Self-polarization and migratory polarization in thin lead zirconate-titanate films

This paper reports on a study of the electrical properties of 0.7–1-μm-thick textured PZT ferroelectric films prepared by rf magnetron sputtering of a PbZr_0.54Ti_0.46O₃ target which additionally contained 10 mol % lead oxide. Such films are shown to feature a combination of a self-polarized state and migratory polarization. The totality of the data obtained suggest that the films had n-type conduction. As shown by the laser beam modulation technique, the polarization was distributed nonuniformly in depth, with most of the poled state localized near the lower interface of the thin-film ferroelectric capacitor. The mechanism underlying the onset of this self-polarization is related to the charging of the lower interface of the structure by electrons, which occurs during the cooling following the high-temperature treatment of the PZT film, and to poling of the bulk of the film by the charged interface. This mechanism of the self-polarization of ferroelectric films is believed to have a universal character. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 4, 2002, pp. 739–744. Original Russian Text Copyright ? 2002 by Pronin, Kaptelov, Tarakanov, Shaplygina, Afanas’ev, Pankrashkin.     

Low-temperature processing of sol–gel-derived lead–zirconate–titanate thin films by oxygen-plasma treatment

Ferroelectric thin films of sol–gel-derived Pb(Zr_x, Ti_1−x)O₃ (lead–zirconate–titanate, PZT) were obtained by the low-temperature processing employing oxygen-plasma treatment. The as-coated PZT films were annealed in oxygen ambience at 450 °C, followed by oxygen-plasma treatment at 200 °C, which gave rise to the ferroelectric hysteresis. Annealing of the as-coated PZT films followed by oxygen-plasma teratment at 200 °C gave rise to the ferroelectric hysteresis.     

Determination of the Steady State Leakage Current in Structures with Ferroelectric Ceramic Films

Steady state leakage currents have been investigated in capacitor structures with ferroelectric solgel films of lead zirconate titanate (PZT) formed on silicon substrates with a lower Pt electrode. It is established that Pt/PZT/Hg structures, regardless of the PZT film thickness, are characterized by the presence of a rectifying contact similar to p–n junction. The steady state leakage current in the forward direction increases with a decrease in the film thickness and is determined by the ferroelectric bulk conductivity.     

Temperature-dependent ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films

To investigate temperature-dependent ferroelectric and dielectric properties of ferroelectric films, Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were prepared on Pt-coated silicon substrates by pulsed laser deposition. The ferroelectric and dielectric behaviors have been studied in a wide temperature range from 80 K to room temperature. The saturated polarization (P_sat) decreases with decreasing temperature and decreasing electric field, whereas remnant polarization (P_r) shows a more complex temperature dependence. These results, which can be well explained based on a temperature-dependent charged defects-domain wall interaction model, might be helpful for further understanding the domain switching behavior. Based on these results, an alternative way to investigate temperature-dependent ferroelectric fatigue is proposed and experimentally carried out. The measured fatigue rate is found to be linearly dependent on temperature, consistent with the report on Pb(Zr,Ti)O₃ films. Temperature-dependent dielectric measurements of the films further confirm the above explanation.     

Lead titanate ferroelectric films on single-crystal silicon

This paper reports on the results of investigations into the phase transformations observed in Pb/Ti/Si and Ti/Pb/Si thin-film heterostructures upon layer-by-layer magnetron sputtering of lead and titanium onto a single-crystal silicon substrate and subsequent annealing in an oxygen atmosphere. It is shown that the dielectric properties of lead titanate films depend on the order of sputtering of lead and titanium metal layers onto the surface of single-crystal silicon. The ferroelectric properties are revealed in 3000-nm-thick lead titanate films prepared by two-stage annealing of the Pb/Ti/Si thin-film heterostructure (with the upper lead layer) at T ₁=473 K and T ₂=973 K for 10 min. These films are characterized by the coercive field E _c=4.8 kV/cm and the spontaneous polarization P _s=16.8 μC/cm². The lead titanate films produced by annealing of the Ti/Pb/Si thin-film heterostructure (with the upper titanium layer) do not possess ferroelectric properties but exhibit properties of a conventional dielectric. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 4, 2002, pp. 745–749. Original Russian Text Copyright ? 2002 by Sidorkin, Sigov, Khoviv, Yatsenko, Logacheva.     

1/<Emphasis Type="Italic">E</Emphasis><Superscript>2</Superscript> Law: Solution of the Unsolved Problem in the Physics of Ferroelectrics

An unusual frequency dependence of a coercive field observed in polycrystalline ferroelectric films of Pb(Zr_{1 ? x}Ti_x)O₃ solid solutions in a wide frequency range has been explained within the model of surface sources emitting thin ferroelectric domains with reversed polarization. This model makes it possible to interpret experimental data as the manifestation of the coercivity paradox in polycrystalline films and predicts the existence of the limiting frequency for switching of domains in agreement with the experiment. The proposed mechanism of polarization switching explains the observed temperature dependence of the activation field E_a ∝ T^?1/2. Furthermore, it predicts an increase in the activation energy for the nucleation of domains with an increase in the size of the source, indicating that the coercive field increases with the degradation of small     

Gas release in the process of thermal treatment of sputtered Pb(Ti<Subscript>0.48</Subscript>Zr<Subscript>0.52</Subscript>)O<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> films

The conditions (regimes of deposition and thermal treatment) for gas bubble formation in ferroelectric Pb(Ti_1–yZr_y)O₃ films have been determined by thermal desorption and electron and optical micros-copy. A mechanism of bubble formation has been proposed. This mechanism rests upon the notion that lead can form oxides of the PbO₂ type with a high oxygen content at relatively low temperatures and that these oxides break down with the release of oxygen to lower oxides of the PbO type upon subsequent heating. These ideas have been taken as the basis of a technique for the fabrication of Pb(Ti_1–yZr_y)O₃ films with a reduced (by an order of magnitude) density of through defects.     

Retention characteristics of Au/Bi3.25La0.75Ti3O12/Si metal-ferro- electric-semiconductor structure

Lanthanum-substituted bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂) (BLT) thin films were deposited on p-type Si(100) substrates using a chemical solution deposition process. The ferroelectric and dielectric properties of the films with an Au/BLT/Si structure were investigated. It was found that retention behaviors of the capacitors after polling with a negative and positive voltage were very different. The capacitor at an accumulation state exhibited a better retention characteristic than that at a depletion state. A rapid loss of memory for the capacitor at depletion state was found and attributed to the depolarization fields inside the ferroelectric film. It is proposed that the interaction between injected charges and ferroelectric polarization plays a role in the retention properties of the MFS capacitors.     

Depolarization field and properties of thin ferroelectric films with inclusion of the electrode effect

The influence of metallic electrodes on the properties of thin ferroelectric films is considered in the framework of the Ginzburg-Landau phenomenological theory. The contribution of the electrodes with different screening lengths l _s of carriers in the electrode material is included in the free-energy functional. The critical temperature T _cl , the critical thickness of the film, and the critical screening length of the electrode at which the ferroelectric phase transforms into the paraelectric phase are calculated. The Euler-Lagrange equation for the polarization P is solved by the direct variational method. The results demonstrate that the film properties can be calculated by minimizing the free energy, which has a standard form but involves the coefficient of the term P². This coefficient depends not only on the temperature but also on the film thickness, the surface and correlation effects, and the electrode characteristics. The calculations of the polarization, the dielectric susceptibility, the pyroelectric coefficient, and the depolarization field show that the ferroelectric state of the film can be destroyed using electrodes from a material whose screening length exceeds a critical value. This means that the electrodes being in operation can induce a transition from the ferroelectric phase to the paraelectric phase. The quantitative criteria obtained indicate that the phase state and properties of thin ferroelectric films can be controlled by choosing the appropriate electrode material.     

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO₃ (BFO) thin films on SrRuO₃ (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2P_r = 59.6 μC/cm² and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO₃ substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO₃ substrates.     

The polarization and dielectric susceptibility of a ferroelectric bilayer film

A ferroelectric bilayer film consisting of two different ferroelectric constituent films with a transition layer within each constituent film and interfacial coupling between two materials is investigated based on the Ginzburg-Landau-Devonshire phenomenological theory. A parameter α, which describing the differences between physical properties of two constituent films is first introduced in our paper, and reflects a more realistic situation compared to the previous treatments assuming the same two constituent films. We study the polarization and dielectric susceptibility properties of the ferroelectric bilayer film with two different constituent films. The results present some interesting phenomena due to the introduction of parameter α.     

Effect of pore size on ferroelectric properties of multiferroic BiFeO3 films prepared on porous silicon

The ferroelectric properties of BiFeO₃ (BFO) films spray deposited on porous silicon have been studied. The analysis of XRD and FESEM investigations show that the crystalline strain in the BFO films increases with pore size. The BFO films on porous silicon substrate showed improvement in ferroelectric fatigue behavior, remanent polarization and ferroelectric switching time. A maximum memory window of 5.54 V at 1 MHz and a large remanent polarization (P_r) of 13.1 μC/cm² have been obtained at room temperature. The improvement in the ferroelectric properties of these films has been correlated to the crystalline strain.     

Geometrical effects in nanodimensional epitaxial films of barium strontium titanate

The effect of the thickness of 6- to 950-nm-thick Ba_0.8Sr_0.2TiO₃ films epitaxially grown on (001)MgO on their ferroelectric properties is investigated. Raman scattering and X-ray diffraction data indicate a structural phase transition taking place at a film thickness of ≈70 nm, which changes drastically the lattice parameters of the film. Raman spectra taken of the films confirm that they are in the ferroelectric state and that their symmetry changes in going over a critical thickness, as revealed by a sharp displacement of the peaks corresponding to the A ₁(TO) and E(TO) components of the soft mode. At film thicknesses of <100 nm, the permittivity versus thickness dependence exhibits two peaks at thicknesses of ～18 and ～36 nm. Near the first peak, the dielectric nonlinearity is considerably higher than near the second one.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Compositional analysis of ferroelectric films coated with carbon layer using laser-induced plasmas spectroscopy

An experimental investigation of the effect of different thickness of carbon layer coated on ferroelectric films on the atomic emission intensity using laser-induced plasmas spectroscopy technique with charge-coupled device (CCD) experimental system has been conducted. The experimental results show that the intensity of the spectra emitted with the carbon layer thickness of 15 μm is much higher than that of pure ferroelectric films. By using this best experimental condition, the atomic concentration ratios of ferroelectric films are evaluated by rationing the integrated intensities of selected spectral emission lines of the plasma produced from the films. And the experimental results show that N_La/N_Co and N_Sr/N_Co atomic concentration ratios are almost in agreement with the corresponding values obtained by traditional compositional analysis techniques of inductively coupled plasma (ICP).     

Room temperature ferroelectricity of tetragonally strained SrTiO3 thin films on single crystal Rh substrates

Epitaxial SrTiO₃ thin films were deposited on single crystalline Rh substrates by pulsed laser deposition. The tetragonally stained structure of the SrTiO₃ thin films with a c/a ratio of 1.04 was confirmed by x-ray diffraction experiments. The SrTiO₃ thin films exhibited good ferroelectric properties with a high remanent polarization (2P_r) of 8 μC/cm² and a canonical ferroelectric piezoresponse hysteresis loop at room temperature. We estimated a high activation electric field of about 6.4 MV/cm for domain wall creeping. This activation electric field is higher than that of typical ferroelectric materials such as PbTiO₃.     

Effect of bilayer structure and a SrRuO3 buffer layer on ferroelectric properties of BiFeO3 thin films

Effects of the BiFe_0.95Mn_0.05O₃ thickness and a SrRuO₃ (SRO) buffer layer on the microstructure and electrical properties of BiFeO₃/BiFe_0.95Mn_0.05O₃ (BFO/BFMO) bilayered thin films were investigated, where BFO/BFMO bilayered thin films were fabricated on the SRO/Pt/Ti/SiO₂/Si(100) substrate by a radio frequency sputtering. All thin films are of a pure perovskite structure with a mixture of (110) and (111) orientations regardless of the BFMO layer thickness. Dense microstructure is demonstrated in all thin films because of the introduction of BFMO layers. The SRO buffer layer can also further improve the ferroelectric properties of BFO/BFMO bilayered thin films as compared with those of these thin films without a SRO buffer layer. The BFO/BFMO bilayered thin film with a thickness ratio of 220/120 has an enhanced ferroelectric behavior of 2P _r??165.23???C/cm² and 2E _c??518.56?kV/cm, together with a good fatigue endurance. Therefore, it is an effective way to enhance the ferroelectric and fatigue properties of bismuth ferrite thin films by constructing such a bilayered structure and using a SRO buffer layer.     

Anisotropic models of polymer ferroelectrics

The phase transitions to an ordered state in three-dimensional polymer systems that consist of flexible and rigid segments with intrachain and interchain orientational-deformational dipole interactions are considered. The statistic properties of the proposed models correspond to the Gaussian and spherical approximations that are used to describe the behavior of anisotropic Heisenberg ferroelectrics and ferromagnets. In the three-dimensional models under consideration, there exists a critical point T _c where a secondorder phase transition from an isotropic state to a state with long-range orientational order occurs. The laws of variation in the temperature T _c as a function of anisotropy of the interactions are established by analytical methods. The temperature dependences of the long-range dipole order parameter for a given chain bending are calculated and the results are compared with the experimental data obtained by the piezoelectric pressure step (PPS) method for thick ferroelectric PVDF and P(VDF-TrFE) polymer films.     

Microwave properties of film Ba x Sr1 ? x TiO3 ferroelectric variconds with a magnesium-containing additive

The electrophysical properties of bulk ceramics based on Ba _x Sr_{1 ? x} TiO₃ solid solutions with a Mg-containing additive and planar variconds based on ferroelectric films obtained by the ion-plasma sputtering of targets with different elemental compositions are studied. Controllability n(U) = C(0)/C(U) and the dielectric loss tangent (tan??) of ferroelectric variconds are measured as functions of the elemental composition of the ferroelectric. The figure of merit of the variconds is estimated, and the film composition providing the best electrophysical parameters is determined.     

Superior electromechanical performance over PZT, in lead zinc niobate (PZN)–lead zirconium titanate (PZT) thin films

Strongly oriented thin films of lead zinc niobate (PZN)–lead zirconium titanate (PZT) with (La,Sr)CoO₃ lower electrodes were grown on MgO {001} substrates by pulsed laser deposition. The films were perovskite-dominated with strong in-plane and out-of-plane orientations. Room-temperature functional characterisation indicated that the films were ferroelectric, with dielectric constant ∼550 and loss tangent ∼0.08 at 1 kHz. The crystallographic strain, as a function of applied dc field, was monitored by in situ X-ray diffraction. The maximum electric-field-induced crystallographic strain (∼0.22%) was comparable to that observed in bulk, but at significantly greater field (∼150 kV cm^-1). The effective d₃₃ value obtained from the crystallographic strain data was around 150 pm V^-1, which is high for ferroelectric thin films 400 nm in thickness. The local polarisation-switching properties of the films were investigated using a piezo-response atomic force microscope. Domain maps for a 5×5 μm² region of material were recorded as a function of dc bias, and confirmed the ferroelectric switching behaviour. PACS 81.05.Je; 81.40.Vw; 68.37.Lp