Dielectric and lattice dynamical properties of SrTiO3 thin films

Abstract

Pulsed-laser-deposited SrTiO₃ thin films are used as all example to illustrate the effects of strain. oxygen vacancies, and interfacial dead layer on the dielectric and lattice dynamical properties in ferroelectric thin films. We found that strain, both lattice mismatch-induced and due to local defects such as oxygen vacancies, dramatically influences the properties of the thin films. The cubic-to-tetragonal structural phase transition is greatly modified by the strain in the films. Raman scattering shows the symmetry-forbidden optical phonons, indicating the reduction of symmetry. The line shape of the polar phonon shows a Fano asymmetry, which we suggest to be due to the micro polar regions or other local polar structures around the oxygen vacancies. The temperature dependence of the low-frequency complex dielectric constant shows markedly distinct quantum fluctuation behaviors from those of the single crystals. The soft mode is considerably hardened in the thin films compared to the single crystals.     

Fatigue and dielectric properties of undoped and Ce doped PZT thin films

Abstract

Ce doping improves the ferroelectric properties of sol-gel derived PZT thin films by facilitating easier domain wall movement. It also decreases the leakage current densities by reducing the concentration of free carriers through a decrease in concentration of Pb and O vacancies. Ce-PZT films retain good dielectric dispersion characteristics since the concentration of defects and defect dipoles are reduced. Ce doping dramatically improves the fatigue resistance of PZT thin films. We have studied the frequency dependence of fatigue behavior and shown that the loss of polarization due to fatigue follows a universal scaling behavior with N/f², where N is the number of the switching cycles and f is the frequency. The origin of the scaling is attributed to the drift of oxygen vacancies, which is the rate limiting process in the growth of the interface layer responsible for fatigue. Empirical fits for both undoped and cerium doped samples show that switchable polarization follows a stretched exponential decay with time or N/f. Cerium doping is believed to improve fatigue resistance by impeding the motion of oxygen vacancies.     

Fatigue properties of Sb doped PZT thin films deposited by dc reactive sputtering

Abstract

Sb doped reactive sputtering-derived Pb(Zr, Ti)O₃ (Zr/Ti=48/52) thin films were investigated with the intention of improving ferroelectric properties. Also, the atomic valence of Sb in PZT thin film was confirmed as trivalent cation (Sb³⁺) by x-ray photoelectron spectroscopy (XPS). According to the tolerance factor t, Sb³⁺ tends to occupy the B-site of ABO₃ perovskite structure and acts as an acceptor that generates oxygen vacancies and holes. Transmission Electron Microscope(TEM) was used to observe the structural changes of PZT thin films by Sb addition. The leakage current densities and P_r of PZT thin films increased as the Sb contents increased. 0.7at% Sb doped PZT(PZST07) thin films exhibited improved fatigue properties (about 10% degradation of the remanent polarization after 10¹⁰ switching cycles).     

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 reactively sputtered IrOx for PZT electrode applications

Abstract

The use of IrO_x for electrodes in PbZr_xTi_1?xO₃ (PZT) capacitors for ferroelectric memory applications has proven to be advantageous in several respects. In comparison with Pt, IrO_x often exhibits improved fatigue and provides resistance to hydrogen induced degradation at elevated temperatures. Since IrO_x is often produced by sputtering in an oxygen containing environment, several forms of IrO_x can be produced depending on the process conditions. This work concentrates on an analysis of the DC reactive sputtering of IrO_x from an Ir metal target. As with other oxidizable metals, Ir exhibits a transition between metal and oxide mode sputtering when sputtered in oxygen containing atmospheres. Variations in the Ar/O₂ gas flow ratio were used to produce Ir and IrO_xfilms on both sides of the metal-to-oxide mode reactive sputtering transition. Changes in the IrO_x film properties were quantified by using a combination of metrics including X-ray diffraction, sheet resistance, and stress. It was found that the, IrO_x crystalline structure and other IrO_x properties could be directly related to the ferroelectric switching performance of PZT capacitors with IrO_xtop electrodes. A relationship between IrO_x deposition processes and resistance to etch induced damage of the ferroelectric properties was also observed.     

Antiferroelectric thin and thick films for high-strain microactuators

Abstract

Niobium-doped or lanthanum-doped lead zirconate titanate stannate antiferroelectric thin and thick films have been prepared on platinum-buffered silicon substrates by a modified sol-gel method and their electric properties were characterized, with emphasis on the field-induced phase transition strains. All the films demonstrate zero remanent polarization and a maximum polarization of more than 30 μC/cm². By choosing the appropriate compositions, the films can have either “square” hysteresis loops with very sharp phase transition or “slanted” hysteresis loops with gradual phase transition. The strain levels of the thin films can reach 0.32% with smaller hysteresis or 0.42% with moderate hysteresis. The thick films can either have a strain level of 0.38% with very small hysteresis or have a strain level of 0.48% with clear digital actuator response. Hence the strain levels of these antiferroelectric films are comparable to that of bulk materials and double that of PZT ferroelectric films, and they are very promising for actuation applications in microelectromechanical systems.     

Effects of temperature on the ferroelectric properties of PbZr0.53Ti0.53O3-based capacitors prepared by pulsed laser-ablation deposition

Abstract

The effects of elevated temperature on the long and short term ferroelectric properties of La_0.5Sr_0.5Co₃(LSCO)/PbZr_0.53 Ti_0.47O₃(PZT)/La_0.5Co₃(LSCO) heterostructure capacitors are presented in this paper. Capacitors are evaluated in the temperature range of 25°C to 100°C. Polarization values (switched and non-switched), coercive voltages, and PZT resistivity are measured as a function of temperature. Rate of fatigue, tendency to imprint, and retention loss are also investigated in the 25–100°C temperature range. An activation energy for the process responsible for loss of remanent polarization is determined from the fatigue data.     

Ferroelectric and dielectric properties of sol-gel and excimer-laser-deposited lead zirconate titanate and barium titanate films

Abstract

Switched remanent polarization was measured as a function of accumulated switching cycles for a variety of ferroelectric films using sinusoidally driven hysteresis loops. Switched remanent polarization and dielectric constant and loss were also obtained as a function of the cycling frequency. PZT films with niobium additives appeared to lose switched remanent polarization with accumulated cycles at a lesser rate than films without niobium. The switched remanent polarization was found to decrease with increasing frequency, which we attribute to the effect of grain size. Also, a decrease of dielectric constant with increasing frequency and an increase of dielectric constant with increased applied voltage are attributed to the effects of domain wall motion contributions to dielectric constant.     

The Effect of La-Doped Bi4Ti3O12 Buffer Layer on Crystallinity and Ferroelectric Properties of PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 Multilayered Thin Films

PbZr_0.58Ti_0.42O₃ (PZT) ferroelectric thin films with Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer of various thickness were fabricated on Pt/TiO₂/SiO₂/p-Si(100) substrates by rf-magnetron sputtering method. The pure PZT film showed (111) preferential orientation in the XRD patterns, and the PZT/BLT films showed (110) preferential orientation with increasing thickness of the BLT layer. There were no obvious diffraction peaks for the BLT buffer layer, for its thin thickness in PZT/BLT multilayered films. There were the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples from the surface images of FESEM. The growth direction and grain size had significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics suggested that 30-nm-thick BLT was just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

Poling of pyroelectric thin films

Abstract

Poling of calcium and zirconium modified lead titanate pyroelelectric thin films has been studied. Poling temperature and electrical field were optimized to achieve high figure of merits. It was found that thermal activation is important to obtain highly poled films. Defect-dipole complexes are supposed to play a major role to retain the polarization and also to enhance it. Porous calcium substituted films achieved an up to three times higer voltage response than zirconate substituted thin films, because porosity decreased the dielectric constant significantly.     

Comments on breakdown,fatigue, and ultrasonic attenuation in ferroelectric films

Abstract

We compare several models of electrical breakdown, fatigue, and attenuation in ferroelectric thin (ca 100 nm) films.     

Precursor design for liquid Injection CVD of lead scandium tantalate thin films

Abstract

Liquid Injection Chemical Vapour Deposition is a technique well suited to the deposition of a range of ferroelectric oxides in thin film form. This paper reports on the design and optimisation of the precursors for the deposition of thin films of the promising pyroelectric material, lead scandium tantalate. The design and use of lead, scandium and tantalum precursors with carefully matched chemical and thermal properties allows the growth of thin films with the required perovskite phase above 550°C. These have the desired 111 orientation when grown on to platinum coated silicon substrates at temperatures above 575°C.     

Texturing and electrical characteristics of sol-gel derived ferroelectric thin films

Abstract

Highly oriented PZT, PLZT and PMN thin films were fabricated on various substrates using sol-gel method. The thin films having different orientation were fabricated by different drying conditions for pyrolysis. The preferred orientations of the PZT, PLZT and PMN thin films were observed using XRD(X-ray diffraction). The hysteresis loops, capacitance-voltage, and fatigue characteristics of the films were investigated using an RT66A standardized ferroelectric test system. The dielectric constant and current-voltage characteristics of the films were investigated using impedance analyzer and pA meter, respectively. The films oriented in particular direction showed better electrical characteristics to the randomly oriented films.     

Lead zirconate titanate thin films on ruthenium dioxide; in situ synthesis using organometallic chemical vapor deposition

Abstract

In this paper the organometallic chemical vapor deposition (OMCVD) of RuO₂ using the precursor ruthenocene, Ru(C₅H₅)₂, will be discussed. Stoichiometric RuO₂ thin films with a specific resistivity of about 50 μΩcm were obtained both on strontium titanate and platinized silicon wafers at temperatures between 300–700°C. PbZr_0.8Ti_0.2O₃ thin films were in situ deposited onto these RuO₂ electrodes. The ferroelectric behavior of the films obtained on the RuO₂ electrodes is compared with those obtained on platinum electrodes. Attempts to deposit in-situ a RuO₂ top electrode on the PbZr_0.8Ti_0.2O₃ were unsuccessful.     

Recent progress in sputtering PZT thin films for ferroelectric memories

Abstract

This paper describes amorphous Pb(Zr, Ti)O₃ (PZT) thin films deposited by cosputtering Pb(Zr_0.5 Ti_0.5)O₃ and PbO targets. By optimizing the amount of the excess Pb and the deposition temperature, PZT thin films with a single perovskite phase were obtained successfully on Ir substrates and Pt substrates at 520°C. 250-nm-thick PZT films crystallized by rapid thermal annealing (RTA) at 600°C for 20 s exhibited excellent ferroelectric properties: a coercive voltage of 1.0 V, a remanent polarization density of about 40 μC/cm², and a polarization switching endurance over 1x109 cycles. Although a heat treatment in a reductive ambient causes degradation of ferroelectric properties of PZT thin films, their degraded ferroelectric properties can be easily recovered from by a 1-min RTA in an oxygen at 400°C.     

Improvement of the electrical properties of PZT thin films using TiO2 buffer layer

Abstract

Thin TiO₂ layers were sputter-deposited on Pt/Ti/SiO₂/Si wafers, as buffer layers for PZT thin film capacitors. It was found that TiO2 buffers of less than 4-nm-thickness could assist in obtaining highly uniform PZT thin films with no second phase. The leakage current behaviors of the PZT based capacitor are improved, while retaining the ferroelectric properties of PZT thin films such as remanent polarization and coercive field. In addition, the uniform distribution of oxygen in PZT on TiO₂/Pt indicates that the TiO₂ buffer layer act as a barrier for lead-platinum reaction, as well as for oxygen diffusion.     

Sputtered ferroelectric thin films for dynamic random access memory applications

Abstract

A low thermal budget (with 550°C annealing) process with Ti-compensation for sputtered ferroelectric PZT thin films has been developed. PZT films with a composition near the morphotrophic phase boundary (Zr/Ti = 53/47) annealed at 550°C for 1 hr in a N₂ ambient exhibits high charge storage density and low leakage current density, which are the important requirements of dielectric materials for ULSI DRAM cells. It was also found that Ti compensated films show good fatigue endurance in comparison with non-Ti compensated films.     

Ferroelectric thin films for Smart Spatial Light Modulator applications

Abstract

System, device and material issues for the use of ferroelectric ceramic thin films in the realization of Smart Spatial Light Modulators are considered. Results show that ferroelectric thin films such as PLZT PBN, KTN, and SBN are particularly attractive.     

Reactive ion etching of sol-gel deposited lead zirconate titanate (pzt) thin films in sf6 plasmas

Abstract

Sol-gel deposited lead zirconate titanate piezoelectric thin films were reactive ion etched in SF₆ plasmas. Etch rates increased approximately linearly with cathode power, reaching 65 nm/min at 300 W, Etch rate was independent of SF₆ flow rate but increased as pressure was reduced, indicating an etch process that is dependent on ion-induced mechanisms. Photoresist masks could be removed subsequent to etching, but mask erosion led to gradually sloped sidewalls. Use of Pt masks provided for anisotropic etching with vertical sidewalls and smooth etched surfaces.     

Conduction mechanism in antiferrolectric PbZrO3 thin films—analysis of charge carrier trapping phenomenon

Abstract

Antiferroelectric compositions, such as PbZrO₃, are attractive candidates in charge storage devices and actuator/transducer applications in MEMs technology. Thin films of PbZrO₃were deposited on Pt coated Si substrates by a pulsed excimer ablation process. The process of field induced ferroelectric phase switching involves the domain wall reorientation in the polycrystalline thin films. The presence of grain boundaries and various defects in the polycrystalline thin films acts as the pinning sources for the various domain walls. These defects capture the charge carriers in the presence of external applied field and hinders further switching of the dipoles in the domains, thereby increases the response times and threshold voltages for the devices operations. Understanding of the trapping phenomenon in these films is very essential. Using Lampert's theory of space charge limited conduction both shallow and deep trap energies were estimated approximately from charge transport analysis.