Vanishing critical thickness in asymmetric ferroelectric tunnel junctions: First principle simulations

Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong Special Administrative Region, China
Journal of Applied Physics (Impact Factor: 2.21). 02/2011; DOI: 10.1063/1.3532000
Source: IEEE Xplore

ABSTRACT The stability of the remnant polarization in the ferroelectric barrier layer is a prerequisite to applications involving ferroelectric tunnel junctions (FTJs) or capacitors. One of the most important issues in the pursuit of further developments in this area is to overcome the limitations due to the critical thickness, below which the ferroelectric polarization disappears. In this paper we report first-principle density-functional calculations of the charge distribution and polarization in an asymmetric FTJ (A-FTJ), i.e., one with dissimilar electrodes. We found that a significant and stable polarization can be retained down to thicknesses as small as 0.8 nm (two unit-cells) in a BaTiO 3 thin film between Pt and SrRuO 3 electrodes, quite unlike the case of symmetric FTJs. We trace this surprising result to the large electric field produced by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectricity, leading to the reduction, or even the complete elimination of the depolarization field, leading to the vanishing of the critical thickness. We speculate that this is a general result for A-FTJs, which could be of importance to applications of ferroelectric thin films and tunneling junctions or capacitors where the presence of the critical thickness is a limiting factor.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Taking into account size effect of the ferroelectric tunnel barrier and interface effect determined by two dissimilar magnetic electrodes, the ferroelectric stability and magnetoelectric effect of asymmetric multiferroic tunnel junctions have been comprehensively calculated. Results of the stable structure, polarization, electrostatic potential, charge density, spin density, and magnetic moments demonstrated a series of the great hidden features in asymmetric multiferroic tunnel junctions and showed significant differences between two polarization states. The potential and magnetic moment are strongly affected by electric and magnetic polarizations, suggesting the possibility of ferroelectric control of magnetization and coexistence of tunnel magnetoresistance and giant electroresistance effects.
    Applied Physics Letters 04/2013; 102(15). · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Theoretical investigations on ferroelectric tunnel junctions (FTJs) with a fixed-thickness composite layer (Pt/MgO/BaTiO3/Pt and Pt/SrTiO3/BaTiO3/Pt) were conducted. It showed that there is an optimal dielectric thickness that can bring the largest tunneling electroresistance (TER) ratio provided that the ferroelectricity does not change with the dielectric thickness. The optimal dielectric thickness is insensitive to ferroelectric polarization in ferroelectric layer and increases linearly with the composite barrier thickness. Considering the size effect of ferroelectricity, the optimal dielectric thickness (unit cells) changes little if the polarization increases slowly with the ferroelectric thickness. Such studies may help to fabricate FTJs with larger TER ratio and put them into practical application.
    Journal of Applied Physics 04/2012; 111(7). · 2.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ferroelectric size effect of BaTiO3 (BTO) tunnel junctions with metal Pt and/or oxide SrRuO3 (SRO) electrodes has been comprehensively investigated by the first-principle calculations. A vacuum layer is included in the supercell calculations, so that full-relaxation is achieved without artificial constraint on the supercell strains. We have constructed all of ten possible types of tunnel junctions with either symmetric or asymmetric geometries to systematically explore the influence of electrode/ferroelectric interfaces. The characteristics of atomic structure, polarization, charge density, and electrostatic potential for different geometries and sizes are revealed. It is found that the ferroelectric stability of a tunnel junction depends significantly on the details of the two electrode/ferroelectric interfaces, which present specific short- and long-range properties, e.g., local bonding environment, electronic screening, built-in field, etc. Result shows that Pt/BTO interfaces have strong coupling with ferroelectric distortion and thus play more dominant roles than the SRO/BTO interfaces in affecting the ferroelectric stability of the tunnel junctions. Particularly, it is found that Pt2/TiO2 interface can induce collective ferroelectric distortion in the initially non-distorted barrier. With a full-relaxation of the strains, an abnormal enhancement of ferroelectricity by Pt2/BaO interface due to Pt-O bonding effect is demonstrated, where a strong interfacial-bonding-related polarizing field is verified. Also importantly, polarization stability of asymmetric tunnel junctions is found dependent on direction, manifested with the appearing of a new critical thickness, below which the tunnel junction loses polarization bistability. Furthermore, it shows that the local features of a specific electrode/ferroelectric interface (e.g., the interfacial atomic structure, local polarization, charge transfer, and potential step) are well kept in different types of tunnel junctions. By analyzing and summarizing the results, our results suggest that traditional phenomenological models need several modifications in order to quantitatively reproduce the size effect of ferroelectric tunnel junctions. Our study provides a comprehensive picture of the ferroelectric size effect in BTO tunnel junctions as a function of electrode/ferroelectric interfaces and should have valuable implications for future studies and applications.
    Journal of Applied Physics 08/2013; 114(6). · 2.21 Impact Factor

Full-text (2 Sources)

Available from
May 20, 2014