Growth of flat SrRuO3 (111) thin films suitable as bottom electrodes in heterostructures

Thin film growth of ferroelectric or multiferroic materials on SrTiO₃(111) with a buffer electrode has been hampered by the difficulty of growing flat electrodes on this polar orientation. We report on the growth and characterization of SrRuO₃ thin films deposited by pulsed laser deposition on SrTiO₃(111). We show that our SrRuO₃(111) films are epitaxial and display magnetic bulk-like properties. Films presenting a thickness between 20 and 30 nm are found to be very flat and therefore suitable as bottom electrodes in heterostructures.     

Large electrocaloric effect of highly (100)-oriented 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 thin films with a Pb(Zr0.3Ti0.7)O3/PbOx buffer layer

0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-PT) thin films with a lead zirconate titanate Pb(Zr_0.3Ti_0.7)O₃ (PZT)/PbO_x buffer layer were deposited on Pt/TiO₂/SiO₂/Si substrates by radio frequency magnetron sputtering technique, and pure perovskite crystalline phase with highly (100)-preferred orientation was formed in the ferroelectric films. We found that the highly (100)-oriented thin films possess not only excellent dielectric and ferroelectric properties but also a large electrocaloric effect (13.4 K at 15 V, i.e., 0.89 K/V) which is attributed to the large electric field-induced polarization and entropy change during the ferroelectric-paraelectric phase transition. The experimental results indicate that the use of PZT/PbO_x buffer layer can induce the crystal orientation and phase purity of the PMN-PT thin films, and consequently enhance their electrical properties.     

Ferroelectric properties of epitaxial Bi3.15Nd0.85Ti3O12 films on SiO2/Si using biaxially oriented MgO as templates

High quality epitaxial Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films with thicknesses from 30 to 80 nm have been integrated on SiO₂/Si substrates. MgO templates deposited by ion-beam-assisted deposition and SrRuO₃ (SRO) buffer layers processed by pulsed laser deposition have been used to initiate the epitaxial growth of BNT films on the amorphous SiO₂/Si substrates. The structural and ferroelectric properties were investigated. Microstructural studies by X-ray diffraction and transmission electron microscopy revealed high quality crystalline with an epitaxial relationship of (001)_BNT||(001)_SRO||(001)_MgO and [100]_BNT||[110]_SRO||[110]_MgO. A ferroelectric hysteresis loop with a remanent polarization of 3.1 μC/cm² has been observed for a 30 nm thick film. The polarization exhibits a fatigue-free characteristic up to 1.44 × 10¹⁰ switching cycles.     

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.     

Nanoscale study by piezoresponse force microscopy of relaxor 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 and 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 thin films grown on platinum and LaNiO3 electrodes

Relaxor 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (70/30 PMN-PT) and 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (90/10 PMN-PT) thin films have been grown by RF-sputtering on platinum (Pt) and lanthanum nickelate (LaNiO₃) bottom electrodes. For both electrodes, macroscopic measurements evidence lower coercive fields, remnant polarizations and piezoelectric coefficients d₃₃ for 90/10 PMN-PT films compared to 70/30 PMN-PT films. For both compositions, coercive fields and remnant polarizations are lower for films grown on LaNiO₃ compared to on Pt while piezoelectric coefficients d₃₃ are higher. For each electrode and composition, a similar behavior is revealed for electromechanical activity at the nanoscale when measuring local piezoelectric hysteresis loops; on the other hand, the voltages required for switching the domains are the highest for 90/10 PMN-PT films grown on LaNiO₃. The existence of large grain boundaries in the films grown on Pt and the presence of local random fields with polar nano-domains for the 90/10 composition could explain the differences measured in domains switching properties at the macroscale and nanoscale levels.     

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.     

Effects of ferroelectric/metal interface on the electric properties of PMN-PT thin films epitaxially grown on Si substrates

Lead magnesium niobate-lead titanate 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-PT) ferroelectric thin films were deposited on SrRuO₃ (SRO), SrTiO₃, and TiN-buffered Si substrates by pulsed laser deposition. X-ray diffraction θ–2θ and Phi scans reveal that PMN-PT films were epitaxially grown on Si substrates. Pt, Al, and Gd metals were employed as top electrodes to investigate the ferroelectric and dielectric properties of these metal/PMN-PT/SRO capacitors. It was found that the coercive field (E _C) of the Gd(or Al)/PMN-PT/SRO capacitor is 4.5 times larger than that of the Pt/PMN-PT/SRO capacitor while the permittivity for the former is only ~27 % of that for the latter, which is analyzed using the model for metal-ferroelectric-metal heterostructures with Schottky contacts. Compared with the Pt/PMN-PT/SRO capacitor, the higher E _C and lower permittivity of the Gd(or Al)/PMN-PT/SRO capacitor are attributed to the stronger space charge field at the Gd(or Al)/PMN-PT interface. The capacitance–voltage characteristics of the metal/PMN-PT/SRO capacitors were also discussed.     

Structural and dielectric properties of (001) and (111)-oriented BaZr0.2Ti0.8O3 epitaxial thin films

We have grown and characterized BaZr_0.2Ti_0.8O₃ (BZT) epitaxial thin films deposited on (001) and (111)-oriented SrRuO₃-buffered SrTiO₃ substrates by pulsed laser deposition. Structural and morphological characterizations were performed using X-ray diffractometry and atomic force microscopy, respectively. A cube-on-cube epitaxial relationship was ascertained from the θ-2θ and φ diffractograms in both (001) and (111)-oriented films. The (001)-oriented films showed a smooth granular morphology, whereas the faceted pyramid-like crystallites of the (111)-oriented films led to a rough surface. The dielectric response of BZT at room temperature was measured along the growth direction. The films were found to be ferroelectric, although a well-saturated hysteresis loop was obtained only for the (001)-oriented films. High leakage currents were observed for the (111) orientation, likely associated to charge transport along the boundaries of its crystallites. The remanent polarization, coercive field, dielectric constant, and relative change of dielectric permittivity (tunability) of (111)-oriented BZT were higher than those of (001)-oriented BZT.     

Magnetoelectric effect in Pb(Zr0.95Ti0.05)O3 and CoFe2O4 heteroepitaxial thin film composite

Multiferroic epitaxial films, include SrRuO₃/Pb(Zr_0.95Ti_0.05)O₃/CoFe₂O₄ has been successfully deposited on SrTiO₃ substrate by pulsed-laser deposition technique. The results show that the prepared films exhibit a single phase. The Pb(Zr_0.95Ti_0.05)O₃ (PZT) film was highly textured with (1 0 0) orientation and gives good ferroelectric properties with saturated polarization of 15 μC/cm². The magnetic coercivity of CoFe₂O₄ film on Pb(Zr_0.95Ti_0.05)O₃ has been dampened to 0.9 kOe. The anisotropic magnetically behavior of CoFe₂O₄ film was changed to isotropic by using high Zr concentrated PZT as underneath layer. Heterostructure films show a good ferromagnetic and ferroelectric coupling that lead to the large magnetoelectricity of 287 mV/cm Oe.     

Effects of bottom electrodes on dielectric properties of epitaxial 2% Mn doped Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> thin films

2 mol% Mn doped Ba(Zr_0.2Ti_0.8)O₃ (Mn-BZT) thin films were prepared by pulsed laser deposition (PLD) on single crystal oxide substrates LaAlO₃(001) and MgO(001), with conductive oxide bottom electrodes LaNiO₃ and SrRuO₃, respectively. Both the Mn-BZT films and the bottom electrode films could be c-axial oriented with a cube-on-cube arrangement on the corresponding substrates. The dielectric properties measured with parallel plate capacitor configurations of Au/Mn-BZT/LNO and Au/Mn-BZT/SRO revealed that the Mn-BZT film on LNO bottom electrode exhibited comparatively higher dielectric constant, larger dielectric tunability and lower dielectric loss than that on SRO. It could be mainly attributed to the better epitaxial growth characteristics and mismatch stress of Mn-BZT thin film on LNO, as well as less misfit dislocation and the better morphology of LNO bottom electrode.     

Fabrication and optical properties of Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) thin films on Si substrates using the PLD method.

Epitaxial 0.67Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3)-0.33PbTiO(3) (PMN-PT) thin films with electro-optic effects were fabricated on (PMN-PT) thin films with electro-optic effects were fabricated on (La0(0.5)Sr0(0.5))CoO(3) (LSCO)/CeO(2)/YSZ-buffered Si(001) substrates using double-pulse excitation pulsed laser deposition (PLD) method with a mask placed between the target and the substrate. Epitaxial growth of PMN-PT thin films was undertaken using the two-step growth method of PMN-PT film. The PMN-PT seed layer was deposited at 500 degrees C on the LSCO/CeO(2)/YSZ/Si, which temperature was the same as that used for LSCO deposition. The PMN-PT thin films were deposited on the PMN-PT seed layer at 600 degrees C, which enables growth of high-crystallinity PMN-PT films with smooth surfaces. We obtained optimum fabrication conditions of PMNPT film with micrometer-order thickness. Resultant films showed high crystallinity with full width at half maximum (FWHM) = 0.73 deg and 1.6 mum thickness. Electro-optic properties and the refractive index value were measured at 633 nm wavelength using the prism coupling method. The obtained refractive index was 2.59. The electro-optic coefficients r(13) and r(33) were determined by applying the electrical field between a semitransparent, thin top electrode of Pt and a bottom LSCO electrode. The electro-optic coefficient was r(13) = 17 pm/V at transverse electric field (TE) mode and r(33) = 55 pm/V at transverse magnetic field (TM) mode.     

Effect of film thickness on ferroelectric domain structure and properties of Pb(Zr0.35Ti0.65)O3/SrRuO3/SrTiO3 heterostructures

Epitaxial Pb(Zr_0.35Ti_0.65)O₃ (PZT) thin films with tetragonal symmetry and thicknesses ranging from 45 to 230 nm were grown at 540 °C on SrRuO₃-coated (001)SrTiO₃ substrates by pulse-injected metalorganic chemical vapor deposition. The effect of the film thickness on the ferroelectric domain structure and the dielectric and ferroelectric properties were systematically investigated. Domain structure analysis of epitaxial PZT films was accomplished with high-resolution X-ray diffraction reciprocal space mapping and high-resolution transmission electron microscopy. Fully polar-axis (c-axis)-oriented epitaxial PZT thin films with high ferroelectric polarization values [e.g., remanent polarization (P _r) ~ 90 μC/cm²] were observed for film thicknesses below 70 nm. Films thicker than 70 nm had a c/a/c/a polydomain structure and the relative volume fraction of c-domains monotonously decreased to about 72% on increasing the film thickness up to 230 nm , and finally P _r diminished to about 64 μC/cm² for the 230-nm-thick epitaxial film. These polarization values were in good agreement with the estimated values taking into account the volume fraction of the c-axis-oriented domains while assuming a negligible contribution of 90° domain reorientation caused by an externally applied electric field.     

Spatial inhomogeneity of ferroelectric domain structure in Pb(Mg1/3Nb2/3)O3–30%PbTiO3 single crystals

Piezoresponse force microscopy studies of domain structures have been performed on (001)-oriented Pb(Mg_1/3Nb_2/3)O₃–30%PbTiO₃ single crystals with a rhombohedral ferroelectric state. Regular (narrow strip-like) and irregular (fingerprint-like) domain patterns were clearly observed on the different surfaces of the unpoled samples. The results reveal that the random internal field due to nanoscale structure irregularity plays an important role in the spatial inhomogeneity of ferroelectric domain structures in PMN-PT single crystals.     

Fabrication and Optical Properties of Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/Thin Films on Si Substrates Using the PLD Method

Epitaxial 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) thin films with electro-optic effects were fabricated on (La_0.5Sr_0.5)CoO₃(LSCO)/CeO₂/YSZ-buffered Si(001) substrates using double-pulse excitation pulsed laser deposition (PLD) method with a mask placed between the target and the substrate. Epitaxial growth of PMN-PT thin films was undertaken using the two-step growth method of PMN-PT film. The PMN-PT seed layer was deposited at 500degC on the LSCO/CeO₂/YSZ/Si, which temperature was the same as that used for LSCO deposition. The PMN-PT thin films were deposited on the PMN-PT seed layer at 600degC, which enables growth of high-crystallinity PMN-PT films with smooth surfaces. We obtained optimum fabrication conditions of PMN-PT film with micrometer-order thickness. Resultant films showed high crystallinity with full width at half maximum (FWHM)=0.73 deg and 1.6 mum thickness. Electro-optic properties and the refractive index value were measured at 633 nm wavelength using the prism coupling method. The obtained refractive index was 2.59. The electro-optic coefficients r₁₃ and r₃₃ were determined by applying the electrical field between a semitransparent, thin top electrode of Pt and a bottom LSCO electrode. The electro-optic coefficient was r₁₃=17 pm/V at transverse electric field (TE) mode and r₃₃=55 pm/V at transverse magnetic field (TM) mode.     

Growth of pseudocubic perovskite-type SrRuO3 thin films on quartz substrate using pulsed laser deposition method

Strontium ruthenium oxide (SrRuO₃) thin films have been grown using pulsed laser deposition technique on silicon, Pt coated silicon and quartz substrates. The effect of substrate temperatures on the structural, microstructure, and electrical properties of the SrRuO₃ films on quartz substrate has been investigated using XRD, SEM, AFM and four-probe method, respectively. The lowest resistivity at room temperature for the SrRuO₃ thin film on quartz substrate has been achieved at substrate temperature of 700 °C. Furthermore, the comparisons of SrRuO₃ thin films deposited on various substrates have been done with respect to structural, microstructural and electrical properties. XRD patterns exhibit that all thin films are a single phase, pseudo-cubic perovskite structure. Study of surface morphology shows that grain size and roughness varies with respect to substrate. It is observed that SrRuO₃ thin films yield larger grain size and root mean square roughness on Pt/Si substrate. Investigation of electrical properties shows that SrRuO₃ thin films can serve the purpose of the bottom electrode in dielectric and ferroelectric devices.     

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.     

Variations of microstructures and electrical properties of Bi4Ti3O12/SrTiO3/(La0.5, Sr0.5)CoO3/MgO epitaxial thin films by annealing

Epitaxial thin films of a heterostructure with Bi₄Ti₃O₁₂(BIT)/SrTiO₃(ST) were successfully grown with a bottom electrode consisting of La_0.5Sr_0.5CoO₃(LSCO) on MgO(001) substrates using pulsed laser deposition. The grown BIT and ST (001) planes were parallel to the growth surface with the orientation relationship of BIT <110>//ST <010>. In the as-deposited film, the BIT (001) plane appeared to expand to relieve a lattice mismatch with the ST (001) plane. However, annealing for 20-40 min induced the BIT (001) plane to contract horizontally with its c-axis expanding, which was associated with a local perturbation in the layer stacking of the BIT structure. This structural distortion was reduced in the film annealed for 1 h, with restoration of the periodicity of the layer stacking. Correspondingly, the dielectric constant of the as-deposited film was increased from 292 to 411 by annealing for 1 h. In parallel, the film was paraelectric but became more ferroelectric, with the remanent polarization and the coercive field changing from 0.1 μC/cm² and 14 kV/cm to 1.7 μC/cm² and 69 kV/cm, respectively.     

Ferroelectric field effect in SrCuO2 and SrRuO3 films

We report on ferroelectric field effect experiments on Pb(Zr_0.52Ti_0.48)O₃/SrCuO₂ and Pb(Zr_0.52Ti_0.48)O₃/SrRuO₃ epitaxial heterostructures with an emphasis on the material characterization. Upon reversing the polarization of the Pb(Zr_0.52Ti_0.48)O₃ ferroelectric layer, we measured a nonvolatile change in the resistivity of ultrathin layers of SrCuO₂ and SrRuO₃. In thin SrRuO₃ films (30 Å) up to a 9% resistivity change has been observed at room temperature.     

Microstructure and electrical properties of Pb(Zr0.52Ti0.48)O3 ferroelectric films on different bottom electrodes

The crystalline quality, dielectric and ferroelectricity of the Pb(Zr_0.52Ti_0.48)O₃ (PZT) films deposited on the LaNiO₃ (LNO), LNO/Pt and Pt bottom electrodes were comparatively analyzed to investigate the possibility for their application. LNO thin films were successfully prepared on Si (100) and Pt(111)/Ti/SiO₂/Si substrates by modified metallorganic decomposition (MOD). The PZT thin films were spin-coated onto the LNO, LNO/Pt and Pt bottom electrodes by sol–gel method. The crystallographic orientation and the microstructure of the resulting LNO films and PZT thin films on the different bottom electrodes were characterized by X-ray diffraction analysis. The dielectric and ferroelectric properties of PZT films on the different bottom electrodes are discussed. The PZT films deposited onto Pt/Ti/SiO₂/Si and LNO/Si substrates show strong (110) and (100) preferred orientation, respectively, while the films deposited onto LNO/Pt/Ti/SiO₂/Si substrates show the peaks of mixed orientations. PZT films on LNO and LNO/Pt bottom electrodes have larger dielectric constant and remnant polarizations compared with those grown on the Pt electrode.     

Electromechanical coupling properties of [001], [011] and [111] poled Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystals

The electric field induced “butterfly” curves and polarization loops, and the stress induced strain and polarization responses of [001], [011] and [111] oriented Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ (PMN-0.32 PT) relaxor ferroelectric single crystals have been systematically investigated by experiment study. The focus is on the effect of constant compressive bias stress on the electromechanical coupling behavior along three crystallographic directions of PMN-0.32 PT single crystals. Dependence of the coercive field, remnant polarization, dielectric constant, and piezoelectric coefficient on the bias stress has been quantified for PMN-0.32 PT single crystals oriented in three different directions. Obtained results show that the large piezoelectric responses under zero compressive stress in [001] and [011] orientation are dominated by intrinsic crystal lattice while the engineered domain structure has a relatively minor effect. It is found that observed responses under stress cycle for [001] oriented crystals are due to polarization rotation and phase transformations. However, those for [011] and [111] oriented crystals are due to domain switching. The “butterfly” curves and polarization loops driven by electric field under different bias compression are described by two non-180° domain switching.