Index of content:
Volume 91, Issue 6, 15 March 2002
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
Microstructure and electric properties of lead lanthanum titanate thin film under transverse electric fields91(2002); http://dx.doi.org/10.1063/1.1448867View Description Hide Description
Polycrystalline leadlanthanumtitanatethin film having perovskite structure was fabricated by metalorganic deposition (MOD) on a substrate at 600 °C for 1 h in atmosphere. Columnar structured buffer layer was also prepared by a MOD process under the same condition. Electrical measurements were conducted on interdigitated electrodes. The crystalline structure and growth behavior of the films have been studied by x-ray diffraction and scanning electron microscopy. It is observed that dielectric response of the film is effected by the cable length used in the measurement and by the values of the ac voltage. Long cable gives rise to an additional resonance peak at high frequency caused by the stray inductance of the contacts and cables. The capacitance and loss tangent over low frequency range shows significant variations due to the trapped charges and space charges in the film. These variations are very dependent on the values of the ac voltage and the length of cable. Meanwhile, the trapped charges and space charges lead to abnormal P–E loops, in which the measured remanent polarization and coercive field increase with increasing frequency.
91(2002); http://dx.doi.org/10.1063/1.1448869View Description Hide Description
In the study of elastic and piezoelectric fields in semiconductors due to buried quantum dots(QDs), the semicoupled piezoelectricmodel is commonly adopted. However, its accuracy and suitability have never been studied. In this article, by developing a fully coupled piezoelectricmodel and deriving the analytical elastic and piezoelectric fields based on this and the semicoupled models, we are able to verify that when the piezoelectric coupling is weak, like GaAs with the electromechanical coupling factor the semicoupled model predicts very accurate results as compared to those based on the fully coupled model. However, if the piezoelectric coupling is relatively strong, like AlN with we have shown that the semicoupled model gives very serious errors or even totally wrong results. Applying these two models to a uniformly strained AlN layer grown along the polar axis has also confirmed our observation. Therefore, for semiconductors like AlN, the fully coupled model presented in this article must be employed in order to give a reliable and accurate prediction for the elastic and piezoelectric fields. Also presented in this article is the distribution of the piezoelectric field on the surface of a half-space GaAs due to a buried QD located at 2 nm below the surface with a volume 4π/3 It is observed that the horizontal electric field on the traction-free and insulating surface shows some special features and its maximum magnitude can be as high as V/m when the uniform mismatch eigenstrain is 0.07. Furthermore, the piezoelectric field on the traction-free and conducting surface exhibits different characters as compared to the traction-free and insulating case.
91(2002); http://dx.doi.org/10.1063/1.1446655View Description Hide Description
Domain structures of unpoled as well as poled (along - and -direction) (PZN)-8% (PT) and (PMN)-29% PT single crystals have been investigated by scanning force microscopy (SFM) in the piezoresponse mode, at room temperature. Antiparallel domain structures have been detected mostly in unpoled crystals of both materials, with a fingerprint pattern in (001)-oriented PZN-8% PT crystal. The ferroelastic domain wall has been identified in poled (110)-oriented PZN-8% PT crystal. “Writing” of ferroelectric domains has been performed by applying a dc voltage to the SFM tip. Local re-poling has been observed for all unpoled as well as for poled (001)-oriented crystals at the voltage Local electrical switching was successful in poled (110)-oriented PMN-29% PT at higher voltage but was not successful in poled (110)-oriented PZN-8% PT crystal. Domain-engineered crystals poled in -direction seem to exhibit more stable (in the sense of local re-poling properties) domain arrangement. Hysteretic dependencies were observed by local application of an ac voltage.
91(2002); http://dx.doi.org/10.1063/1.1448397View Description Hide Description
Based on the mechanism of domain switch, a micromechanics-based model is developed to calculate the hysteresis loop of ferroelectric ceramics under combined electromechanical loading. The development makes use of an extension of the Eshelby-type elastic inclusion problem to a heterogeneous electromechanically coupled ferroelectric medium with distribution of eigenstrain and eigenpolarization generated by domain switch. The ferroelectric ceramic at a generic state is considered to consist of the parent domain and the switched domains whose volume fraction continues to evolve under an increasing electromechanical load. At a given level of applied stress and electric field, the volume fraction of the new domain is determined from a kinetic equation that is derived from consideration of the thermodynamic driving force caused by the reduction of Gibbs free energy and the resistance force associated with the domain wall movement. The theory developed is used first to simulate the hysteresis behavior of a PZT-51 without any superimposed stress, and then with the derived material constants, to independently predict the influence of a superimposed compression. It is found that, consistent with experimental observations, the hysteresis loops flatten out under the axial compression, but that the loops elongate under a transverse compression. Both the remanent polarization and coercive field also tend to decrease with the compressive force regardless of how the compression is applied.
91(2002); http://dx.doi.org/10.1063/1.1446230View Description Hide Description
Variable temperature scanning surface potentialmicroscopy is used to determine thermodynamic and kinetic parameters associated with polarization screening on surfaces. The temperature dependence of the surface potential is indicative of the interplay between the fast dynamics of atomic polarization and slower dynamics of screening charge. The screening charge relaxation kinetics are found to be weakly dependent on temperature with activation energy Equilibrium domain potential difference depends linearly on temperature; the zero potential contrast is observed at ∼110 °C. At room temperature the sign of domain potential is determined by the screening charges rather than polarization charge. A thermodynamic model for screening of ferroelectric surfaces based on Ginzburg–Devonshire theory is developed so that the enthalpy and entropy of charge compensation can be derived from the temperature dependence of surface potential contrast. In the case of in air, the charge compensation mechanism is surfaceadsorption.