PbZrxTi1-xO3 Film Research Articles

Epitaxial growth of full range of compositions of (1 1 1) PbZr1-xTixO3 on GaN

Integrating functional complex oxides with conventional (“non-oxide”) semiconductors emerges to be an important research field and has been attracting great interest. Because of their superior intrinsic material properties, such as a relatively high dielectric constant and polarization, the utilization of PbZr1-xTixO3 (PZT) materials as a dielectric layer is expected to greatly improve the performance of the GaN high electron mobility transistor. The functional PbZr1-xTixO3 exhibits quite different crystal structures and consequently physical properties depending on the composition. In this work we report the growth of full range of compositions of PZT films on MgO buffered GaN substrates. Besides revealing the temperature effect on phase formation and surface morphology, we demonstrated the strong effect of composition on the growth: pure (1 1 1) phase is formed in Ti-rich PZT (x > 0.48) while pyrochlore impurity phase is found in Zr-rich PZT (x < 0.48). By introducing an ultrathin Ti-rich PZT seed layer, we are able to achieve epitaxial growth of Zr-rich PZT. The epitaxial PZT films of different composition all exhibit good ferroelectric properties, showing great promise for future GaN device applications.

Ultraviolet light effect on characteristics of PbZrxTi1-xO3 (PZT) film during poling process

Poling process is one of the widely used strategies to enhance the film properties during the manufacturing processes of ceramic film. A lab-made PbZrxTi1-xO3 (PZT) film by spin-coating on Ti substrate is presented. After heating treatments, the fabricated film was passed corona poling under ultraviolent (UV) light of various power. The characteristics of the PZT film have been evaluated by several instruments, such as X-ray diffractometer (XRD), scanning electron microscopy (SEM) and impedance analyzer. The results indicated that UV light would affect the orientation of crystallization, microstructure and grain size of PZT film surface during poling process under room temperature.

Piezoelectric response and electrical properties of Pb(Zr1-xTix)O3 thin films: The role of imprint and composition

The compositional dependence of the piezoelectric properties of self-polarized PbZr1-xTixO3 (PZT) thin films deposited on Pt/TiO2/SiO2/Si substrates (x = 0.47, 0.49 and 0.50) was investigated by in situ synchrotron X-ray diffraction and electrical measurements. The latter evidenced an imprint effect in the studied PZT films, which is pronounced for films with the composition of x = 0.50 and tends to disappear for x = 0.47. These findings were confirmed by in situ X-ray diffraction along the crystalline [100] and [110] directions of the films with different compositions revealing asymmetric butterfly loops of the piezoelectric strain as a function of the electric field; the asymmetry is more pronounced for the PZT film with a composition of x = 0.50, thus indicating a higher built-in electric field. The enhancement of the dielectric permittivity and the effective piezoelectric coefficient at compositions around the morphotropic phase boundary were interpreted in terms of the polarization rotation mechanism and the monoclinic phase in the studied PZT thin films.

Flexible highly-effective energy harvester via crystallographic and computational control of nanointerfacial morphotropic piezoelectric thin film

Controlling the properties of piezoelectric thin films is a key aspect for designing highly efficient flexible electromechanical devices. In this study, the crystallographic phenomena of PbZr1–x Ti x O3 (PZT) thin films caused by distinguished interfacial effects are deeply investigated by overlooking views, including not only an experimental demonstration but also ab initio modeling. The polymorphic phase balance and crystallinity, as well as the crystal orientation of PZT thin films at the morphotropic phase boundary (MPB), can be stably modulated using interfacial crystal structures. Here, interactions with MgO stabilize the PZT crystallographic system well and induce the texturing influences, while the PZT film remains quasi-stable on a conventional Al2O3 wafer. On the basis of this fundamental understanding, a high-output flexible energy harvester is developed using the controlled-PZT system, which shows significantly higher performance than the unmodified PZT generator. The voltage, current, and power densities are improved by 556%, 503%, and 822%, respectively, in comparison with the previous flexional single-crystalline piezoelectric device. Finally, the improved flexible generator is applied to harvest tiny vibrational energy from a real traffic system, and it is used to operate a commercial electronic unit. These results clearly indicate that atomic-scale designs can produce significant impacts on macroscopic applications.

Effect of Composition on the Physical Properties at Nanoscale of PZT Thin Films

PbZr1−xTixO3 (PZT) thin films (x = 0.46, 0.47, 0.48, 0.49, and 0.50) were deposited on Pt/TiO2/SiO2/Si substrates using a polymeric chemical method to study the effects of the composition on the macroscopic electrical and local piezoelectric properties. Both measurements demonstrate the existence of a self-polarization effect in all studied PZT films. The measurements were discussed in terms of the contribution of the Schottky barriers to the self-polarization effect. It is shown that both Schottky barrier effect and mechanical coupling near the film-substrate interface are not the dominant mechanisms responsible for the observed phenomena.

Phase transition in ferroelectric Pb(Zr0.52Ti0.48)O3 epitaxial thin films

PbZr1−xTixO3 (PZT) has been intensively studied for various ferroelectric applications. Its promising application for micro-electro-mechanical system has reignited the interests due to its outstanding ferroelectric and piezoelectric properties. Most PZT ceramics employed in devices are synthesized with a Zr/Ti ratio close to the tetragonal–rhombohedral morphotropic phase boundary (x=0.48) due to its high electro-mechanical coupling at this composition. Morphotropic phase boundary is particularly interesting to study for the investigation of phase transition. In this work, we report the epitaxial growth and electrical characterization of epitaxial PZT (Zr/Ti=52/48) thin films on Nb-doped SrTiO3. PZT films, with thickness from 30nm to 65nm, were deposited by sol–gel method and eventually crystallized at 700°C by rapid thermal annealing in oxygen. Film ferroelectricity was confirmed by Sawyer–Tower circuit measurement. X-ray diffraction analysis indicates a thickness-dependent structural phase, i.e., a phase transition from tetragonal phase for the thinner film to a biphasic (tetragonal+pseudo-cubic) structure for the thicker film, which is characterized by ellipsometry as a phase separation from the bottom surface of the film to the top one. This phase transition is related to a composition gradient within the film thickness.

$\hbox{PbZr}_{x}\hbox{Ti}_{1 - x}\hbox{O}_{3}$ Ferroelectric Thin-Film Capacitors for Flexible Nonvolatile Memory Applications

This letter reports the fabrication of PbZrxTi1-xO3 (PZT) thin-film capacitors on flexible plastic substrates. The PZT film was formed on a wafer using a sol-gel method and transferred to a thin plastic substrate using an elastomeric stamp. The PZT film on the plastic substrate showed a well-saturated hysteresis loop with a Pr of ~ 20μC/cm2 and a Vc of ~1.1 V at a supplied voltage of 3 V, which are similar to those observed for PZT films on rigid wafers, as well as stable operation under an 8-mm bending radius. These characteristics suggest promising applications in flexible electronic systems.

Improvement of Sol–Gel Derived PbZrxTi1-xO3 Film Properties Using Thermal Press Treatment

A thermal press treatment was introduced in the sol–gel process of PbZrxTi1-xO3 (PZT) thin films for the first time and the crystalline and electrical characteristics of the PZT films were investigated. The thermal press treatment was applied to the amorphous PZT gel film before crystallization annealing. It is found that the crystalline orientation and grain size of the PZT film fabricated with the thermal press treatment are different from those of the film fabricated by the conventional sol–gel process without the thermal press treatment, even though the crystallization conditions are exactly the same. It is demonstrated that the electrical properties, especially leakage current density and breakdown field, are significantly improved for the PZT film fabricated with the thermal press treatment. Furthermore, we also demonstrate that the fatigue property is improved by introducing the thermal press treatment.

Phase diagrams, dielectric response, and piezoelectric properties of epitaxial ultrathin (001) lead zirconate titanate films under anisotropic misfit strains

We develop a nonlinear thermodynamic model to predict the phase stability of ultrathin epitaxial (001)-oriented ferroelectric PbZr1−xTixO3 (PZT) films with x=1.0, 0.9, 0.8, and 0.7 on substrates which induce anisotropic in-plane strains. The theoretical formalism incorporates the relaxation by misfit dislocations at the film deposition temperature, the possibility of formation of ferroelectric polydomain structures, and the effect of the internal electric field that is generated due to incomplete charge screening at the film-electrode interfaces and the termination of the ferroelectric layer. This analysis allows the development of misfit strain phase diagrams that provide the regions of stability of monodomain and polydomain structures at a given temperature, film thickness, and composition. It is shown that the range of stability for rotational monodomain phase is markedly increased in comparison to the same ferroelectric films on isotropic substrates. Furthermore, the model finds a strong similarity between ultrathin PbTiO3 and relatively thicker PZT films in terms of phase stability. The combinations of the in-plane misfit strains that yield a phase transition sequence that results in a polarization rotation from the c-phase (polarization parallel to the [001] direction in the film) to the r-phase, and eventually to an in-plane polarization parallel to the [110] direction (the aa-phase) is determined to be the path with the most attractive dielectric and piezoelectric coefficients resulting in enhancements of 10 to 100 times in the dielectric permittivity and piezoresponse compared to bulk tetragonal ferroelectrics of the same PZT composition.

Nanometer scale crystallographic texture mapping of platinum and lead zirconate titanate thin films by electron backscatter diffraction

Automated high resolution electron backscatter diffraction was used to map the local crystallographic texture of Pt and PbZr1−x Ti x O3 (PZT) thin films with a resolution as high as 5 nm. The Pt and PZT films consisted of 99.9% and 94.3% {111} textured grains (i.e., with (111) planes parallel to the substrate surface), respectively. The average Pt and PZT grain sizes were 46 ± 30 nm and 65 ± 30 nm, respectively. Quantification of misorientation distributions and the fraction of non-{111}-textured grains demonstrates the potential of this local texture measurement method for quantifying the ferroelectric variability limits of PZT-based capacitors.

Comparison of PbZr1−xTixO3 thin films deposited on different substrates by liquid delivery metal organic chemical vapor deposition

In the paper, PbZr1−xTixO3 (PZT) thin films were prepared on different substrates including 8 and 4 in. silicon, Pt, Ir, and IrO2/Ir substrates by liquid delivery metal organic chemical vapor deposition. Through optimizing the preparation condition, the better film uniformity of thickness, composition, and temperature was achieved. The thickness uniformity of PZT films on 8 in. substrate was about ±3.24%. The deposition rate of Pb (15.8 nm/min) and Ti (17.9 nm/min) were much faster than that of Zr (2.5 nm/min). The growth temperature of PZT film was 570–630 °C depending on the substrate used. The growth rate of PZT films deposited on Si (30–35 nm/min) was faster than that on metal substrate. The substrate has a great effect on the ferroelectric properties of PZT films. PZT film growing on Ir was much uniform and denser than that on Pt, highlighting the good microstructure achieved. PZT films based on Ir substrate showed excellent ferroelectric properties. At the applied voltage of 5 V, the remanent polarization (2Pr) and coercive field (Ec) values of Ir/IrO2/PZT/IrO2/Ir capacitors were about 36 μC/cm2 and 50 kV/cm, respectively.

Ferroelectric properties of heteroepitaxial PbTiO3 and PbZr1–xTixO3 films on Nb-doped SrTiO3 fabricated by hydrothermal epitaxy below Curie temperature

PbTiO3 (PTO) and PbZr1-xTixO3 (PZT) films on a (100) Nb-doped SrTiO3 (NSTO) substrate were fabricated at 160 and 210 °C, respectively, by hydrothermal epitaxy below the Curie temperature, TC. The PTO capacitor had a square hysteresis curve compared to the rounded hysteresis curve of the PZT capacitor. These differing behaviors in the polarization-electric hysteresis curves can be explained by the existence of an interfacial layer formed between the PZT film and the NSTO substrate. The PZT capacitor showed almost no polarization fatigue after 1011 switching cycles. However, the PTO capacitor revealed a slightly different fatigue behavior due to the microvoids that formed as a result of the agglomeration of the island growth mode. However, the fatigue behavior of both capacitors revealed that defects, such as the lead or oxygen vacancies, were suppressed by the hydrothermal epitaxy using a very low fabrication temperature below TC.

The memory characteristics of submicron feature-size PZT capacitors with PtOx top electrode by using dry-etching

Dry etching and its effect on the characteristics of submicron feature-size PbZr1−xTixO3 (PZT) capacitors with PtOx top electrode were investigated. The photoresist (PR)-masked PtOx films were etched by an Ar/(20%)Cl2/O2 helicon wave plasma. A fence-free pattern with a significantly high etch rate and sidewall slope was obtained by the addition of O2 into the etching gas mixture, due to the chemical instability of PtOx and the formation of a PtO2 passivation layer to suppress redeposition of the etch by-products on the etched surface. The patterned PtOx electrode can be further used as a hard mask for etching the PZT film, subsequently, with the gas mixture of Ar, CF4 and O2. A high etching rate of PZT and a good etching selectivity to PtOx can be obtained at 30% O2 addition into the Ar/(50%)CF4 plasma. The etched capacitors have a steep, 72°, sidewall angle with a clean surface. Moreover, the addition of O2 into the etching gas can well preserve the properties and the fatigue endurance of PtOx/PZT capacitors.

METAL-ORGANIC CHEMICAL VAPOR DEPOSITION OF PbZrxTi1 –xO3 THIN FILMS ON Ir–Ru ALLOY ELECTRODES

ABSTRACT Surface smoothness and ferroelectric properties of PbZrxTi1-xO3 (PZT) thin films, grown by metal-organic chemical vapor deposition (MOCVD), was improved by applying Ir–Ru alloy electrode. It was found that the specific composition ratio of Ir and Ru could guarantee the best quality of the alloyed films. That is 35∼55% in Ru portion called as the ‘multi-phase' because two different crystallographic structures exist. This multi-phase electrode has high thermal stability under oxygen annealing atmosphere. Moreover, PZT thin films grown on the multi-phase electrodes, have smoother surface roughness, larger remnant polarization and lower current leakage than PZT thin films on Ir electrode. It is thought think that the grain boundaries between different phases of the alloyed electrode could supply high nucleation sites of PZT to restrict large grain growth and regulate grain shape. Therefore, the multi-phase alloyed electrode can be utilized for growing ultra thin PZT film, which is necessary for high-density ferroelectric random access memory.

Piezoelectric effect in epitaxial PbZr1−xTixO3 thin films near morphotropic phase boundary region

The relationship between crystal structure and piezo-response was investigated in epitaxially grown PbZr1−xTixO3 (PZT) thin films on Pt(001)/MgO(001) with a thin PbTiO3 interlayer. Insertion of the interlayer resulted in significant relaxation ofthe strain that could be developed in the course of deposition of the PZT films, consequently leading us to single out only the effect of composition. Composition of the morphotropic phase boundary (MPB), at which tetragonal and rhombohedral phases are mixed with the same volume fraction, was found to be ∼0.55 in Ti/(Zr + Ti) ratio in our films, which is close to the value for bulk polycrystalline PZT (∼0.50). The piezoelectric response peaks were two times higher in the MPB regime than in the single phase regime due to structural instability caused by the coexistence of two phases. The results indicate that epitaxial PZT films having the MPB composition are advantageous over those of other compositions for nano-storage devices based on scanning force microscopy.

A Comparison of PZT-Based and TiNi Shape Memory Alloy-Based MEMS Microactuators

Microactuators based on PbZr1-xTixO3 (PZT) and TiNi shape memory alloys (SMAs) are compared using microelectromechanical systems (MEMS) micropumps and microvalves as typical devices. The advantages of PZT-based microactuation include large actuation forces and high operating frequencies, leading to high operating pressures and flow rates. Disadvantages include high operating voltages and low strains. TiNi SMA-based microactuators can achieve high flow rates at low operating voltages, due to their high recoverable strains; however, they suffer from low operating frequencies and high power consumption. For both microactuation schemes, additional structures are usually required to realize the reciprocating motion required for MEMS devices. When used in MEMS, the thicknesses of PZT and TiNi SMA films range up to several microns. The thickness dependence of properties relevant to microactuation of PZT and TiNi SMA thin films are discussed and compared.

Polarization and self-polarization in thin PbZr1-xTixO3 (PZT) films

The self-polarization effect in ferroelectric thin films has beenstudied for PZT films 0.5-1 µm thick deposited byradio-frequency magnetron sputtering of various ferroelectricceramic targets (Zr/Ti = 54/46, Zr/Ti = 54/46 + 10% PbOand Zr/Ti = 40/60 + 10% PbO). The laser intensity modulationmethod has been applied, together with the methods ofC-V characteristics and dielectric hysteresis loops, todetermine the polarization distribution and evaluate the built-inelectric fields in the films. It is shown that the bottom interfaceof the thin-film Pt-PZT-Pt capacitor structure is the source ofself-polarization for a certain technological sequence of structureformation. The self-polarization effect is caused by two factors:(i) n- or p-type conductivity due to oxygen or lead vacancies orother impurities in the films and (ii) high trap density at thebottom interface of the structure.

Total dose radiation effects of Pt/PZT/Pt ferroelectric capacitors fabricated by PLD method

In order to study total dose radiation effects of PbZrxTi1-xO3 (PZT) film made with the pulsed excimer laser deposition (PLD) technique, hysteresis loops and capacitance-voltage (C-V) curves of PZT film capacitors have been measured before and after -ray irradiation. The results show that, in a range of 0-2 × 105 Gy (Si), with increasing total dose, the remanent polarization 2Pr increased while dielectric constant decreased. This can be explained by charges trapped by some defects during irradiation.

Laser-assisted low temperature processing of Pb(Zr, Ti)O3 thin film

A method for lowering the processing temperature of PbZr1−xTixO3 (PZT) films was developed utilizing a laser-assisted two-step process. In the first step, perovskite phase was initiated in the PZT films to a furnace anneal at low temperatures in the range of 470–550 °C, depending on the Zr/Ti ratio. Later, the films were laser annealed (using KrF excimer laser) at room temperature to grow the perovskite phase, and to improve microstructure and ferroelectric properties. It was found that this two-step process was very effective in producing excellent quality ferroelectric PZT films at low temperatures. It should be noted that although laser annealing of amorphous and/or pyrochlore films directly (one-step process) produced perovskite phase, the ferroelectric properties of these films, irrespective of the composition, were rather unattractive. Some possible reasons for the ineffectiveness of the one-step process were discussed.

Microstructure and properties of PbZr1-xTixO3 thin films made by one and two step metalorganic chemical vapor deposition

Ferroelectric films of PbZr1-xTixO3 (PZT) have been prepared by one and two step MOCVD techniques using an advanced oxide MOCVD tool. The PZT films were deposited onto 6″ Pt/Ti/SiO2/Si and Si wafers to measure their phase formation, microstructure and ferroelectric properties. It was found that nucleation of the PZT perovskite phase started when the deposition temperature exceeded 550°C and grain growth dominated at 650°C and above. The grain size of PbZr0.5Ti0.5O3 thin films increased from 0.04 μm to 0.3 μm with increasing deposition temperature, but the surface roughness and leakage current also increased. Therefore, in order to obtain dense and homogeneous PZT films that have low surface roughness, we have developed a two step MOCVD process. The PZT thin films made by this two step MOCVD process exhibit excellent ferroelectric properties. Typically, 300 nm thick PZT films with a grain size about 0.3 μm have Pr greater than 20–30 μC/cm2 at 5V, a dielectric constant around 1000, and low coercive field (Ec 50–70 kV/cm), fatigue rate about 0.4 after 109 cycles at 5V and leakage current 2 × 10−7 A/cm2 at 100 kV/cm and room temperature on Pt electrodes. The microstructure and ferroelectric properties of PZT thin films made by both one and two step MOCVD techniques were also investigated.