BiFeO3 thin films prepared using metalorganic chemical vapor deposition

BiFeO₃ thin films were grown on (001) SrTiO₃ and (001) ZrO₂(Y₂O₃) substrates by single source metalorganic chemical vapor deposition in the temperature range T = 500 ÷ 800 °C using Fe(thd)₃ and Bi(C₆H₅)₃ as volatile precursors. X-ray diffraction analysis shows cube-on-cube epitaxial growth of BiFeO₃ on (001) SrTiO₃. The strongly reduced bismuth transfer into the film due to the high thermal stability of Bi(C₆H₅)₃ was counterbalanced by the increase of the total pressure as well as of the residence time of the precursor flow in the reactor; the Bi/Fe ratio in the film thus becomes close to that in the precursor mixture. Optical second harmonic generation measurements have evidenced the ferroelectric ordering in BiFeO₃ films and the apparent decrease of the Curie temperature of the strained films as compared to BiFeO₃ single crystal.     

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.     

Improved dielectric properties of chemical solution derived Pb0.5Sr0.5TiO3 thin films by a layer-by-layer annealing method

Pb_0.5Sr_0.5TiO₃ thin films were prepared on Pt/TiO₂/SiO₂/Si and LaNiO₃ (LNO)/Si substrates by using chemical solution deposition technique, and a layer-by-layer annealing method was used in an attempt to improve the dielectric properties of the thin films. The structure, dielectric, and ferroelectric properties of the thin films were investigated. Improved dielectric properties of the thin films were clearly confirmed: the dielectric constant and dielectric loss for the films on Pt/TiO₂/SiO₂/Si substrates annealed at 650 °C were 1064 and 0.027, respectively, at 1 kHz, with a dielectric tunability of more than 50%; similarly, the films prepared on LNO/Si substrates, showed a high dielectric constant of 1280 and a low dielectric loss of 0.023, at 1 kHz. P-E hysteresis loop measurements indicated that the remanent polarization and coercive field for the films on Pt/TiO₂/SiO₂/Si substrates annealed at 650 °C were 15.7 μC/cm² and 51 kV/cm, respectively.     

Enhanced magnetic properties of chemical solution deposited BiFeO3 thin film with ZnO buffer layer

Magnetic properties of BiFeO₃ films deposited on Si substrates with and without ZnO buffer layer have been studied in this work. We adopted the chemical solution deposition method for the deposition of BiFeO₃ as well as ZnO films. The x-ray diffraction measurements on the deposited films confirm the formation of crystalline phase of BiFeO₃ and ZnO films, while our electron microscopy measurements help to understand the morphology of few micrometers thick films. It is found that the deposited ZnO film exhibit a hexagonal particulate surface morphology, whereas BiFeO₃ film fully covers the ZnO surface. Our magnetic measurements reveal that the magnetization of BiFeO₃ has increased by more than ten times in BiFeO₃/ZnO/Si film compared to BiFeO₃/Si film, indicating the major role played by ZnO buffer layer in enhancing the magnetic properties of BiFeO₃, a technologically important multiferroic material.     

Unipolar resistive switching characteristics in Co3O4 films

Unipolar resistive switching behavior has been investigated in Pt/Co₃O₄/Pt stacks. The resistance ratio of the high- and low- resistance states is over 5 × 10³. The “ON/OFF” operation of the memory cells can be repeated more than 200 times at room temperature. The resistance of the two states can be kept for more than 16 h without showing degradation. The temperature dependence of the resistance shows a metallic behavior at the low-resistance state, but a semiconductor-behavior at the high-resistance state. The mechanism responsible for the observed unipolar resistive switching behavior has been discussed.     

The influence of epitaxial strain on magnetic and electrical properties of BiFeO3 thin films

Strained epitaxial BiFeO₃ films deposited on (001) SrTiO₃ substrates by metal organic chemical vapor deposition were studied by optical second harmonic generation (SHG) and SQUID magnetometry. The observed SHG intensity vs temperature dependencies indicate that for less strained films (σ < 0.6 GPa) a strong interplay between the ferroelectric and magnetic subsystems exists, while for the films with larger σ-values strain-induced destruction of the magnetic cycloidal ordering takes place.     

Synthesis of epitaxial BaZrO3 thin films by chemical solution deposition

This work reports on the low temperature preparation and characterization of BaZrO₃ (BZO) epitaxial thin films by chemical solution deposition (CSD). The X-ray θ-2θ scan and φ-scan measurements have demonstrated that the BZO films exhibit cube-on-cube epitaxy on (100) MgO substrates, with the full width at half maximum (FWHM) for the ω-scan and φ-scan of 0.35° and 0.46°, respectively. The SEM and AFM analyses revealed that the morphology of the films is strongly correlated with annealing temperature. The root mean square roughness for the film annealed at 600 °C is 3.63 nm, while for the film grown at 1000 °C is 5.25 nm.     

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.     

Dielectric and structural characterization of KNbO3 ferroelectric thin films epitaxially grown by pulsed laser deposition on Nb doped SrTiO3

KNbO₃ thin films were deposited on SrTiO₃ substrates by pulsed laser deposition. The X-ray diffraction patterns highlight an epitaxial growth according to the (011) orientation. This epitaxial growth was then confirmed by Electron Channeling Pattern. In agreement with the structural characteristics the dense microstructure consists in regular and ordered grains. Dielectric measurements were performed in the 20 Hz to 1 MHz frequency range on a KNbO₃ thin film grown on 2 at.% Nb doped (100)SrTiO₃ substrate in a large range of temperature in order to investigate the paraelectric-ferroelectric transition. Measurements at room temperature revealed a dielectric constant of 450 at 10 kHz and a minimum value of the loss tangent of 0.075 at 100 kHz. Dielectric study in the 20-600 °C temperature range showed a maximum of permittivity at the Curie temperature T_c = 410 °C and evidenced a “progressive” first-order phase transition, different from the classical “diffuse” transition.     

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.     

Ferroelectric properties of Mn-Doped Bi3.15Nd0.85Ti3O12 thin films prepared under different annealing conditions

Mn-doped Bi_3.15Nd_0.85Ti₃O₁₂ (BNTM) thin films were fabricated on Pt/Ti/SiO₂/Si(100) substrates by a chemical solution deposition technique and annealed at different temperatures from 650 to 800 °C. The structures of the films were analyzed using X-ray diffraction, which showed that the BNTM films exhibit polycrystalline structures and random orientations. The surface morphologies of the samples were investigated using scanning electron microscopy. The average grain size of the films increases with increasing annealing temperature. Electrical properties such as remanent polarization (2P_r) are quite dependent on the annealing temperature of BNTM films. It is found that the film annealed at 750 °C exhibits excellent ferroelectricity with a remanent polarization of 2P_r = 89.3 μC/cm² and a coercive field of E_c = 99.2 kV/cm respectively.     

Optimal growth windows of multiferroic BiFeO3 films and characteristics of ferroelectric domain structures

Multiferroic BiFeO₃ films of smooth surface and fully-saturated ferroelectric hysteresis loops have been grown by RF magnetron sputtering. The (001)-oriented epitaxial films showed a large remanent polarisation of 61 µC/cm². A strategy to grow BiFeO₃ films of good ferroelectric property was demonstrated, that was using fast growth rate to achieve accurate stoichiometry for the BiFeO₃ phase and at the same time to avoid the formation of impurity phases associated with the fast growth by accurate control of thermodynamic parameters such as oxygen partial pressure and temperature, as well as proper selection of substrates. Piezoresponse force microscopy revealed fine spontaneous domains for highly resistive epitaxial films, which were switchable under DC biases. For the polycrystalline films of increased density of free carriers, single-domain grains of about 200 nm in diameter were observed due to effective compensation of depolarisation field by free carriers and therefore allowing larger domains.     

Wet chemical route to transparent BiFeO3 films on SiO2 substrates

BiFeO₃ nanoparticles were prepared by a wet chemical synthesis method. Transparent films were deposited on glass and quartz substrates by dip and spin coating processes from the synthesized sol. We obtained thicker films (~ 2 µm) by dip coating process and thinner films (~ 200 nm) by spin coating process. Transmission electron microscopy images confirmed that the particles are nanocrystalline in size. From the optical transmittance spectra the band gap of the BiFeO₃ nanoparticles was determined in the range of ~ 3.03-2.88 eV (~ 410-430 nm). Electrical resistivity, polarization, zero-field-cooled and field-cooled magnetizations versus temperature characteristics were also studied for these films.     

Structure and optical properties of nanocrystalline BiFeO3 films prepared by chemical solution deposition

Bismuth ferric oxide (BiFeO₃) films grown on quartz substrate were prepared by a simple chemical solution deposition. Phase constitution characterization of the films were obtained by X-ray diffraction, surface morphology and transmittance of the films were studied by atomic force microscopy and ultraviolet-visible-infrared light spectrophotometer. The optical constants (refractive index n, extinction coefficient k) were calculated by straightforward method proposed by Swanepoel [Swanepoel R, J. Phys., E J. Sci. Instrum. 1983; 16: 1214-1222. [1]]. The influence of the processing parameters on microstructure and optical properties, especially band gap, were studied.     

Characterization of Al2O3/ZrO2 nano multilayer thin films prepared by pulsed laser deposition

Microstructural characterization of pulsed laser deposited Al₂O₃/ZrO₂ multilayers on Si (1 0 0) substrates at an optimized oxygen partial pressure of 3 × 10⁻² mbar and at room temperature (298 K) has been carried out. A nanolaminate structure consisting of alternate layers of ZrO₂ and Al₂O₃ with 40 bi-layers was fabricated at different zirconia layer thicknesses (20, 15 and 10 nm). The objective of the work is to study the effect of ZrO₂ layer thickness on the stabilization of tetragonal ZrO₂ phase for a constant Al₂O₃ layer thickness of 5 nm. The Al₂O₃/ZrO₂ multilayer films were characterized using high temperature X-ray diffraction (HTXRD) in the temperature range 298–1473 K. The studies showed that the thickness of the zirconia layer has a profound influence on the crystallization temperature for the formation of tetragonal zirconia phase. The tetragonal phase content increased with the decrease of ZrO₂ layer thickness. The cross-sectional transmission electron microscope (XTEM) investigations were carried out on a multilayer thin films deposited at room temperature. The XTEM studies showed the formation of uniform thickness layers with higher fraction of monoclinic and small fraction of tetragonal phases of zirconia and amorphous alumina.     

Structural and electrical characteristics of chemical solution derived (Bi3.2La0.4Nd0.4)Ti3O12 thin films

(Bi_3.2La_0.4Nd_0.4)Ti₃O₁₂ (BLNT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by using chemical solution deposition technique, and the effects of annealing temperatures in the range of 550-750 °C on structure and electrical properties of the thin films were investigated. X-ray diffraction analysis shows that the thin films have a bismuth-layered perovskite structure with preferred (117) orientation. The surface morphology observation by field-emission scanning electron microscopy confirms that films are dense and smooth with uniformly distributed grains. The grain size of the thin films increases with increasing annealing temperature; meanwhile, the structural distortion of the thin films also increases. It was demonstrated that the thin films show good electrical properties. The dielectric constant and dielectric loss are 191 and 0.028, respectively, at 10 kHz for the thin film annealed at 600 °C, and the 2Pr value of the thin film annealed at 700 °C is 20.5 μC/cm² at an electric field of 500 kV/cm.     

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.     

La-substitution Bi2Ti2O7 thin films grown by chemical solution deposition

(La_0.05Bi_0.95)₂Ti₂O₇ (LBTO) thin films had been successfully prepared on P-type Si substrate by chemical solution deposition method. The structural properties of the films were studied by X-ray diffraction. The phase of (La_0.05Bi_0.95)₂Ti₂O₇ is more stable than the phase of Bi₂Ti₂O₇ without La substitution. The films exhibited good insulating properties with room temperature resistivities in the range of 10¹²-10¹³ Ω cm. The dielectric constant of the film annealed at 550 °C at 100 kHz was 157 and the dissipation factor was 0.076. The LBTO thin films can be used as storage capacitors in DRAM.     

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.     

Preparation of AgInS2 chalcopyrite thin films by chemical spray pyrolysis

AgInS₂ thin films were prepared by the spray pyrolysis technique using a water/ethanol solution containing silver acetate, indium chloride and thiourea. We reported our results on the characterization of tetragonal AgInS₂ (chalcopyrite type) films, which were grown from indium deficient spraying solution. The films displayed a n-type conductivity with room temperature resistivities in the range between 10³ and 10⁴ Ω cm. The absorption spectra of sprayed films revealed two direct band-gaps with characteristic energies around 1.87 and 2.01 eV, which are in good agreement with the reported energy values for interband transitions from the split p-like valence band to the s-like conduction band in tetragonal AgInS₂ single crystals.