Dr. Cristina CHIRILA

Ferroelectric-ferromagnetic heterostructures with well-defined polarization orientation are the focus of many research studies owing to their interesting interface-driven phenomena such as magnetoelectric coupling. In most practical electronic applications, capacitor geometry is often used, and in the case of ferroelectric-ferromagnetic heterostructures this can bring additional challenges regarding the overall functionality due to the physical phenomena from the ferroelectric-electrode interface. In this study, it is presented the influence of the top and bottom electrode on the electrical properties of Pb(Zr0,2Ti0,8)O3-La1-xSrxMnO3/SrTiO3(001) epitaxial heterostructures. This was done by growing the thin films with different layer stacking sequences by changing the Sr doping level from the bottom electrode. It was found that both the ferroelectric polarization orientation and tetragonality of the PZT films were significantly affected by the layer stacking sequence and Sr doping level of the bottom electrode. The ferroelectric polarization was oriented either towards or away from the Pb(Zr0,2Ti0,8)O3-La1-xSrxMnO3 interface depending on the layer stacking sequence, and the tetragonality increased when the Sr doping increases from x = 0.3 to x = 0.33. The materials used as the top electrode were Pt and Au/SrRuO3. Electric measurements performed in capacitor geometry show that the hysteresis curves start to be affected by leakage currents, which have a direct impact on the estimation of the ferroelectric polarization values and on the internal built in field. The most severely affected were the measurements performed with top Pt electrodes. The conduction mechanisms and leakage current values obtained by using the top Au/SrRuO3 electrode were found to be dependent on the Sr doping level, despite the fact that the electrical resistivity values and microstructures of the individual La1-xSrxMnO3 films were similar.

Despite the promise of high-k dielectrics, inherent limitations persist in transistor scaling and enhancing energy efficiency, including a fundamental threshold of 60 mV/dec for increasing drain current by an order of magnitude. Proposed solutions involve negative capacitance at the gate oxide to overcome this barrier using ferroelectric structures. Efforts to understand and regulate the switching dynamics and intricate electrostatic configurations of ferroelectric structures towards achieving negative capacitance regimes have intensified. While standalone ferroelectric capacitors cannot stabilize negative capacitance without external fields, multilayered thin films offer a promising solution. Typically, ferroelectric layers are paired with dielectrics/insulator, demonstrating steady-state negative capacitance, often at nanoscale or specific temperature domains. This study aims to stabilize negative capacitance in ferroelectric structures by inducing internal electric fields, aligning the system near coercivity, particularly in bilayer structures formed by two ferroelectric layers with slight differences in polarization values, such as p-n heterojunctions using Pb (Zr,Ti)O3 PZT) with different doping as Fe, Nb, Bi. Most of these structures exhibit evident amplification of capacitance compared to the equivalent series-connected capacitance, across a large temperature domain. The complex capacitance-frequency characteristic of these structures indicates a complex equivalent circuit. Analysis of these complex circuits compared with simple component layers concludes that at least one of the FE layers in these bilayer structures is in a negative capacitance (NC) state.

Carbon monoxide (CO) is reversibly adsorbed on and desorbed from ferroelectric (001) oriented, BaO-terminated barium titanate. All processes are characterized in real time via photoelectron spectroscopy. Adsorption proceeds on different sites/geometries as a function of substrate temperature. Below room temperature, CO is adsorbed on surface Ba. At room temperature, adsorption proceeds on surface oxygen, whereas at high temperatures, "hollow" site adsorption occurs with carbon coordinated with three oxygens, one oxygen initially belonging to CO and two oxygens from the substrate. The amount of CO adsorbed is about one molecule for 10 surface unit cells, which is slightly increased at low temperatures. CO is desorbed if the substrate is heated above the Curie temperature, which is a sign of the definitory role of ferroelectric polarization. The BaTiO3(001) surface is unaffected by repeated cycles of adsorption-desorption.

Pb(Zr,Ti)O-3 (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2Ti0.8)O-3 using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and LaxSrMn1-xO3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Probing of the free surface ferroelectric properties of thin polar films can be achieved either by estimating the band bending variance under the top-most layer or by studying the extent of the extrinsic charge accumulated outside the surface. Photoemitted or incoming low-energy electrons can be used to characterize locally both properties in a spectromicroscopic approach. Thin ferroelectric lead zirco-titanate (PZT) is investigated by combining low energy/mirror electron microscopy (LEEM/MEM) with photoemission electron microscopy (PEEM) and high-resolution photoelectron spectroscopy (XPS). Significant extrinsic negative compensation charge is proven to accumulate on the surface of the outward polarized thin film, indicated by high MEM-LEEM transition values, up to 15.3 eV, and is correlated with the surface electrostatic potential, which can be partially screened either by electrons interacting with the sample or by soft X-rays through the ejection of secondary electrons and generation of positive charge under the surface. A radiation-induced surface charge compensation effect is observed. The study indicates that air-exposed high quality ferroelectric thin films show large negative surface potentials, determined locally on the surface, which are nevertheless sensitive to beam damage and molecular desorption. These values represent a confirmation of previously estimated surface potential energy values determined from the LEED data on clean surfaces.

The p-n junctions are the building blocks of nowadays electronic devices. The n- or p-type conductivity is obtained in classic semiconductors, like Si, by doping with atoms acting as donors or acceptors, respectively. Doping was used in ferroelectrics to influence the transition temperature, magnitude of some physical properties, but not necessarily conduction type. Therefore, comprehensive studies to obtain true ferroelectric p-n junctions by controlled doping are missing. Recently, it has been shown that Pb(Zr0.2Ti0.8)O-3 films doped with & AP;1% atomic Nb (n-type doping) or Fe (p-type doping) have different orientations of polarization in the as-grown state. Knowing that polarization orientation depends on doping type, the next step is to build ferroelectric p-n homojunctions and to study their properties in relation to ferroelectric polarization. p-n and n-p structures were grown for this purpose by successive deposition of Nb-doped and Fe-doped Pb(Zr,Ti)O-3 layers with different thicknesses. We find that these p-n homojunctions are ferroelectric, but the magnitude of the polarization and coercive field, as well as the dominant polarization orientation in the as-grown state, depend on the conduction type of the first grown layer. The I-V characteristics are quasi-linear, although the interfaces with the electrodes behaves as Schottky contacts. The resistance extracted from the I-V characteristics displays an exponential dependence on temperature, with an activation energy in the range of 0.14-0.17 eV. These results are explained assuming that the total current in the junction is the total of electron and hole injections at the electrode interfaces. It is shown that for relatively low doping concentrations, the current density contains a dominant term with a linear voltage dependence and an exponential temperature dependence, as observed experimentally, and a secondary (correction) term that is dependent on the free carrier density and can induce non-linear voltage dependence when this density is significant.

Complex ferroelectric structures with dielectric interlayers may become possible alternatives for neuromorphic computing and low-power field-effect transistors since they exhibit multiple polarization states and negative capacitance. However, the effects on the switching characteristics due to the electric properties of the nonferroelectric circuit element have not been clearly evaluated so far. A high-resistance or low-capacitance element is usually associated with an increased depolarization field and eventually with suppression of polarization but without further consideration of the electrostatic differences. Therefore, we show that switching behavior is dramatically changed if the non-FE element is a resistive component or a capacitive one. This is reflected by either an increased apparent coercive field or imprint, respectively. A negative capacitance regime was observed at different moments but strongly depends on the nature of the nonferroelectric element. The voltage on the ferroelectric component remains constant during switching, which is a fingerprint of the system passing through non-equilibrium states. Therefore, we propose an algorithm to recover the S-shape of polarization dependence on the ferroelectric internal voltage during the slowed transition between the two stable states of polarization.

Fe (acceptor) and Nb (donor) doped epitaxial Pb(Zr0.2Ti0.8)O-3 (PZT) films were grown on single crystal SrTiO3 substrates and their electric properties were compared to those of un-doped PZT layers deposited in similar conditions. All the films were grown from targets produced from high purity precursor oxides and the doping was in the limit of 1% atomic in both cases. The remnant polarization, the coercive field and the potential barriers at electrode interfaces are different, with lowest values for Fe doping and highest values for Nb doping, with un-doped PZT in between. The dielectric constant is larger in the doped films, while the effective density of charge carriers is of the same order of magnitude. An interesting result was obtained from piezoelectric force microscopy (PFM) investigations. It was found that the as-grown Nb-doped PZT has polarization orientated upward, while the Fe-doped PZT has polarization oriented mostly downward. This difference is explained by the change in the conduction type, thus in the sign of the carriers involved in the compensation of the depolarization field during the growth. In the Nb-doped film the majority carriers are electrons, which tend to accumulate to the growing surface, leaving positively charged ions at the interface with the bottom SrRuO3 electrode, thus favouring an upward orientation of polarization. For Fe-doped film the dominant carriers are holes, thus the sign of charges is opposite at the growing surface and the bottom electrode interface, favouring downward orientation of polarization. These findings open the way to obtain p-n ferroelectric homojunctions and suggest that PFM can be used to identify the type of conduction in PZT upon the dominant direction of polarization in the as-grown films.

Photoelectron spectromicroscopy data cubes with sub-micrometer spatial resolution from a lead zirconate titanate thin film are fitted in each spatial point for several areas investigated, and correlations between the results of the fitting parameters are established, enabling one to suggest that surface band bending manifests also in the case of samples with multiple polarization states. These surface band bendings may be used for preliminary assessment of polarization states by using the contrast in binding energy. However, the individual curve fitting of each spectrum and the analysis of correlations between the derived parameters (binding energies and amplitudes) yields results of deeper significance in deriving the distribution of band bending. This allows in some cases discriminating between the origin of the individual components, assessing charging effects and compensation mechanisms, also enabling one to establish correlations between areas with different polarization. It is found that the major part of the investigated areas consist in regions with outwards polarization and without out-of-plane polarization, with the presence of some isolated metal Pb clusters. On a single area, components attributed to inwards polarization are detected, and their formation occurs at the expense of areas without out-of-plane polarization, mostly for a given proportion of the outwards polarization.

Magnetic perovskite films have promising properties for use in energy-efficient spintronic devices and magnetic refrigeration. Here, an epitaxial ferromagnetic La0.67Ba0.33Mn0.95Ti0.05O3 (LBMTO-5) thin film was grown on SrTiO3(001) single crystal substrate by pulsed laser deposition. High-resolution X-ray diffraction proved the high crystallinity of the film with tetragonal symmetry. The magnetic, magnetocaloric and magnetoresistance properties at different directions of the applied magnetic field with respect to the ab plane of the film were investigated. An in-plane uni-axial magnetic anisotropy was evidenced. The LBMTO-5 epilayer exhibits a second-order ferromagnetic-paramagnetic phase transition around 234 K together with a metal-semiconductor transition close to this Curie temperature (T-C). The magnetic entropy variation under 5 T induction of a magnetic field applied parallel to the film surface reaches a maximum of 17.27 mJ/cm(3) K. The relative cooling power is 1400 mJ/cm(3) K (53% of the reference value reported for bulk Gd) for the same applied magnetic field. Giant magnetoresistance of about 82% under 5 T is obtained at a temperature close to T-C. Defined as the difference between specific resistivity obtained under 5 T with the current flowing along the magnetic easy axis and the magnetic field oriented transversally to the current, parallel and perpendicular to the sample plane, respectively, the in-plane magneto-resistance anisotropy in 5 T is about 9% near the T-C.

Polarization switching in ferroelectric films is exploited in many applications, such as non-volatile memories and negative capacitance field affect transistors. This can be inhomogeneous or homogeneous, depending on if ferroelectric domains are forming or not during the switching process. The relation between the polarization switching, the structural quality of the films and the negative capacitance was not studied in depth. Here, Pb(Zr0.2Ti0.8)O-3 (PZT) layers were deposited by pulse laser deposition (PLD) and sol-gel (SG) on single crystal SrTiO3 (STO) and Si substrates, respectively. The structural quality was analyzed by X-ray diffraction and transmission electron microscopy, while the electric properties were investigated by performing hysteresis, dynamic dielectric measurements, and piezo-electric force microscopy analysis. It was found that the PZT layers grown by PLD on SRO/STO substrates are epitaxial while the layers deposited by SG on Pt/Si are polycrystalline. The polarization value decreases as the structure changes from epitaxial to polycrystalline, as well as the magnitude of the leakage current and of the differential negative capacitance, while the switching changes from homogeneous to inhomogeneous. The results are explained by the compensation rate of the depolarization field during the switching process, which is much faster in epitaxial films than in polycrystalline ones.

The discovery of multifunctional properties related to electro-activity of organic systems of biomolecules is important for a variety of applications, especially for devices in the realm of biocompatible sensors and/or bioactuators. A further step towards such applications is to prepare thin films with the required properties. Here, the investigation is focused on the characterization of films of guanine and cytosine nucleobases, prepared by thermal evaporation-an industrial accessible deposition technique. The cytosine films have an orthorhombic non-centrosymmetric structure and grow in two interconnected nanostructured fractal patterns, of nearly equal proportion. Piezoresponse force microscopy images acquired at room temperature on the cytosine films display large zones with antiparallel alignment of the vertical components of the polarization vector. Guanine films have a dense nano-grained morphology. Our studies reveal electrical polarization switching effects which can be related to ferroelectricity in the films of guanine molecules. Characteristic ferroelectric polarization-electric-field hysteresis loops showing large electrical polarization are observed at low temperatures up to 200 K. Above this temperature, the guanine films have a preponderant paraelectric phase containing residual or locally induced nano-scopic ferroelectric domains, as observed by piezoresponse force microscopy at room temperature.

Structural and electrical properties of epitaxial Pb(Zr0.2Ti0.8)O-3 films grown by pulsed laser deposition from targets with different purities are investigated in this study. One target was produced in-house by using high purity precursor oxides (at least 99.99%), and the other target was a commercial product (99.9% purity). It was found that the out-of-plane lattice constant is about 0.15% larger and the a domains amount is lower for the film grown from the commercial target. The polarization value is slightly lower, the dielectric constant is larger, and the height of the potential barrier at the electrode interfaces is larger for the film deposited from the pure target. The differences are attributed to the accidental impurities, with a larger amount in the commercial target as revealed by composition analysis using inductive coupling plasma-mass spectrometry. The heterovalent impurities can act as donors or acceptors, modifying the electronic characteristics. Thus, mastering impurities is a prerequisite for obtaining reliable and reproducible properties and advancing towards all ferroelectric devices.

Epitaxial La0.7Sr0.3MnO3 films with different thicknesses (9-90 nm) were deposited on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The films have been investigated with respect to morpho-structural, magnetic, and magneto-transport properties, which have been proven to be thickness dependent. Magnetic contributions with different switching mechanisms were evidenced, depending on the perovskite film thickness. The Curie temperature increases with the film thickness. In addition, colossal magnetoresistance effects of up to 29% above room temperature were evidenced and discussed in respect to the magnetic behavior and film thickness.

A simple dynamic method was developed to evaluate the components in the equivalent circuit of a ferroelectric capacitor. The method is based on the application of short trapezoidal voltage pulses of variable amplitude and the analysis of the resulting current by considering the ferroelectric capacitor as a parallel R-p-C-p equivalent circuit. The values of R-p and C-p components are obtained in relation to different stages of polarization switching as the amplitude of the applied pulses increases. The most important result obtained by applying the present method is the evidence of an abrupt decrease of the R-p value (about 2 orders of magnitude) at the coercive voltage, while the equivalent C-p does not present a dramatic variation during polarization switching. This is interpreted in the context of negative capacitance obtained for ferroelectrics when polarization passes through zero value. The results obtained by using the proposed dynamic characterization method that will be referred to as "dynamic dielectric characterization", are in good agreement with those obtained by classic capacitance-voltage measurements. A method to stabilize the negative capacitance for longer periods of time is also presented by adding an external resistance in series with the ferroelectric capacitor.

Properties of epitaxial PbZr0.2Ti0.8O3 (PZT) films deposited on Si substrates were investigated for integration in the present CMOS technology. Polarization is downward oriented, in association with the presence of an internal electric field, and has a lower value compared to the PZT films deposited on single crystal perovskite SrTiO3 (STO) substrates (40 mu C/cm(2) versus 80 mu C/cm(2)), while the dielectric constant is larger (180 versus 120). Large value for the pyroelectric coefficient was also found, 1.22 x 10(-3)C/m(2)K, as for PZT grown on single crystal STO. The macroscopic ferroelectric and pyroelectric properties appear to be affected by the structural quality and stoichiometry of the PZT film. The changes in the electric properties are an effect of the strain gradients induced by the large difference between the thermal expansion coefficients of PZT and Si substrate, leading in turn to Pb oxidation and antisite defect formation compared to PZT films deposited on STO substrates.

Gold is deposited on atomically clean, inwards polarized, ferroelectric lead zirco-titanate deposited by pulsed laser deposition on strontium titanate (001) single crystal, then carbon monoxide adsorption and desorption experiments are investigated by in situ fast photoelectron spectroscopy using synchrotron radiation. Atomic force microscopy and high resolution photoelectron spectroscopy are consistent with the formation of 50?100 nm nanoparticles, and their Au 4f core levels point to a negative charge state of gold. As compared with a similar experiment performed on ferroelectric lead zirco-titanate with similar polarization state and without gold, the saturation coverage after exposure to carbon monoxide increases by about 68 %, and also most of the additional carbon is found in oxidized state. Desorption experiments with in situ follow-up by photoelectron spectroscopy are performed as function of temperature, and the neutral carbon intensity decreases when the ferroelectric polarization decreases, while the components corresponding to oxidized carbon remain unchanged. It looks that neutral carbon adsorption is strictly related to the polarization of the ferroelectric film, while carbon still found in molecular form is related to its carbonyl bonding on metal nanoparticles, independent of the polarization state of the substrate. Desorbed carbon at higher temperature uptakes oxygen from the substrate.

We explore the cross coupling between the ferroelectric and ferromagnetic phases in Ni/BaTiO3(001) heterostructures and demonstrate the modulation of the magnetism and incidence of exchange bias in the ultrathin metallic Ni overlayer, depending on the ferroelectric state of the bottom layer. We establish that 5-nm-thick monocrystalline Ni film deposited on BaTiO3 with ferroelectric polarization pointing towards the surface (P+) favors the organization of Ni into uniform ferromagnetic domains. Ni grown on BaTiO3 with opposite ferroelectric polarization is featured by emerging exchange-bias coupling between the ferromagnetic Ni top layers and the antiferromagnetic reacted interface, as theoretically explained by first-principles calculations. We explicitly obtain the morphology of the magnetic domains of the crystalline Ni layer in atomic and magnetic force microscopy measurements (AFM/MFM). The resemblance of AFM and MFM images indicate that, although with radically different morphologies, in both cases all spins orient in the Ni plane. Consequently, the distinct signature of the ferroelectric-ferromagnetic coupling extracted from the magneto-optical Kerr effect measurements encodes all the information of sample magnetism. The peculiar magnetic coupling depending on the ferroelectric state indicates new ways of engineering the functionality of metal/ferroelectric interfaces.

The emerging field of electronics based on ferro-functional materials relies on driving effectively and predictably a ferroelectric system between different polarization states through bias applied to metallic contacts. This requires detailed understanding of the growth mechanisms and electronic properties of the interface, including ferroelectric and material - dependent band alignment and Schottky barrier heights. Whether the major contribution at the interface band alignment comes from the work function difference or from the ferroelectric state is still under debate. Here, using X-ray photoemsion and ab-initio calculations, we derive the complex microscopic picture of metal/ferroelectric interface formation, including growth mechanism, valence alteration, ferroelectric-dependent electrostatic potential and thickness - dependent compensation mechanisms of ferroelectricity, starting from the ultrathin growth of Cu up to 100 angstrom on BaTiO3. One establishes the evolution of the band bending and of the build-in potential from the initial probed thickness of the ferroelectric in the range of 3 lambda (lambda - the inelastic mean free path) while gradually approaching the contact region with the metal at higher thickness of the top layer. We find that the well-defined orientation of the ferroelectric polarization lead to a band bending at the interface, which add at the bending expected from the work function difference of the two joining materials.

Ba0.6Sr0.4TiO3 (BST) ferroelectric thick films were grown on MgO(001) and Al2O3 (0 0 0 1) single-crystal substrates by using a pulsed laser deposition method. Structural, morphological, optical, and terahertz characterization of the BST films were performed by X-ray Diffraction, Atomic Force Microscopy, Spectroscopic Ellipsometry (SE), and Terahertz Time-Domain Spectroscopy (THz-TDS). Single-phase samples with strong preferred (1 1 1) orientation and surface roughness lower than 1.5% of their thicknesses have been obtained for both types of substrates. SE was employed to extract the thickness and optical properties by using a 3-layer optical model (substrate/thin film/roughness). The inferred refractive index @630 nm is around 2.05, while the optical interference is visible until 3.3 eV. The THz-TDS measurements in transmission set-up were carried out one after the other on substrates before and after the BST film deposition. The standard THz-TDS analysis of double-layer samples proved difficult to complete in the cases in which a thin or thick film is deposited on a much thicker substrate of known dielectric properties. However, we have been able to extract the complex dielectric permittivity in the THz domain for BST samples with thicknesses of few microns, by developing a specific procedure of data analysis.

The negative-capacitance effect in devices based on combined ferroelectric-dielectric gate oxides is thought to be a potential solution to break free from the so-called Boltzmann tyranny. To lower the power consumption in field-effect transistors, the subthreshold swing factor S should be reduced below the ther-modynamic limit of 60 mV per decade. Yet, despite numerous studies dedicated to this effect in the past decade, its origin in ferroelectric capacitors or ferroelectric-based superlattices remains unclear, being considered either a transitory product of polarization switching or an intrinsic phenomenon related to the presence of ferroelectric polarization. In this study it is shown, starting from simple electrostatic con-siderations, that negative capacitance is present during polarization switching and is accompanied by a significant increase of the current flowing through the ferroelectric capacitor. Coupled with piezo-force microscopy results, it is shown that the polarization orientation suddenly changes at the coercive voltage, accompanied by a complete reconfiguration of the potential barriers at the Schottky-like contacts present at the electrode-ferroelectric interfaces. A method to estimate the polarization-switching time, as the time associated with the presence of the negative-capacitance effect, is proposed. Values in the range from 100 to 1000 ns are obtained for epitaxial PbZr0.2Ti0.8O3 films. These findings suggest that negative capacitance may be an intrinsic effect in ferroelectrics but that it is a transitory effect, present only when ferroelectric polarization passes through zero (switching).

Carbon layers are deposited on 100 nm thick atomically clean (001) lead zirconate titanate (PZT) in ultrahigh vacuum, ruling out the presence of any contaminants. X-ray photoelectron spectroscopy is used to assess the substrate surface or interface composition, substrate polarization and the thickness of carbon layers, which ranges from less than one monolayer (1 ML) of graphene to several monolayers. Atomically clean PZT(001) exhibit inwards polarization, and this polarization reverses the sign upon carbon deposition. Cationic vacancies are detected near the PZT surface, consistent with heavy p doping of these films near the surface. The carbon layers exhibited a consistent proportion of atoms forming in-plane sp(2) bonds, as detected by near-edge absorption fine structure (NEXAFS) analysis and confirmed partially by scanning tunneling microscopy (STM). In situ poling with simultaneous in-plane transport measurements revealed the presence of resistance anti-hysteresis versus the polarization orientation for films with less than 1 ML carbon amount, evolving towards 'normal' hysteresis for thicker carbon films. The anti-hysteresis is explained in terms of a mixed screening mechanism, involving charge carriers from the sp(2) carbon layers together with holes or ionized acceptors in PZT(001) near the interface. For thicker films, the compensation mechanism becomes extrinsic, involving mostly electrons and holes from carbon, yielding the expected hysteresis.

Manganese is deposited at high temperature on (001) oriented ferroelectric lead zirco-titanate prepared in two different ways: sputter-annealed or just simply annealed in ultrahigh vacuum. Room temperature ferromagnetism (FM) is obtained for Mn deposited on sputter-annealed substrates, while for the other sample preparation a paramagnetic behaviour is obtained. Also, for the first case a clear inwards component of the polarization P( - ) is observed by X-ray photoelectron spectroscopy and piezoresponse force microscopy. Composition analysis evidenced formation of Pb vacancies in the case of FM - P(( - ) )sample, consistent with hole formation near the surface, needed both to stabilize the inwards polarization state and to intermediate ferromagnetism between Mn2+ ions. The indirect exchange ferromagnetism mediated by holes is stronger, most probably because the interaction energy is proportional with the carrier effective mass. Also, whereas in the case of unsputtered substrate a stable surface Mn oxide is formed, defect formation by sputtering seems to favor Mn migration inside the sample. This also induces the formation of a thin film where ferromagnetism and the orientation of ferroelectric polarization might have the same origin, i. e. holes accumulated near the outer surface.

The competition between interface barrier in the Schottky-Mott limit and polarization driven mechanism is established during gradual formation of metal (Au) - ferroelectric (BaTiO3) interface. X-ray photoelectron spectroscopy provides core level energies and valence band positions in the contact region, to monitor the band alignment from the very first stages of metal deposition on BaTiO3. The band bending at metal/ferroelectric (FE) interface is extracted from the shift of core levels (Ba 3d, Ti 2p) as a function of the metal thickness. It is shown that the interface band alignment mechanism involves a well-defined polarization orientation washing out the bending expected from the work function difference. The sudden modification of the binding energies within ferroelectric at the first 2 angstrom Au indicates that the ferroelectric compensation mechanism triggered by the metal overlayer initiates already at ultrathin top layer, while subsequent growth contributes only at a gradual correction of the potential in the FE. The emerging picture is confirmed in first-principle calculation indicating the preferences of Au to grow preferentially to different terminated regions and to stabilize distinct ferroelectric polarization.

A carbon-based layer was deposited by spraying on top of a ferroelectric layer grown by sol-gel on Si (001) substrate and its properties as electrode and absorber for pyroelectric detection were tested. It was found that the electric properties of the ferroelectric capacitor with top carbon-based sprayed electrode (CBSE) are comparable with those of the capacitors with standard top SrRuO3 (SRO)/Au electrode. Pyroelectric measurements show that the pyroelectric signal recorded on ferroelectric capacitors with top CBSE electrode is 2.5 times greater than for top SRO/Au electrode for low frequency range. The value of the pyroelectric coefficient was estimated to 9.73.10(-4) C/m(2)K for CBSE electrodes and 3.36.10(-4) C/m(2)K for SRO/Au respectively. The fabrication process of CBSE is of low cost, easy to implement and with high throughput making it attractive for manufacturing various devices like pyroelectric detector, thermal imaging, solar cells, etc.

Multiple nonvolatile and well-separated capacitive states can be obtained in a two-terminal ferroelectric capacitor setup by fine tuning the polarization switching process. This approach allows for the implementation of memcomputing (same platform for storage and computing) capable ferroelectric structures. Digital and analog storage modes are exemplified in this work together with an algorithm for simple binary computation functions such as OR/NOR and AND/NAND for data processing on the same device. Results are obtained by controlling the polarization switching process in ferroelectric multi-layers such as Pb (Zr0.2Ti0.8)O-3/SrTiO3/Pb (Zr0.2Ti0.8)O-3 and Pb (Zr0.2Ti0.8)O-3/BaTiO3/Pb (Zr0.2Ti0.8)O-3. Besides memcomputing, these results can be used for nondestructive capacitive reading of information in simple ferroelectric capacitors or can open the way toward applications such as neuromorphic and chaotic circuits.

Ferroelectrics are intensively studied materials due to their unique properties with high potential for applications. Despite all efforts devoted to obtain the values of ferroelectric material constants, the problem of the magnitude of static dielectric constant remains unsolved. In this article it is shown that the value of the static dielectric constant at zero electric field and with negligible contribution from the ferroelectric polarization (also called static background dielectric constant, or just background dielectric constant) can be very low (between 10 and 15), possibly converging towards the value in the optical domain. It is also found that the natural state of an ideal, mono-domain, epitaxial ferroelectric is that of full depletion with constant capacitance at voltages outside the switching domain. The findings are based on experimental results obtained from a new custom method designed to measure the capacitance-voltage characteristic in static conditions, as well from Rayleigh analysis. These results have important implications in future analysis of conduction mechanisms in ferroelectrics and theoretical modeling of ferroelectric-based devices.

By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface. (C) 2017 Elsevier B.V. All rights reserved.

Investigations of barium strontium titanate (BST) layers deposited on MgO and Si substrates are presented. Since the Sr content determines the dielectric and optical properties of the BST layers at room temperature, accurate compositional analysis was performed by using Rutherford Backscattering technique at 3.041 Mev.

Here we report a ferroelectric capacitor structure obtained by alternating ferroelectric and insulator thin-film layers which allows an increase of up to 2(n) polarization states, with n the number of ferroelectric layers. Four and up to eight distinct, stable and independently addressed polarization states are experimentally demonstrated in this work. The experimental findings are supported by a theoretical model based on the Landau-Ginzburg-Devonshire theory. The key parameter is the change in the strain conditions of ferroelectric layers induced by the insulating separator. Notably, the 2(n) increase in the storage capacity can be achieved without major changes in the present technology used for FeRAM devices. The test structures demonstrate very good memory characteristics such as retention and fatigue, opening the way towards the design of high density ferroelectric memories.

The stability of thin films of lead zirco-titanate (PZT) under intense soft X-ray beams is investigated by time-resolved photoelectron spectromicroscopy with a lateral resolution below 1 micrometer. Surface dissociation is observed when samples are irradiated with intense (5 x 10(23) photons per s per m(2)) soft X-rays, with promotion of reduced lead on the surface. On areas exhibiting outwards polarization (P(+)), the reduced lead is formed at the expense of P(+)-PZT. On areas presenting co-existing P(+) states with areas without out-of-plane polarization (P-(0)), the reduced lead is formed at the expense of the P-(0)-PZT component, while the P(+)-PZT remains constant. The main dissociation mechanism was found to be triggered by 'hot' electrons in the conduction band, with energies exceeding the surface dissociation energies. Dissociation occurs basically when the electron affinity is larger than the dissociation energy of PbO (for P(+) areas) or PbO- (for P-(0) areas). Such mechanisms may be adapted for dissociation of other molecules on surfaces of ferroelectric thin films or for quantifying the stability of ferroelectric surfaces interacting with other radiation, with applications in the fields of photocatalysis or photovoltaic devices.

We have developed a thin film structure with a maximum magnetoresistance effect (MRE) at room temperature, which is one of the operating requirements for many applications. It is shown that La0.67Ba0.33Ti0.02Mn0.98O3 epilayers obtained by pulsed laser deposition onto (001) SrTiO3 single crystal substrates exhibit the highest MRE, Delta R/R(H) approximate to 150% or Delta R/R(0) approximate to 60% under 5 T, at 300 K, a temperature near to the corresponding Curie temperature (T-C). Both doping with a tiny amount of titanium and induced stress due to lattice mismatch between the thin film and the substrate contribute to a decrease in T-C as compared to the pristine compound and therefore to the decrease in the temperature where the highest MRE is recorded. Published by AIP Publishing.

A hysteresis loop with three polarization states is obtained in the case of a symmetric epitaxial ferroelectric-interlayer-ferroelectric structure with bottom and top SrRuO3 electrodes. The ferroelectric layers are of PbZr0.2Ti0.8O3, while the interlayer is CoFe2O4. It is shown that the three polarization states can be separately accessed, suggesting that this type of structure can be used as building element for a three-state nonvolatile ferroelectric random-access memory (FERAM). The presence of the three-state memory effect is explained through a simple phenomenological model based on Landau-Ginzburg-Devonshire theory. The findings of this study can pave the way to multistate all-oxide FERAM devices, resulting in a 50% increase in the storage density compared to actual nonvolatile memories.

Epitaxial La0.67Ba0.33Ti0.02Mn0.98O3 (denoted as LBTMO hereafter) thin films of approximately 95 nm thickness were deposited by a pulsed laser deposition technique onto SrTiO3 (STO) (001) substrates. High-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM) investigations revealed that the films are epilayers with a four-fold symmetry around the [001] direction. Cross-sectional TEM and the presence of Pendellosung fringes in the XRD profiles demonstrate smooth interfaces. The STO substrate induces an in-plane compressive strain, which leads to a slight tetragonality of the film structure. The epilayers exhibit paramagnetic-to-ferromagnetic phase transitions at the Curie temperature T-C (286 K), close to room temperature. The magnetization easy axis lies in the film plane along the [100] direction of the (001) substrate. The magnetic entropy change (Delta S-M) associated with the second-order magnetic phase transition was determined via magnetization measurements in the temperature range between 210 and 350 K under different magnetic fields. The relative cooling power (RCP) of this film is about 220 J kg(-1), somewhat lower than that of bulk Gd (410 J kg(-1)) for a 50 kOe field change, making the LBTMO ferromagnetic thin films a promising candidate for micro/nanomagnetic refrigeration around room temperature. The proposed universal curve provides a simple method for extrapolating Delta S-M in a wide range of fields and temperatures, thus confirming the order of the magnetic transition in this system. The magnetic entropy (Delta S-M)(max) around T-C is proportional to (mu H-0/T-C)(2/3) in agreement with the mean-field theory, indicating the existence of long-range ferromagnetic interactions in epitaxial LBTMO thin films.

Ferroelectric/electric properties of PbZr0.52Ti0.48O3 (PZT) thin films grown by pulsed laser deposition (PLD) on two different substrates, Si (001) and SrTiO3 (STO) (001), were comparatively analyzed. The structural characterization has revealed the epitaxial relationship between the grown layers and the two types of substrates, with larger density of structural defects for the films deposited on Si (001) with buffer STO layer. The ferroelectric/electric properties are also different, with lower remnant polarization (about half of the value obtained on STO substrate), higher dielectric constant (about two times larger), and lower leakage current (about two orders of magnitude lower) for the PZT films deposited on Si (001) compared to those deposited on (001) STO substrates. Nevertheless, the results show that the use of a STO buffer layer on Si can be a solution to obtain good quality PZT capacitor structures without using expensive single-crystal oxide substrates. In this way, applications based on PZT capacitors (e.g. non-volatile memories, pyroelectric detectors, light switches, etc.) would be more easily integrated directly on Si wafers.

Interfaces formed by gold and copper on single crystal layers of (0 0 1) PbZr0.2Ti0.8O3 (PZT) produced by pulsed laser deposition and exhibiting outwards polarization are analyzed by X-ray photoelectron spectroscopy. The stoichiometry of the layers reproduces reasonably that of the PZT target. The band bending occurring at the interface between PZT and the metals is investigated by analyzing the core level shifts as function on the metal deposition. It is found that for Au/PZT(0 0 1) the gold layer is not continuous and the observed band bendings can be attributed to a Schottky mechanism, whereas for Cu/PZT(0 0 1) the copper layer is continuous; in this latter case, the observed band bendings towards higher energies (lower binding energies) can be attributed to a concomitant bending due to the Schottky effect together with the disappearance of the initial bending due to the outwards polarization of the samples. Metal Pb is observed to segregate only in the case of Cu/PZT(0 0 1), therefore the surface self-reduction might also be connected to the presence of a metal with lower work function, which for larger coverage forms a continuous metal layer, able to provide electrons to the surface. High resolution transmission electron spectroscopy yielded the disappearance of the tetragonal distortion in the case of Cu/PZT(0 0 1), in line with the assumption of disappearance of the polarization-induced band bending. (C) 2015 Elsevier B.V. All rights reserved.

The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O-3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by selfdoping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 10(26) m(-3) for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.

Electrical properties of ferroelectric capacitors based on PbZr0.52Ti0.48O3 thin films grown by pulsed laser deposition on silicon substrate with SrTiO3 buffer layer grown by molecular beam epitaxy were studied. A SrRuO3 layer was deposited as bottom electrode also by pulse laser deposition and Pt, Ir, Ru, SrRuO3 were used as top contacts. Electrical characterization comprised hysteresis and capacitance-voltage measurements in the temperature range from 150 K to 400 K. It was found that the macroscopic electrical properties are affected by the electrode interface, by the choice of the top electrode. However, even for metals with very different work functions (e.g. Pt and SrRuO3) the properties of the top and bottom electrode interfaces remain fairly symmetric suggesting a strong influence from the bound polarization charges located near the interface. (C) 2015 Elsevier B.V. All rights reserved.

The effects of the bonding mechanism and band alignment in a ferroelectric (FE) BaTiO3/ferromagnetic La0.6Sr0.4MnO3 heterostructure are studied using x-ray photoelectron spectroscopy and first-principles calculations. The band lineup at the interface is determined by a combination of band bending and polarization-induced modification of core-hole screening. A Schottky barrier height for electrons of 1.22 +/- 0.17 eV is obtained in the case of downwards FE polarization of the top layer. The symmetry of the bonding states is emphasized by integrating the local density of states +/- 0.2 eV around the Fermi level, and strong dependence on the FE polarization is found: upwards, polarization stabilizes Ti t(2g) (xy) orbitals, while downwards, polarization favors Ti t(2g) (yz) symmetry. It is predicted that the abrupt (La, Sr)vertical bar TiO2 interface is magnetoelectrically active, leading to a A-type antiferromagnetic coupling of the first TiO2 interface layer with the underlying manganite layer through a superexchange mechanism.

The leakage current was studies in epitaxial ferroelectric Pb(Zr0.52Ti0.48)O-3 layer with common SrRuO3 bottom electrode and different metals as top contacts (SrRuO3, Pt, Ir, Ru). It was found that the dominant conduction mechanism in the 200-350 K temperature range and for voltages significantly larger than the coercive value is the thermionic emission governed by the Schottky-Simmons equation. The height of the potential barriers was estimated and was found that this is about the same for negative and positive voltage polarities. No correlation was found between the height of the potential barriers for different top contacts and the work function difference between the bottom and top electrodes. The results suggest that the potential barrier is controlled by the polarization charges in a similar way to the one reported for Pb(Zr0.2Ti0.8)O-3 and BaTiO3 epitaxial films with bottom SrRuO3 electrode and different metals as top contacts. It was also found that above 350 K the conduction mechanism changes to ohmic and/or space charge limited currents.

Electrode interface is a key element in controlling the macroscopic electrical properties of the ferroelectric capacitors based on thin films. In the case of epitaxial ferroelectrics, the electrode interface is essential in controlling the leakage current and the polarization switching, two important elements in the read/write processes of nonvolatile memories. However, the relation between the polarization bound charges and the electronic properties of the electrode interfaces is not yet well understood. Here we show that polarization charges are controlling the height of the potential barriers at the electrode interfaces in the case of Pb(Zr,Ti)O-3 and BaTiO3 epitaxial films. The results suggest that the height is set to a value allowing rapid compensation of the depolarization field during the polarization switching, being almost independent of the metals used for electrodes. This general behavior open a new perspective in engineering interface properties and designing new devices based on epitaxial ferroelectrics.

In this paper, we analyze the structural distortions observed by transmission electron microscopy in thin epitaxial SrRuO3 layers used as bottom electrodes in multiferroic coatings onto SrTiO3 substrates for future multiferroic devices. Regardless of the nature and architecture of the multilayer oxides deposited on the top of the SrRuO3 thin films, selected area electron diffraction patterns systematically revealed the presence of faint diffraction spots appearing in forbidden positions for the SrRuO3 orthorhombic structure. High-resolution transmission electron microscopy (HRTEM) combined with Geometric Phase Analysis (GPA) evidenced the origin of these forbidden diffraction spots in the presence of structurally disordered nanometric domains in the SrRuO3 bottom layers, resulting from a strain-driven phase transformation. The local high compressive strain (-4% divided by -5%) measured by GPA in the HRTEM images induces a local orthorhombic to monoclinic phase transition by a cooperative rotation of the RuO6 octahedra. A further confirmation of the origin of the forbidden diffraction spots comes from the simulated diffraction patterns obtained from a monoclinic disordered SrRuO3 structure. (C) 2014 AIP Publishing LLC.

The band bending at Cu/PZT(001) interfaces is investigated by X-ray photoelectron spectroscopy (XPS) for a PZT(001) layer which exhibits initial outwards ferroelectric polarization. Two competitive processes are identified: (a) formation of the Schottky barrier between the ferroelectric and unconnected Cu islands, and (b) coalescence of the Cu islands, realisation of an electrical contact to the ground of the system, inducing the apparent loss of the component of the ferroelectric polarization perpendicular to the sample surface, at least as it manifests in band bending. Three mechanisms are proposed to explain this loss of band bending when a full metal layer connected to ground is formed on the surface: (i) over-compensation of depolarization field in the sub-surface region, (ii) formation of domains with in-plane orientation of the polarization vector and (iii) loss of polarization in the near-surface layers of the ferroelectric due to electrons provided by the metal. These result in a non-monotonous variation of binding energies with the amount of Cu deposited. High resolution transmission electron microscopy and piezoresponse force microscopy confirmed these hypotheses. The XPS data allowed also to derive the surface PZT composition, its evolution with the deposition of copper and the formation of surface compounds.

The energy band alignment at interfaces between different materials is a key factor, which determines the function of electronic devices. While the energy band alignment of conventional semiconductors is quite well understood, systematic experimental studies on oxides are still missing. This work presents an extensive study on the intrinsic energy band alignment of a wide range of functional oxides using photoelectron spectroscopy with in-situ sample preparation. The studied materials have particular technological importance in diverse fields as solar cells, piezotronics, multiferroics, photoelectrochemistry and oxide electronics. Particular efforts have been made to verify the validity of transitivity, in order to confirm the intrinsic nature of the obtained band alignment and to understand the underlying principles. Valence band offsets up to 1.6 eV are observed. The large variation of valence band maximum energy can be explained by the different orbital contributions to the density of states in the valence band. The framework provided by this work enables the general understanding and prediction of energy band alignment at oxide interfaces, and furthermore the tailoring of energy level matching for charge transfer in functional oxides. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Lead-free piezoelectric (Bi0.5Na0.5)(0.92)Ba0.08TiO3, doped with 5 mol %BiFeO3 (BNT-BT-BFO) nanowires was prepared from its corresponding precursor sol. A polycarbonate membrane with the thickness of 30 mu m and the pore diameter of 100 nm was used as template. Nanowires with average diameter of similar to 80 nm and different lengths of a few microns were prepared from BNT-BT-BFO precursor sol with similar to 0.03 M concentration. Ferroelectric and piezoelectric characterizations of BNT-BT-BFO nanowires were performed using AFM-PFM equipment. The HR-TEM micrograph of nanocrystals, SAED patterns, and XRD pattern indicated that BNT-BT-BFO nanowires crystallized on the lattice of rhombohedral (Na0.5Bi0.5TiO3) phase. The BNT-BT-BFO nanowires showed a significant piezoelectric response, suggesting a good piezoelectric behavior. The saturation magnetization of the nanowires, measured at room temperature and at 5 kOe magnetic field, was Ms = 0.061 emu/g. A saturated ferromagnetic hysteresis loop has been also obtained.

Europium doped zinc oxide (Eu:ZnO) thin films have been obtained by pulsed laser deposition (PLD). 002 textured thin films were achieved on glass and silicon substrates, while hetero-epilayers and homo-epilayers have been attained on single crystal SrTiO3 and ZnO, respectively. X-ray Diffraction (XRD) was employed to characterize the Eu:ZnO thin films. Extended XRD studies confirmed the different thin film structural properties as function of chosen substrates.

This work presents a systematic investigation by X-ray photoelectron spectroscopy of the mechanisms of interface formation and band bending for Au/Pb(Zr,Ti)O-3 (PZT) layers grown on SrTiO3(001) with a SrRuO3 buffer layer, as function on the initial state of the PZT surface. After isolating the chemical effects, such as the formation of metal Pb at some surfaces, the evolution of the core levels with Au deposition allows one to simultaneously investigate the Schottky barrier formation and the built-in potential effects (charging induced by the static ferroelectric polarization). Areas of the sample with outwards P(+) and no polarization perpendicular to the surface P-(0) are identified for all samples. Only the freshly prepared sample exhibited inward polarization areas P(-). The built-in potential is on the order of 0.9 eV, while the Schottky band bending ranges from 0.2 to 0.6 eV towards lower absolute energies, therefore indicating that the work function of PZT exceeds that of Au deposited. We report also a chemically differentiate value of the built-in potential, manifested by a preferential distribution of the charge accumulated at the surface on Ti and O atoms. The O 1s and Ti 2p core levels manifest quite strong variations with the Au thickness for freshly prepared samples, resulting in shifts on the order of 2 eV towards lower binding energies. Au deposited on areas with an outward polarization is positively charged by the same potential as atoms from the PZT film (0.8-0.9 eV), whereas Au deposited on areas with an inward polarization forms a continuous grounded layer, which progressively pumps the accumulated charge and removes the polarization of these areas. (C) 2013 Elsevier B. V. All rights reserved.

The growth of gold layers on Pb(Zr,Ti)O-3 (PZT) deposited on SrTiO3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2-100 angstrom. Two phases are identified, with compositions close to nominal PZT. The 'standard' phase is represented by all binding energies (Pb 4f, Ti 2p, Zr 3d, O 1s) sensibly equal to the nominal values for PZT, whereas the 'charged' phase exhibits all core levels are shifted by similar to 1 eV toward higher binding energies. By taking into account also scanning probe microscopy images together with recent photoemission results, the 'charged' phase belongs to P(+) regions of PZT, whereas the 'normal' phase corresponds to regions with no net ferroelectric polarization perpendicular to the surface. Au deposition proceeds in a band bending of Phi(PZT) - Phi(Au) similar to 0.4-0.5 eV for both phases, identified as similar shifts toward higher binding energies of all Pb, Ti, Zr, O core levels with Au deposition. The Au 4f core level exhibits also an unusually low binding energy component 1 eV below the 'nominal' Au 4f binding energy position (metal Au). This implies the existence of negatively charged gold, or electron transfer from PZT to Au, although the 'normal' PZT phase have a higher work function, as it is derived from the band bending. Most probably this charge transfer occurs toward Au nanoparticles, which have even higher ionization energies. High resolution transmission electron microscopy evidenced the formation of such isolated nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.

The formation of an interface between Bi2O3, Fe2O3, BiFeO3, Bi0.5Na0.5TiO3, and the high work function metallic RuO2 is studied using photoelectron spectroscopy with in situ RuO2 deposition. Schottky barrier heights are derived and the valence band maximum energies of the studied materials are aligned with respect to each other as well as to other functional oxides like SrTiO3 and PbTiO3. The energy band alignment follows systematic trends compared to a large number of oxides, and can be understood in terms of the contribution of Fe 3d and Bi 6s/6p (lone pair) orbitals to electronic states near the valence band maximum. The results indicate that the valence band maxima are largely determined by the local environment of the cations, which allows to estimate valence band maximum energies of oxides with multiple cations from those of their parent binary compounds. The high valence band maximum of BiFeO3 is consistent with reported p-type conduction of acceptor doped material, while the high conduction band minimum makes n-type conduction unlikely.

Barium titanate-substituted bismuth titanate ((Bi0.5Na0.5)(0.92)Ba0.08TiO3, BNT-BT0.08) nanotubes were fabricated using sol-gel chemistry, spin casting, and a porous polycarbonate membrane template. The structure and morphology of the tubes have been investigated by scanning electron microscopy and transmission electron microscopy. The diameter and length of these tubes were about 650 nm and 20 mu m, respectively, and their wall thickness was about 50 nm. The tubes were polycrystalline with average grain size of similar to 40 nm. The BNT-BT0.08 nanotubes exhibited ferroelectric behavior as evidenced by saturated piezoresponse hysteresis loops and phase switching. BNT-BT0.08 piezoelectric nanotubes promise novel device architectures and enhanced electric properties.

Lead-free piezoelectric (Bi0.5Na0.5)(0.92)Ba0.08TiO3, (abbreviated as BNT-BT0.08) wires were prepared using its corresponding precursor sol and, a polycarbonate template membrane. The polycarbonate membrane used has a thickness of 30 mu m and pore diameter of 300 nm. Uniform surface morphology of the wires with average diameter of 145 nm and a length of about 20 mu m was obtained when the concentration of the sol was 0.3 M. The ferroelectric and piezoelectric properties of an individual BNT-BT0.08 wire were investigated using the PFM technique. The hysteresis loops between the PFM phase and DC bias and, the characteristic butterfly loops of the PFM amplitude versus DC bias, indicated a ferroelectric and piezoelectric behavior of as-obtained BNT-BT0.08 wires.

Ba(X1/3Ta2/3)O-3 (X = Mg, Zn) thin films were grown on commercial Pt-coated Si substrates by radiofrequency plasma beam assisted pulsed laser deposition (RF-PLD) method. Single phase Ba(Mg1/3Ta2/3)O-3 and Ba(Zn1/3Ta2/3)O-3 ceramic targets having an ordered hexagonal structure (with P (3) over bar m1 space group) were used for deposition. Structural, morphological and surface characterizations of the Ba(X1/3Ta2/3)O-3 (BXT) films were performed using X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray diffraction studies evidenced a cubic (disordered) perovskite structure, with unit cell typical for Pm3m space group. Scanning electron microscopy investigations showed that BXT films have a columnar microstructure, which is oriented perpendicular to the substrate. The temperature dependence of the dielectric permittivity of the films was recorded at 100 kHz. Unlike targets, the BXT films exhibit positive values of the temperature coefficient of the dielectric permittivity. BaMg1/3Ta2/3O3 and BaZn1/3Ta2/3O3 thin films with dielectric constant of about 22.5 and 25, respectively have been obtained. (C) 2012 Elsevier B.V. All rights reserved.

Electric and magnetic properties of symmetric and asymmetric Pb(Zr-0.2 Ti-0.8)O-3-CoFe2O4 (PZT/CFO) heterostructures, grown by pulsed laser deposition on SrTiO3 (100) substrates with a 25 nm SrRuO3 (SRO) buffer layer as bottom electrode, were investigated by using hysteresis and capacitance measurements. X-ray diffraction, and transmission electron microscopy investigations reveal the high quality crystalline structure and the epitaxial relationship between SRO, PZT and CFO. The electric polarization-voltage hysteresis reveals that the remnant polarization and the coercive field are significantly affected by the CFO layer. The frequency dependence of capacitance suggests a Maxwell-Wagner type relaxation at low frequencies and is also affected by the presence of the PZT/CFO interface(s). The magnetic hysteresis measurements infer the possible presence of another spinel phase (Co3O4) in the CFO film, due to the lattice mismatch at the PZT/CFO interfaces, and with direct influence on the magnetic response of the structure. According to the electric and magnetic characterization, better room temperature multiferroic properties would be expected for the symmetric heterostructure. (C) 2013 Published by Elsevier B. V.

Metal-ferroelectric-metal structures based on epitaxial Pb(Zr0.2Ti0.8)O-3 thin films are prepared by pulsed laser deposition on single crystal SrTiO3 substrates ((001) orientation) with buffer SrRuO3 layer as bottom electrode. Pt, Cu, and SrRuO3 are used as top contacts. The current-voltage (I-V) measurements reveal a strong influence of the top electrode interface on the magnitude of the leakage current and the shape of the I-V characteristics. The lowest current values are obtained for top Cu and the highest for top Pt. Diode-like behavior is obtained for top Cu and Pt, but the forward and reverse biases are opposite in sign. Contrary to the case of BiFeO3 layers deposited on the same type of substrates, it was found that the diode-like behavior is not switchable with the polarization reversal although the polarization values are comparable. It is also shown that the metal-ferroelectric-metal (MFM) structure based on Pb(Zr,Ti)O-3 (PZT) can be simulated and modeled as a back-to-back connection of two Schottky diodes. The diode-like behavior of the MFM structure can be induced by a slight asymmetry of the potential barriers at the electrode interfaces behaving as Schottky contacts. The study ends with a critical discussion of the MFM structures based on PZT and BiFeO3 (BFO) layers. It is shown that the switchable diode-like behavior is not uniquely determined by the polarization reversal and is not a general characteristic for MFM structures. Such behavior may be present only if the polarization induced band-bending at the interface is generating an accumulation layer at the interface. This could be possible in BiFeO3 based MFM structures due to the lower band gap compared to Pb(Zr0.2Ti0.8)O-3 thin films. (C) 2013 AIP Publishing LLC.

The hysteretic properties of metal-ferroelectric-semiconductor (MFS) structures based on Pb(Zr0.2Ti0.8)O-3 (PZT) and ZnO films were studied with respect of the quality of the PZT-ZnO interface. The films were grown by pulsed laser deposition (PLD) on platinized silicon (Pt/Si) substrate and on single crystal, (001) oriented SrTiO3 (STO) substrates. The structural analysis has revealed that the PZT-ZnO stack grown on single crystal STO is epitaxial, while the structure grown on Pt/Si has columnar texture. The temperature change of the capacitance-voltage (C-V) hysteresis direction, from clockwise at low temperatures to counter clockwise at high temperatures, was observed at around 300K in the case of the MFS structure grown by PLD on Pt/Si substrate. This temperature is lower than the one reported for the case of the PZT-ZnO structure grown by sol-gel on Pt/Si substrate (Pintilie et al., Appl. Phys. Lett. 96, 012903 (2010)). In the fully epitaxial structures the C-V hysteresis is counter clockwise even at 100K. These findings strongly points out that the quality of the PZT-ZnO interface is essential for having a C-V hysteresis of ferroelectric nature, with negligible influence from the part of the interface states and with a memory window of about 5V at room temperature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765723]