Compositionally graded bismuth ferrite thin films

The compositionally graded (Bi_0.92La_0.08)(Fe_1−xZn_x)O₃ (x = 0.03, 0.07, and 0.13) thin film was layer-by-layer grown on Pt/Ti/SiO₂/Si(1 0 0) substrates without any buffer layers by radio frequency sputtering. This thin film has a pure polycrystalline perovskite structure with random orientation, a dense microstructure, and a low leakage current density. A large remanent polarization of 2P_r ∼ 142.00 μC/cm² and a good magnetic behavior of 2 M_s ∼ 27.52 emu/cm³ are demonstrated in such a thin film. The applied electric fields and measurement frequencies strongly affect its fatigue endurance, that is, its fatigue endurance was degraded with decreasing frequencies and electric fields.     

Bismuth ferrite bilayered thin films of different constituent layer thicknesses

Bilayered thin films consisting of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ and (Bi_0.90La_0.10)(Fe_0.90Zn_0.10)O₃ layers have been fabricated by radio frequency sputtering. Both multiferroic layers are well retained in these bilayers. Their leakage current, multiferroic properties, and fatigue behavior are largely dependent on the thicknesses of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃. With an increase of the thickness in the (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ layer, the leakage current density of bilayers is degraded due to different grain growth modes and an increase in oxygen vacancies, the dielectric constant (?_r) becomes larger due to the introduction of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ with a high ?_r value, and their magnetic properties are deteriorated with increasing the thickness ratios of (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ with a weaker magnetization. All bilayers exhibit a good ferroelectric behavior regardless of varying thicknesses of the (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ layer, while their coercive field decreases with increasing the thickness of the (Bi_0.90La_0.10)(Fe_0.85Zn_0.15)O₃ layer. An anomalous enhancement in switchable polarization is demonstrated by these bilayers, owing to the involvement of space charges accumulated at the interfaces between two constituent layers.     

Enhanced visible-light-response photocatalytic activity of bismuth ferrite nanoparticles

Multiferroic BiFeO₃ nanoparticles were prepared by a sol-gel rapid calcination technique with average diameter of 35 nm with narrow size distribution. The band gap was determined to be 2.06 eV, indicating their potential application as visible-light-response photocatalyst. The photocatalytic behaviors of BiFeO₃ nanoparticles were estimated by the degradation of Rhodamine B (RhB) under visible light irradiation. And the photocatalytic activities under different pH values were further studied for the first time. The result shows that the BiFeO₃ nanoparticles exhibit the highest photocatalytic activity in the solution with the lowest pH value, almost 100 times higher than that of the bulk.     

Epitaxial growth of tin ferrite thin films using pulsed laser deposition technique

Epitaxial thin films of tin ferrite (SnFe₂O₄) were deposited on (0 0 2) oriented strontium titanate (SrTiO₃) substrate using pulsed laser deposition method. The quality and epitaxial nature of the films were investigated by X-ray diffraction technique. The phi scan of the film and the substrate shows four folds symmetry indicating cube-on-cube epitaxial growth of the film on the substrate. The optical bandgap of the film was estimated to be 2.6 eV using optical transmittance data. Magnetic measurements indicate that the coercive field and remnant magnetization of the film decrease with increase in temperature. The presence of hysteresis loop in M vs. H plot at room temperature indicates the ferromagnetic nature of the film.     

Epitaxial SrRuO3 thin films deposited on SrO buffered-Si(001) substrates for ferroelectric Pb(Zr0.2Ti0.8)O3 thin films

SrRuO₃ thin film electrodes are epitaxially grown on SrO buffered-Si(001) substrates by pulsed laser deposition. The optimum conditions of the SrO buffer layers for epitaxial SrRuO₃ films are a deposition temperature of 700 °C, deposition pressure of 1 × 10^?6 Torr, and thickness of 6 nm. The 100 nm thick-SrRuO₃ bottom electrodes deposited above 650 °C on SrO buffered-Si (001) substrates have a rms (root mean square) roughness of approximately 5.0 Å and a resistivity of 1700 µω-cm, exhibiting an epitaxial relationship. The 100 nm thick-Pb(Zr_0.2Ti_0.8)O₃ thin films deposited at 575 °C have a (00l) preferred orientation and exhibit 2P_r of 40 µC/cm², E_c of 100 kV/cm, and leakage current of about 1 × 10^?7 A/cm² at 1 V. The silicon oxide phase which presents within PZT and SrRuO₃ films, influences the crystallinity of the PZT films and the resistivity of the SrRuO₃ electrodes.     

Enhanced optical modulation due to SPR in gold nanoparticles embedded WO3 thin films

Surface plasmon resonance (SPR) phenomenon of metal-dielectric composite thin films formed by embedding the noble metal nanoparticles in a dielectric matrix offers a high degree of flexibility and enables many applications such as surface enhanced spectroscopes, numerous biological and chemical sensing fields. A remarkable enhancement in optical modulation after embedding the gold nanoparticles in a reticulated mesh like matrix of WO₃ thin films was observed. WO₃ films were prepared onto the conducting ITO coated glass substrates by a novel pulsed spray pyrolysis technique (PSPT). A reticulated mesh like morphology of WO₃ was achieved by optimizing the deposition parameters of PSPT and the gold nanoparticles were embedded in the WO₃ matrix by a drop casting method. Enhancements in electrochromic properties of WO₃ in terms of optical modulation (ΔOD), coloration efficiency (η) and response times (t_c and t_b) were attributed to the assistance of SPR in gold nanoparticles during coloration and electric field induced quenching of SPR during bleaching.     

Hydrothermal growth and characterization of magnetite (Fe3O4) thin films

Well-crystallized magnetite (Fe₃O₄) thin films were successfully prepared by a simple hydrothermal process using hydrazine hydrate as the mineralizer. X-ray diffraction and scanning electron microscopy (SEM) and transmission electron microscopy were employed to characterize the products. SEM images show that the uniform Fe₃O₄ film (∼ 3 μm in thickness) is firmly grown on a nickel substrate. The magnetic property of the Fe₃O₄ particles scraped from the film was measured by Physical Property Measurement System (PPMS) at room temperature, and the magnetization curve reveals a soft ferromagnetic behavior with high saturation magnetization of 85 emu/g. Furthermore, the chemical and growth mechanisms for the hydrothermal formation of the Fe₃O₄ film are discussed.     

Structural, morphological and optical properties of Bi2−xSbxSe3 thin films grown by electrodeposition

Ternary single-phase Bi_2−xSb_xSe₃ alloy thin films were synthesized onto Au(1 1 1) substrates from an aqueous solution containing Bi(NO₃)₃, SbCl₃, and SeO₂ at room temperature for the first time via the electrodeposition technique. The electrodeposition of the thin films was studied using cyclic voltammetry, compositional, structural, optical measurements and surface morphology. It was found that the thin films with different stoichiometry can be obtained by controlling the electrolyte composition. The as-deposited films were crystallized in the preferential orientation along the (0 1 5) plane. The SEM investigations show that the film growth proceeds via nucleation, growth of film layer and formation of spherical particles on the film layer. The particle size and shape of Bi_2−xSb_xSe₃ films could be changed by tuning the electrolyte composition. The optical absorption spectra suggest that the band gap of this alloy varied from 0.24 to 0.38 eV with increasing Sb content from x = 0 to x = 0.2.     

A study of the synthesis of bismuth tellurium selenide nanocompounds and procedures for improving their thermoelectric performance

A bismuth tellurium selenide (Bi₂Te_ySe_3−y) nanocompound for thermoelectric applications was successfully prepared via a water-based chemical reaction in an atmospheric environment. The compound was less that ca. 100 nm in size, with a crystalline structure corresponding to the rhombohedral Bi₂Te_2.7Se_0.3. We sintered the compound via a spark plasma sintering process under the designated sintering conditions and measured the transport properties (i.e., thermal conductivity, resistivity, Seebeck coefficient). The resulting specimens consisted of nanosized grains exhibiting a remarkably low thermal conductivity. Subsequently, we endeavored to improve the other transport properties by adjusting the carrier density of the compound and derived the overall thermoelectric performance by the figure of merit (ZT).     

Effect of lanthanum doping on linear and nonlinear-optical properties of Bi3TiNbO9 thin films

Lanthanum doped Bi₃TiNbO₉ thin films (LBTN-x, La³⁺ contents x = 5%, 15%, 25% and 35 mol.%) with layered perovskite structure were fabricated on fused silica by pulsed laser deposition method. Their linear and nonlinear optical properties were studied by transmittance measurement and Z-Scan method. All films exhibit good transmittance (>55%) in visible region. For lanthanum doping content are x = 5%, 15% and 25 mol.%, the nonlinear absorption coefficient of LBTN-x thin films increases with the La³⁺ content, then it drops down at x = 35 mol.% when the content of La³⁺ in (Bi₂O₂)²⁺ layers is high enough to aggravate the orthorhombic distortion of the octahedra. We found that, 25 mol.% is the optimal La³⁺ content for LBTN-x thin films to have the largest nonlinear absorption coefficient making the LBTN-x film a promising candidate for absorbing-type optical device applications.     

Transparent p-type CuxS thin films

The effect of different mild post-annealing treatments in air, at 270 °C, for 4-6 min, on the optical, electrical, structural and chemical properties of copper sulphide (Cu_xS) thin films deposited at room temperature are investigated. Cu_xS films, 70 nm thick, are deposited on glass substrates by vacuum thermal evaporation from a Cu₂S:S (50:50 wt.%) sulphur rich powder mixture. The as-deposited highly conductive crystalline CuS (covellite) films show high carrier concentration (∼10²² cm⁻³), low electrical resistivity (∼10⁻⁴ Ω cm) and inconclusive p-type conduction. After the mild post-annealing, these films display increasing values of resistivity (∼10⁻³ to ∼10⁻² Ω cm) with annealing time and exhibit conclusive p-type conduction. An increase of copper content in Cu_xS phases towards the semiconductive Cu₂S (chalcocite) compound with annealing time is reported, due to re-evaporation of sulphur from the films. However, the latter stoichiometry was not obtained, which indicates the presence of vacancies in the Cu lattice. In the most resistive films a Cu₂O phase is also observed, diminishing the amount of available copper to combine with sulphur, and therefore the highest values of optical transmittance are reached (65%). The appearance on the surface of amorphous sulphates with annealing time increase is also detected as a consequence of sulphur oxidation and replacement of sulphur with oxygen. All annealed films are copper deficient in regards to the stoichiometric Cu₂S and exhibit stable p-type conductivity.     

Laser annealing of flash-evaporated CuInSe2 thin films

In this paper, the impact of laser annealing on the structural, electrical, and optical properties of CuInSe₂ (CIS) thin films has been investigated. The films were deposited using a modified flash evaporation system onto glass substrates. Structural analysis using x-ray diffraction (XRD) showed that the films have a strong preferred growth direction in the 〈112〉 plane. After laser annealing with a diffused beam of 20 ns width, the structure was relaxed and an increase in the intensity of 〈112〉 diffraction line was observed. A gas-microphone-type, high-resolution, near-infared (IR) photoacoustic spectrometer was used for the analysis of nonradiative defect states in as-grown and laser-annealed CIS thin film samples at room temperature. The absorption coefficient has been derived from photoacoustic spectra to establish activation energies for several defect-related energy levels. The calculated intrinsic defect ionization energies were also compared with the existing data available in the literature. The changes in the optical properties of the films have been explained in terms of the variations in the structural characteristics within the material. This paper was presented at the fourth International Surface Engineering Congress and Exposition held August 1–3, 2005 in St. Paul, MN.     

Nano-grain Al2O3 crystals grown by sputtering of aluminium on CoCrPt thin films for potential application in ultra-high-density recording media

Designing supraceramic assemblies based on Al₂O₃ has remained a challenge due to the problems associated with the suitable dispersion in neat compounds and ability to control the preferred orientation in a unique fashion. Herein, granular HCP-(CoCrPt)_100−X(Al₂O₃)_X (X represents the percent weight) thin films with Si(1 0 0) substrates have been fabricated using sputtering technique followed by annealing treatment. Structural and magnetic properties of thin film have been investigated for potential application in magnetic recording media. It was shown that coercivity increased from 0.5 to 2.5 kOe by increasing the nano-grain Al₂O₃ content in the CoCrPt magnetic layers. In CoCrPt-Al₂O₃ thin films coercivity of 2.5 kOe has been obtained with increasing the Al₂O₃ content from 3 to 13 wt.% in the annealed thin films. The structural properties of the samples were studied using X-ray diffraction (XRD) and transmission electron microscope (TEM) equipped with selected area electron diffraction (SAED). The magnetic properties of the samples were measured with a vibrating sample magnetometer (VSM). The VSM results showed that the HCP-CoCrPt-Al₂O₃ granular films are a promising candidate for ultra-high-density recording media because of its low Al₂O₃ content and simple manufacturing process.     

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca₃Co₄O₉ thin films with nominal composition of Ca_3−xAg_xCo₄O₉ (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca₃Co₄O₉ thin films could reach 0.73 mW m⁻¹ K⁻² at around 700 K, which was about 16% higher than that of the pure Ca₃Co₄O₉ thin film.     

Surface morphology of nanostructured anatase thin films prepared by pulsed liquid injection MOCVD

Nanostructured anatase thin films were prepared by chemical vapor deposition feeding a metal organic precursor in a pulsed liquid injection mode. The films were deposited at 500 °C on stainless steel substrates using a single molecular titanium isopropoxide liquid solution as the precursor, without any reactant gas. An effective pulsing injection mechanism for the precursor supply provides uniform concentration of the precursor in the vapor phase, allowing the formation of titanium dioxide layers from small micro-doses of the metal organic precursor. Energy-dispersive spectroscopy, scanning electron microscopy and X-ray diffraction studies show the formation of crack-free nanostructured anatase thin films, highly oriented, formed basically by stepwised nanostructures with a uniform coverage and no detectable carbon contamination. Surface crystallinity, composition, nanostructure and morphology are discussed in terms of the experimental parameters used in the deposition process.     

Phase transition and mechanical properties of ZrNxOy thin films on AISI 304 stainless steel

ZrN_xO_y thin films were deposited on AISI 304 stainless steel (304SS) substrates by reactive magnetron sputtering. The specimens were produced by sputtering a Zr target at 500 °C and the reactive gasses were N₂ and O₂ at various flow rates (ranging from 0 to 2 sccm). The purpose of this study was to investigate the effect of oxygen flow rate on the phase transition and accompanying mechanical properties of the ZrN_xO_y thin films. The oxygen contents of the thin films increased significantly with increasing oxygen flow rate. X-ray diffraction (XRD) revealed that the characteristics of the films can be divided into three zones according to the major phase with increasing oxygen content: Zone I (ZrN), Zone II (Zr₂ON₂) and Zone III (m-ZrO₂). The hardness of the ZrN_xO_y films decreased with increasing oxygen content due to the formation of the soft oxide phase. Modified XRD sin²ψ method was used to respectively measure the residual stresses of ZrN, Zr₂ON₂ and m-ZrO₂ phases. The results showed that the residual stress in ZrN was relieved as the oxygen content increased, and Zr₂ON₂ and m-ZrO₂ were the phases with lower residual stress. Compositional depth profiles indicated that there was a ZrO₂ interlayer near the film/substrates interface for all samples except the mononitride ZrN specimen. Contact angle was used as an index to assess the wettability of the film on substrate. The contact angles of ZrN, Zr₂ON₂ and m-ZrO₂ on stainless steel were indirectly measured using Owens-Wendt method. The results showed that ZrO₂ possessed the lowest wettability on 304SS among the three ZrN_xO_y phases, indicating that the ZrO₂ interlayer may account for the spallation of the ZrN_xO_y films after salt spray tests.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.     

Oxygen partial pressure dependence of memory effect of sputtered nc-Al/α-Al2O3 thin films

Nanocrystalline aluminum embedded in amorphous dielectric alumina matrix thin films (nc-Al/α-Al₂O₃) was synthesized via reactive magnetron sputtering. The nc-Al/α-Al₂O₃ films at different oxygen partial pressures were sputtered on p-type Si substrates from a pure Al target in the mixed ambient of Ar and O₂. Both deposition rate and aluminum concentration increase as the oxygen partial pressure decreases. X-ray photoelectron spectroscopy and high-resolution transmission electron microscope studies give the confirmation of nanocrystalline Al embedded in amorphous Al₂O₃ matrix. This nanocomposite thin film exhibits memory effect as a result of charge trapping. The flat band voltage value depends on the Al nanocrystal concentration which is related to oxygen partial pressure.     

Dependence of lattice distortion of monoclinic phase on film thickness in Pb(Zr0.58Ti0.42)O3 thin films

In this paper, the chosen composition of PZT film falls in rhombohedral phase region and the dependence of lattice distortion on film thickness in sol-gel derived Pb(Zr_0.58Ti_0.42)O₃ thin films was systematically investigated. The results confirm that the Pb(Zr_0.58Ti_0.42)O₃ films have monoclinic phase even though the composition falls in the rhombohedral phase region. The mixed textures of (1 0 0) and (1 1 1) occur in the PZT films. In the case of mixed textures, a method using ψ-scan XRD to characterize the phase type of Pb(Zr_0.58Ti_0.42)O₃ film is presented. It is found that the phase type of (1 0 0)-oriented grains is M_A phase, and that of (1 1 1)-oriented grains is M_B phase. Moreover, the lattice constants of both M_A and M_B phases are sensitive to the film thickness. The lattice distortion of monoclinic phase becomes smaller as film thickness increases.