Properties Of Ferroelectric Films Research Articles

Model for ferroelectric semiconductors thin films accounting for the space varying permittivity

Reducing thickness of ferroelectric films typically comes with an apparent degradation of their ferroelectric and dielectric properties. The existence of low-dielectric interfacial layers is often invoked to explain such behaviors. Much work has been done on modeling ferroelectric thin films by considering a ferroelectric layer between two layers with low dielectric constant. In these models it is necessary to introduce extrinsic parameters; the dielectric constant and the polarization are step functions of the depth inside the film. We have developed a model for ferroelectric semiconductors based on the inhomogeneous Landau–Devonshire theory, including surface effects. The local electric field is determined by solving Poisson’s equation in which a differential permittivity replaces the dielectric constant. We have found that the hysteresis loops were strongly influenced by the correlation length. That point is discussed by examining the electric field, polarization, and differential permittivity profile inside the film. It is shown that the differential permittivity strongly decreases with the correlation length increases. That phenomena leads to high electric fields inside the film and especially close to the surface. As a consequence, an explanation for thin film ferroelectric properties can be given by invoking space charge density many orders of magnitude lower than those usually considered.

Size effects in thin films of order–disorder ferroelectrics allowing for the depolarization field

AbstractFilms of order–disorder type ferroelectrics have been considered in the mean field approximation taking into account a depolarization field. It has been shown that size effects in this system could be described on the base of a bulk system equation of state with Curie temperature dependent on the film thickness. A comparison of different type ferroelectric film properties has been performed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ferroelectric and Dielectric Properties of Lanthanum-Modified Bismuth Titanate Thin Films Obtained by the Polymeric Precursor Method

Lanthanum-modified bismuth titanate, Bi4 − xLaxTi3O12 (BLT), thin films with a La concentration of 0.75 was grown on Pt/Ti/SiO2/Si substrates by using the polymeric precursor solution and spin-coating method. The scanning electron microscopy (SEM) showed rounded grains, which is not typical for these system. The BLT films showed well-saturated polarization-electric field curve which 2Pr = 41.4 μC/cm2 and Vc = 0.99 V. The capacitance dependence on the voltage is strongly nonlinear, confirming the ferroelectric properties of the film resulting from the domains switching. These properties make BLT a promising material for FERAM applications.

The internal electric field originating from the mismatch effect and its influence onferroelectric thin film properties

The influence of the mismatch effect on thin ferroelectric film properties has been studiedin the phenomenological theory framework. The polarization dependent part of the surfaceenergy that defined the boundary conditions for the Euler–Lagrange differentialequation was written as a surface tension energy. The latter was expressed viathe surface polarization and the tension tensor related to the mismatch of thesubstrate and film lattice constants and thermal expansion coefficients. The interfacialstrain caused by the mismatch effect induces the additional surface polarizationPm via a piezoelectric effect that arises near the surface in any film.The new parameter Pm/PS (PS is the known value of the spontaneous polarization in the bulk ferroelectric material atT = 0 K)appeared in the derived phenomenological equations. So we calculate the influence of the parameterPm/PS on the depth distribution of the dimensionless film polarizationP/PS, its dependence on temperature, film thickness and applied electric field,as well as that on the hysteresis loop shape, coercive field values, phasediagram and average dielectric susceptibility temperature dependence. Non-zeroPm/PS values cause a mismatch induced thickness dependent internal electric fieldEm.We have shown that this field drastically influences all the properties. In particular, the polarizationprofile becomes asymmetrical, the polarization temperature dependence resembles that in theexternal electric field and there is a possibility of external field screening by the internal fieldEm. The asymmetry obtained for the hysteresis loop suggests that it is possible that theself-polarization phenomenon recently observed in some films is related to the mismatch effect.The thickness induced ferroelectric–paraelectric phase transition has been shown to exist when|Pm|/PS<1. A largeenough ratio |Pm|/PS>1 could be the physical reason for the ferroelectric phase conservation in ultrathin film. Thepossibility of observing the peculiarities of the film property temperature and thicknessdependences related to the mismatch effect is discussed.

Surface Segregation Mechanisms in Dielectric Thin Films

It is often assumed that evaporation causes lead and bismuth depletion at the surface of ferroelectric films. However, this is in contrast with evidence that shows surface enrichment of Pb and Bi. A segregation mechanism that is analogous to the diffusion processes observed during the oxidation of metals is described. The presence of an electrochemical potential is discussed, together with the implications on the electrical properties of ferroelectric films.

The Influence of Mismatch-Induced Field on Thin Ferroelectric Film Size Effects

The influence of the mismatch effect on the thin ferroelectric film properties has been studied in phenomenological theory framework. The polarization-dependent part of surface energy was expressed via surface polarization and tension tensor related to mismatch of a substrate and a film lattice constants and thermal expansion coefficients. The interfacial strain caused by the mismatch effect induces the additional surface polarization P m via piezoelectric effect that exists near the surface in any film. P m causes mismatch-induced thickness dependent internal electric field E m which influences drastically all the film properties. In particular the polarization profile and hysteresis loop become asymmetrical, polarization temperature dependence resembles the one in the external electric field, and there is possibility of external field screening by the field E m . The obtained results mak it possible to suppose that mismatch effect could be the physical reason of the ferroelectric phase conservation in ultrathin film.

Position-thickness-dependent stresses and stress-induced diffuse dielectric anomaly in perovskite ferroelectric films

A set of position-thickness-dependent stress distribution functions in a film grown on a cubic substrate is obtained by solving elastic mechanical equations. A concise phenomenological theory has been developed to investigate phase transitions and dielectric properties of ferroelectric films under stress. The calculated results show that the tensile and compressive stresses coexist in a periodic array, accordingly the continuous stress originated from the misfit dislocation at the interface causes the diffuse dielectric anomaly. The influences of both the film thickness and grain size on dielectric permittivity are discussed. Numbers of experimental results is well explained by the theory.

Low Temperature PZT Film by MOCVD

Lead Zirconium Titanium Oxide (PZT) thin films were prepared at low temperature using a SMI Metal Organic Chemical Vapor Deposition SpinCVD™ tool. Films prepared by this technique were evaluated for multiple device applications. The low temperature PZT film process addresses problems with device manufacture encountered by techniques that rely on high thermal budgets to obtain superior materials properties. Film growth processes were developed based on the following criteria: low defect density, a high necked and dense microstructure, and highly uniform properties across the wafer. The growth process, ferroelectric film properties, and suitability for IR sensing and FRAM applications are discussed.

Modification of (Pb,La)(Zr,Ti)O3 thin films during pulsed laser liftoff from MgO substrates

Pulsed excimer laser irradiation of an absorbing thin film through a transparent substrate has been employed to separate epitaxial (Pb,La)(Zr,Ti)O3 (PLZT) thin films from their MgO growth substrates. The morphology, microstructure, and dielectric properties of the laser-modified PLZT layer generated at the original PLZT/MgO interface by this laser liftoff (LLO) process was investigated. Atomic force microscopy measurements of the film surface after LLO indicated average surface roughness on the order of 90–100 nm, complementing previously reported scanning electron microscopy studies that revealed what appeared to be a rough glassy layer. The surface roughness is thought to form as the original growth substrate is ejected from the films transferred towards the end of the 38 ns pulse, pulling the molten near-interfacial portion of the PLZT film via surface tension, and followed by quenching of the liquid phase. Transmission electron microscopy of PLZT films transferred to Si wafers showed the surface layer consists of an irregular amorphous layer ranging in thickness from 50 to 100 nm, although some surface features protruded as far as 300 nm. Enhanced levels of lead were found to be present in the amorphous phase by Rutherford backscattering spectrometry, implying PbO evaporation and subsequent condensation during LLO. Electrical measurements determined the relative dielectric constant of the surface layer to be in the range of 40 – 70. Subsequent ion milling of the layer demonstrated that the ferroelectric and dielectric properties of the original PLZT films were largely recovered, while the surface roughness was reduced to below 40 nm. The observed differences between the dielectric and ferroelectric properties of the film transferred after complete removal of the amorphous layer compared to those of the film on the original MgO substrate are attributed to the removal of substrate clamping.

Properties of ferroelectric films based on Nb-modified PZT produced by PLD technique

According to the general formula Pb 1− X/2 (Zr 0.65 Ti 0.35) 1− X Nb X O 3+4 mol% PbO excess ( X=1 and 4%) lead niobium zirconate titanate (PZTN) ferroelectric films were deposited by ablation technique on Pt coated Si, and MgO(1 0 0) substrates using either a Nd:YAG (355 nm) or a XeCl (308 nm) excimer laser. X-ray diffraction was used to determine the crystallographic structure as well as to identify the presence of secondary phases, i.e. non-perovskite phases. The frequency dependent effective dielectric function of PZTN samples produced using different lasers was measured experimentally and modeled. Also, the ferroelectric behaviour of the as-deposited films was studied using a modified Sawyer–Tower bridge.

MOCVD of Ferroelectric Thin Films

ABSTRACTWe have examined the growth of a number of important ferroelectric oxides by MOCVD using a rotating disk reactor. Highly uniform and reproducible films over 6” wafers have been routinely achieved. Materials include Lead Zirconate Titanate (PZT, PbZrxTi1-xO3), Lead Lanthanum Zirconate Titanate (PLZT), Strontium Bismuth Tantalate (SBT), CeMnO3 (CMO), and others. Emphasis has been on achieving highly crystalline and oriented films at the lowest deposition temperatures possible, for compatibility with other integrated device materials and processing; and the achievement of optimum ferroelectric and pyroelectric performance. The effects of varying growth parameters, barrier and/or template layers, and post-growth annealing have been studied. The growth process, physical characterization, and ferroelectric film properties will be discussed.

AC conduction mechanism of SrBi2Ta2O9 thin films below room temperature

Ferroelectric thin films have tremendous application potential for data storage, sensors, uncooling infrared detectors, classes of tunable microwave devices, and microelectromechanical system technologies [1–4]. Carrier transport plays an essential role in determining some characteristics of the ferroelectric thin films such as fatigue and leakage to meet commercial requirements. In recent years, different transport mechanisms in ferroelectric materials, such as space charge limited conduction (SCL), thermionic emission limited conduction (TEL), and Poole-Frenkel emission limited conduction (PFL) have been reported [5–10]. Further, current-voltage (IV) characteristics may contain contributions from SCL and PFL conductions, and even TEL conduction at high direct current (dc). The latter TEL part can usually be distinguished by its dependence on the selection of electrode material. To date, most of the studies on the transport properties of ferroelectric films are based on dc measurements, such as dc IV measurements. In direct current conductivity measurements, all types of carrier contributions to conductance are superposed. Thus, the various transport mechanisms cannot be easily distinguished. It is well known, however, that each type of carrier (or mechanism) possesses an independent relaxation time. Based on this feature, it is easy to distinguish the contributions of various mechanisms to electrical transport from frequency dependent conductivity measurements. However, there has been little experimental work on alternating current (ac) transport properties of the thin films below room temperature, although data collected at a broader temperature range would be helpful in understanding the conduction mechanisms of these films. The electrical properties of ferroelectrics are known to be closely associated with their crystallographic structure. An important example is SrBi2Ta2O9 (SBT), a representative of a new class of ferroelectrics in which, the structure is based on perovskite octahedron layers (SrTa2O7)2− sandwiched between (Bi2O2)2+ layers. SBT exhibits almost no fatigue and low dc leakage on Pt electrodes at room temperature (up to 1012 cycles at saturation), accompanied by other superior properties [11–17]. These unusual properties have generated tremendous interests in the fundamental conduction mechanisms of SBT. In this letter, we report the ac transport properties from 100 Hz to 15 MHz of SBT ferroelectric thin films in the temperature range of 10 to 300 K. The dominant conduction mechanisms at different frequency ranges were analyzed. Most importantly, results indicated the existence of conduction through the perovskite octahedron layers of the SBT structure. SBT thin films were prepared by pulsed laser deposition (PLD) on heated Pt/Ti/SiO2/Si substrates (400 ◦C), followed by a post-annealing process at 750 ◦C for 1.5 h in oxygen [16] 30 nm thick Pt electrodes with an area of 2.0 × 10−4 cm2 were deposited on the top surface of the films through a shadow mask by a UHV electron beam evaporator (UMS 500p made by Balzers), giving the capacitive multilayered structure; Pt/SBT/Pt. The crystallographic structure of the SBT films was characterized by X-ray diffraction along the (008) and (115) direction. The microstructure of the films was observed by transmission electron microscopy, with the mean grain size of about 200 nm, thickness of about 200 nm, dense film morphology and abrupt interfaces. Ferroelectric properties of the thin films were determined with an RT66A ferroelectric tester from Radiant Technologies. A typical hysteresis loop showed that the remanent polarization and coercive field were 10 μC/cm2 and 57 kV/cm at room temperature, respectively. The electrical conductivity of the SBT thin films was measured by a HP4194A Impedance/Gain-Phase Analyzer in the temperature range of 10 to 300 K. These measured values of conductivity in the range between 100 Hz and 15 MHz were simulated by an equivalent circuit using an ideal capacitor shunted with an ideal resistor. The frequency dependence of the electrical conductivity is illustrated in Fig. 1 with temperature as a

Dependence of threshold thickness of crystallization and film morphology on film processing conditions in poly(vinylidene fluoride–trifluoroethylene) copolymer thin films

In spin-cast films of poly(vinylidene fluoride–trifluoroethylene) on metalized silicon substrate, there exists a threshold thickness of crystallization dth, below which the crystallinity drops precipitously. Due to the direct link between the crystallinity and functional properties in the polymer, there is a corresponding large change in the film ferroelectric properties, including the dielectric constant, the polarization level, and polarization switching speed, as the thickness is reduced to below dth. Detail microstructure studies show that this threshold thickness is controlled by the stable crystal lamellar size along the film thickness direction. By varying the film processing condition to reduce the crystal lamellar size in the thickness direction, dth can be reduced markedly. As a result, better ferroelectric responses were obtained in ultrathin films.

The influence of depolarization field on dielectric and pyroelectric properties of ferroelectric films

Calculations of the spontaneous polarization (Ps), dielectric susceptibility (χ) and pyroelectric coefficient (Π) of the ferroelectric films have been performed in the thermody- namic phenomenological theory framework. The Euler-Lagrange equation determining po- larization dependence on the film parameters and the external electric field was solved ana- lytically under the boundary conditions with different extrapolation lengths at two surfaces, respectively. The depolarization field contribution was taken into account in the model of short-circuited mono-domain ferroelectric film, treated as perfect insulator. The detailed analy- sis of the aforementioned quantities, space distribution and their average values in two cases with and without depolarization field was carried out. It was shown that the depolarization field shifts critical temperature to smaller values and the critical thickness to bigger value in comparison to those obtained without accounting the depolarization field. Meanwhile aver- age values of Ps, χ and Π dependences on the film parameters and temperature are similar to the corresponding dependences obtained without accounting the depolarization field. The depolarization field was shown to flatten Ps, χ and Π space distributions, which have the peculiarities otherwise (e.g. small maxima in χ and Π coordinate profiles near the film sur- faces). It was shown that depolarization field influence in short-circuited film could be ne- glected when the film thickness or the extrapolation lengths in the boundary conditions are larger than the correlation length value.

Investigation of Electrical Degradation Effects in Ferroelectric Thin Film Based Tunable Microwave Components

The impact of electrical stressing on the microwave performance of the SrTiO 3 films deposited onto different substrates (MgO and LaAlO 3 ) by pulsed laser deposition has been investigated. The most important characteristics of the tunable microwave components like tunability and loss tangent have been monitored. It has been found that electrical degradation effects, being sensitive to the device operating conditions and ferroelectric film properties, are large enough to affect the performance of ferroelectric-based tunable microwave devices within the expected operating time.

Investigation of Electrical Degradation Effects in Ferroelectric Thin Film Based Tunable Microwave Components

The impact of electrical stressing on the microwave performance of the SrTiO 3 films deposited onto different substrates (MgO and LaAlO 3 ) by pulsed laser deposition has been investigated. The most important characteristics of the tunable microwave components like tunability and loss tangent have been monitored. It has been found that electrical degradation effects, being sensitive to the device operating conditions and ferroelectric film properties, are large enough to affect the performance of ferroelectric-based tunable microwave devices within the expected operating time.

Characterization of the effective electrostriction coefficients in ferroelectric thin films

Electromechanical properties of a number of ferroelectric films including PbZrxTi1−xO3(PZT), 0.9PbMg1/3Nb2/3O3–0.1PbTiO3(PMN-PT), and SrBi2Ta2O9(SBT) are investigated using laser interferometry combined with conventional dielectric measurements. Effective electrostriction coefficients of the films, Qeff, are determined using a linearized electrostriction equation that couples longitudinal piezoelectric coefficient, d33, with the polarization and dielectric constant. It is shown that, in PZT films, electrostriction coefficients slightly increase with applied electric field, reflecting the weak contribution of non-180° domains to piezoelectric properties. In contrast, in PMN-PT and SBT films electrostriction coefficients are field independent, indicating the intrinsic nature of the piezoelectric response. The experimental values of Qeff are significantly smaller than those of corresponding bulk materials due to substrate clamping and possible size effects. Electrostriction coefficients of PZT layers are shown to depend strongly on the composition and preferred orientation of the grains. In particular, Qeff of (100) textured rhombohedral films (x=0.7) is significantly greater than that of (111) layers. Thus large anisotropy of the electrostrictive coefficients is responsible for recently observed large piezoelectric coefficients of (100) textured PZT films. Effective electrostriction coefficients obtained by laser interferometry allow evaluation of the electromechanical properties of ferroelectric films based solely on the dielectric parameters and thus are very useful in the design and fabrication of microsensors and microactuators.

Effects of domain formation on the dielectric properties of ferroelectric thin films

A Landau-Ginsburg-Devonshire-type thermodynamic theory is used to describe dense laminar domain structures in epitaxial ferroelectric thin films grown on “tensile” substrates and to compute their small-signal dielectric responses. For PbTiO3 and BaTiO3 films, it is found that the ferroelectric phase transition results in the formation of a pseudo-tetragonal polydomain a 1/a 2/a 1/a 2 state. The stability range of the a 1/a 2/a 1/a 2 structure in the misfit strain-temperature phase diagram is determined. Effects of domain formation on the dielectric properties of ferroelectric films are analyzed by comparing permittivities of polydomain and single-domain films. Theoretical results are compared with the experimental data on (Ba1−xSrx)TiO3 films grown on platinum coated silicon substrates.

Electromechanical properties of ferroelectric films for MEMS

The possibility of miniaturization and integration with Si technology makes ferroelectric films promising for microelectromechanical systems (MEMS). Electromechanical properties of thin-layer ferroelectrics, which determine their performance in micro devices, are still poorly understood as compared to the properties of bulk materials. Small grain size, preferred orientation of the films and their strong clamping to the rigid substrate are the main factors, which are responsible for the large difference in electromechanical responses of thin-film and bulk piezoelectrics. This paper reviews author's recent investigations on electromechanical properties of ferroelectric films. The emphasis is given to the contribution of domain wall motion to piezoelectric properties, which is shown to be strongly suppressed in thin films as compared to bulk materials. Nevertheless, this contribution is found to be essential at sufficiently high ac electric field in PZT films of the morphotropic phase boundary and rhombohedral compositions. Large anisotropy of the piezoelectric properties is found in PZT films having different textures. This anisotropy is similar to that recently observed in relaxor single crystals and is explained based on the anisotropy of electrostriction coefficients. Electrostrictive properties of bulk and thin film ferroelectrics are compared and analyzed.

Landau theory of interfacestress effect on the phase transition properties of ferroelectric films

A coupling term between stress and polarization has been introduced into Landau free energy to describe the interface stress effect of a thin ferroelectric film. The stress is assumed decreases exponentially away from the interface. Profiles of polarization and susceptibility have obtained for both tensile and compressive stress. We find that the compressive stress can enhance the polarization near the interface and reduce the dielectric constant, and tensile can reduce the polarization and enhance the dielectric polarization near the interface. Temperature dependence of polarization and dielectric constant have also obtained The compressive interface stress can shift the Curie temperature to a higher temperature, even higher than the bulk Curie temperature, however tensile shifts the Curie temperature to a lower temperature.