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Pb(Zr,Ti)O

Graded structures with different architectures were obtained by spark plasma sintering from (Ba1-xSrx)TiO3 (BST, x = 0.10; 0.20; 0.30) powders. The presence of the composition gradient was confirmed by structural and compositional investigations using X-ray diffraction and electron microscopy combined with energy dispersive X-ray spectroscopy. The concentration gradient was either asymmetric (3 layers, starting with x = 0.10 and ending with x = 0.30) or symmetric (5 or 6 layers, starting and ending with x = 0.10, and with a single or double x = 0.30 layer in the middle, respectively). Electrical measurements reveal a decrease of the dielectric constant with increasing the number of the layers. It was found that the symmetric graded structure with 6 layers has the best thermal stability of both, the dielectric constant (variation of only 8% between zero and 100 °C) and the pyroelectric coefficient (6% variation between zero and 80 °C). In addition, an enhancement of the pyroelectric signal for frequencies above 100 Hz is obtained in symmetric structures, an effect that is attributed to the additive contributions of the signals originated from the layers with different Sr content.

Single-phase Ba1-xSrxTiO3 (BST) perovskite ceramics with 0.3 ≤ x ≤ 0.4 were prepared from powders synthesized via sol-gel route. The compositions have the ferroelectric-paraelectric phase transition close to room temperature. At 20 °C the BST ceramics are ferroelectric for 0.3 ≤ x ≤ 0.35 and paraelectric for x = 0.375 and x = 0.40. The study follows the relation between the structural changes produced when increasing the Sr content and the dielectric properties at low intensity electric fields. It is found that the grain size and tetragonality decreases as the Sr content increases. Analyses of complex permittivity and impedance spectroscopy reveal the temperature and frequency dependencies of the dielectric properties. The phase transitions seem to be of first order for all compositions, with a thermal hysteresis that decreases with increasing the Sr content, fact attributed to the corresponding increase of the grain boundaries weight allowing a more efficient stress relaxation in the structure during the change of the symmetry from cubic to tetragonal. The diffusiveness degree during the phase transition is increasing with Sr content, suggesting some relaxor-type contribution attributed to smaller grain size. The ac conductivity follows the universal Jonscher law, with an ac component having the power parameter s independent of Sr content, and a dc component that it is thermally activated with an activation energy of about 0.7–0.77 eV attributed to oxygen vacancies acting as donor-like defects. The fit of impedance spectra at different temperatures and frequencies is obtained by using an equivalent circuit accounting the grains, grain boundaries, electrode interfaces and the local contributions produced by reorientation of defect dipoles or defect clusters. All the component circuits have significant variations around phase transitions. These are discussed in relation to structural changes occurring during transition and considering the changes in the distribution of various charges when polarization vanishes.

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.

Cu–Fe–Sn–S have been electrodeposited on indium tin oxide coated glass (ITO/glass) substrates, varying only the deposition time, followed by sulfurization in argon atmosphere at a temperature of 500 °C. X-ray diffraction patterns confirmed the formation of polycrystalline CFTS and other secondary phases. The Raman spectroscopy results confirm the formation of stannite phase, by the existence of the most intense peak at 330 cm−1 corresponding to A-symmetry vibrational mode, while the SnS2 surface phase reduces upon increasing deposition time. The inferred bandgaps by specular transmission are in 1.4–1.7 eV range, influenced by the detected orthorhombic Cu4SnS4 and rhodostannite secondary phases. The electrical measurements confirm the p-type nature of the films, while density of free carriers is relatively high (~ 1019 cm−3), leading to extremely low resistivity in the Ω cm range.

Fe (acceptor) and Nb (donor) doped epitaxial Pb(Zr

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.

Composites of magnetite (Fe3O4) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix were prepared by a molding process. Two types of samples were obtained by free polymerization with randomly dispersed particles and by polymerization in an applied magnetic field. The magnetite nanoparticles were obtained from magnetic micrograins of acicular goethite (α-FeOOH) and spherical hematite (α-Fe2O3), as demonstrated by XRD measurements. The evaluation of morphological and compositional properties of the PDMS:Fe3O4 composites, performed by SEM and EDX, showed that the magnetic particles were uniformly distributed in the polymer matrix. Addition of magnetic dispersions promotes an increase of thermal conductivity compared with pristine PDMS, while further orienting the powders in a magnetic field during the polymerization process induces a decrease of the thermal conductivity compared with the un-oriented samples. The shape of the magnetic dispersions is an important factor, acicular dispersions providing a higher value for thermal conductivity compared with classic commercial powders with almost spherical shapes.

The frequency and temperature dependence of dielectric properties of CH3NH3PbI3 (MAPI) crystals have been studied and analyzed in connection with temperature-dependent structural studies. The obtained results bring arguments for the existence of ferroelectricity and aim to complete the current knowledge on the thermally activated conduction mechanisms, in dark equilibrium and in the presence of a small external a.c. electric field. The study correlates the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes, as well as with the different transport regimes indicated by the Nyquist plots. The different energy barriers revealed by the impedance spectroscopy highlight the dominant transport mechanisms in different frequency and temperature ranges, being associated with the bulk of the grains, their boundaries, and/or the electrodes’ interfaces.

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.

Multi-layered structures, composed of thin films from materials with different compositions or physical properties, represents a way to obtain enhanced properties or even new functionalities. In this work, lead zirconate titanate PbZrxTi1-xO3 (PZT; x = 0.20, 0.52, 0.80) multilayers were grown by pulsed laser deposition (PLD) on a single crystal strontium titanate (SrTiO3, STO) substrate, using a strontium ruthenate (SrRuO3, SRO) film as buffer layer for epitaxial growth, and also as back electrode. Up and down multi-layers were grown and their physical and structural properties were compared, up being the structure in which Zr concentration was varied from 20% near the STO substrate to 80% at the surface, while down is for the structure in which the Zr concentration starts with 80% near the substrate and ends with 20% at the surface. It was found that the electric and pyroelectric properties of the two graded structures are significantly different. The up structure presents electric properties that are comparable with those of single composition PZT films while the properties of the down structure are deteriorated, especially in terms of the leakage current magnitude. Pyroelectric signal could be measured only for the up structure. These differences were attributed to larger density of structural defects in the down structure compared to the up one. This is due to the different growth sequence: up structure starts with tetragonal PZT on cubic substrate (lower lattice mismatch, 1.1%) while down structure starts with rhombohedral PZT on cubic substrate (larger lattice mismatch, almost 5%).

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.

Ferroelectric field effect transistors (FeFETs) based on lead zirconate titanate (PZT) ferroelectric material and amorphous-indium-gallium-zinc oxide (a-IGZO) were developed and characterized. The PZT material was processed by a sol-gel method and then used as ferroelectric gate. The a-IGZO thin films, having the role of channel semiconductor, were deposited by radio-frequency magnetron sputtering, at a temperature of ~50°C. Characteristics of a typical field effect transistor with SiO2 gate insulator, grown on highly doped silicon, and of the PZT-based FeFET were compared. It was proven that the FeFETs had promising performances in terms of Ion/Ioff ratio (i.e., 106) and IDS retention behavior.

Ba0.75Sr0.25TiO3 (BST) and PbZr0.68Fe0.14Nb0.14Ti0.04O3 (PZFNT) ceramic pellets were obtained by ceramic technology and their structural, ferroelectric and pyroelectric properties were investigated. The relative density of BST and PZFNT is about 93% and 90%, respectively, with an average grain size of 102 ?m and 6.45 ?m. Both materials have similar room temperature dielectric constants (~2000), but PZFNT shows higher remnant polarization (~15?C/cm2) and better pyroelectric properties (~1.69?10?4 C/m2K), which recommend it for pyroelectric detectors, infrared radiation- and laser pulse energy-meters.

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.

International audience; 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.

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 thermodynamic limit of $60\phantom{\rule{0.2em}{0ex}}\mathrm{mV}$ 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 considerations, 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 $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}$ 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).

Germanium Metal-Ferroelectric-Semiconductor (MFS) capacitors based on ferroelectric Hf1−xZrxO2 (HZO) with clean, oxide free Ge/HZO interfaces emerge as an interesting layer structure for the fabrication of ferroelectric field effect transistor (FeFET) non-volatile memory devices. It is shown that, at low temperature (

This work involves the synthesis and characterization of Cu2ZnSnS4 (CZTS) layers. The films were prepared on ITO/glass substrate by ecofriendly and simple single-step electrodeposition method followed by sulfurization and annealing at 500 °C under Argon flow. By using two different complexing agents, the electrodeposition process can give better results. Therefore, the effect of combining the trisodium citrate - TC to multiple complexing agents (cetyl trimethyl ammonium bromide - CTAB, ethylene diamine tetra acetic acid - EDTA, Boric Acid - BA, Glutamic Acid - GA and Tartaric Acid - TA) is investigated. The characterization of the absorber films was done by X-ray diffraction (XRD) analysis, Raman spectroscopy, Scanning Electron Microscopy and Diffuse Reflectivity. The combination of TC and CTAB is suggested to be the best pair of complexing agents within the combinations used in this work.

In this work, we report on the variability of the Schottky effect in solution processed Ba1-xSrxTiO3 films (BST, x = 0, 0.5) grown on 0.5% Nb doped SrTiO3 substrates with top Pt electrodes (NSTO/BST/Pt). The films display leakage currents accompanied by varying degrees of hystereses in the current-voltage measurements. The magnitude of the leakage and hystereses depend on the Sr content. We focus on the current-voltage (I-V) behavior of our samples in the light of thermodynamic theory of ferroelectrics coupled with equations of semiconductors. Our calculations allowed us to unambigously determine the electronic character of the defects and related band bending effects in our samples. The extent of asymmetry and the hystereses in the I-V curves for x = 0 and 0.5 are shown to be controlled by the polarization in qualitative agreement with our calculations. Amplitude of the ferroelectric polarization, which is a function of composition here, has a strong impact on leakage currents in forward bias while this effect is much weaker under negative bias. The latter occurs as polarization pointing away from the NSTO semiconducting substrate causes depletion of carriers at the NSTO side of the NSTO/BST interface, increasing resistance to current flow through the stack. Such an occurence also increases the energy gap between the Fermi level and the conduction bands of the films, thereby reducing the bulk conduction through the film as well. The dependence of leakage currents on polarization direction points out to the possibility of a non-destructive read-out route in ferroelectric films much thicker than tunnel junctions.

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.

Un-doped ZnO (UDZO) and Li-doped ZnO (LZO) polycrystalline thin films were grown on platinized silicon by pulsed laser deposition (PLD). The electrical properties were investigated on as-grown and annealed UDZO and LZO films with capacitor configuration, using top and bottom platinum electrodes. In the case of the as-grown films it was found that the introduction of Li increases the resistivity of ZnO and induces butterfly shape in the C-V characteristic, suggesting ferroelectric-like behavior in LZO films. The properties of LZO samples does not significantly changes after thermal annealing while the properties of UDZO samples show significant changes upon annealing, manifested in a butterfly shape of the C-V characteristic and resistive-like switching. However, the butterfly shape disappears if long delay time is used in the C-V measurement, the characteristic remaining non-linear. Pyroelectric signal could be measured only on annealed films. Comparing the UDZO results with those obtained in the case of Li:ZnO, it was found that the pyroelectric properties are considerably enhanced by Li doping, leading to pyroelectric signal with about one order of magnitude larger at low modulation frequencies than for un-doped samples. Although the results of this study hint towards a ferroelectric-like behavior of Li doped ZnO, the presence of real ferroelectricity in this material remains controversial.

The integration of ferroelectrics in perovskite solar cells is proposed as possible way to enhance charge collection efficiency. First results on solar cell manufactured with PbTiO3 (PTO) instead of TiO2 have shown negligible values for the power conversion efficiency (PCE). This is explained by the high serial resistance of sol-gel deposited PTO on F:SnO2 electrodes (FTO). Although PTO layer has remnant polarization of 22 μC/cm2, the high potential barrier (0.25 ± 0.05 eV) at the FTO/PTO interface and low carrier mobility (10−8 cm2 V−1 s−1) compared to TiO2 leads to high serial resistance. Better results were obtained with thinner PTO layers grown by pulsed laser deposition, with PCE values up to 0.6%. Further enhancement was obtained by replacing PTO with BaTiO3 (BTO), with PCE value reaching about 0.8% after poling the cell with +3 V. The most important finding was that the magnitude of the short circuit current increases with the amplitude of the poling voltage while the value of the open-circuit voltage remains about the same, around 0.9 V. This is explained through more efficient collection of the charges generated under illumination in the absorber layer due to the polarization that is present in the ferroelectric film. Keywords: Ferroelectric, Perovskite, Solar cell, Polarization