Surface charge dynamics on air-exposed ferroelectric Pb(Zr,Ti)O3(001) thin films.

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.

Experimental Band Structure of Pb(Zr,Ti)O3 : Mechanism of Ferroelectric Stabilization.

Pb(Zr,Ti)O3 (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(Zr0.2 Ti0.8 )O3 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 Lax SrMn1- x O3 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.

Magnetocaloric and Giant Magnetoresistance Effects in La-Ba-Mn-Ti-O Epitaxial Thin Films: Influence of Phase Transition and Magnetic Anisotropy.

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 (TC). 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/cm3 K. The relative cooling power is 1400 mJ/cm3 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 TC. 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 TC.

Controlling polarization direction in epitaxial Pb(Zr0.2Ti0.8)O3 films through Nb (n-type) and Fe (p-type) doping.

Fe (acceptor) and Nb (donor) doped epitaxial Pb(Zr0.2Ti0.8)O3 (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.

Influence of Thickness on the Magnetic and Magnetotransport Properties of Epitaxial La0.7Sr0.3MnO3 Films Deposited on STO (0 0 1).

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.

Homogeneous versus Inhomogeneous Polarization Switching in PZT Thin Films: Impact of the Structural Quality and Correlation to the Negative Capacitance Effect.

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)O3 (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.

Electro-active properties of nanostructured films of cytosine and guanine nucleobases.

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.

Accidental Impurities in Epitaxial Pb(Zr0.2Ti0.8)O3 Thin Films Grown by Pulsed Laser Deposition and Their Impact on the Macroscopic Electric Properties.

Structural and electrical properties of epitaxial Pb(Zr0.2Ti0.8)O3 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.

Resistance hysteresis correlated with synchrotron radiation surface studies in atomic sp2 layers of carbon synthesized on ferroelectric (001) lead zirconate titanate in an ultrahigh vacuum.

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 sp2 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 sp2 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.

Low value for the static background dielectric constant in epitaxial PZT thin films.

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.

Multiple polarization states in symmetric ferroelectric heterostructures for multi-bit non-volatile memories.

Here we report a ferroelectric capacitor structure obtained by alternating ferroelectric and insulator thin-film layers which allows an increase of up to 2n 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 2n 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.

Polarization landscape effects in soft X-ray-induced surface chemical decomposition of lead zirco-titanate, evidenced by photoelectron spectromicroscopy.

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 × 1023 photons per s per m2) 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.

Structural, magnetic and magnetocaloric effects in epitaxial La0.67Ba0.33Ti0.02Mn0.98O3 ferromagnetic thin films grown on 001-oriented SrTiO3 substrates.

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 TC (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 (ΔSM) 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 ΔSM in a wide range of fields and temperatures, thus confirming the order of the magnetic transition in this system. The magnetic entropy (ΔSM)max around TC is proportional to (µ0H/TC)2/3 in agreement with the mean-field theory, indicating the existence of long-range ferromagnetic interactions in epitaxial LBTMO thin films.

Polarization induced self-doping in epitaxial Pb(Zr0.20Ti0.80)O3 thin films.

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)O3, 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 self-doping 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.

Polarization-control of the potential barrier at the electrode interfaces in epitaxial ferroelectric thin films.

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)O3 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.