TiO3 Ferroelectric Thin Films Research Articles

Nanoscale defects and microwave properties of (BaSr)TiO3 ferroelectric thin films

Thin film ferroelectrics may have important applications in microwave devices but in general have significantly worse properties than bulk material. This is principally because of secondary and point defects. The natures of the defects are reviewed and strategies to study and remove them outlined.

Specific features of the formation of Ba x Sr1 − x TiO3 ferroelectric thin films on different substrates by radio-frequency sputtering

Epitaxial thin films of Ba x Sr1 − x TiO3 solid solutions on MgO(001) and NdGaO3(011) single-crystal substratesare prepared by radio-frequency cathode sputtering of a ceramic target with stoichiometric composition on a magnetron-sputtering apparatus. The main qualitative differences in optical, structural, and electrical properties of the ferroelectric films prepared are determined, and their substantial dependence on the choice of the substrate is revealed.

Proton redistribution and pseudoantiferroelectricity in H+ implanted Pb(Zr,Ti)O3 thin films

Hydrogen ions were implanted into 500-nm-thick Pb(Zr,Ti)O3 ferroelectric thin films with different doses and energies. An antiferroelectric behavior was confirmed in the implanted thin films with proper H+ injection energies from independent measurements of polarization-electric hysteresis loops and capacitance-voltage curves. With the increase in the H+ doping concentration and implanting energy up to 25 keV, the characteristic pseudoantiferroelectric behavior becomes more evident in the films along with the concomitant reduction in the remnant polarization. However, the above antiferroelectricity is weakened for the restoration of a preferred ferroelectric state, once the implanting energy is higher than 35 eV. The consequent “Trim94” simulation of the H+ distribution as well as the induced oxygen vacancies (VO⋅⋅) indicates the almost linear shift in the depth for the maximum charge density within the film with the enhanced implanting energy until the depth moves out of the film thickness above 40 keV. Beyond the antiferroelectric dependence on the implanting energy in thin films, the previous ferroelectric state can also be rejuvenated under a bipolar-field stressing through the redistribution of the H+ and VO⋅⋅ concentrations. The rejuvenation process is accelerated upon heating due to the increased charge mobility. The doping charges are immobile during short-time domain switching but movable under a long-time negative/positive field with the estimated activation energy of 0.23/0.29 eV. This study implies the potential application of high-density charge storage of the implanted ferroelectric capacitor with the property comparable to a genuine antiferroelectric capacitor.

Ferroelectric Properties of Bi3.25La0.75 Ti3O12 Thin Films Crystallized in Different N2/O2 Ambients

Bi3.25La0.75 Ti3O12 (BLT) ferroelectric thin films are deposited by sol-gel method and annealed for crystallizaion in total 1sccm N2/O2 mixed gas with various ratio at 750°C for 30 min. The effect of crystallization ambient on the structural and ferroelectric properties of the BLT films is studied. The growth direction and grain size of BLT film are revealed to affect ferroelectric properties. After the BLT film is annealed in 20% O2, the largest Pr value is obtained, which is ascribed to an increase of random orientation and large grain size. The fatigue property is improved with the concentration of oxygen in the ambient increasing, which is ascribed to annealing in the ambient with high concentrated oxygen adequately decreasing the defects related to lack of oxygen.

Ferroelectric tunable balanced right- and left-handed transmission lines

The tuning of the dispersion characteristics of a balanced composite right- and left-handed transmission lines is demonstrated at centimeter and millimeter wavelengths. To this aim, coplanar waveguides, periodically loaded by interdigitated capacitances and stublike inductances, were fabricated onto a Ba0.5Sr0.5TiO3 ferroelectric thin film. The scattering parameter measurements show that the lines are left handed from 8 to 18 GHz and right handed above. The results are interpreted by the retrieval of the effective permittivity and permeability, which exhibit Drude-like dispersion characteristics with a magnetic plasma frequency, which can be tuned via the voltage-controlled permittivity variation in the BST thin film.

Deposition of BaxSr1–xTiO3 thin Films by Double RF Hollow Cathode Plasma Jet System

AbstractLow‐pressure plasma jet system with two hollow cathodes was used for deposition of Bax Sr1–x TiO3 (BSTO) ferroelectric thin films. Two RF hollow cathode plasma jet guns were pointed to the substrate where BSTO thin films were deposited. High density RF hollow cathode discharge was generated in ceramic nozzles made of SrTiO3 (STO) and BaTiO3 (BTO). Pulse modulated RF power was applied on water‐cooled copper block insulated from the plasma by ceramic and grounded stainless steel shield. In the experiment, the composition of BSTO films was controlled by magnitude of average RF power applied on particular plasma jet guns with STO and BTO nozzles. By this method, BSTO films were deposited on single crystal silicon substrate and on Pt top of Pt/TiO2/SiO2/Si multi‐layer system. X‐ray diffraction proved that BSTO thin films were polycrystalline with perovskite structure and measurement of dielectric properties showed ferroelectric hysteresis loop at room temperature. Time resolved Langmuir probe system and plasma impedance monitor were used for in situ plasma diagnostics and control of deposition conditions. Furthermore, plasma jets were analyzed during the deposition process by spatially resolved optical emission spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

PLD and RF-PLD synthesis of Ba0.6Sr0.4TiO3 ferroelectric thin films for electrically controlled devices

Ba0.6Sr0.4TiO3 (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance–voltage (C–V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.

Heterostructure and Compositional Depth Profile of Low-Temperature Processed Lead Titanate-Based Ferroelectric Thin Films Prepared by Photochemical Solution Deposition

The heterostructure and compositional depth profile of low-temperature processed (Pb0.76Ca0.24)TiO3 (PCT24) ferroelectric thin films have been studied in the present work. The films were prepared by ultraviolet (UV) sol–gel photoannealing (also called photochemical solution deposition, PCSD) onto platinized silicon substrates and crystallized at 450 °C in air and oxygen atmospheres. Despite using such a low temperature, analysis carried out by X-ray photoelectron spectroscopy (XPS) revealed the total lack of organic rests within the bulk film. Complementary information about the heterostructure of the films was also obtained by Rutherford backscattering spectroscopy (RBS). Both analytical techniques detected the presence of a lead gradient in the films, together with small fluctuations on the concentration of this element along the bulk film. The RBS study also showed that the films of this work develop a PtxPb interface between the ferroelectric layer and the Pt bottom electrode. The thickness of this in...

Effects of annealing schedule on orientations of Bi3.2Nd0.8Ti3O12 ferroelectric films prepared by metalorganic solution deposition

Fatigue-free Bi3.2Nd0.8Ti3O12 ferroelectric thin films were successfully prepared on p-Si(111) substrate using metalorganic solution deposition process. The orientation and formation of thin film under different annealing schedules were studied using XRD and AFM. XRD analysis indicated that (200)-oriented films with degree of orientation of I(200)/I(117)=2.097 and 0.466 were obtained by preannealing the film at 400°C for 10min followed by rapid thermal annealing at 700°C for 3min, 10min and 20min, respectively, (008)-oriented film with degree of orientation of I(008)/I(117)=1.706 were obtained by rapid thermal annealing the film at 700°C for 3min without preannealing, and (008)-oriented film with degree of orientation of I(008)/I(117)=0.719 were obtained by preheating the film from room temperature to 700°C at 20°C/min followed by annealing for 10min. The a-axis and c-axis orientation decreased as increase in annealing time due to effects of (111)-oriented substrate. AFM analysis further indicated that preannealing at 400°C for 10min followed by rapid thermal annealing at 700°C for 3min resulted in formation of platelike crystallite parallel to substrate surface, however rapid thermal annealing at 700°C for 3min without preannealing resulted in columnar crystallite perpendicular to substrate surface.

Electron emission characteristics of sol-gel (Ba0.67Sr0.33)TiO3 thin film coated silicon tips

Field emission characteristics of sol-gel (Ba0.67Sr0.33)TiO3 (BST) ferroelectric thin film coated silicon emitters have been systemically investigated. The effects of BST coating, BST annealing temperature, and biasing at BST-silicon junction on field emission characteristics of BST coated silicon tips have been studied. The authors have found that field emission characteristics of silicon tips can be significantly improved by BST coating. The turn-on electric field required between the silicon tips and the anode significantly reduced from 70to4–20V∕μm by BST coating. The theoretical studies have shown that the improved emission characteristics by BST coating may be attributed to the lower electron affinity and the lower work function properties of BST material. Emission stability was also improved by BST coating. BST coated silicon emitter has a relatively stable emission current with 20% fluctuation compared to 50% current fluctuation of uncoated silicon tips. It was also found that the emission characteristics of BST coated silicon tips strongly depended on BST annealing temperature. The samples with 750°C BST annealing temperature were found to have the best emission characteristics with low turn-on electric field of 4–9V∕μm. A higher annealing temperature results in higher activation of ionic conduction, increasing carrier concentration, raising the Fermi level toward the conduction band, and thus reducing the effective work function. Finally, the electron field emission from BST coated silicon tips was improved significantly by the positive bias on BST-silicon junction. The theoretical studies have shown that the effective work function is reduced with a positive bias on BST-silicon junction and hence the turn-on electric field is significantly reduced by the bias on BST-silicon junction.

Probing fatigue in ferroelectric thin films with subnanometer depth resolution

The authors report the study of polarization fatigue in Pb(Zr,Ti)O3 (PZT) ferroelectric thin films using in situ high-resolution grazing incidence x-ray specular reflectivity of synchrotron radiation. The results demonstrate that there is no formation of a region of different electron densities in the film growth direction with subnanometer depth resolution during fatigue. The upper bounds on the theoretically predicted interfacial accumulation of oxygen vacancies at the interfaces between PZT and Pt electrodes are determined by the comparison of experimental results and theoretical simulations.

Structural and optical characterization of Ba0.8Sr0.2TiO3 PLD deposited films

Structural and optical properties of Ba0.8Sr0.2TiO3(BST) ferroelectric thin films, deposited by the pulsed laser ablation (PLD) technique on Si/SiO2/Ti/Pt, Si/SrRuO3 and Si substrates, were performed by X-ray diffraction, micro Raman, atomic force microscopy (AFM), and optical reflectometry measurements. Temperature dependences of the Raman spectrum, and room temperature refractive and extinction indices, and surface roughness were evaluated.

Structural and optical properties of Bi3.25Nd0.75Ti3O12 ferroelectric thin films

Ferroelectric Bi3.25Nd0.75Ti3O12 (BNT) thin films were grown on (111)Pt/Ti/SiO2/Si substrates by a chemical solution method. The films were composed of large rod-like grains. XRD and Raman spectroscopy measurements showed they were polycrystalline perovskite structure with a good crystallinity. Pt/BNT/Pt capacitors had been fabricated and showed good ferroelectricity. The optical constants (n, k) of BNT thin films in the wavelength ranges of 0.2–1.7 μm and 2.5–11.4 μm were obtained by spectroscopic ellipsometry measurements. The dispersion of the refractive index in the interband transition region followed the single electronic oscillator model. The optical band gap was found to be about 3.61 eV.

Microstructure and electrical properties of BaTiO3 and (Ba,Sr)TiO3 ferroelectric thin films on nickel electrodes

Nickel thin films have been sputtered on standard Si/SiO2 substrates with TiO2 as an adhesive layer. The thermal stability of these substrates was analyzed. SEM images show an increase in grain size with annealing temperature. They were found to be stable till 800°C, beyond which the nickel layer disintegrated. These substrates were used for deposition of BaTiO3 and (Ba,Sr)TiO3 dielectric thin films under a reducing atmosphere. The dielectric thin films were processed with various pyrolysis and annealing temperatures in order to optimize the dielectric properties. Increased pyrolysis temperatures showed an increase in the grain size. Results on these nickelised substrates were finally compared with dielectric films deposited on platinized silicon substrates under identical conditions but crystallized in an oxygen atmosphere.

Low-temperature electrical characteristics of Bi3.15Nd0.85Ti3O12 thin films

Ferroelectric and dielectric characteristics of Bi3.15Nd0.85Ti3O12 ferroelectric thin films were studied at temperatures from 296 down to 100K. The temperature dependence of these characteristics was discussed in terms of the Rayleigh model. Although the Rayleigh law fits well the dielectric data below 140K, it cannot describe the data at higher temperatures, where a mechanism contributes to the dielectric constant without producing losses. This mechanism may probably be related to bending of domain walls pinned by two nearest obstacles. The contribution from such bending is frozen out at low temperatures due to enhanced domain pinning.

Linear and nonlinear optical properties of (Pb,La)(Zr,Ti)O3 ferroelectric thin films grown by radio-frequency magnetron sputtering

Linear and nonlinear optical properties of lanthanum-modified lead zirconate titanate (Pb0.92La0.08)(Zr0.65Ti0.35)O3 (PLZT 8/65/35) ferroelectric thin films are presented in this paper. The PLZT ferroelectric thin films were grown on quartz substrates by radio-frequency magnetron sputtering at 650 °C. Their crystalline structure and surface morphologies were examined by x-ray diffraction and atomic force microscopy, respectively. It was found that the PLZT thin films exhibit well-crystallized perovskite structure and good surface morphology. The fundamental optical constants (the band gap energy, linear refractive index and absorption coefficient) were obtained through the optical transmittance measurements. A Z-scan technique was used to investigate the optical nonlinearity of the PLZT thin films on quartz substrates. The films display a strong third-order nonlinear optical effect. A large and negative nonlinear refractive index n2 is determined to be 1.21 × 10−6 esu for the PLZT thin films. All results show that the PLZT ferroelectric thin films have potential applications in nonlinear optical devices.

Enhanced ferroelectric loop asymmetry of lead zirconate titanate thin films under nanoindentation

The effect of mechanical stress on the local switching behavior and hysteresis properties of Pb(Zr,Ti)O3 and Mn-doped Pb(Zr,Ti)O3 ferroelectric thin films has been studied using spherical nanoindentation. A conductive WC–Co cermet indenter tip of 500μm nominal radius was employed in a modified nanoindentation system to allow the simultaneous application of driving voltage and mechanical loading, and consequently to collect the ferroelectric charge developed on thin films without a top electrode. Instrumented progressive indentation caused an enhancement of hysteresis loop deformations. The charge released versus applied voltage (Q-V) hysteresis loops gradually shifted along the voltage axis with increasing indentation force. Also, an enhanced vertical shift and hysteresis gap of the Q-V loops was observed. The parameter of horizontal loop asymmetry (δ) increased almost linearly with the force by an increment of about (0.4–0.5)×10−3∕100mN with a 50V peak drive voltage and a 50Hz sinusoidal wave form. The effect of nanoindentation on polarization reversal in thin ferroelectric films is suggested to result from a variation in residual stress state, asymmetric distribution of charged defects, and asymmetric lattice distortion produced by the inhomogeneous indentation stress field.

Crystal orientation dependence of the dielectric properties for epitaxial BaZr0.15Ti0.85O3 thin films

Epitaxial Ba0.15Zr0.85TiO3 (BZT) ferroelectric thin films with (001), (011), and (111) orientations were, respectively, grown on La0.67Sr0.33MnO3 (LSMO) buffered LaAlO3 substrates by pulsed laser deposition method. The dc electric-field dependence of permittivity and dielectric loss of (001)-, (011)-, and (111)-oriented BZT/LSMO heterostructures obeys the Johnson formula, and the ac electric-field dependence of that obeys the Rayleigh law under the subswitching field region. The anisotropic dielectric properties are attributed to the higher mobility of the charge carriers, the concentration of mobile interfacial domain walls, and boundaries in the (111)-oriental films than in the (110)- and (100)-oriented films.

Large third-order optical nonlinearity in (Pb,La)(Zr,Ti)O3 ferroelectric thin film

Ferroelectric (Pb0.92La0.08)(Zr0.65Ti0.35)O3 (PLZT) film with good surface morphology and perovskite structure was grown in situ on quartz substrate by radio-frequency magnetron sputtering at 650°C. The fundamental optical constants (the band gap energy, linear refractive index, and absorption coefficient) of the film are obtained by optical transmittance measurement. The third-order nonlinear optical properties of the films were investigated by the Z-scan technique. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. A large nonlinear refractive index as high as 1.21×10−6esu is displayed in the film. All results show that PLZT ferroelectric thin film has potential applications in nonlinear optical devices.

Linear and nonlinear optical properties of RF sputtered (Pb,La)(Zr,Ti)O3 ferroelectric thin films

Linear and nonlinear optical properties of (Pb,La)(Zr,Ti)O-3 (PLZT) ferroelectric thin films were presented in this paper. The PLZT ferroelectric thin films have been in situ grown on quartz substrates by radio-frequency (RF) magnetron sputtering at 650 degrees C. Their crystalline structure and surface morphologies were examined by X-ray diffraction and atomic force microscopy, respectively. It can be found that the PLZT thin films exhibit well-crystallized perovskite structure and good surface morphology. The fundamental optical constants (the band gap energy, linear refractive index, and linear absorption coefficient) were obtained through the optical transmittance measurements. A Z-scan technique was used to investigate the optical nonlinearity of the PLZT thin films on quartz substrates. The films display the strong third-order nonlinear optical effect. A large and negative nonlinear refractive index n(2) is determined to be 1.21 x 10(-6) esu for the PLZT thin films. All results show that the PLZT ferroelectric thin films have potential applications in optical limiting, switching, and modulated-type optical devices.