Pb(Mg1/3Nb2/3)O3 and (1 − x)Pb(Mg1/3Nb2/3)O3 − xPbTiO3 Relaxor Ferroelectric Thick Films: Processing and Electrical Characterization

The lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃ or PMN], and its solid solutions with lead titanate (PbTiO₃ or PT), are of great interest because of their high electromechanical properties. At large PMN content, these materials exhibit relaxor characteristics with large electrostrictive strains and a large permittivity, while compositions near the morphotropic phase boundary present very interesting piezoelectric properties. So far, properties of these materials in ceramic, thin film and single-crystal form have been investigated. In this paper, we report on preparation and properties of pyrochlore free PMN and 0.65PMN-0.35PT thick films (thickness = 10 to 20 m). The films were prepared from ethyl cellulose ink by screen printing on alumina substrate. The influence of various parameters, such as powder characteristics, inks formulation and films sintering conditions, on films densification are discussed. The dielectric and electromechanical properties of the films were examined. Relaxor-like behaviour was clearly demonstrated in PMN films. The maximum relative permittivity for PMN film was 10000 (at 0.1 kHz), which is lower than in bulk ceramics (17800 at 0.1 kHz) prepared under the same conditions. For 0.65PMN-0.35PT, the maximum relative permittivity was around 15500 against 24000 in the bulk. Several parameters, which might be responsible for the lower permittivity, are discussed. Poled 0.65PMN-0.35PT thick films exhibit relatively large piezoelectric response (d ₃₃ up to 200 pm/V) and unipolar strains approaching 0.1%, making these films of interest for various actuator and transducer applications.     

Pb(Mg1/3Nb2/3)0.97Ti0.03O3 Ferroelectric Thin Films, Deposited by Laser Ablation on TiN Bottom Electrodes

Pb(Mg_1/3Nb_2/3)_0.97Ti_0.03O₃ (PMNT) polycrystalline thin films were deposited on Titanium Nitride electrode at different temperatures by laser ablation, using a wavelength of 248 nm. The morphology of the films was analyzed by scanning electron microscopy (SEM). The nature of the ferroelectric layer-electrode interface is studied by transmission electron microscopy (TEM) as well as the effect of its characteristics in the performance of the multilayer system. The influence of the annealing temperature on the dielectric properties was studied by hysteresis and fatigue measurements.     

DIELECTRIC BEHAVIOUR OF A NANOGRAIN PMN POWDERS

ABSTRACT

Dielectric permittivity of 150 nm mean grain size Pb(Mn_1/3Nb_2/3)O₃ (PMN) nanopowder has been investigated by dielectric spectroscopy in the 20 Hz–1 MHz frequency range and 100 K–400 K temperature range. The broad and diffused dielectric dispersion has been observed, but the dispersion region and the maximum of the real part of dielectric permittivity is shifted to lower temperatures compare to PMN single crystal and ceramics. The mean relaxation time, obtained from fits of the frequency dependences of the dielectric permittivity with Cole-Cole formula, changes according to the Vogel-Fulcher law with the freezing temperature T ₀ = 88 K which is much lower than in bulk PMN materials.     

Electrical and optical properties of microwave dielectric thin films prepared by pulsed laser deposition

Abstract

Bi₂(Zn_1/3Nb_2/3)₂O₇, BiZN, materials possess high dielectric constant and low loss factor in microwave frequency region. They have good potential for device application, especially in the form of thin films. However, the microwave dielectric properties of a thin film are very difficult to be accurately measured. Evaluation on the dielectric behavior of the films through the performance of the microstrip line devices made of these films involves metallic conduction and stray field losses. A novel measuring technique, which can directly evaluate the microwave dielectric properties of a thin film is thus urgently needed.

In this paper, BiZN thin films were grown on [100] MgO single crystal substrates using pulsed laser deposition process. The high-frequency dielectric properties of thus obtained thin films were determined using optical transmission spectroscopy (OTS). The [100] preferentially oriented films with pyrochlore structure can be obtained for the thin films deposited at 400–600°C substrate temperature under 0.1 mbar oxygen pressure. OTS measurements reveal that the index of refraction (n=1.95–2.35) and absorption coefficient (k=0.28x10^?4-2.25 × 10^?4 nm^?1) of the films vary insignificantly with the crystallinity of the BiZN films.     

Improved ferroelectricity of strained SrTiO3 thin films on sapphire

Magnetron sputtered and laser deposited SrTiO₃ thin films are deposited on CeO₂ buffered sapphire substrates. Their structural properties are investigated and correlated to the dielectric properties of the SrTiO₃ films. It is shown, that the biaxial compressive strain imposed by the substrate on the ferroelectric films leads to a considerable increase of the permittivity and tunability of SrTiO₃ thin films in technically relevant temperature regimes. Generally, the permittivity and tunability decreases with increasing strain. However, the ferroelectric phase transition of the SrTiO₃ films is shifted to higher temperatures compared to that of single crystalline SrTiO₃. As a consequence, the permittivity of the films is larger than that of undistorted SrTiO₃ single crystals for small strain (Δa/a < 0.005) and temperatures above the Curie temperature. Furthermore, a linear dependence of the loss tangent and the tunability on the permittivity is observed, which indicates, that all three properties are affected by the same mechanism that itself is affected by the lattice strain.     

Grain-Boundary Effect on the Curie-Weiss Law of Ferroelectric Ceramics and Polycrystalline Thin Films: Calculation by the Method of Effective Medium

The influence of grain boundaries on the dielectric properties of ferroelectric ceramics and polycrystalline thin films is described theoretically by the method of effective medium. Grain boundaries are modeled by low-permittivity (dead) layers, which do not exhibit ferroelectric instability. The effective permittivity of a polycrystalline material is calculated in the paraelectric regime above the transition temperature. The calculations are based on the solution of electrostatic problem for a spherical dielectric inclusion separated from the surrounding dissimilar matrix by a low-permittivity interface layer. For isotropic bulk ceramics, an analytic expression is derived for the effective permittivity as a function of the grain size, dead-layer thickness, and its permittivity. Temperature dependence of the aggregate dielectric response is calculated for BaTiO₃ (BT) ceramics of different grain sizes and found to be in good agreement with measurements. It is shown that grain boundaries not only renormalize the Curie-Weiss temperature and constant, but may also cause deviations from the Curie-Weiss law. For BT polycrystalline thin films grown on dissimilar substrates, numerical calculations of the effective dielectric constants are performed, taking into account both the grain-boundary and substrate effects on the film anisotropic dielectric response. Theoretical predictions are compared with the grain size dependence of the permittivity of BT films grown on Pt-coated Si.     

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba,Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Ba_x,Sr_{1 ? x})TiO₃ [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO₃ [LAO] and Nb-doped SrTiO₃ [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ～ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.     

Excimer laser ablation processed ferroelectric and antiferroelectric thin films

Abstract

The dielectric and electrical properties of excimer laser ablated processed paraelectric (Ba_0.5, Sr_0.5)TiO₃, ferroelectric Bi-layered SrBi₂(Ta_0.5Nb_0.5)₂O₉, and antiferroelectric (PbZrO₃) thin films have been investigated. The effect of processing parameters on the microstructure of the films and the functional properties has been presented in detail. Some of the recent studies of stress induced effects, dielectric, hysteresis and ac and dc electrical properties have been highlighted in conjunction with microstructures of the films.     

Characterization in a Wide Frequency Range (40 MHz–67 GHz) of a KTa0.65Nb0.35O3 Thin Film for Tunable Applications

A (100) oriented KTa_0.65Nb_0.35O₃ 400 nm-thin film has been deposited by Pulsed Laser Deposition on MgO substrate. Microwave measurements, performed on InterDigitated Capacitors, show a paraelectric phase at room temperature with a tunability for the devices of 64% under an electric field of 400 kV/cm. Then, using a specific de-embedding method, the complex permittivity of the KTN thin film has been extracted from 40 MHz up to 67 GHz on coplanar waveguides. As promising applications are pointed out at 60 GHz, such as indoor communications, material characterizations are expected in this spectrum.     

Ferroelectric switching and fatigue behavior for PZT/YBCO thin film heterostructures

Abstract

The dielectric and ferroelectric properties for Au/Pb(Zr,Ti)O₃/YBa₂Cu₃O_7?x heterostructures at low temperatures are reported. The fatigue behavior and the ferroelectric switching effect for the structures are also investigated. The PZT/YBCO thin film heterostructures were deposited on MgO(100) substrates by laser ablation. The ferroelectric and dielectric properties and optical response of the oriented PZT films with different thicknesses have been studied over the temperature range from 20 K to 300 K. The dielectric loss of the structure was found to decrease by an order of magnitude when the YBCO bottom electrode became superconducting. A very low fatigue rate of the structure has also been obtained below T _c of YBCO layer.     

Properties of (001)-textured Pb(Mg1/3Nb2/3)O3–PbTiO3 thin films

Relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) thin films with [001] preferential orientation were deposited on platinized silicon wafers by a sol–gel method, in which a PbO seeding layer was involved. The influences of annealing temperature on the crystal phase, microstructure, and electrical properties of the PMN–PT films were investigated. Pyrochlore-free perovskite PMN–PT films could be formed on PbO-seeded Pt(111)/Ti/SiO₂/Si wafers at 800 °C, which was also the optimal annealing temperature for endowing the film with the best ferroelectric and dielectric properties. The enhanced properties were attributed to the improved crystallinity and microstructure. The leakage behaviors of the PMN–PT films annealed at different temperatures were also measured and discussed.     

Ferroelectric properties of heterolayered lead zirconate titanate thin films

Heterolayered Pb(Zr_{1 − x} Ti _x )O₃ thin films consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ layers were successfully deposited via a multistep sol-gel route assisted by spin-coating. These heterolayered PZT films, when annealed at a temperature in the range of 600–700^∘C show (001)/(100) preferred orientation, demonstrate desired ferroelectric and dielectric properties. The most interesting ferroelectric and dielectric properties were obtained from the six-layered PZT thin film annealed at 650^∘C, which exhibits a remanent polarization of 47.7 μC/cm² and a dielectric permittivity of 1002 at 100 Hz. Reversible polarization constituents a considerably high contribution towards the ferroelectric hysteresis of the heterolayered PZT films, as shown by studies obtained from C-V and AC measurement.     

Effect of AC&DC Field on the Dielectric Response of (Pb,La)TiO3 Thin Films

Lanthanum doped lead titanate thin films are the potential candidates for the capacitors, actuators and pyroelectric sensor applications due to their excellent dielectric, and ferroelectric properties. Lanthanum doped lead titanate thin films are grown on platinum coated Si substrates by excimer laser ablation technique. A broad diffused phase transition with the maximum dielectric permittivity (?_max) shifting to higher temperatures with the increase of frequency, along with frequency dispersion below T_c, which are the signatures of the relaxor like characteristics were observed. The dielectric properties are investigated from ?60°C to 200°C with an application of different dc fields. With increasing dc field, the dielectric constant is observed to reduce and phase transition temperature shifted to higher temperature. With the increased ac signal amplitude of the applied frequency, the magnitude of the dielectric constant is increasing and the frequency dispersion is observed in ferroelectric phase, whereas in paraelectric phase, there is no dispersion has been observed. The results are correlated with the existing theories.     

Bilayered Pb(Zr,Ti)O3/(Bi,Nd)4Ti3O12 thin films

Bilayered thin films consisting of Pb(Zr_0.52Ti_0.48) O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) layers are successfully deposited on Si(100)/SiO₂/Ti/Pt substrate by a combined process involving sol-gel and RF-sputtering. Their dielectric properties cannot be described by the simple rule of mixture on the basis of the series connection model. There occurs a dielectric layer of lower dielectric permittivity in the bilayered thin film, which degrades the polarization behaviors. The bilayered film gives rise to an improvement in fatigue resistance up to 10¹⁰ switching cycles. Moreover, the domain pinning effect after polarization switching is reduced greatly as compared to that of single layered PZT and BNT thin films.     

The Enhanced Dielectric and Insulating Properties of Al2O3 Modified Ta2O5 Thin Films

Future-generation memory devices will require materials with higher dielectric constants compared to conventional dielectric materials such as silicon oxide and silicon nitride. Tantalum oxide (Ta₂O₅) is one of the most promising high dielectric constant materials because of its ease of integration into conventional VLSI processes compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were previously reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1 – x)Ta₂O₅-xAl₂O₃ was not clearly understood. The present research attempts to explain the higher dielectric constant of (1 – x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅-0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. Systematic study of dielectric and insulating properties of (1 – x)Ta₂O₅-xAl₂O₃ thin films indicate improved leakage current properties and reliability characteristics such as temperature coefficient of capacitance and bias stability with increase in Al₂O₃ concentrations.     

Random-Site Cation Ordering and Dielectric Properties of PbMg1/3Nb2/3O3-PbSc1/2Nb1/2O3

(1 ? x)PbMg_1/3Nb_2/3O₃-(x)PbSc_1/2Nb_1/2O₃ (PMN-PSN) solid solution crystals have been grown by the flux method in the whole concentration range. X-ray supercell reflections due to B-cation ordering were observed for as-grown crystals from the 0.1 ≤ x ≤ 0.65 compositional range. Though the ordered domains are rather large (～50 nm) the relaxor-like dielectric behavior is observed for compositions with x < 0.6. The diffusion of the dielectric permittivity maximum in as-grown crystals is the lowest at x = 0.6 and increases towards the end members of solid solution. Such behavior is explained within a Bragg-Williams approach by employing the random layer model. At x ～ 0.6 the excitation energy determined from the Vogel-Fulcher relation exhibits a jump which we regard to changing the kind of the polar regions from PbMg_1/3Nb_2/3O₃ to PbSc_1/2Nb_1/2O₃ related type.     

Electromagnetic Wave Absorbing Properties of High-Permittivity Ferroelectrics Coated with ITO Thin Films of 377 Ω

For the aim of thin electromagnetic wave absorbers used in quasi-microwave frequency band, this study proposes the high-permittivity ferroelectrics of quarter wavelength thickness (/4 spacer) coated with ITO thin film of 377 /sq (impedance transformer). For high-permittivity dielectrics, BaTiO₃ (BT), 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (PMN-PT) and 0.8Pb(Mg_1/3Nb_2/3)O₃-0.2Pb(Zn_1/3Nb_2/3)O₃ (PMN-PZN) are prepared by conventional ceramic processing technique. The ferroelectric materials show high dielectric constant and dielectric loss in microwave frequency range and their dominant loss mechanism is considered to be domain wall relaxation or dynamics of polar clusters. The microwave absorbance (determined at 2 GHz) of BT, PMN-PT and PMN-PZN are found to be 65% (at a /4 thickness of 3.5 mm), 20% (2.5 mm) and 37% (2.5 mm), respectively. By coating ITO thin films on the ferroelectric substrates with a thickness of /4, the microwave absorbance is greatly improved. Particularly, when the sheet resistance of ITO films is closed to 377 /sq, the reflection loss is reduced to –20 dB (99% power absorption). This is attributed to the wave impedance matching led by ITO thin film combined with a /4 thickness of high-permittivity dielectric spacer. It is, therefore, successfully proposed that the ITO/ferroelectrics structure with controlled electrical properties and thickness can be useful as thin microwave absorbers used in quasi-microwave frequency band.     

Preparation and properties of sputtered lead titanate thin films on MgO single crystals and mgo buffer layers

Abstract

By means of planar multitarget sputtering (001) oriented PbTiO₃ films were deposited onto highly preferred (100) oriented platinum electrodes on (100) MgO single crystal substrates. Single phase perovskite type films with a degree of (001) orientation between 60% and 70% have been sputtered at substrate temperatures as low as about 470°C. The as grown films exhibit a dielectric constant in the range of 120 to 140 and a pyroelectric coefficient of about 20 nCcm^?2K^?1 at room temperature. The dielectric loss is about 0.01 at frequencies from 1 to 10 kHz. (100) GaAs substrates with an evaporated, highly oriented (100) MgO buffer layer were also used as substrates. However, on these substrates the platinum bottom electrode did not grow highly oriented though the same deposition parameters for Pt deposition as in the case of the single crystalline MgO substrate were used. That's why PbTiO₃ was produced with a lower (001) preferred orientation. Therefore, the dielectric constant is higher (170–190) and the pyroelectric coefficient is lower (12 nCcm^?2K^?1).     

Structural and Electrical Properties of Ba(ZrxTi1-x)O3 Films Prepared by RF-magnetron Sputtering Using Metal Targets

Ba(Zr_xTi_1-x)O₃ (BZT) thin films with different Zr contents were deposited on (100)MgO and (100)Pt/(100)MgO substrates by RF-magnetron sputtering using metal targets. The BZT thin films had a single perovskite phase with only (001)/(100) orientation. In all cases, the ratio of Ba/Ti was stoichiometry and BZT films possess a dense microstructure. The grain size was decreased and BZT thin films showed ferroelectric-to-paraelectric properties with increasing Zr content. At room temperature, the tunability of nearly 30% was achieved at 1 MHz; meanwhile, a relatively low dielectric loss was obtained. These results indicated that we succeeded in depositing high-quality and potential tunable ferroelectrics.     

Chemical solution processing and properties of tungsten bronze (ba,la)nb2o6 thin films

Abstract

Highly oriented (Ba,La)Nb₂O₆ thin films have been synthesized by a chemical solution deposition method. A homogeneous and stable (Ba_0.75La_0.167)Nb₂O₆ (BLN) precursor solution was prepared by controlling the reaction of metal alkoxides. BLN precursor films crystallized in the tetragonal tungsten bronze phase at 700°C. BLN thin films on MgO(100) and Pt(100)/MgO(100) substrates showed the prominent c-axis preferred orientation. BLN thin films on Pt(100)/MgO(100) exhibited the diffuse phase transition depending upon the frequency.