Ferroelectric field effect in (La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 heterostructures

Abstract

Epitaxial LaCoO₃/Pb(Zr,Ti)O₃/(La,Sr)CoO₃ (LSCO) heterostructures have been grown on LaAlO₃ by pulsed laser deposition for investigating ferroelectric field effect. In the heterostructure, semiconducting LaCoO₃ was used as a conducting channel layer, instead Si. The resistivity of the LaCoO₃ (LCO) channel layer was found to be dependent on an oxygen ambient, primarily the ambient oxygen pressure during deposition. The resistivity of the LCO layer varied in the range of 0.1 – 100 Ω cm. Ferroelectric field effect induced in LaCoO₃ layer was observed by measuring the resistance modulation of the LCO layer with respect to the polarized state of the PZT layer. The resistance modulation of 9% was obtained in the 680 Å thick LCO layer. Further the resistance modulation was improved up to 45% after applying dc bias. It is suggested that the LCO/PZT/LSCO heterostructure can be used as a ferroelectric field effect transistor.     

Processing and Properties of Ferroelectric Pb(Zr,Ti)O3/Silicon Carbide Field-Effect Transistor

Metal-ferroelectric-(insulator)-semiconductor MF(I)S structures have been fabricated and the properties of pulsed laser-deposited PZT/Al₂O₃ gate stacks have been studied on n- and p-type 4H-SiC. Among several polytypes of SiC, 4H-SiC is considered as the most attractive one because of its wider bandgap (E _g ? 3.2 eV) as well as higher and more isotropic bulk mobility than other polytypes. Single PZT phase without a preferred orientation was confirmed by x-ray diffraction. The interface trap densities N _IT, fixed oxide charges Q _F, and trapped oxide charges Q _HY have been estimated by C-V curves with and without photo-illuminated measurements at room temperature. It is found that the charge injection from SiC is the dominant mechanism for C-V hysteresis. Importantly, with PZT/Al₂O₃ gate stacks, superior C-V characteristics with negligible sweep rate dependence and negligible time dependence under the applied bias were obtained compared to PZT directly deposited on SiC. The MFIS structures exhibited very stable capacitance-voltage C-V loops with low conductance (<0.1 mS/cm², tan δ ～ 0.0007 at 400 kHz) and memory window as wide as 10 V, when 5 nm-thick Al₂O₃ was used as a high bandgap (E _g ～ 9 eV) barrier buffer layer between PZT (E _g ～ 3.5 eV) and SiC (E _g ～ 3.2 eV). The structures have shown excellent electrical properties promising for the gate stacks as the SiC field-effect transistors (FETs). Depletion mode transistors were prepared by forming a Pb(Zr_0.52Ti_0.48)O₃/Al₂O₃ gate stack on 4H-SiC. Based on this structure, ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films have been integrated on 4H-silicon carbide (SiC) in a SiC field-effect transistor process. Nonvolatile operation of ferroelectric-gate field-effect transistors in silicon carbide (SiC) is demonstrated.     

Epitaxial La0.5Sr0.5CoO3 electrode films for ferroelectric device applications

Abstract

We have grown high quality La_0.5Sr_0.5CoO₃ thin films by Pulsed Laser Deposition. This material is a electrically conducting perovskite which can be used as structurally compatible electrodes for devices using ferroelectric materials such as Pb-La-Zr-Ti-O (PLZT). Oxygen stability was investigated under various annealing conditions. LSCO/PLZT/LSCO heterostructures were grown epitaxially on SrTiO₃ (STO) and MgO substrates. P-E hysteresis loop was observed with P_r = 20 μC/cm² and E_c = 24 kV/cm. Direct observation of the piezoelectric effect in PLZT was made by measuring the PLZT c-axis lattice constant under various electric fields. LSCO/STO superlattice was also grown by PLD. The superlattice showed excellent crystallinity with a Rutherford Backscattering (RBS) channeling minimum yield of only 3.6%.     

Piezoelectric and Pyroelectric Properties of Pb(Zr,Ti)O3 Films for Micro-Sensors and Actuators

The piezoelectric and the pyroelectric properties of PZT films are systematically investigated for tetragonal (Zr/Ti = 30/70), morphotropic (52/48), and rhombohedral (70/30) compositions. The magnitude of the effective longitudinal piezoelectric coefficient (d₃₃) and pyroelectric coefficient (p) of these films is measured by atomic force microscopy and Byer-Roundy method, respectively. All films are consistently highly textured (111) orientation and have dense microstructures. Slightly less degree of texture in higher Zr-rich composition is observed due to the lattice mismatch between PZT and Pt bottom electrode and higher activation energy for nucleation. Squareness of polarization hysteresis loops is optimized in tetragonal composition, which indicates the tetragonal PZT is closer to the ideal hysteresis behavior than other compositions. It is shown that the piezoelectric coefficient and the pyroelectric figure of merit are dependent on the dielectric properties of the films. The morphotropic PZT films with high dielectric constant and low pyroelectric figure of merit show the largest piezoelectric coefficient values, while the tetragonal PZT films with low dielectric constant and high remanent polarization values show the largest pyroelectric figure of merit compared to other compositions, which indicate the suitability for PIR sensor devices.     

Enhancement of Ferroelectric Properties of Pb(Zr,Ti)O3 Films Grown by Metal-Organic Chemical Vapor Deposition on Noble-Alloy Bottom Electrodes

Pb(Zr,Ti)O₃ (PZT) films grown on Ir electrodes by a metal-organic chemical vapor deposition (MOCVD) have suffered from high leakage and rough surface. We sputtered Pt and Ir simultaneously onto Ti/SiO₂/Si substrates and formed Ir-Pt alloy bottom electrodes with various compositions. With an optimal composition of Ir and Pt, PZT films grown by MOCVD on this substrate showed smoother surface and suppressed leakage via the bottom interface. At the specific composition of Ir and Pt, two different phases seemed to be acquired. They constituted the electrodes and affected the PZT grain nucleation independently so that the grains with different origins grew and restrained the vicinal grains, and finally soothed the faceted-grain-formation. No fatigue was observed even in PZT on Ir-Pt alloy with much Pt content.     

The bulk photovoltaic effect in ferroelectric Pb(Zr,Ti)O3 thin films

Abstract

The bulk photovoltaic effect (BPE) has been investigated in lead zirconate titanate (PZT) thin films. Measurements of the kinetics, spectral distribution and photocurrent hysteresis loops have been made. In the extrinsic spectral region, the steady-state photocurrent is primarily due to the BPE, where the photovoltaic tensor component has been determined to be G₃₁ = 10_?9 cm/V. However, in the intrinsic region, the BPE has not been determined due to the strong contribution from photoinjection currents. Finally, it is shown that the BPE may be the driving force for photoinduced hysteresis changes in PZT thin films, particularly in the extrinsic spectral region.     

Preferentially oriented (La,Sr)CoO3/PbLa0.1TiO3 (La,Sr)CoO3 tri-layers on lithium-fluoride and sodium-chloride substrates

Abstract

(La, Sr)CoO₃/PbLa_0.1TiO₃/(La, Sr)CoO₃ tri-layers have been grown in situ using pulsed laser deposition on single crystal LiF and NaCl substrates because of their potential to serve as substrates and sacrificial layers for building suspended infrared detector structures. At growth temperatures around 525°C to 615°C and oxygen partial pressures of ∽25 mTorr, the LaSrCoO₃/PbLaTiO₃/LaSrCoO₃ tri-layers on LiF substrates exhibit preferential orientation, as indicated by x-ray diffraction data. The surface morphology of each successive layer is smooth, as shown in scanning electron microgram. Higher growth temperatures result in better orientations but suffer from substrate out-gassing, while lower growth temperatures lead to polycrystalline film growth. On the NaCl substrates, buffer layers, such as Pt and Pd, are necessary to achieve oriented films by suppressing severe substrate out-gassing at the growth conditions required for stoichiometric (La, Sr)CoO₃/PbLa_0.1TiO₃/(La, Sr)CoO₃ tri-layer growth.     

Water-Immersible Micromachined Pb(Zr, Ti)O3 Thin Film Actuators

We demonstrate a water-immersible thin film lead zirconate titanate, Pb(Zr, Ti)O₃, [PZT] actuator, without special passivation layer, towards in-vivo or in-vitro scanning probe microscope (SPM) measurements of living cells in water or biological fluids. In order to be water-immersible, the electrodes need to be electrically insulated and the piezoelectric layer needs to be protected against direct water contact. This paper describes our design solution with a simple fabrication process for a water-immersible piezoelectric device, which separates the bottom electrode from the top electrode by having a narrow ditch covered with PZT film. The PZT film is then encapsulated with the top metal electrode without insulation layer. In this structure, the PZT is sandwiched between the top and bottom metal electrodes to prevent water permeation. The device is fabricated using lift-off processing for the bottom and top electrodes, sol-gel spinning for the PZT thin film and wet etching for the PZT patterning. The piezoelectric constant, d₃₁, is about –100 pC/N. The dielectric polarization and fatigue properties of the devices were measured in air and water. The spontaneous polarization, remnant polarization, coercive field and dielectric constant are 54 C/cm², 15 C/cm², 60 kV/cm and 1200, respectively. The polarization property of the device was unchanged in either air or water up to 1 × 10⁹ continuous cycles.     

Fabrication and Electromechanical Properties of Pb(Zr0.52,Ti0.48)O3 Micro-Diaphragm

ABSTRACT

Multilayered piezoelectric micro-diaphragms have been successfully fabricated by micro-electro-mechanical-system (MEMS) processing. The micro-diaphragms consisted of diol based sol-gel derived Pb(Zr_0.52Ti_0.48)O₃ (PZT) capacitor, sputtered Pt electrode, and low temperature oxide(LTO)/SiN_x/Si substrate. The PZT film exhibited (111) oriented structure. The dielectric constant and loss of the PZT thin films were 800 and 3% at 100～ 100 kHz, respectively. The remanent polarization was 20 μ C/cm². The lateral dimension of the PZT film was varied relative to the square-shaped supporting membrane with 300 or 400 μ m length. The relative size (ratio of lateral dimensions) of the PZT film to the supporting membrane was varied from 0.7 to 1.1 to investigate its influence on the system performance. The micro-diaphragm exhibited mechanical displacement from 0.067 to 0.135 μ m at 15 V and had a maximum displacement at a ratio of relative size of 0.8, regardless of the lateral size of the supporting membrane. The fundamental resonant frequency of the micro-diaphragm which has 300 μ m length supporting membrane was in the range of 348 kHz to 365 kHz, depending on the relative size. As the PZT size increased relative to the supporting membrane, the resonant frequency decreased and reached a minimum at the relative size of 0.8. The micro-diaphragm with the supporting membrane (400 μ m length) had a lower resonant frequency, i.e., 251～270 kHz, but showed a similar behavior to the micro-diaphragm with the supporting membrane (300 μ m length) in relation to the resonant frequencies with the relative size.     

Epitaxial thin film heterostructures of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3

Abstract

We have grown stoichiometric pure perovskite phase Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) thin films and PMN-PT/SrRuO₃ heterostructures on miscut (100) SrTiO₃ substrates by pulsed laser deposition. X-ray diffraction θ–2θ scans show that the PMN-PT films are purely c-axis oriented. The off-axis Φ scans show that the heterostructures grow “cube-on-cube” with an in-plane epitaxial arrangement of PMN-PT[100], [010] // SrRuO₃[001] // SrTiO₃[100] and PMN-PT[010], [100] // SrRuO₃[110] // SrTiO₃[010]. The crystalline quality of the films found to be comparable to that of bulk single crystals. The AFM images show that the SrRuO₃ and PMN-PT layers have smooth surfaces with root mean square roughness of 9Å. These epitaxial heterostructures can be used for the fabrication of piezoelectric devices.     

Slow Dielectric Relaxation in Pb(Zr,Ti)O3 and (Ba,Sr)TiO3 Ferroelectric Thin Films

An impedance analysis of Ni/Pb(Zr,Ti)O₃/Pt thin-film structures based on measurements at the frequencies from 100 Hz to 100 MHz, along with the data of Grazing Angles XRD, TEM and photo-electric study, is used to obtain electronic structure of the PZT thin films deposited by sol-gel method on silicone substrates. Both slow capacitance relaxation and charging/discharging currents versus time under step-voltage excitation have been studied in (Ba,Sr)TiO₃ thin films between SrRuO₃ electrodes.     

Low-temperature sintering of ferroelectric Pb(Zr,Ti)O3 thick films derived from stable sol-gel solutions

Abstract

We have studied sintering and densification of PbZr_0.52Ti_0.48O₃ (PZT) films derived from diol-based sol-gel solutions. We found that densification by sintering began at below 750°C and completed at 850°C in 5 min. Initially, 0.83- μm-thick PZT single-coated films were prepared on Pt/Ti/SiO₂/Si substrates from stable propylene-glycol (l,2-propanediol)-based solutions by crystallization at 700°C. The crystallized films consisted of fine perovskite grains and voids. We studied the firing temperature dependence of various properties such as microstructure, crystallinity, and ferroelectric properties for the single-coated films. Finally, 0.54- μm-thick PZT single-coated dense films were prepared by firing at 850°C for 5 min. In order to prepare thicker PZT dense films, we studied low-temperature sintering of PZT multicoated thick films. Using this approach, 1.7- μm-thick PZT dense films were prepared by firing at 850°C for 5 min.     

Pulse width dependent polarizations of ferroelectric (Pb,La)(Zr,Ti)O3 thin film capacitors

Abstract

We report the measurements of pulse polarizations of (Pb, La)(Zr, Ti)O₃ ferroelectric capacitors as a function of pulse width. Pulse polarizations were measured from the switching current responses in the pulse width range from 1 μs to 1 s. The relation between pulse polarizations(switched(P?) and non-switched polarization(P?)) and hysteresis loop parameters was studied. In the case of all write/read pulse widths are same, P? increased but P? decreased with increasing pulse width. These results are explained by poling effects and confirmed by measuring pulse width dependencies with different patterns of pulse trains.     

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.     

Synthesis of Pb(Zr, Ti)O3 Nanopowders by Milling Coprecipitation Method

Nano-size powders of lead zirconate titanate (PZT) were fabricated by a new milling coprecipitation method (MCP) improved from the conventional wet ball milling and precipitation. This method consists of slurry preparation from nanoparticles of TiO₂ with aqueous solution of ZrO(NO₃)₂ and Pb(NO₃)₂ with zirconia ball mill media, followed by precipitation with NH₄OH as precipitant. Milling media (1mm and 3mm balls) improves the precipitation homogeneity during processing. Single-phase perovskite structure of PZT was formed at a calcination temperature of 500C and powders of 50 nm particle size were obtained. Powders were characterized using TG-DTA, SEM and XRD methods. Sintering ability of powders and piezoelectric properties of the ceramics were also investigated.     

Pb(Zr,Ti)O3 thin film growth on yttrium-treated Si(100)

Abstract

Pulsed laser ablation has been used to deposit ferroelectric Pb(Zr, Ti)O₃ (PZT) thin films on Si(100) and on yttrium-treated Si(100) substrates. The yttrium (Y) treatment of a Si surface followed by oxidation resulted in formation of a very thin, Y-enhanced SiO₂ antidiffusion barrier layer, thereby suppressing the undesirable PZT/Si interdiffusion. The best PZT film grown on Y-treated Si(100) had a breakdown voltage of 0.6 MV/cm, a coercive field of 71 KV/cm, and a remanent polarization of 18 μC/cm².     

Preparation of Pb(Ti,Zr)O3/RuO2 multilayers in situ by pulsed laser ablation deposition

Abstract

RuO₂/PbZr_0.52Ti_0.48O₃ multilayers have been prepared by pulsed laser deposition and characterised by Grazing Incident Angle Bragg Diffraction and scanning electron microscopy. The results are correlated with the different deposition conditions and post-growth thermal annealing. The structure of the paraelectric pyrochlore phase (Pb₂(Zr,Ti)₂O₆) and its stability relative to the ferroelectric perovskite is discussed.     

Growth and electrical properties of Pb(Zr,Ti)O3 thin films by sputtering using an alloy target

Abstract

Pb(Zr_x,Ti_1?x)O₃ thin films were deposited on Pt/SiO₂/Si substrates by the rf magnetron sputtering using an alloy target consisting of Zr-Ti alloy and Pb metal. The dependence of electrical properties on film thickness and sputtering gas pressure was investigated. The dielectric constant and the remanent polarization decreased and the coercive field increased with the decrease of the film thickness. In the dependence of gas pressure, the relative dielectric constant of the film with only a perovskite phase were in the range of 235–280, which were higher than those of the film with only a pyrochlore phase, 20. The asymmetry of hysteresis loops increased with the decrease of the gas pressure.     

PHOTO INDUCED CURRENT IN (Pb0.97La0.03) (Zr0.52Ti0.48)O3 THIN FILMS OF DIFFERENT THICKNESSES

ABSTRACT

Photovoltaic effect in ferroelectric materials exhibits potential for applications of sensors and remote controls in micro-electro-mechanical systems, and a systematic evaluation on the photovoltaic behavior in ferroelectric materials becomes important. However, as a critical parameter that determines the photovoltaic output in ferroelectric thin films, the film thickness effect on photocurrent output has not been investigated for thin film samples. In this work, a theoretical model has been developed to describe the thickness-dependent photocurrent in (Pb_0.97La_0.03)(Zr_0.52Ti_0.48)O₃(PLZT) thin films with a sandwich electrode structure. This model indicates that photocurrent increases exponentially with the decrease in film thickness. Therefore, a significantly enhanced photocurrent can be expected in thinner PLZT films. The predicted thickness dependence of the short circuit photocurrent was also supported by our experimental results.     

Self-Organized (100)-/(001)-Preferred Orientation in Pb(Zr,Ti)O3 Films Grown on Polycrystalline Substrates by Metalorganic Chemical Vapor Deposition

Lead zirconate titanate [Pb(Zr _x Ti_{1 ? x} )O₃, PZT] films were grown on (100), (110) and (111)SrRuO₃//SrTiO₃ substrates at 600°C by metalorganic chemical vapor deposition (MOCVD). The crystal orientation dependence of the growth rate was investigated for these films. The growth rate of (100)-/(001)-oriented epitaxial films was approximately 1.7 and 2.0 times higher than that of (110)-/(101)- and (111)-/(11&1macr;)-oriented epitaxial films, respectively. On the other hand, the growth rate of (100)-/(001)-preferred oriented PZT films grown on (111)Pt/TiO₂/SiO₂/(100)Si substrates was almost the same with that of (100)-/(001)-oriented epitaxial films. The deposition rate of these films was approximately 1.5 μm/h. High growth rate of (100)-/(001)-oriented PZT grains makes (100)-/(001)-preferred orientation on (111)Pt/TiO₂/SiO₂/(100)Si substrate. From transmission electron microscopy observation, (100)-/(001)-oriented grains were found to be directly grown on (111)-oriented Pt grains without obvious another oriented grains. As a result, orientation-controlled PZT films were successfully grown on (100)Si substrates having (111)-oriented Pt bottom electrodes.