Noncontact Atomic Force Microscopy - Yale School of Engineering ...

P.II-31
Polarization-dependent electron tunneling into ferroelectric surfaces
Peter Maksymovych 1 , Stephen Jesse 1 , Pu Yu 2 , Ramamoorthy Ramesh 2 , Arthur P.
Baddorf 1 , Sergei V. Kalinin 1
1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
2 Department of Physics and Dept. of Materials Science, University of California, Berkeley
Electron tunneling underlies the operation of numerous devices relevant to
information technology and has been proposed in futuristic applications for energy
harvesting and quantum computing. Replacing a conventional insulator in the tunnel
junction with electronically correlated materials can yield new types of electronic
functionality. In one such concept, dubbed ferroelectric tunneling, the tunneling barrier
height is controlled by the polarization of a ferroelectric oxide, enabling non-volatile
conduction states that can be switched with electric field. Although ferroelectric
tunneling has been a recent focus of a number of theoretical studies, aiming to unveil the
interfacial behaviors of the ferroelectric order parameter, a convincing experimental
demonstration of this phenomenon has been lacking. The key challenges are to find a
material system that simultaneously satisfies the dimensional constraints for tunneling
and ferroelectricity, as well as to assure that the conductance is not dominated by
extrinsic effects due to charge injection and filamentary conduction, which is ubiquitous
in complex oxides.
In this talk we will demonstrate a highly reproducible polarization control of local
electron transport through thin Pb(Zr0.2Ti0.8)O3 film. Despite being 30 nm thick,
conductive atomic force microscopy revealed that the film possessed spatially and
temporally reproducible local conductivity. Fowler-Nordheim electron tunneling was
identified as the conductance mechanism, likely enabled by strong electric field in the
sub-surface region (excess of 10 6 V/cm) created by the relatively sharp metal tip. Local
conductance exhibited strong hysteresis depending on the probed bias range. Using a
newly-developed combination of piezoresponse force and conductive atomic force
microscopy, we have, for the first time, directly correlated local events of ferroelectric
and resistive switching [1]. The large polarization of the studied film resulted in as high
as 500-fold enhancement of the FN-tunneling conductance upon ferroelectric switching,
sufficient to demonstrate a local non-volatile memory function.
The physical mechanism of the observed effect was traced to the polarizationdependence
of the height and possibly width of the metal-ferroelectric Schottky barrier.
Our measurements have thus extended a well-known polarization dependence of surfacepotential
on ferroelectric materials from Kevlin probe force microscopy to demonstrate
local control of electron transport through such materials. Variable-temperature
measurements and local effects due to dielectric non-linearities will also be discussed.
[1] P. Maksymovych, S. Jesse, P. Yu, R. Ramesh, A. P. Baddorf, S. V. Kalinin, Science (2009) in review.
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