Peculiarities of crystallization of thin ferroelectric films of lead zirconate titanate

The thermodynamics of processes involved in the growth and annealing of ferroelectric films of lead zirconate titanate Pb(Ti,Zr)O₃ (PZT) has been studied using the method of synchronous thermal analysis (STA). Thin PZT films were grown by the RF magnetron sputtering and then annealed in air or in an inert gas (argon) at atmospheric pressure and various temperatures within 20–600°C. It was found that the annealing in an oxygen-containing medium (air) is accompanied by changes in the enthalpy and mass of the system, which is due to the interaction with oxygen, while the heat treatment in an inert medium did not lead to any changes in the PZT film. It is established that the observed changes are related to the conversion of lead monoxide into orthoplumbate in the PZT film volume. STA experiments demonstrate that this transformation leads to a significant change in the structure of the ferroelectric film. Based on these data, a mechanism is proposed that accounts for the structural changes involved in the annealing of PZT films.     

Effect of microwave annealing temperatures on lead zirconate titanate thin films

Lead zirconate titanate (Pb(1.1)(Zr(0.52)Ti(0.48))O(3)) thin films of thickness 260?nm on Pt/Ti/SiO(2)/Si substrates were densified by 2.45?GHz microwave annealing. The PZT thin films were annealed at various annealing temperatures from 400 to 700?°C for 30?min. X-ray diffraction showed that the pyrochlore phase was transformed to the perovskite phase at 450?°C and the film was fully crystallized. The secondary (again pyrochlore) phase was observed in the PZT thin films, which were annealed above 550?°C. The surface morphologies were changed above 550?°C of the PZT thin films due to the secondary phase. Higher dielectric constant (ε(r)) and lower dielectric loss coercive field (E(c)) were achieved for the 450?°C film than for the other annealed films.     

Dielectric properties of lead zirconate titanate thin films seeded with barium strontium titanate nanoparticles

A low temperature synthetic method recently proposed by the authors was applied to the fabrication of lead zirconate titanate (PZT) thin films containing crystalline seeds of barium strontium titanate (BST) nanoparticles. PZT precursor and the BST particles were prepared with complex alkoxide methods. Precursor solution suspending the BST particles was spin-coated on Pt/Ti/SiO₂/Si substrate to film thickness of 500–800 nm at particle concentrations of 0–25.1 mol%, and annealed at various temperatures. Seeding of BST particles prevented the formation of pyrochlore phases, which appeared at temperatures above 400 °C in unseeded PZT films, and induced crystallization of PZT into perovskite structures at 420 °C, which was more than 100 °C below the crystallization temperature of the unseeded PZT films. Measurement of dielectric properties at 1 kHz showed that the 25.1 mol% BST-seeded PZT films annealed at 450 °C had a dielectric constant as high as 300 with a dissipation factor of 0.05. Leakage current density of the film was less than 1×10⁻⁶ A/cm² at applied electric field from 0 to 64 kV/cm.     

Dip-coating conditions and modifications of lead titanate and lead zirconate titanate films

The modifications of dip-coated lead titanate (PT) and lead zirconate titanate (PZT) films strongly depend on the film thickness and the substrate in addition to the heat-treatment temperature. At 500 to 600 ° C, metastable paraelectric pyrochlore grew on glass plates (amorphous plates) when the thickness of the coated films produced by one coating cycle was below 100 nm, while ferroelectric perovskite formed on crystalline substrates or when thick films were coated on amorphous plates. This tendency is discussed in terms of an inhomogeneous reaction and the epitaxial effect. The perovskite PT films coated on single-crystal SrTiO₃ plate at 700 ° C were strongly oriented to thec-axis.     

Role of precursors in the formation of lead zirconate titanate thin films

Thin ferroelectric Pb(Zr_0.48Ti_0.52)O₃ lead zirconate titanate films have been prepared on platinized silicon substrates via chemical solution deposition. The starting film-forming solutions were prepared by reacting lead acetate with Ti and Zr alkoxides in 2-methoxyethanol. The electrical properties of the films were shown to depend on the Ti and Zr alkoxide precursors used in synthesis. The nature of the zirconium alkoxide had the strongest effect on the properties of the films. The best properties were obtained with zirconium n-propoxide.     

Micropore formation in lead zirconate titanate films

This paper presents an experimental study of the pyrochlore-to-perovskite phase transition in ferroelectric lead zirconate titanate (PZT) films grown on silicon substrates by rf magnetron sputtering and annealed in air or in an inert (argon) atmosphere at temperatures of up to 600°C and atmospheric pressure. Simultaneous thermal analysis results demonstrate that annealing in air leads to release of the latent heat of the phase transition, which is due to the conversion of lead oxide to lead orthoplumbate in the bulk of the PZT film. This transition is accompanied by changes in the densities of the perovskite phase and parent (pyrochlore) phase. In this case, the possibility of phase transformation should be ensured by changes in the volume of the system. The change in the volume of the film leads to the formation of micropores in the bulk of thin PZT films. The micropore size has been determined by scanning electron microscopy, and the phase composition of the films has been assessed by X-ray diffraction. We present experimental evidence that micropore nucleation and growth at the interface between the new and old phases in thin PZT films is due to the difference in density between these phases.     

Heat treatment effects on the formation of lanthanum-modified lead zirconate titanate thin films

The microstructural and compositional properties of lanthanum-modified lead zirconate titanate (PLZT) thin films deposited on platinum coated Si substrates by RF magnetron sputtering have been studied. The heat treatment processes of substrate heating during deposition and post deposition furnace and rapid thermal annealing were compared as processes for obtaining the desired pervoskite phase. PLZT thin films deposited with in-situ substrate heating showed little evidence of micro-cracking. The XRD data obtained showed the formation of pervoskite phase at 550 °C and indicated the suppression of the pyrochlore phase for increasing temperatures. The RBS analysis revealed a film thickness of 140 nm and composition of (Pb_0.91La_0.09)(Zr_0.6Ti_0.4)O₃. Deposition performed with in-situ substrate heating at 650 °C resulted in highly (110) pervoskite orientated thin films with an average grain size around 160 to 200 nm and an RMS roughness of 3 nm.     

Improved dielectric properties of lead lanthanum zirconate titanate thin films on copper substrates

Thin films of lead lanthanum zirconate titanate (PLZT) were directly deposited on copper substrates by chemical solution deposition and crystallized at temperatures of ~ 650 °C under low oxygen partial pressure (pO₂) to create film-on-foil capacitor sheets. The dielectric properties of the capacitors formed have much improved dielectric properties compared to those reported previously. The key to the enhanced properties is a reduction in the time that the film is exposed to lower pO₂ by employing a direct insertion strategy to crystallize the films together with the solution chemistry employed. Films exhibited well-saturated hysteresis loops with remanent polarization of ~ 20 μC/cm², dielectric constant of > 1100, and dielectric loss of < 0.07. Energy densities of ~ 32 J/cm³ were obtained at a field of ~ 1.9 MV/cm on a ~ 1 μm thick film with 250 μm Pt electrodes.     

Interfacial mechanisms controlling phase formation in sol-gel derived lead zirconate titanate thin films

An RBS study has been carried out to study the relative extent of Pb diffusion in the substrate from PZT films. It is found that extensive diffusion of Pb occurs into the quartz substrate making the film severely deficient in Pb. No such diffusion occurs in the case of PZT films on sapphire and the concentration of Pb is near stoichiometric, except for a small loss due to volatilization. Excessive Pb deficiency, rather than any epitaxial effect, has earlier been proposed by us to be the crucial factor responsible for the existence of the pyrochlore phase in PZT thin films on substrates such as Si, glass, quartz etc. The present results confirm this. The effects of other process variables such as thickness and chemical composition (Zr/Ti ratio) of the film can also be understood in terms of the same phenomenon.     

不锈钢衬底上锆钛酸铅膜的制备

以改性二氧化锆为过渡层,用溶胶－凝胶法在不锈钢衬底上成功制备了锆钛比为52／48的锆钛酸铅膜,扫描电镜分析表明,膜的表面平整,无裂纹,X射线分析结果说明,经650℃退火30min后,膜为纯的钙钛矿相,以衬底为下电极、直径为0.32mm的金上电极,测量了0.8μm膜的电滞回线及其它性能,膜的剩余极化为18μC/cm^2。     

Lead zirconate titanate thin films on GaAs substrates

Thin films of the piezoceramic lead zirconate titanate (PZT) of composition Pb(Zr0.53Ti0.47)O3 have been prepared on a platinized GaAs substrate system using a propanediol based sol-gel technique. A Si3N4 buffer layer was deposited onto the GaAs by plasma-enhanced chemical vapour deposition so as to minimize Ga and As diffusion during film fabrication. Rapid thermal processing (RTP) techniques were used to thermally decompose the sol-gel layer to PZT in a further effort to avoid problems of Ga and As diffusion. Adhesion between the electrode and substrate was found to improve when an intermediate Ti layer deposited between the Pt and Si3N4 was oxidized prior to depositing the Pt layer. A crystalline PZT film was produced on the Pt/TiO2/Si3N4/GaAs by firing the sol-gel coating at 350°C for 1 min and then at 650°C for 10 s using RTP. A single deposition of precursor sol resulted in a film 0.5 μm thick. Measured average values of remanant polarization and coercive field were 14 μC cm-2 and 47 kV cm-1, respectively. The polarization value is rather low, as conventionally fired films on silicon have remanent polarization values of 20–30 μC cm-2; the lower values may be due to incomplete crystallization during RTP, but a degradation of properties due to Ga-As diffusion, despite the precautions, cannot be ruled out at this stage. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thickness and erbium doping effects on the electrical properties of lead zirconate titanate thin films

Thin films of erbium doped lead zirconate titanate (PZT) of different thickness were deposited by sol–gel technique on Pt/TiO₂/SiO₂/Si substrates. Capacitance–voltage measurements show that the dielectric constant continuously increases with the thickness. This is interpreted in terms of effects due to a low permittivity interfacial layer in series with the ferroelectric bulk. The linear fit of the reciprocal of capacitance vs. thickness leads to a true dielectric constant of the ferroelectric of 774 and interfacial capacitance of 14.6 nF. The leakage current properties also depend on thickness and temperature. The calculated interfacial potential barrier height amounts to 0.81 and 0.74 eV, respectively for erbium doped and pure PZT thin films.     

Preparation of lead zirconate titanate thin films with a combination of self-assembly and spin-coating techniques

A low-temperature synthetic method for preparing lead zirconate titanate (PZT) perovskite film on a Pt substrate is proposed. The method consists of the self-assembly of PZT particles on a substrate and successive spin coating with a precursor of PZT. The PZT particles that had sub-micron sizes and perovskite structures were prepared by annealing amorphous PZT particles formed from a complex alkoxide precursor. The PZT particles were deposited on a Pt substrate that was surface-modified with (3-mercaptopropyl) trimethoxysilane to chemically fix the particles on the substrate. Another PZT precursor solution was used for the spin-coating on the PZT-deposited substrate, and then the spin-coated film was annealed at 350 °C to remove organic residues left in the film. The spin-coated PZT film prepared at 350 °C had a dielectric constant of 118 at a frequency of 1000 Hz.     

Low-temperature crystallization of sol-gel-derived lead zirconate titanate thin films using 2.45 GHz microwaves

Lead zirconate titanate (PZT) thin films of thickness 420 nm were deposited on Pt/Ti/SiO₂/Si substrate using a spin coating sol-gel precursor solution, and then annealed using 2.45 GHz microwaves at a temperature of 450 °C for 30 min. The film has a high perovskite content and high crystallinity with a full width at half maximum of 0.35°. The surface roughness of the PZT thin film was 1.63 nm. Well-saturated ferroelectric properties were obtained with a remanent polarization of 46.86 μC/cm² and coercive field of 86.25 kV/cm. The film also exhibited excellent dielectric properties with a dielectric constant of 1140 and a dissipation factor of 0.03. These properties are superior to those obtained by conventional annealing at a temperature of 700 °C for 30 min.     

Scaling and disorder analysis of local I-V curves from ferroelectric thin films of lead zirconate titanate

Differential analysis of current-voltage characteristics, obtained on the surface of epitaxial films of ferroelectric lead zirconate titanate (Pb(Zr(0.2)Ti(0.8))O(3)) using scanning probe microscopy, was combined with spatially resolved mapping of variations in local conductance to differentiate between candidate mechanisms of local electronic transport and the origin of disorder. Within the assumed approximations, electron transport was inferred to be determined by two mechanisms depending on the magnitude of applied bias, with the low-bias range dominated by the trap-assisted Fowler-Nordheim tunneling through the interface and the high-bias range limited by the hopping conduction through the bulk. Phenomenological analysis of the I-V curves has further revealed that the transition between the low- and high-bias regimes is manifested both in the strength of variations within the I-V curves sampled across the surface, as well as the spatial distribution of conductance. Spatial variations were concluded to originate primarily from the heterogeneity of the interfacial electronic barrier height with an additional small contribution from random changes in the tip-contact geometry.     

Corona poling of highly (001)/(100)-oriented lead zirconate titanate thin films

Corona poling is an interesting non-contact poling process for ferroelectric materials which, has in the past, been extensively applied to ferroelectric polymers, but not to other types of ferroelectric materials. Here, it has been investigated as an alternative technique to the conventional direct-contact poling of ferroelectric thin films.

Contact-poling and corona poling techniques were applied to highly (001)/(100) oriented lead zirconate titanate thin films of composition 52% Zr, 48% Ti (PZT52/48). Different poling voltages and durations were used to pole the samples, and the effects on the piezoelectric coefficients e_31,f and d_33,f were explored. From the magnitudes of piezoelectric coefficients and suppression of e_31,f relative to d_33,f, it was concluded that corona poling is a more suitable technique for poling PZT52/48 thin films than direct poling, since piezoelectric coefficients were higher and sample damage was less for corona poling when compared with contact-poling.     

Ultra-thin and isolated dots in polycrystalline lead zirconate titanate films

Size effects with critical thickness or minimum volume for ferroelectricity are of importance in the application of polycrystalline PZT thin films as future memory devices and as storage media. Isolated dots of perovskite phases in the matrix of pyrochlore were synthesized by isothermal annealing through transformation from amorphous to perovskite. Control of the transformation kinetics allows us to produce the isolated ferroelectric dots with a diameter of 50 nm. Domain structure of the isolated dots is also studied by piezoresponse force microscopy. As prepared, all isolated dots contain perpendicularly polarized monodomains. Domain structures and switching behaviors of the isolated dots are similar to those of the single crystalline PZT films. Polycrystalline PZT films with a thickness of 50 nm were also investigated. They show excellent piezoresponse properties and switching behaviors. Ultra-thin polycrystalline PZT films can play a major role in the application of future ferroelectric memories and field-effect transistors as well as for storage media using the local probe technique.