Multiferroic properties of Co and Nd co-substituted Bi5Ti3FeO15 thin films

Co and Nd co-substituted Bi₅Ti₃FeO₁₅ thin films were prepared on Pt/Ti/SiO₂/Si substrates via metal organic decomposition method. The structural and multiferroic properties of the films were investigated. It was found that Co ions enter into the lattice and occupy the Fe site. The Bi_4.15Nd_0.85Ti₃Fe_0.5Co_0.5O₁₅ films simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature, and its 2P_r and 2M_r are 38 μC/cm² and 3 kA/m, respectively. Moreover, substitutions create local ferromagnetic order and antiferromagnetic order depending on whether the local bonding is FeOCo or FeOFe/CoOCo, respectively. The competing interaction of the ferromagnetic and antiferromagnetic phases results in an interesting magnetic behavior of the films.     

Size effects of polycrystalline lanthanum modified Bi4Ti3O12 thin films

The film thickness dependence on the ferroelectric properties of lanthanum modified bismuth titanate Bi_3.25La_0.75Ti₃O₁₂ was investigated. Films with thicknesses ranging from 230 to 404 nm were grown on platinum-coated silicon substrates by the polymeric precursor method. The internal strain is strongly influenced by the film thickness. The morphology of the film changes as the number of layers increases indicating a thickness dependent grain size. The leakage current, remanent polarization and drive voltage were also affected by the film thickness.     

Multiferroic Bi0.7Dy0.3FeO3 thin films directly integrated on Si for integrated circuit compatible devices

Magnetoelectric multiferroic Bi_0.7Dy_0.3FeO₃ (BDFO) thin films deposited on p-type Si (100) substrate using pulsed laser deposition technique demonstrated a saturated ferroelectric and ferromagnetic hysteresis loop at room temperature. More interestingly, the observed change in electric polarization with applied magnetic field in these films indicated the presence of room temperature magnetoelectric coupling behavior. Using high-frequency capacitance-voltage measurements, the fixed oxide charge density, interface trap density and dielectric constant were estimated on Au/BDFO/Si capacitors. These results suggest the integrated circuit compatible application potential of BDFO films in the field of micro-electro-mechanical systems and non-volatile memories.     

Studies on chemically deposited CuInSe2 thin films

Nanocrystalline thin films of CuInSe₂ have been prepared by chemical bath deposition technique at temperatures below 60 °C. X-ray diffraction of the films confirmed the identity of CuInSe₂ and with largely broadened peaks indicated the nanocrystalline nature of the films. Images from scanning electron microscope represented spherical nanoparticles.     

Ag and O ion irradiation induced improvement in dielectric properties of the Ba(Co1/3Nb2/3)O3 thin films

We present the preliminary results of temperature and frequency dependent dielectric measurements on Ba(Co_1/3Nb_2/3)O₃ (BCN) thin films. These films were prepared on indium tin oxide (ITO) coated glass substrates by the pulse laser deposition (PLD) technique. It exhibits single-phase hexagonal symmetry. These films were irradiated with Ag¹⁵⁺ (200 MeV) and O⁷⁺ (100 MeV) beams at the fluence 1 × 10¹¹, 1 × 10¹², and 1 × 10¹³ ions/cm². On irradiating these films, its dielectric constant (?′) and dielectric loss (tan δ) parameters improve compared to un-irradiated film. Compared to O⁷⁺ irradiation induced point/cluster defects Ag¹⁵⁺ induced columnar defects are more effective in reducing/pinning trapped charges within grains. The present paper highlights the role of swift heavy ion irradiation in engineering the dielectric properties of conductive samples to enable them to be useful for microwave device applications.     

Structural and morphological characterization of Pr and Er-containing SiO2-P2O5 sol-gel thin films

Present work is focused on obtaining and characterization of sol-gel thin films belonging to SiO₂-P₂O₅-Er₂O₃ (I) and SiO₂-P₂O₅-Pr₂O₃ (II) systems. The films have been obtained by spin coating technique for three rotation speeds: 2000, 3500 and 5000 rpm. The deposition of the films was performed at different periods of time, i.e. 24 h, 96 h, 120 h, 144 h and 168 h after instant preparation of the precursor sols. FTIR (Fourier transform infrared spectroscopy) and Raman characterization aimed at investigating the structural changes that occurred in silicophosphate network in dependence on the spin rate of the substrate as well as on the time period elapsed since the sol preparation till the deposition day. FTIR spectra recorded in the 400-4000 cm⁻¹ range revealed Si-O-Si, P-O-P and Si-O-P vibration modes and optical phonons specific for OH units. Raman spectra collected in the 100-4000 cm⁻¹ range put in evidence stretching, bending and mixed vibration modes specific for silicophosphate network as well as rare-earth ion peaks specific to certain electronic transitions. Morphological investigation made by AFM (atomic force microscopy) on Er and Pr-doped silicophosphate sol-gel films evidenced specific features depending on the parameters mentioned above and SEM (scanning electron microscopy) analysis revealed micron sphere structural units, exfoliation of the films and micro cracks.     

Effect of the microwave oven on structural, morphological and electrical properties of SrBi4Ti4O15 thin films grown on Pt/Ti/SiO2/Si substrates by a soft chemical method

Thin films of SrBi₄Ti₄O₁₅ (SBTi), a prototype of the Bi-layered-ferroelectric oxide family, were obtained by a soft chemical method and crystallized in a domestic microwave oven. For comparison, films were also crystallized in a conventional method at 700 °C for 2 h. Structural and morphological characterization of the SBTi thin films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Using platinum coated silicon substrates, the ferroelectric properties of the films were determined. Remanent polarization P_r and a coercive field E_c values of 5.1 μC/cm² and 135 kV/cm for the film thermally treated in the microwave oven and 5.4 μC/cm² and 85 kV/cm for the film thermally treated in conventional furnace were found. The films thermally treated in the conventional furnace exhibited excellent fatigue-free characteristics up to 10¹⁰ switching cycles indicating that SBTi thin films are a promising material for use in non-volatile memories.     

Carrier induced magnetism in dilute Fe doped Ge1−xSbx thin films

Thin films of Fe_0.01Ge_1−xSb_x (x = 0.01, 0.05, 0.10) alloys were prepared by thermal evaporation technique. Characterization of these thin films was done using High Resolution X-Ray Diffraction (HRXRD), Two Probe Resistivity measurement, Atomic Force Microscopy (AFM) and Magnetic Force Microscopy (MFM) respectively. The resistivity results show that activation energy increases with increase in Sb concentration. The low temperature conduction is explained by Variable Range-Hopping mechanism, which fits very well for the whole temperature range. The Arrhenius plot reveals semiconducting behavior. The AFM images of alloys show almost uniform particle size distribution with average particle size varying from 35 to 60 nm with increase in Sb concentration. The MFM images corresponding to the AFM images show the films exhibiting ferromagnetic interactions at room temperature. The average magnetic domain sizes were observed to increase from 43 to 68 nm with increase in Sb concentration from x = 0.01 to x = 0.10.     

Optimization of Bi2Te3 and Sb2Te3 thin films deposited by co-evaporation on polyimide for thermoelectric applications

The optimization of the deposition process of n-type Bismuth Telluride and p-type Antimony Telluride thin films for thermoelectric applications is reported. The films were deposited on a 25 μm-thick flexible polyimide (kapton) substrate by co-evaporation of Bi and Te, for the n-type element, and Sb and Te, for the p-type element. The evaporation rate of each material was monitorized by an oscillating crystal sensor and the power supplied to each evaporation boat was controlled with a PID algorithm in order to achieve a precise user-defined constant evaporation rate.The influence of substrate temperature (in the range 240-300 °C) and evaporation rates of Bi, Te and Sb on the electronic properties of the films was studied and optimized to obtain the highest Seebeck coefficient. The best n-type Bi₂Te₃ films were deposited at 300 °C with a polycrystalline structure, a composition close to stoichiometry, electrical resistivity ∼20 μΩ m and Seebeck coefficient −195 μV/°C. The best p-type Sb₂Te₃ films were deposited at 240 °C, are slightly Te-rich, have electrical resistivity ∼20 μΩ m and Seebeck coefficient +153 μV/°C. These high Seebeck coefficients and low electrical resistivities make these materials suitable for fabrication of Peltier coolers and thermopile devices.     

Phase transition behaviors and thermal conductivity measurements of nitrogen-doped Sb2Te3 thin films

Crystallization temperature of nitrogen-doped Sb₂Te₃ (ST) thin films increased with increasing nitrogen doping concentration, which indicates that the long-term stability of the metastable amorphous state can be improved by nitrogen doping. The root-mean-square (rms) roughness values of the films showed a significant decrease with nitrogen doping. Thermal conductivity of nitrogen-doped ST thin films was measured using a transient thermoreflectance (TTR) technique. It was found that the thermal conductivity decreased with increasing nitrogen doping concentration and increased with increasing annealing temperature. Nitrogen-doped ST thin films are suitable phase-change materials for low programming power consumption applications of phase-change random access memory (PCRAM).     

Dislocations in Bi0.4Ca0.6MnO3 epitaxial film grown on (110) SrTiO3 substrate

Bi_0.4Ca_0.6MnO₃ (BCMO) film with a thickness of 110 nm was epitaxially grown on a (110) SrTiO₃ (STO) substrate using pulsed laser ablation technique. The microstructure of the epitaxial films was investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) in details. Two different kinds of dislocations, one being perpendicular to the BCMO/STO interface, the other being parallel to the interface, have been commonly observed. The formation mechanism for these dislocations has been discussed. All the dislocations are thought to relieve the local strain in the epitaxial film.     

Effects of Mn doping on structural and dielectric properties of sol-gel-derived (Ba0.835Ca0.165)(Zr0.09Ti0.91)O3 thin films

We reported the effects of Mn doping on the structure and dielectric properties of (Ba_0.835Ca_0.165)(Zr_0.09Ti_0.91)O₃ (BCZT) thin films prepared by sol-gel method. The (Ba_0.835Ca_0.165)Mn_x(Zr_0.09Ti_0.91)_1 − xO₃ (x = 0, 0.002, 0.005, and 0.01) thin films exhibited a pure pseudo-cubic perovskite structure with random orientation. Scanning electron microscopy and atomic force microscopy observation showed that increasing Mn-doping amount caused a decrease in particle size and a cluster of the particles, while the film surface remained smooth and crack-free. Compared with the undoped film, Mn doped BCZT thin films exhibited smaller dielectric constant and lower dielectric loss. The figure of merit reached the maximum value of 16.7 with a tunability of 53.6% for the film with 0.5 mol % Mn doping, when a bias electric field of 400 kV/cm was applied at 100 kHz. The results indicated that the Mn doped BCZT thin films are suitable for tunable microwave device applications.     

A simple route to synthesize nanocrystalline TiO2 films at room temperature

At room temperature, TiO₂ films were deposited by direct current pulse magnetron sputtering. Varying the O₂/Ar flow ratio, TiO₂ films with different crystalline structures and surface morphologies were obtained. X-ray diffraction spectra and atomic force microscopy showed TiO₂ films mainly existed as amorphous and crystalline structures when the O₂/Ar flow ratio was less and more than 6/14, respectively. With these results, a critical point was found at O₂/Ar = 6/14. The film growth behavior was controlled by the shadowing effect and the mixture of crystallographic orientation and grain growth effects, when the O₂/Ar flow ratio was less and more than the critical point, respectively. The present work reveals a simple way to prepare the high quality anatase phase TiO₂ films at room temperature, as well as the growth mechanism behind it.     

Nanoscale domain switching in Bi3.15Eu0.85Ti3O12 thin films annealed at different temperature by scanning probe microscopy

The nanoscale domain switching behavior of Bi_3.15Eu_0.85Ti₃O₁₂ (BET) thin films annealed at temperatures of 600, 650 and 700 °C is investigated by scanning probe microscopy (SPM) via the direct observation on their domain structure. It is shown that most ferroelectric domains are clearly detectable and confined in the grains. Some domains opposite to the polarizing electric field were observed by SPM, and their formations are attributed to the grain boundaries playing an active roles in pinning a preferential polarization state. The remnant polarization (2P_r) values of BET thin films increase with the annealing temperature due to the increase of nanoscale domains switched into the direction of polarizing electric field.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

Effect of precursor solution on the formation of perovskite phase of Pb(Mg1/3Nb2/3)O3 thin films

Precursor solutions for Pb(Mg_1/3Nb_2/3)O₃ (PMN) synthesis were obtained by Pechini's method. The influence of the concentration of organic materials on the phase formation has been studied. For this purpose, PMN solutions were prepared with different precursors and were characterized by thermogravimetric and differential thermal analysis. The obtained solutions were deposited onto a Si (100) substrate by dip coating and pre-treated in a hot plate at 300 °C for 1 h. The films were annealed at 600, 700, 800 and 900 °C for 1 h and characterized by X-ray diffraction. The perovskite phase was formed after annealing at 600 and 700 °C when the solution of PMN was prepared with a lower amount of organic material and starting with niobium oxide. By increasing the temperature to 800 or 900 °C, only the formation of pyrochlore phase was observed. With the solution prepared from niobium ethoxide, only the presence of pyrochlore phase was observed independently of the annealing 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.     

Photoelectrochemical properties of thin Bi2WO6 films

Thin Bi₂WO₆ film prepared from an amorphous heteronuclear complex via dip-coating method is investigated as a visible light-driven photoelectrode material. Photoelectrochemical properties of the resultant film are investigated on the basis of linear sweep voltammetry and current-time curves, and conduction and valence band edges of the film electrode are determined from the photocurrent voltage response. Anodic photocurrent associated with the oxidation of water is obtained under visible light irradiation. Furthermore, the film as a photoanode can degrade rhodamine B (RhB) and methylene blue (MB) in aqueous solution under visible light irradiation slowly. The application of bias potential further improves the photodegradation efficiency of RhB and MB. Based on the analytic result of current-time curve, the stability of the film electrode is confirmed.     

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.     

The effect of heat treatment on physical properties of nanograined (WO3)1-x-(Fe2O3)x thin films

Thin films of (WO₃)_1-x-(Fe₂O₃)_x composition were deposited by thermal evaporation on glass substrates and then all samples were annealed at 200-500 °C in air. Optical properties such as transmittance, reflectance, optical bangap energy, and the optical constants of the “as deposited” and the annealed films were studied using ultraviolet-visible spectrophotometry. It was shown that the annealing process changes the film optical properties which were related to Fe₂O₃ concentration. Moreover, using X-ray photoelectron spectroscopy, we have indicated that WO₃ is stoichiometric, while iron oxide was in both FeO and Fe₂O₃ compositions so that the FeO composition converted to Fe₂O₃ after the annealing process. Using atomic force microscopy, it was observed that surface of the “as deposited” films were smooth with a nanometric grain size. The film surface remained unchanged after annealing up to 300 °C. Surface roughness and the grain size of the films with x = 0, 0.05, and 0.75 highly increased at higher annealing temperatures (400 and 500 °C), but were nearly unchanged for medium x-values (0.3 and 0.4).