• Issue 2  |
  • January 2010

Polarization Control of Electron Tunneling into Ferroelectric Surfaces

Recent work by Petro Maksymovych and his colleagues at the Center for Nanophase Materials Sciences on ferroelectric oxides that has led to the discovery that polarization switching in 30-50 nm oxide films of lead-zirconate and bismuth ferrite can abruptly change their local electrical conductivity by as much as 50,000% [1].  Polarization-dependent electron tunneling was first hypothesized by Leo Esaki almost 30 years ago, but has so far been elusive due to the dominance of extrinsic conductance mechanisms in complex oxides such as oxygen vacancy diffusion and formation of localized conductive filaments.  Using a unique scanning force microscope developed at the CNMS, the researchers investigated the nanoscale conductivity of epitaxially grown perovskite ferroelectrics.  The strong electric field of a sharp metal probe was used to confine the ferroelectric phase transition and electron transport in a local defect-free environment, and to simultaneously detect both properties.  Despite the large thickness of the studied films, they measured spatially and temporally reproducible local conductivity in the regime of high-field (Fowler-Nordheim) electron tunneling and its enhancement by as much as 500-fold when spontaneous polarization changes direction.  It was demonstrated that this effect can implement a simple binary memory function utilizing resistive read-out of the polarization direction, and that the magnitude of the electroresistance can be tuned through the electrostatic control of ferroelectric switching. 

Using electrical current to detect spontaneous polarization enables fast, non-destructive read-out of the polarization direction, while the information density can be substantially increased by shrinking the memory material to nanoscale.  Intrinsic ferroelectric electroresistance will thus take these materials well beyond existing FeRAM technology.  When compared to other memristor-based memories relying on defect-induced filamentary conduction, intrinsic electroresistance has the advantage of being tunable through the properties of underlying phase transition.  The on-off ratio, switching rate and spatial extent of the resistive switching in ferroelectrics can be controlled through the choice of the top and bottom electrodes, the size of the ferroelectric film, strain and doping, while the size of the polarization domains controls the information density.  Ferroelectric materials will also very likely go beyond bi-stable resistive switches.  For example, bismuth ferrite exhibits eight polarization directions, and multiaxial switching may enable multi-state memory functionality.  On the other hand, coupling between ferroelctricity and ferromagnetism in multiferroic materials can produce switchable spin-polarized current.  In addition to demonstrating the new property of ferroelectrics, it is believed this study is a seminal example of where the nanoscale phase transitions coupled to conductivity or magnetism enable new low-dimensional phenomena relevant to applications.  

In Summary

  • Resistive switching driven by the intrinsic phase transition rather than defects or local filaments
  • Switching voltage and on-off current is controlled by electrostatic boundary conditions
  • Observed in 5 nm to 50 nm epitaxial films
  • Fruitful playground for transport properties of correlated oxides:  size-effects, coupled order parameters, non-linearities

Giant electroresistance in 30 nm of ferroelectric Pb(Zr0.2Ti0.8)O3

The graph shows a simultaneous change in local strain and conductivity
measured as the electrical bias on the metal tip of the force microscope is swept from -5 V to 5 V and back (blue and red arrow). The abrupt event of ferroelectric switching (bottom curve) coincides with a similarly abrupt, > 15000% enhancement of local conductivity (top curve). Giant electroresistance has excellent repeatability and reproducibility across the surface [1].


Non-volatile memory function based on the ferroelectric control of Fowler-Nordheim tunneling

The blue curve is a voltage pulse sequence used to record (w) and read-out (r) the up (1) and down (0) polarization direction on the ferroelectric surface. The red curve is a current read-out, the magnitude of which clearly and repeatedly differentiates between the polarization directions.

Publication & Reference
[1] P. Maksymovych, S. Jesse, P. Yu, R. Ramesh, A. P. Baddorf, S. V. Kalinin, Polarization Control of Electron Tunneling into Ferroelectric Surfaces, Science 324, 1421 (2009)..

This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.