Comparison of the effect of PLT and PZT buffer layers on PZT thin films for ferroelectric materials applications

In order to study the effect of different buffer layers on the Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films, 10-nm thick (Pb_0.72La_0.28)Ti_0.93O₃ (PLT) and Pb(Zr_0.52Ti_0.48)O₃ buffer layers have been deposited on the Pt(1 1 1)/Ti/SiO₂/Si substrates by pulsed laser deposition, respectively. The top buffer layers were also deposited on PZT thin films with the same thickness of the seed layers in order to enhance the fatigue characteristics of PZT thin films. We compared the results of dielectric constant, hysteresis loops and fatigue resistance characteristics. It was found that the dielectric properties of PZT thin films with PLT buffer layers were improved by comparing with PZT thin films with PZT buffer layers. The polarization characteristics of PZT thin films with PLT buffer layers were observed to be superior to those of PZT thin films using PZT buffer layers. The remanent polarization of PZT thin films showed 36.3 μC/cm² and 2.6 μC/cm² each in the case of use PLT and PZT buffer layers. For the switching polarization endurance analysis, PZT thin films with PLT buffer layers showed more excellent result than that of PZT thin films with PZT buffer layers.     

Effect of buffer layers on the property of Pb(Zr,Ti)O3-Pb(Mn,W,Sb,Nb)O3 thin films grown by pulsed laser deposition

New ferroelectric Pb(Zr,Ti)O₃-Pb(Mn,W,Sb,Nb)O₃ (PZT-PMWSN) thin film has been deposited on a Pt/Ti/SiO₂/Si substrate by pulsed laser deposition. Buffer layer was adopted between film and substrate to improve the ferroelectric properties of PZT-PMWSN films. Effect of a Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Pb_0.72La_0.28)Ti_0.93O₃ (PLT) buffer layers on the stabilization of perovskite phase and the suppression of pyrochlore phase has been examined. Role of buffer layers was investigated depending on different types of buffer layer and thickness. The PZT-PMWSN thin films with buffer layer have higher remnant polarization and switching polarization values by suppressing pyrochlore phase formation. The remnant polarization, saturation polarization, coercive field and relative dielectric constant of 10-nm-thick PLT buffered PZT-PMWSN thin film with no pyrochlore phase were observed to be about 18.523 μC/cm², 47.538 μC/cm², 63.901 kV/cm and 854, respectively.     

Ferroelectric properties of bilayer structured Pb(Zr0.52Ti0.48)O3/SrBi2Ta2O9 (PZT/SBT) thin films on Pt/TiO2/SiO2/Si substrates

Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films with large remanent polarization and SrBi₂Ta₂O₉ (SBT) thin films with excellent fatigue-resisting characteristic have been widely studied for non-volatile random access memories, respectively. To combine these two advantages_, bilayered Pb(Zr_0.52Ti_0.48)O₃/SrBi₂Ta₂O₉ (PZT/SBT) thin films were fabricated on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition method. X-ray diffraction patterns revealed that the diffraction peaks of PZT/SBT thin films were completely composed of PZT and SBT, and no other secondary phase was observed. The electrical properties of the bilayered structure PZT/SBT films have been investigated in comparison with pure PZT and SBT films. PZT/SBT bilayered thin films showed larger remanent polarization (2P_r) of 18.37 μC/cm² than pure SBT and less polarization fatigue up to 1 × 10⁹ switching cycles than pure PZT. These results indicated that this bilayered structure of PZT/SBT is a promising material combination for ferroelectric memory applications.     

PbTiO3/PbZr0.3Ti0.7O3/PbTiO3夹心结构铁电薄膜子晶层的优化

用Sol-Gel法制备了Pb_(1+x)TiO₃/PbZr_03Ti_07O₃/Pb_(1+x)TiO₃(PT/PZT/PT)夹心结构及PZT铁电薄膜,为了获得高质量的PT/PZT/PT夹心结构铁电薄膜,使用不同过量Pb配比(x)的PbTiO₃ (PT)层进行制备,以获得优化的PT子晶层.X射线 关键词： PT/PZT/PT 夹心结构 子晶 铁电薄膜     

Fatigue-free Pb(Zr0.52Ti0.48)O3 thin films on indium tin oxide coated substrates by a sol-gel process

Fatigue-free Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin films were successfully prepared on indium tin oxide (ITO) coated glass substrates using the sol-gel method combined with a rapid thermal annealing process (RTA). The films post-annealed at a temperature of 700 °C for 2 min by RTA process formed (110)-oriented Pb(Zr_0.52Ti_0.48)O₃ thin films with pure perovskite structure, and had a good morphology as well. The good ferroelectricity of the prepared PZT films was confirmed by P–E hysteresis loop measurements. Fatigue characteristics showed stable behavior. Degradation in polarization was not found while the repeating cycles were up to 10¹¹, and a low leakage current density of 10⁻⁸ A/cm² was also obtained from the highly fatigue-resisted PZT thin films on ITO/glass substrates. Received: 19 October 1998 / Accepted: 29 March 1999 / Published online: 26 May 1999     

Excimer laser-induced transformation in laser ablated Pb(Zr0.52Ti0.48)O3 amorphous thin films

Pb(Zr_0.52Ti_0.48)O₃ (PZT) amorphous thin films were deposited on Si substrates at room temperature and 573?K by pulsed laser deposition. The as-deposited films were subsequently annealed at various laser power densities using a KrF pulsed excimer laser irradiation to induce the phase transformation from amorphous to ferroelectric perovskite structure. Structural analysis shows the possibility of transformation from pyrochlore to perovskite transformation when irradiated above a laser power density of 1.4?MW/cm², which is in agreement with the thermal simulation. The surface quality of the PZT films deposited on 573?K is remarkably superior to that deposited at room temperature due to the enhanced thin structure and composition homogeneity. Almost all the pyrochlore phase transformed into perovskite structure after annealing at 2.8?MW/cm² for 120?s for both PZT films deposited at room temperature and 573?K, respectively. P-E hysteresis measurement of the laser-treated PZT shows relatively low remnant polarization P _r of about 1.2?μC/cm².     

Pb(Zr0.53Ti0.47)O3 thin films with different thicknesses obtained at low temperature by microwave irradiation

Pb(Zr_0.53Ti_0.47)O₃ (PZT) thin films with different thicknesses (99-420 nm) were prepared on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by sol-gel method and films were annealed at 450 °C for 30 min using a single-mode cavity of 2.45 GHz microwaves. X-ray diffraction analysis indicated that the pyrochlore phase was transformed to the perovskite phase at above 166 nm films. The grain sizes were increased, surface roughnesses were decreased, and electrical properties were improved with film thickness. The leakage current density was 9 × 10⁻⁸ A/cm² at an applied electrical field of 100 kV/cm. The ohmic and field-enhanced Schottky emission mechanisms were used to explain leakage current behavior of the PZT thin films. These results suggest that microwave annealing is effective for obtaining low temperature crystallization of thin films with better properties.     

Phase compositions and ferroelectric properties of Pb(Zr0.52,Ti0.48)O3 thin films with different highly orientations prepared by a sol-gel route

It is shown that different highly oriented Pb(Zr_0.52,Ti_0.48)O₃ films can be obtained on Pt/Ti/SiO₂/Si substrate using a sol-gel technique. The effects of pyrolysis temperature on the orientation, phase composition and ferroelectric properties of the films are investigated. It is found the ferroelectric hysteresis loops of (1 1 1)-oriented film, (1 1 1) and (1 0 0) mix-oriented film can both be saturated when the external electric field is large enough, whereas the hysteresis loop of (1 0 0)-oriented film is difficult to saturate. The analysis of X-ray diffraction indicated the possibility of different phase composition in different oriented films under large film residual stress. Higher remnant polarization (53 μC/cm²) for (1 0 0)-oriented film can be attributed to its more tetragonal phase composition, which results in that the in-plane domain switching can continuously occur with external electric field increasing.     

(111)择优取向的Pb(Zr0.52Ti0.48)O3铁电薄膜的制备及研究

选用不同浓度的Pb(Zr_0.52Ti_0.48)O₃溶胶，用Sol-gel法在Pt/Ti/SiO₂/Si基片上沉积一层厚度不同的Pb(Zr_0.52Ti_0.48)O₃ (PZT52)过渡层，经400℃烘烤、550℃退火等程序后，再用Sol-gel法在PZT52过渡层上沉积Pb(Zr_0.52Ti_0.48)O _{关键词： PZT铁电薄膜 择优取向 过渡层 剩余极化强度}     

Structure evolution and stoichiometry control of Pb(Zr, Ti)O3 thick films fabricated by electrospray assisted vapour deposition

Well-crystallized and stoichiometric Pb(Zr, Ti)O₃ (PZT) films, typically ∼5 μm thick, with pure perovskite-type rhombohedral structures have been successfully prepared via an electrospray assisted vapour deposition (ESAVD) method. Control of the deposition temperature within a narrow range of 300-400 °C resulted in films with the most desirable phases. PZT films with close stoichiometric match with the expected composition ratio and uniform element distribution were obtained by adding the appropriate levels of excess Pb in the precursor solutions. The annealed films were uniform, dense, compact and adherent to the substrates. The dielectric constant, ?_r, and loss tangent, tan δ, of the fabricated PZT films measured at 10 kHz were 442 and 0.09, respectively. The ESAVD deposited PZT films showed a remanent polarization, P_r, of 15.3 μC/cm² and coercive field, E_c, of 86.7 kV/cm. These results demonstrate the clear potential of the ESAVD method as a promising technique for the fabrication of thick PZT films.     

Preparation and characterization of poled nanocrystal and polymer composite PZT/PC films

Pb(Zr_0.52Ti_0.48)O₃ (PZT) nanocrystals and transparent polycarbonate (PC) composite thin films with useful properties for ferroelectric, piezoelectric and electro-optic devices were prepared by a spin-coating technique. Ultra-fine PZT (∼40–50 nm) nanocrystals with pure perovskite tetragonal phase were synthesized by a hydrothermal method. The structure and morphology of the composite thin films were studied by means of X-ray diffraction and scanning electron microscopy. To obtain the optimum electro-optic properties of poled composite films, the poling condition under an external electric field was optimized through the dielectric properties of PZT and PC polymer and effective field intensity theory. The electro-optic coefficient of the poled PZT/PC composite film is estimated to be 30.5 pm/V. The transparency spectra were measured and the optical band gaps of the unpoled, poled at 145 °C and poled at 165 °C composite thin films are estimated to be 4.26 eV, 4.21 eV and 4.18 eV, respectively. The measured dielectric constants of PZT/PC are in good agreement with the calculated values for the composite with a very small PZT volume fraction, based on the Onsager effective-field theory. This offers a reliable and indirect way to predict the dielectric constant of nanocrystals. PACS 81.40.Tv; 78.66.Sq; 78.66.Vs     

Influence of plasma pressure on the growth characteristics and ferroelectric properties of sputter-deposited PZT thin films

PZT thin films of thickness (320-1040) nm were synthesized on Si/SiO₂/Ti/Pt multilayered substrates by radio frequency magnetron sputtering. The influence of plasma pressure in the range of (0.24-4.9) Pa, during deposition, on the structural, electrical and ferroelectric properties of the PZT films was systematically studied. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and cross-sectional transmission electron microscopy (XTEM) were employed for structural study. Nano-probe Energy Dispersive (EDX) line scanning was employed to investigate the elemental distribution across the film-bottom electrode interface. I-V characteristics and polarization-electric field (P-E) hysteresis loop of the films were measured. The study reveals that the plasma pressure has a strong influence on the evolution and texture of the ferroelectric perovskite phase and microstructure of the films. At an optimum plasma pressure of 4.1 Pa, PZT films are grown with 93% perovskite phase with (1 1 1) preferred orientation and uniform granular microstructure. These films show a saturation polarization of 67 μC/cm², remnant polarization of 30 μC/cm² and coercive field of 28 kV/cm which, according to the literature, seem to be suitable for device applications.Transmission electron microscopy (TEM) study shows that at a plasma pressure of 4.1 Pa, the PZT/bottom Pt interface is sharp and no amorphous interlayer is formed at the interface. At a higher plasma pressure of 4.9 Pa, poor I-V and P-E hysteresis loop are observed which are interpreted as due to an amorphous interlayer at the film-bottom electrode interface which is possibly enriched in Pb, Zr, O and Pt.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.     

Correlation between local hysteresis and crystallite orientation in PZT thin films deposited on Si and MgO substrates

(0 0 1)-Oriented tetragonal ferroelectric PbZr_0.53Ti_0.47O₃ (PZT) thin films (90 nm of thickness) have been grown on TiO_x/Pt/TiO₂/SiO₂/Si and TiO_x/Pt/MgO substrates. The existence of (1 0 0)-oriented crystallites in the c-axis matrix of the (0 0 1)-oriented films has been evidenced by using four circles X-ray diffraction. Depending on the substrate, the ratio of the lattice parameters c/a was found to be 1.02 (Si) and 1.07 (MgO) and this was correlated with the coercive field values. Local piezoelectric hysteresis loops produced by atomic force microscopy have been taken with profit to characterize the switching properties of the ferroelectric domains at the scale of individual crystallites. In each case, (1 0 0)-oriented crystallites require much higher voltage than (0 0 1)-oriented crystallites for switching. These results are explained by taking into account the strain imposed by the substrate in the film. We conclude that piezoelectric hysteresis loops produced by atomic force microscopy provide very rich information for addressing the local switching property of individual crystallites in PZT thin films.     

Low temperature crystallization behavior of multi-walled carbon nanotubes/Pb(Zr0.52Ti0.48)O3 nanocomposite thin films through annealing in various atmosphere and duration control

We report a successful fabrication of 300 nm thick carbon nanotubes and Pb(Zr_0.52Ti_0.48)O₃ (CNT–PZT) nanocomposite thin films with annealing temperature as low as 500 °C in H₂/N₂ atmosphere. Realizing the thickness of CNT–PZT nanocomposite thin films down to few hundred nanometers is one way to reduce the operating voltage of its application to micro- or nano-electromechanical system. The field emission scanning electron microscopic and atomic microscopic analysis revealed that the nanocomposite thin films annealed in H₂/N₂ atmosphere exhibits the most favorable surface morphology with adequate perovskite (111) reflection of PZT based on X-ray diffraction analysis. The measured dielectric constant and loss tangent of the nanocomposite thin films show that the annealing duration of 30 min promotes the optimum dielectric properties of the nanocomposite thin films. Our observations suggest that the annealing atmosphere and duration are important parameters in controlling the crystallization behavior hence the dielectric properties of the nanocomposite thin films, which can be readily applicable to other nanocomposite thin films.     

Phase evolution studies of sol�Cgel derived lead zirconate titanate (PZT) nanopowder using X-ray diffraction and X-ray photoelectron spectroscopy

We report here the formation of single phase lead zirconate titanate (PZT) nanopowder with composition Pb(Zr_0.52Ti_0.48)O₃ and average crystallite size 12?C20?nm, synthesized by sol?Cgel process. The phase evolution of PZT gel powder, heat treated at temperatures 550, 650 and 800°C was monitored by X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS). The high resolution XPS spectra of Pb4f, Zr3d, Ti2p and O1s show that PZT with pure perovskite structure is obtained at 800°C while at lower temperatures pyrochlore phase co-exists with perovskite phase. The XRD results also support this analysis. We have also identified the pyrochlore phase using XPS by analyzing the corresponding variations in the FWHM values, peak positions and the separation between the spin doublets of Pb, Zr and Ti associated with it. The composition of the final powder obtained with pure perovskite structure is calculated and is close to the designed value.     

Crystallization kinetics and growth mechanism of Pb(Zr0.52·Ti0.48)O3 nanopowders

In this study, Pb(Zr_0.52·Ti_0.48)O₃ nanopowders were synthesized via sol–gel process. Particle morphology, crystalline phases and thermal behavior were characterized by scanning electron microscopy, X-ray diffraction and simultaneous thermal analyzer, respectively. The X-ray diffraction pattern showed perovskite phase clearly. The non-isothermal activation energy for the perovskite crystallization in Pb(Zr_0.52·Ti_0.48)O₃ gel powders was 224.91 kJ mol^?1. Both growth morphology parameter (n) and crystallization mechanism index (m) are close to 3.0, indicating that the bulk nucleation is dominant in the perovskite PZT formation. To determine dielectric properties, the calcined Pb(Zr_0.52·Ti_0.48)O₃ nanopowders were pressed using uniaxial press. It was found that the Pb(Zr_0.52·Ti_0.48)O₃ disks, by sintering at 1,200 °C for 2 and 10 h, and at 1 kHz frequency, had 966 and 1,490 of the dielectric constant, respectively.     

Influence of the substrate on ferroelectric properties of 〈1 1 1〉 oriented rhombohedral Pb(Zr0.6Ti0.4)O3 thin films

〈1 1 1〉-oriented Pb(Zr_0.6Ti_0.4)O₃ thin films were elaborated in the same run by RF multitarget sputtering on Si/SiO₂/TiO₂/Pt(1 1 1) and LaAlO₃/Pt(1 1 1) substrates. PZT thin films were textured, exhibiting 〈1 1 1〉 fibre texture on silicon substrates whereas epitaxial relationships were found when grown on LaAlO₃/Pt(1 1 1). On the latter substrate, values of spontaneous polarization and of dielectric permittivity were measured close to that calculated previously along the 〈1 1 1〉 direction of PZT rhombohedral single crystal. On the contrary, spontaneous polarization and dielectric permittivity measured on PZT thin films deposited on platinized silicon were found deviating from calculated values. These different electrical results are attributed to different ferroelectric domain configurations.     

Electric and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript> bilayer thin films prepared by pulsed-laser deposition

CoFe₂O₄ (CFO) thin film with highly (111)-preferential orientation was first deposited on the silicon substrate by a pulsed-laser deposition, and then Pb(Zr_0.52Ti_0.48)O₃ (PZT) layers were deposited with different oxygen pressures to form the bilayer CFO/PZT nanocomposite thin films. X-ray diffraction showed that the PZT preferential orientation was strongly dependant on the oxygen pressure. The smooth film surface was obtained after depositing the CFO and PZT layers. The bilayer thin films exhibit good ferromagnetic and ferroelectric properties, and a low leakage current density of 0.004 μA/cm² at 50 kV/cm. The leakage current density curves show loops for the electric polarized field when the electric field reverses. PACS 77.84.Lf; 75.80+q; 81.05.Zx; 81.15.Fg     

Polarization fatigue resistance of Ca-doped Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript> thin films prepared by the sol–gel method

The ferroelectric and polarization fatigue characteristics of Pb_1-xCa_x(Zr_0.52Ti_0.48)O₃ (PCZT) thin films prepared using the sol–gel method were studied. The Ca-doping slightly suppresses the ferroelectricity of Pb(Zr_0.52Ti_0.48)O₃ (PZT) because of the quantum paraelectric behavior of CaTiO₃. Compared with PZT thin films, the PCZT (x=0.2) thin films show enhanced fatigue resistance at room temperature, further emphasized by the almost fatigue-free behavior at 100 K. The temperature-dependent dc-conductivity suggests a decrease of the oxygen vacancy density by almost 20 times and a slightly declined activation energy U for oxygen vacancies, upon increasing of the Ca-doping content from 0.0 to 0.2. It is argued that the improved fatigue endurance is ascribed to the decreasing density of oxygen vacancies due to the Ca-doping, although the lowered activation energy of oxygen vacancies is unfavorable. PACS 77.84.Dy; 66.30.-h; 68.35.Fx