Properties of La-substituted Na0.5Bi4.5Ti4O15 ferroelectric thin films

Ferroelectric Na_0.5La_0.5Bi₄Ti₄O₁₅ (NaLaBTi) thin films were prepared by a chemical solution deposition method. The NaLaBTi thin films annealed at 750 °C under oxygen atmosphere were randomly oriented polycrystalline. Electrical properties of the NaLaBTi thin films were compared to Na_0.5Bi_4.5Ti₄O₁₅ thin films and better properties were observed in the NaLaBTi thin films. Remnant polarization (2P_r) and coercive electric field (2E_c) were 43 µC/cm² and 204 kV/cm at an applied electric field of 478 kV/cm, respectively. Leakage current density was 1.95 × 10^− 6 A/cm² at 100 kV/cm. Dielectric constant and dielectric loss were 805 and 0.05 at 1 kHz, respectively. Switchable polarization was suppressed by 15% after 1.44 × 10¹⁰ switching cycles.     

Multiferroic properties of BiFeO3/Bi4Ti3O12 double-layered thin films fabricated by chemical solution deposition

Multiferroic BiFeO₃/Bi₄Ti₃O₁₂ (BFO/BTO) double-layered film was fabricated on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The effect of an interfacial BTO layer on electrical and magnetic properties of BFO was investigated by comparing those of pure BFO and BTO films prepared by the same condition. The X-ray diffraction result showed that no additional phase was formed in the double-layered film, except BFO and BTO phases. The remnant polarization (2P_r) of the double-layered film capacitor was 100 μC/cm² at 250 kV/cm, which is much larger than that of the pure BFO film capacitor. The magnetization-magnetic field hysteresis loop revealed weak ferromagnetic response with remnant magnetization (2M_r) of 0.4 kA/m. The values of dielectric constant and dielectric loss of the double-layered film capacitor were 240 and 0.03 at 100 kHz, respectively. Leakage current density measured from the double-layered film capacitor was 6.1 × 10^− 7 A/cm² at 50 kV/cm, which is lower than the pure BFO and BTO film capacitors.     

Effect of yttrium doping on the dielectric properties of CaCu3Ti4O12 thin film produced by chemical solution deposition

Pure and yttrium substituted CaCu₃Ti_4 − xY_xO_{12 − x / 2} (x = 0, 0.02, 0.1) thin films were prepared on boron doped silica substrate employing chemical solution deposition, spin coating and rapid thermal annealing. The phase and microstructure of the sintered films were examined using X-ray diffraction and scanning electron microscopy. Dielectric properties of the films were measured at room temperature using electrochemical impedance spectroscopy. Highly ordered polycrystalline CCTO thin film with bimodal grain size distribution was achieved at a sintering temperature of 800 °C. Yttrium doping was found to have beneficial effects on the dielectric properties of CCTO thin film. Dielectric parameters obtained for a CaCu₃Ti_4 − xY_xO_{12 − x / 2} (x = 0.02) film at 1 KHz were k ∼ 2700 and tan δ ∼ 0.07.     

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.     

The properties of transparent semiconductor Zn1 − xTixO thin films prepared by the sol-gel method

Transparent semiconductor thin films of Zn_1 − xTi_xO (0 ≦ x ≦ 0.12) were deposited on alkali-free glass substrates by the sol-gel method. The effects of Ti addition on the crystallization, microstructure, optical properties and resistivity of ZnO thin films were investigated. The as-coated films were preheated at 300 °C, and then annealed at 500 °C in air ambiance. X-ray diffraction results showed all polycrystalline Zn_1 − xTi_xO thin films with preferred orientation along the (002) plane. Ti incorporated within the ZnO thin films not only decreased surface roughness but also increased optical transmittance and electrical resistivity. In the present study, the Zn_0.88Ti_0.12O film exhibited the best properties, namely an average transmittance of 91.0% (an increase of ~ 12% over the pure ZnO film) and an RMS roughness value of 1.04 nm.     

Studying trivalent/bivalent metal ion doped TiO2 as p-TiO2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO₂ thin films (M_xTi_1−xO₂, M = Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Mn²⁺ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti⁴⁺ oxidation state of the Ti2p band in the doped p-TiO₂. The homogenous M_xTi_1−xO₂ was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current (I)–voltage (V) characteristics for the Ag/n-ZnO/M_xTi_1−xO₂/ITO/glass assembly showed rectifying behavior with small turn-on voltages (V₀) < 1 V. The ideality factor (η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.     

The synthesis of lead-free ferroelectric Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 thin films by sol-gel method

(1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ [BNT-BKT-100x] thin films have been successfully deposited on Pt/Ti/SiO₂/Si substrates by a sol-gel process together with rapid thermal annealing. A morphotropic phase boundary (MPB) between Bi_0.5Na_0.5TiO₃ and Bi_0.5K_0.5TiO₃ was determined around x ∼ 0.15. Near the MPB, the film exhibits the largest grain size, the highest ε value (360) and the largest P_r value (13.8 μC/cm²). The BNT-BKT thin film system is expected to be a new and promising candidate for lead-free piezoelectric applications.     

Ferroelectric and pyroelectric properties of (Na0.5Bi0.5)TiO3-BaTiO3 based trilayered thin films

Trilayered Bi_3.25La_0.75Ti₃O₁₂ (25 nm)/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (300 nm)/Bi_3.25La_0.75Ti₃O₁₂ (25 nm) and Pb(Zr_0.4Ti_0.6)O₃ (25 nm)/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (300 nm)/Pb(Zr_0.4Ti_0.6)O₃ (25 nm) thin films without undesirable phases have been deposited on Pt/Ti/SiO₂/Si substrates. It was found that the Bi_3.25La_0.75Ti₃O₁₂ and Pb(Zr_0.4Ti_0.6)O₃ layers are very effective to inhibit the charge transport in the trilayered films. Much better insulating properties than those of (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ films have been achieved in the trilayered films. The trilayered films show good dielectric, ferroelectric and pyroelectric properties. Remnant polarizations 2P_r of 16 µC/cm² and 34 µC/cm², pyroelectric coefficients of 4.8 × 10^− 4 C m^− 2 K^− 1 and 7.0 × 10^− 4 C m^− 2 K^− 1 have been obtained for the Bi_3.25La_0.75Ti₃O₁₂/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃/Bi_3.25La_0.75Ti₃O₁₂ and Pb(Zr_0.4Ti_0.6)O₃/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃/Pb(Zr_0.4Ti_0.6)O₃ thin films, respectively. The trilayered films are promising candidates for sensor and actuator applications.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Leakage current characteristics of Bi3.15Nd0.85Ti3 − xZrxO12 thin films

Thin films of Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) and Bi_3.15Nd_0.85Ti_3 − xZr_xO₁₂ (BNTZ_x, x = 0.1 and 0.2) were fabricated on Pt/TiO₂/SiO₂/Si(100) substrates by a chemical solution deposition (CSD) technique at 700 °C. Structures, surface morphologies, leakage current characteristics and Curie temperature of the films were studied as a function of Zr ion content by X-ray diffraction, atomic force microscopy, ferroelectric test system and thermal analysis, respectively. Experimental results indicate that Zr ion substitution in the BNT film markedly decreases the leakage current of the film, while almost not changing the Curie temperature of the film, which is at about 420-460 °C. The decrease of the leakage current in BNTZ_x films is that the conduction by the electron hopping between Ti⁴⁺ and Ti³⁺ ions is depressed because Zr⁴⁺ ions can block the path between two adjacent Ti ions and enlarge hopping distance.     

Composition dependence of optical constants in ferroelectric Ba(Ti1 − x,Nix)O3 thin films by optical transmittance technique

Ba(Ti_1 − x,Ni_x)O₃ ferroelectric thin films with perovskite structure are prepared on fused quartz substrates by a sol-gel process. Optical transmittance measurements indicate that Ni-doping has an obvious effect on the energy band structure of BaTiO₃. It has been found that the refractive index n, extinction coefficient k, and band gap energy E_g of the films are functions of the film composition. The E_g of Ba(Ti_1 − x,Ni_x)O₃ decreases approximately linearly as the Ni content increases, which is attributed to the decline of conduction band energy level with increasing the Ni content. On the other hand, n and k both increase linearly with increasing the Ni content because of the increase of packing density. These results indicate that thin films might have potential applications in BaTiO₃-based thin-film optical devices.     

Dependence of Zr content on electrical properties of Bi3.15Nd0.85Ti3-xZrxO12 thin films synthesized by chemical solution deposition (CSD)

The Bi_3.15Nd_0.85Ti_3-xZr_xO₁₂ (BNTZ) thin films with Zr content (x = 0, 0.05, 0. 1, 0.15, and 0.2) were prepared on Pt/Ti/SiO₂/Si (100) substrates by chemical solution deposition (CSD) technique. The crystal structures of BNTZ films were analyzed by X-ray diffraction (XRD). The effects of Zr contents on the ferroelectric, dielectric properties, and leakage current of BNTZ films were thoroughly investigated. The XRD results demonstrated that all the films possessed a single phase bismuth-layered structure and exhibited the highly preferred (117) orientation. Among these films, the film with Zr content x = 0.1 held the maximum remanent polarization (2P_r) of 50.21 μC/cm² and a low coercive field (2E_c) of 210 kV/cm.     

Novel transparent conducting oxide: Anatase Ti1−xNbxO2

Single-crystalline Ti_1−xNb_xO₂ (x = 0.2) films of 40 nm thickness were deposited on SrTiO₃ (100) substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction measurement confirmed epitaxial growth of anatase (001) film. The resistivity of Ti_1−xNb_xO₂ films with x ≥ 0.03 is 2-3 × 10^− 4 Ω cm at room temperature. The carrier density of Ti_1−xNb_xO₂, which is almost proportional to the Nb concentration, can be controlled in a range of 1 × 10¹⁹ to 2 × 10²¹ cm^− 3. Optical measurements revealed that internal transmittance in the visible and near-infrared region for films with x = 0.03 was more than 97%. These results demonstrate that the presently developed anatase Ti_1−xNb_xO₂ is one of the promising candidates for the practical TCOs.     

Crystal structure and ferroelectric properties of Ca(Cu3−xMx)Ti4O12 (M = Fe and Ni) ceramics

M-substituted Ca(Cu_3−xM_x)Ti₄O₁₂ (CCMTO) ceramics, where M = Fe and Ni, were synthesized and the influence of M substitutions for Cu on the crystal structure and ferroelectric properties of CCMTO ceramics were investigated in this study. From the variations in the lattice parameters of CCMTO ceramics, the solubility limit of Ni substitution for Cu in CaCu_3−xNi_xTi₄O₁₂ (CCNTO) ceramics was x = 0.2, whereas that of CaCu_3−xFe_xTi₄O₁₂ (CCFTO) ceramics was x = 0.05. The crystal structural analysis of CCMTO ceramics revealed that the single phase of CCMTO ceramics belongs to the I23 non-centrosymmetric space group of I23; as a result, the P_r and E_c values of CCFTO ceramics at x = 0.05 were 1.8 μC/cm² and 40 kV/cm, respectively. The ferroelectric behavior of CCMTO ceramics by the M substitutions for Cu may be related to the displacement of a Ti⁴⁺ cation in the TiO₆ octahedra and tilting of the Ti–O–Ti angle because of the non-centrosymmetric space group.     

Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature

Bi_3.25Eu_0.75Ti₃O₁₂ (BET) thin films were deposited on Pt/Ti/SiO₂/Si(111) substrates by a metal-organic decomposition method. The effects of annealing temperatures 600-800 °C on microstructure, ferroelectric, dielectric and piezoelectric properties of BET thin films were studied in detail. The spontaneous polarization (87.4 × 10^− 6 C/cm² under 300 kV/cm), remnant polarization (65.7 × 10^− 6 C/cm² under 300 kV/cm), the dielectric constant (992.9 at 100 kHz) and the effective piezoelectric coefficient d₃₃ (67.3 pm/V under 260 kV/cm) of BET thin film annealed at 700 °C are better than those of the others. The mechanisms concerning the dependence of the enhancement d₃₃ are discussed according to the phenomenological equation, and the improved piezoelectric performance could make the BET thin film a promising candidate for piezoelectric thin film devices.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Structural, magnetic and dielectric properties of Bi5−xLaxTi3Co0.5Fe0.5O15 ceramics

The Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4) ceramics were successfully synthesized by a modified Pechini process. The samples were characterized by X-ray diffraction and no impurity phase has been detected. The cell volume of the composites increases monotonously with the increase of La content, which indicates that La ions have been incorporated into the lattice of Bi₅Ti₃Co_0.5Fe_0.5O₁₅. The magnetic measurements show that La doping on Bi sites has enhanced the magnetization of Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4). Both the dielectric constants and loss tangent of all the samples decrease on increasing frequency and then become almost constant at room temperature. The La doped Bi₅Ti₃Co_0.5Fe_0.5O₁₅ samples exhibit improved dielectric and ferroelectric properties, with higher dielectric constant enhanced remnant polarization and lower losses at room temperature.     

Ferroelectric and fatigue-free properties of Au/(Bi,La)4Ti3O12/ITO thin film capacitors by pulsed laser deposition

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were deposited on indium-tin-oxide (ITO) coated glass substrates by pulsed-laser-ablation method. Films deposited at 400 °C and annealed at 650 °C resulted in remnant polarization and coercive field values of 14-16 μC/cm² and 90-100 kV/cm, respectively. The fatigue measurements were conducted until 1 × 10¹¹ cycles but the individual switched and unswitched polarizations showed unequal magnitudes. Such an unequal switching polarization proves that an extrinsic effect mainly associated with the electrode exists in this thin film capacitor. The overall switching polarizations showed no polarization degradation, suggesting that BLT films are fatigue resistive even on hybrid-metal-oxide electrodes.     

Microfabrication of dilute magnetic semiconducting Ti1−xCoxO2 films using photosensitive sol-gel method

Photosensitive Ti_1−xCo_xO₂ gel films are prepared by chemical modification with benzoylacetone, and then their Fourier transform infrared spectra and ultraviolet visible spectra are measured. Results show that the chelate rings of benzoylacetone with Ti ions are formed in the Ti_1−xCo_xO₂ gel films. With irradiation of UV light, the chelate rings are photolyzed, which leads to a change of the solubility of the Ti_1−xCo_xO₂ gel films in methanol. Based on this, the micro-patterns of Ti_1−xCo_xO₂ gel films are obtained. After the patterned gel films are dried at 200 °C and are fired at 550 °C for 1 h, negative patterned Ti_1−xCo_xO₂ inorganic films are obtained. The Ti_1−xCo_xO₂ films have an anantase structure and exhibit a room temperature ferromagnetic property.     

Structural and physical properties of Mg-doped CuAlO2 thin films

The CuAl_1−xMg_xO₂ (x = 0, 0.01, 0.02 and 0.05) thin films were successfully deposited on quartz substrate by using the RF magnetron sputtering technique. XRD patterns indicate that the delafossite structure could be guaranteed for all CuAl_1−xMg_xO₂ films. The conductivity measured at room temperature for CuAl_0.98Mg_0.02O₂ film is three orders of magnitude higher than that of undoped CuAlO₂ film and the band gaps of CuAl_1−xMg_xO₂ (x = 0, 0.01, 0.02 and 0.05) thin films decrease with the increase of the doping concentration, which is related to the formation of impurity energy levels with increasing the doping concentration.