even in the high electric field region. It is also demonstrated that the cut crystal poled along the direction has a polar multidomain structure composed of two kinds of 90° domains, not four kinds of equivalent domains, and that it exhibits almost the same piezoelectric properties as those of single-domain crystals.
91(2002); http://dx.doi.org/10.1063/1.1473672View Description Hide Description
Epitaxialheterostructures of ordered perovskitethin films were grown by pulsed laser deposition using MgO and single-crystal substrates, and bottom electrode layers. The small-signal dielectric response of the heterostructures was measured in a broad range of temperatures (290–625 K) and frequencies The contribution of the film–electrode interfaces was evaluated, and the true properties of the films were analyzed. Both relaxor and normal ferroelectric types of behavior were identified in the films. The evolution of behavior and shift of dielectric maxima were found to correlate with the temperature of deposition and the type of the substrate. This was suggested to refer to the microstructural factors such as in-plane strain, grain size, and misfit dislocations.
Phase diagrams and dielectric response of epitaxial barium strontium titanate films: A theoretical analysis91(2002); http://dx.doi.org/10.1063/1.1473675View Description Hide Description
We develop phase diagrams for single-domain epitaxialbariumstrontiumtitanatefilms on cubic substrates as a function of the misfit strain based on a Landau–Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. The biaxial epitaxy-induced internal stresses enable phase transformations to unusual ferroelectric phases that are not possible in single crystals and bulk ceramics. The dielectric response of the films is calculated as a function of the misfit strain by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. It is shown that by adjusting the misfit strain via substrate selection and film thickness, a high dielectric response can be obtained, especially in the vicinity of structural instabilities. Theoretical estimation of the dielectric constant of (001) and films on (001) Si, MgO, and substrates as a function of misfit strain and film thickness is provided. An order-of-magnitude increase in the dielectric constant with increasing film thickness is expected for films on and substrates. A structural instability around 40 nm is predicted in films on MgO substrates accompanied by a substantial increase in the dielectric constant. For films on MgO substrates thicker than 40 nm, the analysis shows that the dielectric constant decreases significantly. We show that the theoretical approach not only predicts general trends but is also in good quantitative agreement with the experimental data reported in literature.
91(2002); http://dx.doi.org/10.1063/1.1470249View Description Hide Description
This article examines the thickness effects of ferroelectricfilms on gate tunneling suppression and charge control in metal-ferroelectric-insulator-semiconductor field-effect transistors (MFISFETs). The formalism used is based on a blocking-layer model for the ferroelectricfilm and a self-consistent solution of the Poisson and Schrödinger equation. We show that with a polar ferroelectric the threshold voltage of the FET can be altered by controlling the ferroelectricfilm thickness. We also study the thickness dependence of the capacitance–voltage curve and the surface charge density and the effects of ferroelectrichysteresis. The tunneling probability and leakage current calculation in a MFISFET device are provided in this article. Ferroelectrics-based transistors show higher sheet charges and lower tunneling currents than oxide-based devices.
Effects of coercive voltage and charge injection on memory windows of metal-ferroelectric-semiconductor and metal-ferroelectric-insulator-semiconductor gate structures91(2002); http://dx.doi.org/10.1063/1.1467629View Description Hide Description
We have prepared (SBT)/Si metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-insulator-semiconductor (MFIS) ferroelectric gate structures and investigated the changes in memory window with different thickness of SBT and in the MFS and MFIS. As a result, it is found that the memory window increases with increasing thickness of SBT and decreasing thickness of The experimental and theoretical analysis reveals that the memory window equals to the difference between the effective coercive voltage applied to the ferroelectricfilm and the flat band voltage shift due to charge injection Increasing the thickness of SBT, the seems to be saturated at higher voltage, whereas the starts to increase exponentially at the higher gate voltage. In contrast, the decreases with decreasing thickness of resulting in the enhancement of the memory window due to the reduction of charge injection.