Session H32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Conducting Domain Walls and Conduction Mechanisms

Conduction of topologically-protected charged ferroelectric domain walls

We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO$_3$ protected by the topology of multiferroic vortices using \textit{in situ} conductive atomic force microscopy, piezo-response force microscopy, and kelvin-probe force microscopy at low temperatures. Conductance spectra reveal that only negatively charged tail-to-tail walls, in contrast to positively charged head-to-head walls, exhibit ohmic-like conduction in addition to Schottky-like rectification. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.   

Electrical dressing of domain walls in hexagonal ErMnO$_3$

Domain walls are natural mobile nanoscale objects that can exhibit structural, physical, and chemical properties which drastically differ from the surrounding bulk material. This applies to a large variety of phenomena including chemical/electrical transport, multiferroicity, or superconductivity. Unfortunately, in contrast to bulk materials, very little is known about involved length scales and control parameters when it comes to domain walls and experimental evidence is highly desirable. Here, we report on electrical dressing of trimerization-polarization walls in ErMnO$_3$. Using piezoforce-response microscopy and conductive atomic force microscopy we reveal that two characteristic length scales are to be distinguished: A first one corresponding to the structural / ferroelectric changes occurring at the wall and a second one referring to the associated electric properties. Furthermore, we demonstrate the response of the electrically dressed walls to external electric fields and develop a model that explains this response. Our results are expected to generally apply to domain walls in ferroelectric semiconductors and provide new insight into the interplay of charge and lattice degrees of freedom at domain walls.   

Anisotropic conductance at improper ferroelectric domain walls

Domain walls in ferroelectric oxides hold great potential for the development of new device paradigms in oxide nanoelectronics due to their field-tunable functionality. They are also of fundamental interest for studies of ferroic and low-dimensional systems physics. We investigate the electronic conductance of ferroelectric domain walls in the improper ferroelectric ErMnO3. We show that the conductance is a continuously tunable function of the domain wall orientation, with a range of an order of magnitude. We explain the observed behavior using first-principles density functional and phenomenological theories, and relate it to the extraordinary stability of the inherent head-to-head and tail-to-tail domain walls in hexagonal manganites, which is a direct consequence of the improper ferroelectric character of these materials.   

Conduction at domain walls in insulating Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ thin films

Ferroic domain walls are intrinsically nanoscale and often present functional properties beyond those of their parent material. One of the most striking examples is the recent discovery of electrical conduction\footnote{Seidel et al., Nat. Mat. {\bf8}, 229 (2009)} at domain walls in multiferroic BiFeO$_3$. Different scenarios have been proposed to explain the observed conduction, generally relating it to the complex nature of domain walls specific to BiFeO$_3$.\footnote{Lubk et al., PRB {\bf80}, 104110 (2009); Chiu et al., Adv. Mat. {\bf23}, 1530 (2011); Farokhipoor et al., PRL {\bf107}, 127601 (2011)} Here, we report on scanning probe microscopy studies of domain-wall-specific conduction in thin films of tetragonal ferroelectric (PZT). Our measurements show nonlinear asymmetric current-voltage characteristics with strong thermal activation at $T>150$ K. Moreover, the average current signals remain stable over the duration of measurement (up to four days). In light of recent transmission electron microscopy measurements at 180$^{\circ}$ domain walls in PZT,\footnote{Jia et al., Sci. {\bf331}, 1420 (2011)} we discuss the possible conduction mechanisms, highlighting the role of electrode asymmetry and microscopic domain wall structure promoting local defect segregation.   

Interplay between polarization and conductivity in BiFeO$_{3}$ thin films

BiFeO$_{3}$ (BFO) is a rhombohedrally distorted, ferroelectric, antiferromagnetic perovskite and one of the few room temperature multiferroics. We've previously reported on conduction at 71$^{o}$ domain walls in BFO thin films grown on SrRuO$_{3}$-buffered SrTiO$_{3}$ substrates. For clarifying the origin of conductivity in domain/domain walls, the conduction mechanisms have been extensively studied. The large current regime is determined by Schottky emission from the tip. The migration of oxygen vacancies to the domain walls lowers the Schottky barrier heights at the interface with the metallic tip compared to that in the domains, which results in the observed difference of conductivity in domains and domain walls. In this work we investigate the tunability of the conductivity upon changes in the electrode's work function, as well as the interplay between polarization and conductivity in BFO thin films.   

Tunable Metallic Conductivity in Ferroelectric Nanodomains

Domain wall conductivity in ferroelectric and multiferroic oxides is an essential example of new electronic properties created by topological defects. So far electron transport through domain walls in canonical BiFeO$_{3}$ and PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT) ferroelectrics has been dominated by thermally activated hopping, concealing the enabling physics and limiting potential applications. We will present a pioneering observation of metallic conductivity in nanoscale ferroelectric domains in PZT, that unambiguously identifies a new conduction channel created through the bulk of the oxide film [1]. From a corollary theoretical analysis, we conclude that metallic conductance is enabled by the interplay of charging and flexoelectric effects at tilted and curved walls of the nanodomain. Furthermore, both type and density of carriers can be tuned by manipulation of the order parameter. Thus, a new family of electronic properties may be found in multiferroic and topologically nanostructured complex oxides. [1] Maksymovych et al, \textit{Nano Lett}. in review (2011). Research conducted at the Center for Nanophase Materials Sciences, sponsored by BES, U. S. DOE.   

Ultrafast p-d charge-transfer carrier dynamics of multiferroic BiFeO3

We report first comprehensive understanding of ultrafast carrier dynamics in bulk single crystal BiFeO$_{3}$. From a wavelength dependent optical pump-probe measurement, we find that the photoexcited carriers relax to the bottom of band through electron-phonon coupling with a $\sim$1 ps time constant that does not significantly change across the antiferromagnetic transition. Following relaxation, carriers leave the conduction band or original excited electronic configuration and decay via radiative recombination, which is supported by our photoluminescence spectroscopy, reported for the first time.   

Photocurrent effect of epitaxial tetragonal-like BiFeO$_{3}$ thin film

Photovoltaic effect in ferroelectrics has recently received many attentions due to potential applications related to optoelectronic devices and solar cells. Here we report photocurrent effect of highly elongated ``tetragonal-like'' BiFeO$_{3}$ thin films grown on LaAlO$_{3}$ (001) substrates using pulsed laser deposition technique. Spatially resolved photocurrent measurements are performed with varying photon wavelength and polarization. Being combined with local ferroelectric domain structure by piezoresponse force microscopy, the spatially resolved techniques make a pathway to explore inter-relation between electric polarization and photon polarization. This study might deepen our understating of light induced conduction phenomena in ferroelectrics.   

Intrinsic defects in BiFeO3: Energetics and implication for magetism

We investigate energetics of the intrinsic defects in bulk multiferroic BiFeO3 and explore their implication for magnetization in this compound using a first-principles approach based on density functional theory. We find that dominant defects in oxidizing conditions are Bi and Fe vacancies and in reducing conditions are O and Bi vacancies. When enforcing charge neutrality, the calculated carrier concentration shows that the BiFeO3 grown in oxidizing conditions has p-type conductivity. The conductivity decreases with oxygen partial pressure and the material becomes insulating with tendency for n-type conductivity. We find that the Bi and Fe vacancies produce a magnetic moment of $\sim $1 $\mu $B and $\sim $5 $\mu $B per vacancy, respectively, for p-type BiFeO3 and none for insulating BiFeO3. O vacancies do not introduce any moment for both p-type and insulating BiFeO3. Calculated magnetic moments due to intrinsic defects are consistent with those reported experimentally for the bulk BiFeO3, however do not explain the large magnetization observed in some experiments on thin-film BiFeO3.   

Dielectric screening enhanced Hall mobility in doped ferroelectrics

A low electron mobility is the key limitation that prevents widespread device applications of complex oxide materials. However, in some perovskites, for example SrTiO$_3$ and KTaO$_3$, high mobilities in excess of 10,000 cm$^2$ s$^{-1}$ V$^{-1}$ are measured. Together with this dramatic increase in mobility as temperature is lowered, their dielectric constants also increase from a few hundred at room temperature to near 20,000 at low temperatures, suggesting a correlation between the dielectric constant and the mobility. By using electron-doped ferroelectric crystals of composition KTa$_{1-x}$Nb$_x$O$_3$, where the ferroelectric transition temperature can be tuned by changing the Ta:Nb ratio, we demonstrate an enhancement of the Hall mobility by a factor of 2-3 at the Curie temperature up to room temperature. We conclude that the mobility in these doped ferroelectrics peaks at the Curie temperature due to the increased dielectric constant, which reduces charge carrier scattering by impurities. Enhanced mobility could result in faster oxide transistors, boost the performance of thermoelectric devices, and enable more efficient photovoltaic materials. Supported by ORNL's LDRD program (W.S., H.M.C., V.C., G.E.J.), U.S. DOE, BES, MSED (M.A.M., B.C.S.) and SUFD (M.D.B., I.N.I.).   

The persistence of ferroelectric distortions in electron-doped BaTiO3: microscopic origins and critical behavior

To explore possible novel applications of the prototypical ferroelectric oxides we perform theoretical studies of electron-doping in BaTiO3. The presence of conduction electrons in a ferroelectric opens the possibility of bi-stable behavior directly in a conducting material which may lead to new functionalities. It is known, however, that conduction electrons screen the long range Coulomb interactions responsible for polar instabilities. Interestingly though, our first-principle density functional calculations reveal that ferroelectric distortions can persist in electron-doped BaTiO3 up to 0.01 e/unit cell, consistent with experimental results [1], suggesting that ferroelectricity and conductivity can coexist. To elucidate the competition between the long range Coulomb interactions and the short range bonding effects we have developed an adequate electrostatic model. Using this model, we reproduce the polarization vs. doping behavior obtained from first-principles and derive an analytical expression for the critical doping above which ferroelectric distortions disappear. [1] T. Kolodiazhnyi et al, Phys. Rev. Lett. 104, 147602 (2010).   

Multiphysics model of semiconducting ferroelectrics and its application to memory devices

Ferroelectrics are used in many electronic devices, in particular as transistors for ferroelectric memory devices. The behavior of these materials are often described via the classic time-dependent Ginzburg Landau model, where they are treated as insulators. However, it is well known that ferroelectrics are in fact wide band-gap semiconductors. It then follows that capturing the key aspects of semiconductor physics--band bending at the interface, Fermi levels, depletion layers, require ferroelectrics to be treated as semiconductors. In this work, we introduce a model that addresses these difficulties, yet at the same time is consistent with both the time-dependent Ginzburg Landau model and the classic drift-diffusion model in semiconductors. Unlike other models, our model makes no a priori assumptions on the space charge and polarization distributions and is not restricted to equilibrium profiles. We first demonstrate that charge carriers migrate to neutralize electric fields across 90\r{ } domain walls. Finally, we attempt a full simulation of a ferroelectric transistor and model current flow, electric field and polarization distributions.   

Conduction mechanism in BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ columnar nanostructure

Multiferroic materials, which possess interaction between more than one ferroic ordering parameters, had attracted great scientific and technological interests. Among the bi-phase magneto-electric nanostructures, BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ (BFO-CFO) is a model system with ferroelectricity and ferrimagnetism coupling to each other through stress mediation. In this study, we investigated the electron transport behavior and the leakage-current mechanism in high quality nano-composite BFO-CFO thin films. The CFO nanopillars were heteroepitaxially embedded in a BFO matrix grown on SrTiO$_{3}$ substrates. Macroscopic vertical transport result showed the interface limit model was the dominant mechanism of the large leakage. Local conduction in epitaxial BFO-CFO nanostructures was studied by conducting atomic force microscope (C-AFM) while the nature of band structure variation was demonstrated by scanning tunneling microscope (STM). This study provides a basic explanation of leakage mechanism in self-assembled composite material system.   

Current-Controlled Negative Differential Resistance Due to Joule Heating In Tio$_2$

We show that Joule heating causes current-controlled negative differential resistance (CC-NDR) in TiO2 memristive systems by constructing an analytical model of the current-voltage characteristics based on polaronic transport for Ohm's law and Newton's law of cooling and fitting this model to experimental data. This threshold switching is he ``soft breakdown'' observed during electroforming in TiO2 and other transition-metal oxide based memristors, as well as a precursor to ``ON'' or ``SET'' switching of unipolar memristors from their high to their low resistance states. The shape of the V-I curves is a sensitive indicator of the nature of the polaronic conduction, which apparently follows an adiabatic regime [1]. \\[4pt] [1] A.S. Alexandrov, A.M.Bratkovsky, B.Bridle, S.E.Savel'ev, D. Strukov, and R.S.Williams, Appl. Phys. Lett. 99, xxx (2011).   

Physical and Electrical Characterization of HfO$_{2}$, HfSiO$_{4}$, and ZrSiO$_{4}$ Memristors Based on Sol-Gel Synthesis

To date, most memristive devices have been fabricated by using TiO$_{2}$ or TaO$_{x}$ dielectric films. In order to explore the possible advantages of other high-$\kappa$ dielectrics in memristive devices, memristors were fabricated with HfO$_{2}$, HfSiO$_{4}$, and ZrSiO$_{4}$ layers synthesized from sol-gels. X-ray photoelectron spectroscopy measurements are consistent with reported spectra of HfO$_{2}$, HfSiO$_{4}$, and ZrSiO$_{4}$ films, but contain significant amounts of carbon. The films also have low densities and are flat, as measured by vacuum ultraviolet spectroscopic ellipsometry and optical profilometry measurements, respectively. This flat morphology is different from previous solution-processed dielectric films that exhibited rough surfaces [1]. Transmission electron microscopy measurements were also used to characterize these dielectric films. Current-voltage measurements indicate that, despite the contamination, the memristors exhibit nonvolatile bipolar resistive switching. The retention times measured for these memristors are $\sim$10$^{6}$ s. Capacitance and conductance measurements of these memristors indicate differences between the ON and OFF states, which will be discussed further. \\ \newline [1] J.L. Tedesco, et al., ECS Trans. $\textbf{35}$, 111 (2011).