A possible mechanism of anomalous shift and asymmetric hysteresis behavior of ferroelectric thin films

Abstract

We studied theoretically the hysteresis behavior of ferroelectric thin films. The anomalous ferroelectric response is discussed by use of a bilayer model. Electrical conductivities of the films have been taken into account. To model the effects of the inhomogeneity of polarization and permittivity across the interface, the film is assumed to possess a secondary dielectric/ferroelectric phase ͑a dead or passive layer͒ with asymmetric conductivity. This configuration is found to produce large shifting ͑along the field axis͒ and deformation of the measured hysteresis loop. This is a manifestation of the asymmetric conductivity of the material. Theoretical calculation based on this model shows that the observed phenomena of shifted and skewed hysteresis loop in ferroelectric thin films can be explained in this way. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1853520͔The hysteresis behaviors of ferroelectric thin films have attracted great research interest for many years. Their bistable polarization as well as small size are promising characters for developing compact nonvolatile memories. Research studies of the anomalous effects and degradation mechanisms in the hysteresis behavior of ferroelectric thin films are essential for device applications. A most notable phenomenon is the large voltage offset along the horizontal ͑electric field͒ axis found in the hysteresis loop measurements. [1][2][3][4][5][6][7][8][9][10][11] The result is a deformed hysteresis loop with asymmetric switching. 6 Many researchers attributed the shift effect to the internal field caused by trapped charge carriers. [1][2][3][4] Other explanations included domain pinning, 5,6 rectifying effects formed at the ferroelectric-electrode interface 7-9 and the effect of passive layers. [10][11][12] It seems that the definitive mechanism is still not fully understood and the voltage shift may arise from multiple sources. Theoretical models to gain deeper insight using different approaches have been proposed. [11][12][13][14][15] In the absence of a unifying theory to account for all possible origins, we believe that the current theoretical understanding of the shift phenomenon can benefit from further input in terms of ideas and models. This work attempts to model this offset phenomenon by introducing a passive dielectric/ferroelectric layer at the filmelectrode interface. In the framework of the LandauGinzburg theory, the variation of polarization, which is usually more pronounced in thinner films, is modeled by a gradient term and extrapolation lengths in the free energy expression. This approach is suggested to be effectively the same as the existence of a passive layer ͑or dead layer͒. 16 It has been demonstrated that polarization gradient across a ferroelectric film can lead to asymmetric conduction. 17 The spatially varied polarization near the thin film surfaces is then thought to possess asymmetric electrical conductivity, while asymmetric conductivity may not emerge from the "normal" region of the film since the polarization shoul...

Improving the Electrical Properties of Zr-Doped Bi3.15Nd0.85Ti3O12 Thin Films by Engineering Polarization Rotation

“…This phenomenon has been explained by introducing a dead layer interface between the electrode and bulk film, [23][24][25] in which the shift effect has a dynamic accumulation process of an internal direct-current (DC) electric field; that is, the shift degree of the hysteresis loop should depend on time development. 25 Under an applied electric field of 450 kV/cm, the 2P r of the BNTZ thin films with various Zr contents (x = 0, 0.05, 0.1, 0.3, and 0.5) were 31 lC/cm 2 , 35 lC/cm 2 , 46 lC/cm 2 , 28 lC/cm 2 , and 26 lC/cm 2 , respectively. These 2P r values demonstrate that Ti-site substitution by Zr could enhance the 2P r value of BNT thin film.…”

Effect of electrode configurations on the process-induced imprint behavior of epitaxial Pb(Zr0.52Ti0.48)O3 capacitors

“…10,11 Grossmann and co-workers proposed that a strong electric field within a weak-ferroelectric or nonferroelectric surface layer can drive the voltage shift, 12 and with a single surface layer Wong and Shin have theoretically modeled the shifted and deformed P-E hysteresis loops. 13 Although the origin of such a layer is still under debate, Abe et al 14 have attributed it to an asymmetrical relaxation of the epitaxial lattice-misfit strain between BaTiO 3 and the SrRuO 3 bottom electrode, and we also showed that an enlarged misfit can increase the voltage shift in epitaxial Pb͑Zr 0.52 Ti 0.48 ͒O 3 ͑PZT͒ capacitors. 15 This strain-related scenario seems consistent with the imprint induced by mechanical stress, where a simultaneous backswitching of ferroelectric domains, as revealed by the local piezoresponse method, was observed during the polarization reversal.…”

Effects of Stress and Depolarization on Electrical Behaviors of Ferroelectric Field-Effect Transistor

“…The electrical behaviors of FeFET determine the information storage capability and stability, and they are affected by various degradation effects, such as fatigue, relaxation, and imprint [2]. To understand these mechanisms, a lot of models based on mathematical function, such as the Miller model [3], Preisach model [4], Lue model [5], and Zheng model [6], were proposed to simulate hysteresis behavior and degradation effect of ferroelectrics.…”