Electro-Optic Properties of Heteroepitaxial Pb(Zr,Ti)O3/La0.5Sr0.5CoO3 Film Structures

Vertical ferroelectric Pb(Zr,Ti)O₃ (PZT) 1 μm thick film capacitor was fabricated by pulsed laser deposition technique (PLD) onto conducting La_0.5Sr_0.5CoO₃(LSCO) 100 nm thick bottom electrode on both side polished YAlO₃ + 1% Nd₂O₃ (Nd:YAlO₃) single crystal substrate to operate as a Pockels cell optical modulator. On top of the PZT film, semitransparent 30 nm thick Au electrode was deposited by thermal evaporation. Intensity of the chopped 670 nm polarized laser radiation transmitted through the Au/PZT/LSCO/Nd:YAlO₃ cell was measured at various temperatures and bias voltage applied. Applying 20 V (200 kV/cm) across the capacitive cell, modulation of the transmitted light as high as 3% was achieved while the voltage tunability measured at 1 kHz from C-V characteristics was about 70%. Thermo-optical measurements performed for PZT/Nd:YAlO₃ sample in the range up to 400°C showed the phenomenon of critical opalescence in the vicinity of Curie temperature at 208°C. Optical transmission through the PZT film biased with electric field was studied in the range 400 to 1000 nm. Film thickness, refraction index and absorption coefficient have been determined from the interference pattern observed in the PZT transmission spectrum. A simple model yields the dispersion relation for the electro-optic coefficient.     

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.     

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.     

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.     

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.     

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.     

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.     

Self-polarization effect in Pb(Zr,Ti)O3 thin films

Abstract

The self-polarization effect is investigated in Pb(Zr,Ti)O₃ (PZT) thin films deposited by sol-gel and magnetron sputtering techniques. The effective piezoelectric coefficient of as-grown films, which is proportional to their initial polarization (self-polarization), is measured by a sensitive interferometric technique as a function of the annealing temperature, PZT composition, film thickness and bottom electrode material. The results indicate that the films are self-polarized by an internal bias field upon cooling through the phase transition temperature. It is suggested that a built-in field of a Schottky barrier between the PZT film and the bottom electrode is responsible for the observed effect. Self-polarization of the films is found to be very stable and in some cases to be as high as 90% of that produced by the subsequent room temperature poling. This property is very useful for piezoelectric and pyroelectric applications of PZT films since the poling procedure can be avoided. The properties of self-polarization are found to be similar for the films produced by sol-gel and sputtering techniques, suggesting that the same mechanism is operative in both cases.     

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.     

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.     

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

Bilayered ferroelectric thin films consisting of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) have been successfully synthesized on Pt/Ti/SiO₂/Si substrates, via a combined sol–gel and rf-sputtering route. Their ferroelectric and dielectric properties are critically dependent on the phases present, film texture and in particular layer and film thicknesses. Due to the coupling of PZT and BNT bilayers, there requires an optimized thickness combination of the two ferroelectric layers, in order to give rise to the wanted ferroelectric and dielectric properties, while the phenomenon can not be accounted for by the simple series connection model.     

Preparation of Pb(Zr,Ti)O3 Thin Films on Silica Substrate by Metallo-Organic Decomposition

In this study, we report the deposition of crack-free transparent PZT films (up to ～859 nm) by metallo-organic decomposition (MOD) process on amorphous silica substrate. Effect of SrTiO₃ (STO) buffer layer on the growth behavior of PZT thin films deposited on SiO₂-coated silicon substrates was systematically studied. Perovskite phase, which cannot be formed directly on SiO₂/Si substrates, has been obtained when a thin STO film (～150 nm) was used as buffer layer. A SIMS examination indicates that the upward diffusion of Si-species into PZT layer is minimal, although downward diffusion of Pb-species into the SiO₂ layer is still observable.     

Comparison Study of (001)-/(100)-Oriented Epitaxial and Fiber-Textured Pb(Zr,Ti)O3 Thick Films Prepared by MOCVD

(001)-/(100)-oriented epitaxial PZT films and fiber-textured PZT films with a thickness of 2 μ m were deposited on (100) _c SrRuO₃//(100)SrTiO₃ and (111)Pt/TiO₂/SiO₂/(100)Si substrates, respectively, by metalorganic chemical vapor deposition (MOCVD). Crystal structure and the electrical properties were compared for epitaxial PZT films with those of fiber-textured ones, which had the same out-of-plane orientation but different in-plane orientation. The constituent phase change from the single phase of tetragonal PZT, mixture phases of tetragonal and rhombohedral ones and the single phase of rhombohedral one for both films when the Zr/(Zr + Ti) ratio increased. The out-of-plane lattice parameter of (001)- and (100)-axes of PZT films were almost the same value for both films. This indicates the residual strain in the PZT films was almost the same. Dielectric constant (? _r) took the maximum value around the MPB composition for (001)-/(100)-oriented fiber-textured films but was almost independent of the Zr/(Zr + Ti) ratio for epitaxial ones. Moreover, there was no significantly dependence of remanent polarization (P _r) value on the Zr/(Zr + Ti) ratio for (001)-/(100)-oriented fiber-textured films, while it took minimum value near the morphotropic phase boundary (MPB) for epitaxial ones.     

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.     

Ferroelectric Properties and Memory Characteristics of Pb(Zr0.52 Ti0.48)O3 Thin Films Crystallized by Hot Isostatic Pressing

Pb(Zr_0.52Ti_0.48)O₃ films with highly uniform c-axis orientation were fabricated on PbTiO₃ (PT)/ Pt(111)/SiO₂/Si(100) substrates by hot isostatic pressing (HIP) from the amorphous state. All the PZT samples HIP-treated 500°C for 1 h under gas pressures of 1.0–2.0 MPa showed the preferred (001) orientation with c-axis orientation, α > 0.80. The relative permittivity tended to decrease gently with increasing HIP pressure, whereas the dielectric loss increased almost linearly in the 1.5–100 MPa. The PZT sample treated at 1.5 MPa had a symmetric and slim hysteresis loop shape with a remanent polarization, P _r = 15 μ C/cm² and coercive field, E _c = 60 kV/cm. Both samples treated at 10 and 100 MPa exhibited almost the fatigue-free behavior that resisted degradation even after 3 × 10¹⁰ cycles.     

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.     

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.     

Microwave Dielectric Properties of Ferroelectric Pb(Zr1 − x Ti x )O3 Thin Films Using Interdigital Capacitors

Ferroelectric Pb(Zr_{1 ? x} Ti _x )O₃ (PZT) films were deposited on (001) MgO single crystals using sol-gel method. Structural properties and surface morphologies of PZT films were investigated using an X-ray diffractometer and a scanning electron microscopy, respectively. The dielectric properties of PZT films were investigated with the dc bias field of 0–135 kV/cm using interdigitated capacitors (IDC) which were fabricated on PZT films using a thick metal layer by photolithography and etching process. The small signal dielectric properties of PZT films were calculated by a modified conformal mapping method with low and high frequency data, such as capacitance measured by an impedance gain/phase analyzer at 100 kHz and reflection coefficient (S-parameter) measured by a HP 8510C vector network analyzer at 1–20 GHz. The IDC on PZT films exhibited about 67% of capacitance change with an electric field of 135 kV/cm at 10 GHz. These PZT thin films can be applied to tunable microwave devices such as phase shifters, tunable resonators and tunable filters.     

Defect chemistry and transport properties of Pb(Zr1/2Ti1/2)O3

Abstract

In order to gain insight into the degradation mechanisms associated with ferroelectric thin films, such as fatigue and imprint, an understanding of the defect chemistry and transport properties of the material is needed. In this study several complimentary techniques have been used to either measure or calculate indirectly the various thermodynamic parameters governing defect formation and transport in Pb(Zr_1/2Ti_1/2)O₃, (PZT). By combining the results of DC equilibrium conductivity, thermoelectric power and the sealed cell techniques, “constant composition oxygen activity” and “constant composition conductivity,” values for the oxidation enthalpy (ΔH_ox), hole trapping energy (E _A) and the enthalpy of motion for holes (ΔE _A) have been determined to be ?0.49 eV, ≤0.9 eV and ≥0.1 eV, respectively. From these results, it is apparent that PZT is an oxygen excess p-type semiconductor in the experimental regime of 500°–700°C and P(O₂) ≥ 10^?4 atm. Furthermore, the results indicate that there is a significant concentration of trapped holes at high temperatures and hole conduction appears to be an activated process (i.e. small polaron conduction).