195 research outputs found
Theoretical Approach to Electroresistance in Ferroelectric Tunnel Junctions
In this paper, a theoretical approach, comprising the non-equilibrium Green's
function method for electronic transport and Landau-Khalatnikov equation for
electric polarization dynamics, is presented to describe polarization-dependent
tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using
appropriate contact, interface, and ferroelectric parameters, measured
current-voltage characteristic curves in both inorganic
(Co/BaTiO/LaSrMnO) and organic (Au/PVDF/W)
ferroelectric tunnel junctions can be well described by the proposed approach.
Furthermore, under this theoretical framework, the controversy of opposite TER
signs observed experimentally by different groups in
Co/BaTiO/LaSrMnO systems is addressed by
considering the interface termination effects using the effective contact
ratio, defined through the effective screening length and dielectric response
at the metal/ferroelectric interfaces. Finally, our approach is extended to
investigate the role of a CoO buffer layer at the Co/BaTiO
interface in a ferroelectric tunnel memristor. It is shown that, to have a
significant memristor behavior, not only the interface oxygen vacancies but
also the CoO layer thickness may vary with the applied bias.Comment: 12 page
Polarization imaging in ferroelectric polymer thin film capacitors by pyroelectric scanning microscopy
A Pyroelectric Scanning Microscopy system, which uses laser-induced thermal modulation for mapping the pyroelectric response, has been used to image a bipolar domain pattern in a ferroelectric polymer thin film capacitor. This system has achieved a resolution of 660±28 nm by using a violet laser and high f-number microscope objective to reduce the optical spot size, and by operating at high modulation frequencies to reduce the thermal diffusion length. The results agree well with a thermal model implemented numerically using finite element analysis
Piezoresponse force microscopy for polarity imaging of GaN
The polarity distribution of GaN based lateral polarity heterostructures is investigated by piezoresponse force microscopy (PFM). Simultaneous imaging of surface morphology, as well as the phase and magnitude of the piezoelectric response, is performed by PFM on a GaN film with patterned polarities on a c-Al2O3 substrate. We demonstrate that the polarity distribution of GaN based lateral polarity heterostructures can be deduced from the phase image of the piezoresponse with nanometer scale spatial resolution
Domain structure and polarization reversal in ferroelectrics studied by atomic force microscopy
The ferroelectric domain structure and its dynamics under applied electric field have been studied with nanoscale resolution by atomic force microscopy (AFM). Two mechanisms responsible for the contrast between opposite domains are proposed: large built-in domains are delineated in friction mode due to the tip–sample electrostatic interaction, and small domains created by an external field are imaged in topography mode due to piezoelectric deformation of the crystal. The ability of effective control of ferroelectric domains by applying a voltage between the AFM tip and the bottom electrode is demonstrated. It is experimentally confirmed that the sidewise growth of domain proceeds through the nucleation process on the domain wall
Domain structure and polarization reversal in ferroelectrics studied by atomic force microscopy
The ferroelectric domain structure and its dynamics under applied electric field have been studied with nanoscale resolution by atomic force microscopy (AFM). Two mechanisms responsible for the contrast between opposite domains are proposed: large built-in domains are delineated in friction mode due to the tip–sample electrostatic interaction, and small domains created by an external field are imaged in topography mode due to piezoelectric deformation of the crystal. The ability of effective control of ferroelectric domains by applying a voltage between the AFM tip and the bottom electrode is demonstrated. It is experimentally confirmed that the sidewise growth of domain proceeds through the nucleation process on the domain wall
On the persistence of polar domains in ultrathin ferroelectric capacitors
The instability of ferroelectric ordering in ultra-thin films is one of the
most important fundamental issues pertaining realization of a number of
electronic devices with enhanced functionality, such as ferroelectric and
multiferroic tunnel junctions or ferroelectric field effect transistors. In
this paper, we investigate the polarization state of archetypal ultrathin
(several nanometres) ferroelectric heterostructures: epitaxial
single-crystalline BaTiO films sandwiched between the most habitual
perovskite electrodes, SrRuO, on top of the most used perovskite substrate,
SrTiO. We use a combination of piezoresponse force microscopy, dielectric
measurements and structural characterization to provide conclusive evidence for
the ferroelectric nature of the relaxed polarization state in ultrathin
BaTiO capacitors. We show that even the high screening efficiency of
SrRuO electrodes is still insufficient to stabilize polarization in
SrRuO/BaTiO/SrRuO heterostructures at room temperature. We identify
the key role of domain wall motion in determining the macroscopic electrical
properties of ultrathin capacitors and discuss their dielectric response in the
light of the recent interest in negative capacitance behaviour.Comment: 13 pages, 4 figure
Polarization imaging in ferroelectric polymer thin film capacitors by pyroelectric scanning microscopy
A Pyroelectric Scanning Microscopy system, which uses laser-induced thermal modulation for mapping the pyroelectric response, has been used to image a bipolar domain pattern in a ferroelectric polymer thin film capacitor. This system has achieved a resolution of 660±28 nm by using a violet laser and high f-number microscope objective to reduce the optical spot size, and by operating at high modulation frequencies to reduce the thermal diffusion length. The results agree well with a thermal model implemented numerically using finite element analysis
Domain wall saddle point morphology in ferroelectric triglycine sulfate
Ferroelectric domain walls, across which there is a divergence in polarization, usually have enhanced electrical conductivity relative to bulk. However, in lead germanate, head-to-head and tail-to-tail walls are electrically insulating. Recent studies have shown that this is because, when oppositely oriented domains meet, polar divergence is obviated by a combination of domain bifurcation and suspected local dipolar rotation. To explore the uniqueness, or otherwise, of this microstructure, we have used tomographic piezoresponse force microscopy to map three-dimensional domain morphologies in another uniaxial ferroelectric system: triglycine sulfate. This mapping reveals an abundance of domain wall saddle points, which are characteristic of interlocking bifurcated domains. Conducting atomic force microscopy, performed close to the saddle points, showed no evidence for highly localized conducting domain wall sections, across which a divergence in polarization might be implied; this supports the notion that localized dipolar rotation occurs to minimize any potential polar discontinuity. Overall, our study, therefore, confirms that mutual domain bifurcation and suspected local dipolar rotation are not unique to lead germanate and instead may be widely present in other uniaxial ferroelectrics
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