Multiferroic properties of Bi3.15Nd0.85Ti3O12-CoFe2O4 bilayer films derived by a sol-gel processing

Multiferroic Bi_3.15Nd_0.85Ti₃O₁₂ (BNT)-CoFe₂O₄ (CFO) bilayer films with different preferential orientations and thickness fractions for the BNT layer were prepared on Pt/Ti/SiO₂/Si substrate by a sol-gel processing. The experimental results showed that the bilayer films with preferentially a-axis oriented and thicker BNT layer have better ferroelectric properties. The magnetoelectric coupling response is weak when the degree of a-axis orientation of the BNT layer is low or the leakage current is high, while it is mainly controlled by the thickness fraction in other cases.     

Magnetoelectric CoFe2O4/Pb(Zr0.53Ti0.47)O3 composite thin films of 2-2 type structure derived by a sol-gel process

CoFe₂O₄/Pb(Zr_0.53Ti_0.47)O₃ (CFO/PZT) magnetoelectric composite thin films of 2-2 type structure have been prepared onto Pt/Ti/SiO₂/Si substrate by a sol-gel process and spin coating technique. The optimal annealing process of composite thin films was determined by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It is found that the amount of the citric acid and concentration of CFO starting precursor solution have great impact on morphologies of composite thin films. Subsequent scanning electron microscopy (SEM) investigations show that the prepared thin films exhibit good morphologies and compact structure, and cross-sectional micrographs clearly display a multilayered nanostructure of multilayered thin films. The purpose of this work is to determine the optimal annealing processes of composite thin films and to prepare magnetoelectric composite thin films with good microstructure. It is shown that the films exhibit both good magnetic and ferroelectric properties, as well as a magnetoelectric effect.     

热解温度对镧、锰共掺杂铁酸铋薄膜铁电性能的影响(英文)

在不同热解温度下,采用溶胶-凝胶法在Pt/Ti/SiO2/Si衬底上制备镧、锰共掺杂铁酸铋铁电薄膜Bi0.9La0.1Fe0.95Mn0.05O3(BLFMO)。利用热失重仪(TGA)分析BLFMO原粉的质量损失,用 X 射线衍射仪(XRD)和原子力显微镜(AFM)分析 BLFMO 薄膜的晶体结构和表面形貌。在热解温度为420℃时,得到BLMFO薄膜的剩余极化值为21.2μC/cm2,矫顽场为99 kV/cm,漏电流密度为7.1×10-3 A/cm2,说明薄膜在此热解温度下具有较好的铁电性能。     

Structure, ferroelectric/magnetoelectric properties and leakage current density of (Bi0.85Nd0.15)FeO3 thin films

In this paper, we report on the structure, ferroelectric/magnetoelectric properties and improvement of leakage current density of (Bi_0.85Nd_0.15)FeO₃ (BNFO) thin films deposited on Pt(1 1 1)/Ti/SiO₂/Si substrates from the polymeric precursor method. X-ray patterns and Rietveld refinement indicated that BNFO thin films with a tetragonal structure can be obtained at 500 °C for 2 h in static air. Field emission scanning electron, atomic force and piezoelectric force microscopies showed the microstructure, thickness and domains with polarization-oriented vectors of BNFO thin films. Ferroelectric and magnetoelectric properties are evident by hysteresis loops. The magnetoelectric coefficient measurement was performed to show the magnetoelectric coupling behavior. The maximum magnetoelectric coefficient in the longitudinal direction was close to 12 V/cm Oe. Piezoresponse force microscopy micrographs reveal a polarization reversal with 71° and 180° domain switchings and one striped-domain pattern oriented at 45° besides the presence of some nanodomains with rhombohedral phase involved in a matrix with tetragonal structure. The cluster models illustrated the unipolar strain behavior of BNFO thin films. The leakage current density at 5.0 V is equal to 1.5 × 10⁻¹⁰ A/cm² and the dominant mechanism in the low-leakage current for BNFO thin films was space-charge-limited conduction.     

Enhanced dielectric and piezoelectric response in PZT superlattice-like films by leveraging spontaneous Zr/Ti gradient formation

Spontaneous Zr/Ti gradient formation during crystallization in sol-gel-processed Pb(Zr_xTi_1−x)O₃ films is used to prepare superlattice-like (SL), highly (1 0 0)-oriented thin films on Pt/Ti/SiO₂/Si substrates. SLs with stacking periodicity ranging from 13 up to 60 nm are synthesized with compositional gradient normal to the film surface and composition centered at x ≈ 0.53. X-ray diffraction (XRD) shows high order satellite peaks and no secondary phases. XRD structural refinement, along with XPS depth profile chemical analysis, reveals that the crystal structure alternates between rhombohedral and in-plane polarized tetragonal phases, effectively corresponding to “artificially created” phase boundaries. SL films have ∼45% and ∼20% higher d_33,f piezoelectric coefficient and dielectric permittivity, respectively, with respect to compositional-gradient-free films of similar thickness, possibly due to enhanced reorientation of electrical dipoles and higher extrinsic contributions due to the motion of the “artificially created” phase boundaries in SL films. Dielectric nonlinear studies indicate a higher amount of extrinsic contributions to the dielectric response in SL and gradient-enhanced films than in conventional films of similar average composition. This processing method provides a simple chemical route to create thin ferroelectric films with enhanced dielectric and piezoelectric properties suitable for a range of miniaturized applications.     

Structural and electrical properties of SrBi2(Ta0.5Nb0.5)2O9 thin films

SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBTN) thin films were obtained by polymeric precursor method on Pt/Ti/SiO₂/Si(1 0 0) substrates. The film is dense and crack-free after annealing at 700 °C for 2 h in static air. Crystallinity and morphological characteristic were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The films displayed rounded grains with a superficial roughness of 3.5 nm. The dielectric permittivity was 122 with loss tangent of 0.040. The remanent polarization (P_r) and coercive field (E_c) were 5.1 μC/cm² and 96 kV/cm, respectively.     

Structure and electrical properties of PbZrO3 antiferroelectric thin films doped with barium and strontium

In this paper, we report on the structure and electrical properties of lead zirconate (PbZrO₃) thin films doped with barium (Ba²⁺) and strontium (Sr²⁺) deposited on platinum-buffered silicon substrates by a sol-gel method. Effects of Ba²⁺ and Sr²⁺ dopants on microstructure and electrical properties of the PbZrO₃ antiferroelectric thin films were investigated in details. X-ray diffraction patterns and scanning electron microscope micrographs illustrated that orientation and surface microstructure of these antiferroelectric films were dopant-dependent. The dielectric measurements showed that Sr²⁺ doping stabilized the antiferroelectric phase, while Ba²⁺ doping destabilized the antiferroelectric phase. It was also found that fatigue property of the antiferroelectric PbZrO₃ thin films was improved remarkably by the dopants.     

Thickness effects of Bi0.89Ti0.11FeO3 thin films deposited on PbZr0.2Ti0.79Nb0.01O3 buffer layers

Bi_0.89Ti_0.11FeO₃ thin films with the thicknesses of 200-440 nm were fabricated on the 40-nm-thick PbZr_0.2Ti_0.79Nb_0.01O₃ (PZTN)-buffered Pt(1 1 1)/Ti/SiO₂/Si substrates using a metal organic decomposition process. As a result of the good insulating property and high breakdown characteristic of the PZTN buffer layer, the leakage currents in the Bi_0.89Tb_0.11FeO₃ films are significantly reduced. All the films show well-saturated and rectangular P-E hysteresis loops without any evident leaky behavior. The remnant polarization P_r and coercive field E_c for all Bi_0.89Ti_0.11FeO₃ films are around 45-50 μC/cm² and 200 kV/cm, respectively, and show weak dependent on the film thickness. The 200-nm-thick Bi_0.89Ti_0.11FeO₃ film exhibits better fatigue-free characteristic and charge-retaining ability, and the domain backswitching is significantly restrained due to the strong anti-aging ability of the PZTN buffer layer.     

Optical properties of thermally evaporated Bi2Se3 thin films treated with AgNO3 solution

Bismuth selenide (Bi₂Se₃) thin films have been prepared onto clean glass substrates by the thermal evaporation technique. The deposited films were then immersed in silver nitrate solution for different periods of time, followed by annealing in Argon atmosphere at 473 K for 1 h, to obtain Ag/Bi₂Se₃ samples. The prepared films have been examined by X-ray and transmission electron microscopy for structural determination. The optical transmission and reflection spectra of the deposited films have been recorded within the wavelength range 400-2500 nm. The variation of the optical parameters of the prepared films, such as refractive index, n, and the optical band gap, E_g as a function of the immersion duration times has been determined. The refractive index dispersion in the transmission and low absorption region is adequately described by the well-known Sellmeier dispersion relation, whereby the values of the oscillator strength, oscillator position, the high-frequency dielectric constant, ε_∞ as well as the carrier concentration to the effective mass ratio, N/m* were calculated as a function of the immersion duration time.     

Structural, ferroelectric and dielectric properties of In2O3:Sn (ITO) on PbZr0.53Ti0.47O3 (PZT)/Pt and annealing effect

Ferroelectric indium tin oxide (ITO) on PbZr_0.53Ti_0.47O₃ (PZT)/Pt structure, prepared by RF sputtering onto SiO₂/Si substrates, is studied in order to investigate the effect of ITO as a top electrode in these systems. X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments were performed to study the structure and the surface morphology of the samples. From X-ray diffraction, we observe that the ITO thin film grows with the (1 1 1) texture and the peaks attributed to PZT are all from the perovskite phase. The average roughness (RMS) of the PZT surface is found to be 1.650 nm from AFM experiment. The ferroelectric and dielectric properties were inferred from polarization hysteresis loops, capacitance and dielectric constant measurements. These properties have been compared to those of the widely studied Pt/PZT/Pt system prepared under the same conditions. The effect of ITO/PZT/Pt annealing has been studied. Annealing at 400 °C leads to 13% increase in the dielectric constant ?_r.     

Preparation and electrical properties of Bi0.5Na0.5TiO3-BaTiO3-KNbO3 lead-free piezoelectric ceramics

Lead-free (1 − x)Bi_0.47Na_0.47Ba_0.06TiO₃-xKNbO₃ (BNBT-xKN, x = 0-0.08) ceramics were prepared by ordinary ceramic sintering technique. The piezoelectric, dielectric and ferroelectric properties of the ceramics are investigated and discussed. The results of X-ray diffraction (XRD) indicate that KNbO₃ (KN) has diffused into Bi_0.47Na_0.47Ba_0.06TiO₃ (BNBT) lattices to form a solid solution with a pure perovskite structure. Moderate additive of KN (x ≤ 0.02) in BNBT-xKN ceramics enhance their piezoelectric and ferroelectric properties. Three dielectric anomaly peaks are observed in BNBT-0.00KN, BNBT-0.01KN and BNBT-0.02KN ceramics. With the increment of KN in BNBT-xKN ceramics, the dielectric anomaly peaks shift to lower temperature. BNBT-0.01KN ceramic exhibits excellent piezoelectric properties and strong ferroelectricity: piezoelectric coefficient, d₃₃ = 195 pC/N; electromechanical coupling factor, k_t = 58.9 and k_p = 29.3%; mechanical quality factor, Q_m = 113; remnant polarization, P_r = 41.8 μC/cm²; coercive field, E_c = 19.5 kV/cm.     

Rhombohedral-monoclinic phase transition related to grain orientation in Zr-rich Pb(ZrxTi1−x)O3 thin films

Pb(Zr_xTi_1−x)O₃ thin films with mixed orientations of (1 1 1) and (1 0 0) were prepared on Pt/Ti/SiO₂/Si substrate by sol-gel technique. The compositions of PZT thin films are chosen as x = 0.55 and x = 0.58. Both of the compositions are in the rhombohedral phase region of the Pb(Zr_xTi_1−x)O₃ phase diagram, but the former is near the monoclinic phase existence region, and the latter is far from the monoclinic phase existence region. Rhombohedral-monoclinic phase transitions are reported in both of the thin films. The results show that the phase transition is related to the grain orientation. Phase transitions in the films are clearly identified: rhombohedral phase transforms to M_B phase in (1 1 1)-oriented grains, and rhombohedral phase transforms to M_A phase in (1 0 0)-oriented grains. The remnant polarization is determined by the content of (1 1 1)-oriented grains. It is shown that the remnant polarization is greater in the film with higher content of (1 1 1)-oriented grains.     

Preparation of delafossite-type CuCrO2 films by sol-gel method

High-quality c-axis oriented delafossite-type CuCrO₂ films were successfully prepared by a simple sol-gel method. The microstructure, optical properties as well as room temperature resistivity were studied. It was found that the grain sizes of CuCrO₂ films pretreated with different temperatures are different; the films were smooth and consisted of fine particles. The maximal transmittance of CuCrO₂ films can reach 70% in the visible region. Optical transmission data of CuCrO₂ films indicate a direct band gap and an indirect-gap of about 3.15 eV and 2.66 eV, respectively. The carrier mobility of the films pretreated at 300 °C is smaller than that of the films pretreated at a higher temperature, because of the stronger carrier scattering.     

Thermal stability and electric properties of Ba0.7Sr0.3TiO3 parallel plate capacitor with nano-Cr interlayer

A novel sandwich structure of Ba_0.7Sr_0.3TiO₃/Cr/Ba_0.7Sr_0.3TiO₃ (BST/Cr/BST) was sputtered onto Pt/Ti/SiO₂/Si substrate. With the insertion of a Cr layer, the leakage currents are decreased and the thermal stability of the specimens is enhanced. Temperature coefficient of capacitance (TCC) of specimens with BST(200 nm)/Cr(2 nm)/BST(200 nm) multifilms can achieve about 83% lower than those with BST (400 nm) monolayer. However, the dielectric constant of the BST(200 nm)/Cr(2 nm)/BST(200 nm) multifilms decreases to about 37% of that BST monolayer. The leakage current densities under an electric field of 125 kV/cm at 90 °C are 4 × 10^− 4 A/cm² and 9 × 10^− 1 A/cm² for BST (200 nm)/Cr (2 nm)/BST (200 nm) and monolayer BST (400 nm), respectively. X-ray diffraction results indicate the formation of a CrO₃ secondary phase after annealing at 700 °C or above in O₂ atmosphere. The root causes for the improvement of leakage currents and thermal stability with the insertion of nano-Cr interlayer are explored. The results show the insertion of Cr-nanolayer improves the electric properties for application in capacitors.     

Enhanced piezoelectric properties of Ta substituted-(K0.5Na0.5)NbO3 films: A candidate for lead-free piezoelectric thin films

Piezoelectric (K_0.5Na_0.5)NbO₃ (KNN) and (K_0.5Na_0.5)(Nb_0.7Ta_0.3)O₃ (KNNT) thin films were prepared via chemical solution deposition. An analysis of X-ray diffraction revealed that Ta⁵⁺ diffuses into the KNN to form a single perovskite structure. Compared to KNN films, KNNT films exhibited a low leakage current density due to their fine-grain nonporous structures. The partial substitution of Ta⁵⁺ for the B-site ion Nb⁵⁺ in the KNNT films decreased the Curie temperature (T_C). This in turn led to the existence of a polymorphic phase transition near room temperature and further improvement in the piezoelectric properties. Lead-free KNNT films exhibited a well-saturated piezoelectric hysteresis loop with a effective piezoelectric coefficient (d_33,eff) value of 61 pm/V, comparable to that of PZT thin films.     

Effects of lanthanum doping on the preferred orientation, phase structure and electrical properties of sol-gel derived Pb1-3x/2Lax (Zr0.6Ti0.4)O3 thin films

Pb_1-3x/2La_x (Zr_0.6Ti_0.4)O₃ thin films (0 ≤ x ≤ 0.08) were prepared on the Pt (1 1 1)/Ti/SiO₂/Si (1 0 0) substrates by a sol-gel method. The morphology, preferred orientation, phase structure, dielectric and ferroelectric properties of the films have been investigated. Our results show that lanthanum doping is favorable to enhance crystalline and obtain (1 0 0)-preferred orientation of the films. Meanwhile, it is suggested that the films undergo a structure change from “rhombohedral” phase to monoclinic phase as the lanthanum-doped content is increased to x ≈ 0.05. Results of dielectric properties and ferroelectric properties indicate that lanthanum doping contributes to improve film dielectric constant and dielectric loss while it brings about a striking decrease in remnant polarization value. Possible explanations for the variations of electrical properties have been discussed in terms of preferred orientation, phase structure and large lattice distortion.     

Effect of Nb doping on the phase transition and optical properties of sol-gel TiO2 thin films

Various content Nb-doped TiO₂ thin films were prepared by sol-gel process. XRD analysis shows that the existence of crystalline TiO₂ in anatase and rutile form depends on the Nb content in the examined samples. It is observed that Nb promotes the anatase to rutile phase transition but has a depression effect on the anatase grain growth. It is found that incorporation of about 4 at.% of Nb completely transforms anatase TiO₂ to the rutile form at a calcination temperature as high as 900 °C. The mechanism is proposed. Optical analyses show that the films have an average of 60% transmission in visible region. The energy gap values using Tauc's formula have also been estimated. The band gap of rutile Ti_1−xNb_xO₂ solid solutions increases with increasing x.     

Structural and piezoelectric properties of LiNbO3-modified BiScO3–PbTiO3 ceramics

Piezoelectric perovskite materials based on the solid solution (1 − x)BiScO₃–xPbTiO₃ (BSPT) have been attracting attention for their high Curie temperature (T_c = 450 °C) and excellent piezoelectric properties. The LiNbO₃ (LN), which has a T_c as high as 1150 °C, has been recently reported forming a phase pure perovskite solid solution with some perovskite structure compounds. In the current work, the effects of LN substitution on the structural and electrical properties of BSPT ceramics were investigated in the 0.36BiScO₃–0.64{(1 − x)PbTiO₃–xLiNbO₃} (BSPTLNx) system. The results of LN addition in the BSPT ceramics show significant enhancement of the piezoelectric properties. The piezoelectric constant d₃₃, planar electromechanical coupling coefficient and remnant polarization P_r values reached 465 pC/N, 0.57 and 48 μC/cm², respectively, for x = 0.04. The T_c gradually decreases with increasing LN content in the BSPTLNx system, due to the structure transform from the tetragonal to the rhombohedral. A typical relaxor behavior is also produced with the LN substitution in the BSPTLNx system.