Epitaxial Pb Research Articles

Shifted Morphotropic Phase Boundary in [111]-Oriented Nb-Doped Pb(ZrxTi1–x)O3 Epitaxial Films: Insights into Piezoelectricity and Domain Variation

Morphotropic phase boundary (MPB) in Nb-doped Pb(Zr,Ti)O3 thin films grown epitaxially on Nb:SrTiO3 (111) surfaces was studied by investigating their local piezoelectric response and domain structure using piezoresponse force microscopy. A sharp peak of piezoelectric coefficients was observed at Zr/Ti = 40/60, supporting the fact that the substrate constraint shifted the MPB in [111]-epitaxial Pb(Zr,Ti)O3 thin films toward the PbTiO3 side of composition from Zr/Ti = 52/48 for bulk materials. The domains at MPB also show a distinguishing structure with shrinking size and remarkably larger vertical piezoresponse compared to the single-phase samples. This work provided a deeper understanding about the effect of substrate constraint on the phase structures and electrical properties of epitaxial ferroelectric films.

Ultra-Compact On-chip Electro-Optic Waveguide Ring Resonators Based on Asymmetric Au-(Pb, La)(Zr, Ti)O3-Au Structure

Waveguide ring resonators (WRRs) operating at a telecom window of 1.4∼1.6 μm based on asymmetric Au-(Pb, La)(Zr, Ti)O3-Au structure is proposed and investigated. Electro-optical control of such WRRs is conducted by introducing epitaxial (Pb, La)(Zr, Ti)O3 (PLZT) thin films as the core layer deposited between the upper and lower gold slabs with different widths. The long-term reliability, large electro-optic (EO) coefficients, and spontaneous polarization properties of PLZT thin films have ensured high performance of our WRRs. It has been demonstrated that the footprint of WRR can be fabricated less than 20 μm2 with the radius of ring resonator around 1 μm through combining PLZT thin films with surface plasmons (SPs). Furthermore, the large extinction ratio (∼43 dB), as well as comparatively low insertion loss (<10.5 dB), and moderate modulation sensitivity (0.8 nm/V) can be achieved with proposed WRRs in the wavelength range of 1.4∼1.6 μm. Besides, the strong mode confinement capacity of such asymmetric metal-insulator-metal (MIM) structure can support high bending waveguides, paving the way for the fabrication of more compact electro-optic WRRs with nanometer-scale radius that can work in the other wavelength ranges.

Ferroelectric and piezoelectric responses of (110) and (001)-oriented epitaxial Pb(Zr0.52Ti0.48)O3 thin films on all-oxide layers buffered silicon

Epitaxial ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films were fabricated on silicon substrates using pulsed laser deposition. Depending on the buffer layers and perovskite oxide electrodes, epitaxial films with different orientations were grown. (110)-oriented PZT/SrRuO3 (and PZT/LaNiO3) films were obtained on YSZ-buffered Si substrates, while (001)-oriented PZT/SrRuO3 (and PZT/LaNiO3) were fabricated with an extra CeO2 buffer layer (CeO2/YSZ/Si). There is no effect of the electrode material on the properties of the films. The initial remnant polarizations in the (001)-oriented films are higher than those of (110)-oriented films, but it increases to the value of the (001) films upon cycling. The longitudinal piezoelectric d33,f coefficients of the (110) films are larger than those of the (001) films, whereas the transverse piezoelectric d31,f coefficients in the (110)-films are less than those in the (001)-oriented films. The difference is ascribed to the lower density (connectivity between grains) of the former films.

Orientation and film thickness dependencies of (100)- and (111)-oriented epitaxial Pb(Mg1/3Nb2/3)O3 films grown by metal organic chemical vapor deposition

(100)- and (111)-oriented epitaxial Pb(Mg1/3Nb2/3)O3 films with 500 and 1300 nm in thickness were grown by metal organic chemical vapor deposition. Remained strain was almost relaxed because the crystal structure of the films was almost the same as that of bulk Pb(Mg1/3Nb2/3)O3. Relative dielectric constant showed the maximum value against the temperature that depended on the measurement frequency. Maximum relative dielectric constant, εr(max.), and the temperature showing εr(max.), T(max.), decreased and increased with the frequency, respectively, are in good agreement with reported data for the bulk. εr(max.) and T(max.), respectively increased and decreased with the film thickness and (111)-oriented films showed larger value than that of the (100)-oriented one. Ferroelectricity was observed for all films up to 297 K and monotonously decreased with increasing temperature. Saturation polarization value increased with the film thickness and (111)-oriented films showed larger value than (100)-oriented ones. On the other hand, the coercive field decreased with increasing film thickness, but was almost independent with the film orientation.

Orientation control of epitaxial tetragonal Pb(ZrxTi1−x)O3 thin films grown on (100)KTaO3 substrates by tuning the Zr/(Zr + Ti) ratio

Pulsed metal organic chemical vapor deposition is used to epitaxially grow 30-nm thick tetragonal Pb(ZrxTi1−x)O3 thin films on (100) KTaO3 single crystal substrates. The in-plane misfit strain, which is controlled by the Zr/(Zr+Ti) ratio of Pb(ZrxTi1−x)O3 thin films, tunes the film orientation. When Zr/(Zr+Ti) = 0, which leads to the tensile strain from the substrate, a perfectly (100)-oriented film is formed, whereas when Zr/(Zr+Ti) = 0.45, which leads to the compressive strain from the substrate, a completely (001)-oriented film is realized. The volume fraction of the (001)-oriented domain almost linearly increases as the Zr/(Zr+Ti) ratio increases for films with (100) and (001) mixed orientations; this change in the volume fraction can be explained by considering the average lattice parameter matching between the Pb(ZrxTi1−x)O3 thin film and the KTaO3 substrate. These results demonstrate a method to systematically control the film orientation using the Zr/(Zr+Ti) ratio to tune the in-plane strain of Pb(ZrxTi1−x)O3 films grown on (100) KTaO3 single crystal substrates.

Tuning structure in epitaxial Pb(Mg1/3Nb2/3)O3–PbTiO3 thin films by using miscut substrates

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) is one of the most promising ferroelectric material for actuator, dielectric and electrocaloric applications. However, oriented and phase pure thin films are essential to use the outstanding properties of these compounds. In this work it is demonstrated that the use of miscut substrates influences the growth mechanism leading to a significantly broader deposition window to achieve the required film quality. Therefore, epitaxial 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 films were grown by pulsed laser deposition on (001)-oriented single crystalline SrTiO3 (STO) substrates with a miscut angle between 0 and 15° towards the [100] direction using a conducting La0.7Sr0.3CoO3 buffer layer. The influence of the vicinal angle on the PMN–PT structure was studied by high resolution X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. A nearly pure perovskite phase growth with a cube-on-cube epitaxial relationship was obtained on all miscut STO substrates, whereas a significant volume fraction of the pyrochlore phase was present on the standard substrate. Reciprocal space measurements revealed a peak split of the perovskite reflections indicating structural variants of PMN–PT with different c/a ratios. An additional tilting of the PMN–PT planes with respect to the buffer layer was observed on some samples, which might be explained with the incorporation of dislocations according to the Nagai model. Polarization loops were measured in a temperature range between room temperature and 150°C showing a sharp drop of the remanent polarization above 65°C on vicinal substrates.

Towards reversible control of domain wall conduction in Pb(Zr0.2Ti0.8)O3 thin films

Control over the localised conductance recently observed at ferroelectric domain walls is key for their integration into potential nanoelectronics devices. Using a combination of piezoresponse force microscopy and local conductance mapping, we demonstrate switching between conducting and insulating behavior at 180° domain walls in epitaxial Pb(Zr0.2Ti0.8)O3 thin films subjected to ultrahigh vacuum thermal annealing or exposed to ambient conditions, respectively. The reversibility of this behavior is demonstrated in repeated annealing-exposure cycles. To explain these observations, we propose a mechanism based on changes in electrostatic and chemical boundary conditions through oxygen vacancy redistribution and the removal of surface adsorbates.

Fabrication of High-Efficiency Piezoelectric Energy Harvesters of Epitaxial Pb(Zr,Ti)O3 Thin Films by Laser Lift-off

Abstract We fabricated piezoelectric vibration energy harvesters of c-axis-oriented epitaxial Pb(Zr,Ti)O3 (PZT) thin films on stainless steel (SS304) cantilevers in an effort to improve their power-generation efficiency and toughness. Using radio-frequency magnetron sputtering, we deposited the epitaxial PZT thin films on the MgO substrates, and then transferred the PZT films onto microfabricated SS304 cantilevers using laser lift-off (LLO). LLO did not degrade the transferred epitaxial PZT thin films, which exhibited a high piezoelectric coefficient (e 31,f =–4.8 C/m2) and a low relative dielectric constant (ε r=340), comparable to those of the original PZT thin film on MgO. At a resonance frequency of 143 Hz, the energy harvesters generated large output power of 1.8 μW at an acceleration of 1.0 m/s2, and the output power reached a maximum of 49 μW at an acceleration of 7.5 m/s2.

Subcritical switching dynamics and humidity effects in nanoscale studies of domain growth in ferroelectric thin films

Ferroelectric domain switching in c-axis-oriented epitaxial Pb(ZrTi)O3 thin films was studied using biased scanning probe microscopy tips. While linear and logarithmic dependence of domain size on tip bias and writing time, respectively, are well known, we report an additional linear dependence on relative humidity in the 28–65% range. We map out the switched domain size as a function of both the tip bias and the applied pulse time and describe a growth-limited regime for very short pulses and a nucleation-limited regime for very low tip bias. Using ‘interrupted-switching’ measurements, we probe the nucleation regime with subcritical pulses and identify a surprisingly long relaxation time on the order of 100 ms, which we relate to ionic redistribution both on the surface and within the thin film itself.

Investigation of the polarization dependence of the transient current in polycrystalline and epitaxial Pb(Zr,Ti)O3 thin films

The polarization dependence of the current in epitaxial and polycrystalline (with conductive grain boundaries) Pb(Zr,Ti)O3 (PZT) films is studied using direct-current (dc) measurements and scanning spreading current microscopy. Both methods show identical results in micro- and nanoscale ranges. The current response from the film to the applied bias contains a long relaxation component that depends on the bias rise rate and polarization direction, exhibiting current peaks near the coercive force value. The polarization dependences of the current for polycrystalline and epitaxial films are found to be fundamentally different. The current of the polycrystalline film is much higher when the bias is directed against the polarization, whereas the current of the epitaxial film is higher if the bias and polarization directions coincide. All films exhibit current hysteresis of non-ferroelectric (clockwise) direction with decreasing bias. It is also shown that the polarization dependences of the transient current in both polycrystalline and epitaxial films are similar to the polarization dependence of the photovoltaic current in these films.

Effect of elastic compliances and higher order Landau coefficients on the phase diagram of single domain epitaxial Pb(Zr,Ti)O3 (PZT) thin films

We report the qualitative study of the influence of both elastic compliances and higher order terms of Landau free energy potential on the phase diagram of Pb(Zr0.5Ti0.5)O3 thin films by using a single domain Landau theory. Although the impact of elastic compliances and higher order terms of the Landau free energy potential on the phase diagram of ferroelectric thin films are known, the sensitivity of the phase diagram of PZT thin film on these parameters have not been reported. It is demonstrated that, while values of elastic compliances affect the positions of the phase boundaries including phase transition temperature of the cubic phase; higher order terms can potentially introduce an a1a2-phase previously predicted in PbTiO3 phase diagram.

Enhancement of pyroelectric signal by continuous ultraviolet illumination of epitaxial Pb(Zr0.2Ti0.8)O3 films

The pyroelectric signal generated by an epitaxial Pb(Zr0.2Ti0.8)O3 film can be enhanced by continuous illumination with ultraviolet (UV) light. The measured signal increases more than 2 times at low modulation frequencies of the incident infrared (IR) radiation (∼10 Hz) and at wavelengths where the short-circuit photocurrent presents the maximum value (∼280–300 nm). The tentative explanation is that the changes in polarization induced by the temperature variation under modulated IR illumination are generating a variable internal electric field, able to collect the photogenerated carriers produced under continuous UV illumination leading to an additional signal in phase with the pyroelectric one. This finding could be exploited for designing pyroelectric detectors with enhanced characteristics by combining both UV and IR responses.

General equivalent circuit derived from capacitance and impedance measurements performed on epitaxial ferroelectric thin films

Voltage and frequency dependent capacitance measurements were performed on epitaxial BaTiO3 and Pb(Zr0.2Ti0.8)O3 thin films deposited on single crystal SrTiO3 substrates with (001) and (111) orientations. The measured capacitors have common bottom SrRuO3 contact and different metals as top electrodes: SrRuO3, Pt, Cu, Al, and Au. The capacitance-voltage characteristics were used to extract information regarding the density of the free carriers and the linear contribution to the static dielectric constant. The frequency dependent impedance was used to develop a suitable equivalent circuit for the epitaxial ferroelectric capacitors. It was found that the frequency dependence of the imaginary part of the impedance can be well simulated, in all cases, using a circuit composed of Schottky-type capacitance related to electrode interfaces, contact resistance, and the R-C parallel connection related to the ferroelectric volume of the film. Values for the components of the equivalent circuit were obtained by fitting the experimental data with the simulated curves. These were then used to extract quantities such as dielectric constant in the ferroelectric volume, the width of the depletion layers, and the apparent built-in potential. It was found that, although the investigated capacitors are of different ferroelectric materials, grown on substrates with different orientations, and having different metals as top electrodes, the values for the capacitance associated with the Schottky contacts and the apparent built-in potential are not very different. The results suggest a strong influence of ferroelectric polarization on the electrode interface properties in the case of epitaxial ferroelectric films.

The effects of strain relaxation on the dielectric properties of epitaxial ferroelectric Pb(Zr0.2Ti0.8)TiO3 thin films

We study the effects of strain relaxation on the dielectric properties of epitaxial 40 nm Pb(Zr0.2Ti0.8)TiO3 (PZT) films. A significant increase in the defect and dislocation density due to strain relaxation is observed in PZT films with tetragonality c/a &amp;lt; 1.07 grown on SrTiO3 (001) substrates, which results in significant frequency dispersion of the dielectric constant and strong Rayleigh type behavior in those samples. This combined structural-electrical study provides a framework for investigating strain relaxation in thin films and can provide useful insights into the mechanisms of fatigue in ferroelectric materials.

Epitaxial Pb(Mg1/3Nb2/3)O3-PbTiO3 (67/33) thin films with large tunable self-bias field controlled by a PbZr1−xTixO3 interfacial layer

Epitaxial Pb(Mg1/3Nb2/3)O3-PbTiO3 200 nm thick, (001) oriented, perovskite phase-pure films were grown on a range of PbZr1−xTixO3 buffer layers (x = 0.2–0.8) and sandwiched between SrRuO3 electrodes on (001) SrTiO3 substrates to form a ferroelectric capacitor structure. Devices without a buffer layer or with a buffer layer of highly tetragonal PbZr1−xTixO3 show very large self-bias fields up to 1.0 × 107 V/m. These self-bias fields correlate with strain gradient layers near the bottom electrode observed in these devices only. The large self-bias was explained quantitatively in terms of the flexoelectric effect.

Compliant ferroelastic domains in epitaxial Pb(Zr,Ti)O3 thin films

Ordered patterns of highly compliant ferroelastic domains have been created by use of tensile strained epitaxial Pb(Zr,Ti)O3 thin films, of very low defect density, grown on DyScO3 substrates. The effect of 180° switching on well-ordered a/c 90° domain patterns is investigated by a combination of transmission electron microscopy, piezoelectric force microscopy, and X-ray diffraction. It is shown that ferroelastic a-domains, having an in-plane polarization, can be created and completely removed on a local level by an out-of-plane electric field. The modifications of the ferroelastic domain pattern can be controlled by varying the parameters used during switching with a piezoresponse force microscope to produce the desired arrangement.

Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)

The unoccupied electronic structure of epitaxial Pb films on Si(111) is analyzed by angle-resolved two-photon photoemission in the Γ¯→M¯ direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6pz states with the Si substrate. The experimentally obtained dispersion E(k||) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k||) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6pz states with the Si band. However, the momentum range at which mixing of 6pz and 6px,y derived subbands is found to occur in the presence of the Si substrate is closer to Γ¯ than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a px,y mediated interaction of the pz states with the Si conduction band.

Enhanced dielectric nonlinearity in epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 thin films

High quality c-axis oriented epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 films were fabricated using pulsed laser deposition on (001) LaAlO3 substrates with conductive LaNiO3 buffers. Besides confirmation of the in-plane and out-of-plane orientations using X-ray diffraction, transmission electron microscopy study has revealed columnar structure across the film thickness with column width around 100 nm. Characterization of ferroelectric properties was carried out in comparison with polycrystalline Pb0.92La0.08Zr0.52Ti0.48O3 films to extract the effect of epitaxial growth. It is found that the ratio between the irreversible Rayleigh parameter and reversible parameter increased up to 0.028 cm/kV at 1 kHz on epitaxial samples, which is more than twice of that on their polycrystalline counterparts. While this ratio decreased to 0.022 cm/kV with increasing frequency to100 kHz, a much less frequency dependence was observed as compared to the polycrystalline case. The epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 films exhibited a higher mobility of domain wall and the higher extrinsic contribution to the dielectric properties, as well as reduced density of defects, indicating that it is promising for tunable and low power consumption devices.

Epitaxial Growth of Ferroelectric Pb(Zr,Ti)O3 Layers on GaAs

ABSTRACTFerroelectric epitaxial Pb(Zr,Ti)O3 (PZT) layers were grown by pulsed laser deposition on SrTiO3/GaAs templates fabricated by molecular beam epitaxy. The templates present an excellent structural quality and the SrTiO3/GaAs is abrupt at the atomic scale thanks to surface Ti pre-treatment. The PZT layers contain a- and c- domains, as shown by X-Ray diffraction analyses. Piezoforce microscopy experiments and macroscopic electrical characterizations indicate that PZT is ferroelectric. A relative dielectric permittivity of 164 is extracted from these measurements.

Programmable ferroelectric tunnel memristor

We report an analogously programmable memristor based on genuine electronic resistive switching combining ferroelectric switching and electron tunneling. The tunnel current through an 8 unit cell thick epitaxial Pb(Zr[0.2]Ti[0.8])O[3] film sandwiched between La[0.7]Sr[0.3]MnO[3] and cobalt electrodes obeys the Kolmogorov-Avrami-Ishibashi model for bidimensional growth with a characteristic switching time in the order of 10^-7 seconds. The analytical description of switching kinetics allows us to develop a characteristic transfer function that has only one parameter viz. the characteristic switching time and fully predicts the resistive states of this type of memristor.