Polarization-electric field relations of ferroelectric/antiferroelectric layered ceramics in Pb(Nb, Zr, Sn, Ti)O3 system

Ferroelectric/antiferroelectric bi-layer ceramics with different ferroelectric and antiferroelectric phase thickness ratio (FE/AFE thickness ratio) in Pb(Nb, Zr, Sn, Ti)O₃ system were prepared and characterized. With increasing the maximum external electric field from 0 to 40 kV/cm, polarization-electric field relation was always ferroelectric-like or underwent an antiferroelectric-ferroelectric transition, depending on the FE/AFE thickness ratio. All layered ceramics showed ferroelectric-like hysteresis loops with maximum external electric field of 40 kV/cm, and much higher remanent polarizations were attained than those of the ferroelectric/antiferroelectric heterostructures reported previously.     

Growth and characterization of (Pb,La)(Zr,Ti)O3 thin film epilayers on SrTiO3-buffered Si(001)

Ferroelectric Pb_0.92La_0.08Zr_0.4Ti_0.6O₃ (PLZT) thin films were deposited on SrTiO₃-buffered Si(001) substrate by on-axis radio frequency magnetron sputtering. X-ray diffraction analysis revealed epitaxial growth of monocrystalline PLZT films, with an (001) rocking curve full width at half maximum of ∼ 0.3°. φ-scans showed 45° in-plane orientation of the perovskite unit cell relative to that of silicon. The elemental composition of the thin film heterostructure was examined by Auger sputter depth profiling measurements. The recorded profiles suggest that the SrTiO₃ buffer layer serves not only as a template for epitaxial growth, but also as a barrier suppressing Pb-Si interdiffusion between the PLZT layer and the Si substrate. The surface roughness of the PLZT layer was measured at ∼ 4 nm for films with ∼ 500 nm thickness. Wavelength dispersions for the refractive index (n) and the extinction coefficient (k) were obtained from spectroscopic ellipsometry measurements, with n ∼ 2.48 at the main communication wavelength λ = 1550 nm and k < 0.001 for λ > 650 nm. Recorded polarization vs. electric field loops for the PLZT epilayer, with a SrRuO₃ electrode layer interposed between PLZT and SrTiO₃, showed a remnant polarization P_r ≈ 40 µC/cm² and coercive field E_c ≈ 100 kV/cm. These findings suggest that the sputter-deposited PLZT thin films retain the functional properties critical to ferroelectric and electro-optic device applications, also when integrated on a semiconductor substrate.     

Microstructure and ferroelectric properties of epitaxial BaTiO3/SrRuO3/SrTiO3 (001) films grown by pulsed laser deposition under high oxygen pressure

BaTiO₃ films were epitaxially grown on SrTiO₃ (001) substrates buffered with SrRuO₃ films as bottom electrode by pulsed laser deposition under high oxygen pressure of 30 Pa. The quality of the BaTiO₃/SrRuO₃/SrTiO₃ multilayer films was analyzed by means of X-ray diffraction, atomic force microscopy and transmission electron microscopy. BaTiO₃ films were found to be highly c-axis-oriented tetragonal phase with c/a = 1.002. The dielectric constant first increased with increasing temperature, and showed a peak at the Curie temperature of about 356 K. The films had well-saturated hysteresis loops with a remnant polarization of 7.3 μC/cm² and a coercive field of 29.5 kV/cm at room temperature.     

Enhancement of the dielectric properties of Pb(La,Ti)O3 thin films fabricated by pulsed laser deposition

PLT thin films with a thickness of 600 nm were grown by pulsed laser deposition (PLD) using different laser wavelengths of 355, 532 and 1064 nm, respectively. We have systematically investigated the variation of grain sizes depending on the process condition. A two-step process to grow (Pb_0.72La_0.28)Ti_0.93O₃ (PLT) films was adopted and verified to be useful to enlarge the grain size of the film and to enhance leakage current characteristics. Structural and electrical properties including dielectric constant, ferroelectric characteristics, and leakage current of PLT thin films were shown to be strongly influenced by grain size.     

Molecular beam epitaxy growth of BaTiO3 thin films and crucial impact of oxygen content conditions on the electrical characteristics

In this study, high quality crystalline BaTiO₃ films were fabricated in different oxygen content conditions on Nb-doped SrTiO₃ (001) substrates by molecular beam epitaxy (MBE). BaTiO₃ epitaxial film presents a critical thickness of 6.4 nm and is almost entirely relaxed on SrTiO₃ with a thickness of ~ 50 nm. The electrical properties of 50 nm strain-free BaTiO₃ films were investigated by both macro- and microscopic measurements. Their electrical characteristics were found to be strongly influenced by different oxygen content conditions used during the preparation or annealing of the samples. Limited molecular oxygen partial pressure in a MBE chamber probably leads to a great amount of oxygen vacancies in the oxide film and results in rectification behavior of the oxygen-deficient BaTiO_3-x/SrTiO₃:Nb interface. Several approaches such as using atomic oxygen ambiance during the growth, annealing under elevated oxygen pressure were employed in order to decrease the oxygen vacancy density and these approaches eventually permit obtaining BaTiO₃ films with good ferroelectric characteristics.     

Epitaxial growth and electrical measurement of single crystalline Pb(Zr0.52Ti0.48)O3 thin film on Si(001) for micro-electromechanical systems

We report in this work the epitaxial growth and the electrical characteristics of single crystalline Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin film on SrTiO₃(STO)-buffered Si(001) substrate. The STO buffer layer deposited by molecular beam epitaxy allows a coherent oxide/Si interface leading enhanced PZT crystalline quality. 70 nm-thick PZT (52:48) layer was then grown on STO/Si(001) by sol-gel method. X-ray diffraction demonstrates the single crystalline PZT film on Si substrate in the following epitaxial relationship: [110] PZT (001)//[110] STO (001)//[100] Si (001). The macroscopic electrical measurements show a hysteresis loop with memory window of 2.5 V at ± 7 V sweeping range and current density less than 1 μA/cm² at 750 kV/cm. The artificial domains created by piezoresponse force microscopy with high contrast and non-volatile properties provide further evidence for the excellent piezoelectric properties of the single crystalline PZT thin film.     

Needle-like LiFePO4 thin films prepared by an off-axis pulsed laser deposition technique

Lithium iron phosphate (LiFePO₄) thin films were prepared by pulsed laser deposition with an off-axis geometry. Amorphous, needle-like and crystallized granular thin films were prepared on Si and titanium substrates. The preferred orientation of these crystallized LiFePO₄ thin films is (120). Microstructures of the deposited films are dependant on the substrate temperature (room temperature, 500 °C and 700 °C) and Ar pressure (5 Pa and 30 Pa) in the chamber. The needle-like film grows following a self-shadowing mechanism. LiFePO₄ thin film with high crystallinity shows a voltage plateau which is a typical feature of the phase transition reaction for bulk material, while the sloped profiles are observed clearly in the charging and discharging curves of LiFePO₄ thin films with low cystallinity.     

Enhanced ferroelectric properties of Mg doped (Ba,Sr)TiO3 thick films grown on (001) SrTiO3 substrates

Highly (001)-oriented 1 mol% Mg doped (Ba_0.67,Sr_0.33)TiO₃ (BST) films with a thickness of 1.25 μm were grown on (110) SrRuO₃/(001) SrTiO₃ substrates by pulsed laser deposition. X-ray diffraction measurements reveal that the BST thick films have very high crystalline quality, and have a distorted lattice with a large tetragonality a/c = 1.012. The BST thick films have a remanent polarization (P_r) value as large as 10.1 μC/cm² and a coercive electric field (E_c) value of 65.0 kV/cm. The films possess dielectric constant and loss values of ε_r = 385.36 and tgδ = 0.038 at 1 kHz and room temperature. The leakage currents of the films are on the order of 10^− 5 A/cm² at ± 150 kV/cm. The mechanism for enhancing electric properties of the Mg doped BST films was also discussed.     

Structural and thermal characterization of La5Ca9Cu24O41 thin films grown by pulsed laser deposition on (1 1 0) SrTiO3 substrates

In the present study stoichiometric, b-axis oriented La₅Ca₉Cu₂₄O₄₁ thin films were grown by pulsed laser deposition on (1 1 0) SrTiO₃ substrates in the temperature range 600-750 °C. High resolution transmission electron microscopy was employed to investigate the growth mechanism and the epitaxial relationship between the SrTiO₃ substrates and the La₅Ca₉Cu₂₄O₄₁ films grown at 700 °C. The 3-ω method was used to measure the cross-plane thermal conductivity of La₅Ca₉Cu₂₄O₄₁ films in the temperature range 50-350 K. The observed glass-like behavior is attributed to atomic-scale defects, grain boundaries and an interfacial layer formed between film and substrate.     

Distribution of pyrochlore phase in Pb(Mg1/3Nb2/3)O3-PbTiO3 films and suppression with a Pb(Zr0.52Ti0.48)O3 interfacial layer

Thin films of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) on Pt/Ti/SiO₂/Si (Pt/Si) substrates both with and without a Pb(Zr_0.52Ti_0.48)O₃ (PZT) interfacial layer were investigated. Perovskite and pyrochlore coexistence was observed for PMN-PT thin films without a PZT interfacial layer. Interestingly, most of the pyrochlore phase was observed in single-coated films and in the first layer of multi-coated films. The pyrochlore phase exhibited grains with an average size of about 25 nm, which is smaller than those of the perovskite phase (about 90 nm). In contrast, for PMN-PT thin films grown on a PZT interfacial layer, the formation of a pyrochlore phase at the interface between PMN-PT layers and the substrate is completely suppressed. Moreover, small grains are not observed in the films with a PZT interfacial layer. The measured polarization-electric field (P-E) hysteresis loops of PMN-PT films with and without PZT layers indicate that enhanced electrical properties can be obtained when a PZT interfacial layer is used. These enhanced properties include an increase in the value of remanent polarization P_r from 2.7 to 5.8 μC/cm² and a decrease in the coercive field E_c from 60.5 to 28.0 kV/cm.     

Structural and magnetic properties of epitaxial SnFe2O4 thin films

Epitaxial thin films of SnFe₂O₄ are deposited on sapphire substrate by ablating the sintered SnFe₂O₄ target with a KrF excimer laser (λ = 248 nm and pulsed duration of 20 ns). X-ray diffraction study reveals that SnFe₂O₄ films are epitaxial along (222) direction. The optical bandgap of SnFe₂O₄ film is estimated using transmittance vs. wavelength data and is observed to be 2.71 eV. The presence of hysteresis loop at room temperature in magnetization vs. field plot indicates the ferromagnetic behavior of the film. It is observed that the coercive field and remnant magnetization decrease with increase in temperature.     

Electric response of Pb0.99[(Zr0.90Sn0.10)0.968Ti0.032]0.98Nb0.02O3 ceramics to the shock-wave-induced ferroelectric-to-antiferroelectric phase transition

Shock-wave-enforced ferroelectric (FE)-to-antiferroelectric (AFE) phase transition releases a large electrical polarization, having application in pulse power technology. In the present work, the depoling currents under shock wave compression were investigated in Pb_0.99[(Zr_0.90Sn_0.10)_0.968Ti_0.032]_0.98Nb_0.02O₃ (PZST) ceramics with composition close to the FE/AFE phase boundary. Shock wave was generated by gas-gun and propagated in a direction perpendicular to the remanent polarization. It was found that the shock pressure promoted the phase transition under the short-circuit condition. The shock pressure dependence of the released charge was associated with the evolution of FE-to-AFE phase transition. The onset of phase transition was about 0.40 GPa and complete transformation occurred at 1.23 GPa. However, the released charge decreased with increasing load resistance. The reason may be that the electric field suppresses the phase transition in uncompressed zone and/or shock induces conductivity in compressed zone. Results lay the foundation for application of PZST ceramics in shock-activated power supply.     

Effect of lattice-mismatch strain on the structural and dielectric properties of (Pb,Sr)TiO3 thin films grown by pulsed laser deposition

Pb_0.35Sr_0.65TiO₃ (PST) thin films have been fabricated on LaAlO₃ (LAO) and MgO substrates using the pulsed laser deposition technique. The microstructure characteristics of the films were examined by means of X-ray diffraction, atomic force microscopy, scanning electron microscopy and Raman spectroscopy, and the results indicate that the films are epitaxially grown and show good crystallinity. The dielectric constant dependence on DC bias voltage and temperature were measured in a planar capacitor configuration for these films. Compared to the PST thin film grown on LAO, the film grown on MgO showed a higher temperature of the capacitance maximum and a higher dielectric constant at zero bias. We explain the results by taking into account the lattice-mismatch strain between the substrate and the film. In contrast to the in-plane compressive strain induced by the LAO substrate, the in-plane tensile strain induced by the MgO substrate enlarges the unit cell of PST and enhances the magnitude of dipole moments, which increases the dielectric constant. These results indicate that a reasonable in-plane tensile strain could improve the dielectric properties of PST thin films.     

Lead-free piezoelectric thin films of Mn-doped NaNbO3-BaTiO3 fabricated by chemical solution deposition

Lead-free piezoelectric thin films of NaNbO₃-BaTiO₃ were fabricated on Pt/TiO_x/SiO₂/Si substrates by chemical solution deposition. Perovskite NaNbO₃-BaTiO₃ single-phase thin films with improved leakage-current and ferroelectric properties were prepared at 650 °C by doping with a small amount of Mn. The 1.0 and 3.0 mol% Mn-doped 0.95NaNbO₃-0.05BaTiO₃ thin films showed slim ferroelectric P-E hysteresis and field-induced strain loops at room temperature. The 1.0 and 3.0 mol% Mn-doped 0.95NaNbO₃-0.05BaTiO₃ films showed remanent polarization values of 6.3 and 6.2 μC/cm², and coercive field of 41 and 55 kV/cm, respectively. From the slope of the field-induced strain loop, the effective piezoelectric coefficient (d₃₃) was found to be 40-60 pm/V.     

Phase developments in the Pb(Zn1/2W1/2)O3-Pb(Zn1/3Ta2/3)O3-Pb(Zn1/3Nb2/3)O3 pseudo-ternary system

The phase stability ranges in the B-site precursor (Zn_1/2W_1/2)O₂-(Zn_1/3Ta_2/3)O₂-(Zn_1/3Nb_2/3)O₂ were determined by X-ray diffraction (XRD), where wolframite, tri-αPbO₂, and columbite phases were identified. Next attempts were carried out (with the addition of PbO) for the system Pb(Zn_1/2W_1/2)O₃-Pb(Zn_1/3Ta_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃, where the perovskite phase did not develop in the entire compositions investigated. Instead, only the Pb₂WO₅ and pyrochlore phases (along with ZnO) resulted.     

Improved dielectric and ferroelectric properties in Ti-doped BiFeO3-PbTiO3 thin films prepared by pulsed laser deposition

Ti-modified thin films of multiferroic 0.72Bi(Fe_1 − xTi_x)O₃-0.28PbTiO₃ (BFPT, x = 0 and 0.02) solid solution were prepared by pulsed laser deposition. The BFPT (x = 0 and 0.02) films possess a tetragonal structure with highly preferential (001) orientation. The effects of the ionic substitution on the properties of BFPT (x = 0 and 0.02) films have been investigated. The leakage current of the BFPT (x = 0.02) thin film is significantly reduced, and the dielectric and ferroelectric properties greatly improved by the aliovalent ionic substitution of Ti⁴⁺ for Fe³⁺. The BFPT (x = 0.02) thin film exhibits a reasonably high remnant polarization P_r with 2P_r up to 90 μC/cm² at 312 kV/cm and a switchable polarization up to 92 μC/cm² at 417 kV/cm.     

Texture control and ferroelectric properties of Pb(Nb,Zr,Sn,Ti)O3 thin films prepared by chemical solution method

A chelating and spin-coating procedure was used to prepare antiferroelectric thin films of Pb_0.99Nb_0.02[(Zr_0.84Sn_0.16)_0.982Ti_0.018]_0.98O₃. By controlling substrate and thermal processing conditions, films with strong (100) and (111) textures, as well as without textures, were prepared. These antiferroelectric films showed the characteristic double hysteresis polarization vs. electric field loops. An orientation dependence of the critical field for the antiferroelectric-to-ferroelectric phase transition was also observed. In addition, when the applied voltage exceeded a critical level during the first “ramp up,” an abnormal reduction in the area of the hysteresis loop was noticed in the (100) and (111) textured films.     

Composition controlling of KNbO3 thin films prepared by pulsed laser deposition

The composition of KNbO₃ thin films prepared by pulsed laser deposition is crucially influenced by the deposition configuration. In the present study, the composition of KNbO₃ thin films grown on Si (100) substrates by pulsed laser ablation was tried to be controlled by adjusting the target-substrate distance and the oblique angle of substrate from the plume axial direction. It was found that the K deficiency in the films can be effectively avoided by setting the substrate at an appropriate oblique angle from the plume axial direction. The stoichiometric KNbO₃ thin films with a K/Nb molar ratio of 0.98 were successfully obtained, where the substrates were set at an oblique angle of 3-12° from the plume axis while the target-substrate distance was kept at 40 mm.     

Phase diagrams and polarization of single-domain epitaxial Pb(Zr1-xTix)O3 thin films

Within the framework of modified Landau-Devonshire thermodynamic theory, the equilibrium polarization states and physical properties of single-domain Pb(Zr_1-xTi_x)O₃ (PZT) thin films epitaxially grown on dissimilar cubic substrates are investigated. The “misfit strain-temperature” phase diagrams are obtained for several different compositions (x = 0.75, 0.65 and 0.55) of PZT films. The presence of stability range of the monoclinic phase is the characteristic feature of these phase diagrams, which separates the stability ranges of the tetragonal phase and the orthorhombic phase. Further analysis shows that the monoclinic phase widens with the increase of Zr content, and meanwhile the center shifts from positive values of the misfit strain towards zero.