Volume 86, Issue 2, 15 July 1999
Index of content:
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
86(1999); http://dx.doi.org/10.1063/1.370849View Description Hide Description
Without external fields,ferroelectric materials will have multidomain configuration in the ferroelectric state. Detailed analysis found that twinning may not be treated as random since the number of orientations for the domain walls are limited in a given symmetry change during a ferroelectric phase transition. In each finite region of a large crystal or in small crystallites, a particular set of twins is favored under certain boundary conditions, which consists of only two of the low temperature variants. Statistic models of random distribution of domains do not apply for calculating the physical properties of such twin structures. However, one could derive the two domain twin properties by using the constitutive equations and appropriate mechanical boundary considerations. This paper presents a theoretical analysis on such a two-domain twin system, including its global symmetry and effective material properties resulting from different twinning configurations. Numerical results are derived for and
86(1999); http://dx.doi.org/10.1063/1.370850View Description Hide Description
Reaction mechanisms for chemical-vapor deposition (CVD) of (BST) films have been studied by modeling reactions that should occur in the gas phase and on the film surfaces. We considered a conventional model and two other kinds of models: a copolymerization and a surface reflection model. The conventional model consisted of thermal decomposition of dipivaloylmethanato (DPM) source materials in the gas phase, followed by deposition of the decomposition products on the film surfaces. In the copolymerization model, and were assumed to preferentially copolymerize with in the gas phase, and the resulting products behave as dominant film precursors. In the surface reflection model, moreover, each source material was assumed to thermally decompose in the gas phase, and the products from adsorbed on film surfaces prevent the other Ba and Sr precursors from sticking onto the surfaces. Numerical simulations were performed for CVD of films, and the atomic incorporation rates of Sr and Ti into films were calculated as a function of the titanium source flow rate. The conventional model gave numerical results that the Sr incorporation rate was independent of the titaniumflow rate, while the experiments showed a decrease with increasing flow rate. This implies that the conventional model cannot explain the reaction mechanisms. On the other hand, the incorporation rates of Sr obtained from the two experimental models decreased with increasing titaniumflow rate, being in agreement with the experiments. Moreover, the overall sticking probabilities for film deposition calculated using the experimental models also fitted the experiments, where the surface reflection model gave a better agreement. These results indicate that the experimental models, especially the surface reflection model is suitable to explain the CVDreaction mechanisms for and thus, BST film deposition.
86(1999); http://dx.doi.org/10.1063/1.370851View Description Hide Description
Thermally stimulated polarization and depolarization current (TSPC and TSDC, respectively) measurements were exploited to characterize the dielectric relaxation of single crystals of different origin, i.e., grown by different methods and containing different amount of impurities. In addition to the dc conduction current, measured by the TSPC runs, at least one TSDC band was detected for the investigated specimens. Activation parameters characterizing the polarization and conduction processes were evaluated, and the possible nature of the related defects has been discussed. The results obtained for nominally pure crystals show the presence of orientation polarization related to some permanent dipoles. In the less pure samples, in addition to the high-temperature interfacial polarization, a low-temperature band was tentatively ascribed to a space charge polarization related to the impurities.
Theoretical investigation of the influence of space charges on ferroelectric properties of thin film capacitor86(1999); http://dx.doi.org/10.1063/1.371147View Description Hide Description
During ion etching (IE) processes used for making a Pt electrode in the fabrication of ferroelectriccapacitors, the film is exposed to radiation and energetic ion bombardment. The influence of such processes on polarization-electric field characteristics of lead zirconate titanatethin filmcapacitor has been reported by E. G. Lee [Appl. Phys. Lett. 69, 1223 (1996)]. A large field shift and a constriction in the hysteresis loop are experimentally observed. For these authors, this behavior is probably due to the existence of space charges trapped near electrodes. For a better understanding of these phenomena, we have developed a model of ferroelectriccapacitor based on the phenomenological Landau–Devonshire theory. A two-dimensional lattice of dipoles is assumed to describe the film. By solving Poisson’s equation, the electric-field distribution inside the film is calculated, which allows us to locally determine the evolution of the polarization by minimizing free energy. IE effects and probably the existence of space charges in the film are introduced by means of a doping layer in the film near the electrode. The influence of important parameters such as doping level, and thickness of the doped layer on the hysteresis loop are investigated. Main experimental thin film electric behaviors are well reproduced and explained considering dipoles switched inside the lattice. Results are interpreted by splitting the hysteresis loop deformation into two effects with different origins: a constriction effect related to the number of impurities inside the film and a shift effect related to the asymmetrical impurities distribution.