Effects of co-substitution on the electrical properties of BiFeO3 thin films prepared by chemical solution deposition

Ferroelectric BiFeO₃ thin films with Nd-Cr (or Sm-Cr) co-substitution (denoted by BNdFCr and BSmFCr, respectively) were deposited on the Pt(2 0 0)/TiO₂/SiO₂/Si(1 0 0) substrates by a chemical solution deposition method. X-ray diffraction patterns revealed the formation of BNdFCr and BSmFCr thin films without any secondary phases. The co-substituted BNdFCr (or BSmFCr) thin films, which were annealed at 550 °C for 30 min in N₂ atmosphere, exhibited enhanced electrical properties compared to BFO thin films with the remanent polarization (2P_r) and coercive electric field (2E_c) of 196, 188 μC/cm² and 600, 570 kV/cm with the electric field of 800 kV/cm, respectively. The leakage current densities of BNdFCr and BSmFCr thin films measured at room temperature were approximately three orders of magnitude lower than that of BFO thin film, and the leakage current at room temperature of the thin films exhibited three distinctive conduction behaviors. Furthermore, the values of pulse polarizations [i.e., +(P*-P^{^}) or −(P*-P^{^})] of BNdFCr and BSmFCr thin films were reasonably unchanged up to 1.4 × 10¹⁰ switching cycles.     

Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

Effects of vanadium doping on structure and electrical properties of SrBi4Ti4O15 thin films

The effects of vanadium(V) doping into SrBi₄Ti₄O₁₅ (SBTi) thin films on the structure, ferroelectric, leakage current, dielectric, and fatigue properties have been studied. X-ray diffraction result showed that the crystal structure of the SBTi thin films with and without vanadium is the same. Enhanced ferroelectricity was observed in the V-doped SrBi₄Ti₄O₁₅ (SrBi_4−x/3Ti_4−xV_xO₁₅, SBTiV-x (x = 0.03, 0.06, and 0.09)) thin films compared to the pure SrBi₄Ti₄O₁₅ thin film. The values of remnant polarization (2P_r) and coercive field (2E_c) of the SBTiV-0.09 thin film capacitor were 40.9 μC/cm² and 105.6 kV/cm at an applied electric field of 187.5 kV/cm, respectively. The 2P_r value is over five times larger than that of the pure SBTi thin film capacitor. At 100 kHz, the values of dielectric constant and dielectric loss were 449 and 0.04, and 214 and 0.06 for the SBTiV-0.09 and the pure SBTi thin film capacitors, respectively. The leakage current density of the SBTiV-0.09 thin film capacitor measured at 100 kV/cm was 6.8 × 10⁻⁹ A/cm², which is more than two and a half orders of magnitude lower than that of the pure SBTi thin film capacitor. Furthermore, the SBTiV-0.09 thin film exhibited good fatigue endurance up to 10¹⁰ switching cycles. The improved electrical properties may be related to the reduction of internal defects such as bismuth and oxygen vacancies with changes in the grain size by doping of vanadium into SBTi.     

Room-temperature ferromagnetism in Sn1−xMnxO2 nanocrystalline thin films prepared by ultrasonic spray pyrolysis

Sn_1−xMn_xO₂ (x=0.01-0.05) thin films were synthesized on quartz substrate using an inexpensive ultrasonic spray pyrolysis technique. The influence of doping concentration and substrate temperature on structural and magnetic properties of Sn_1−xMn_xO₂ thin films was systematically investigated. X-ray diffraction (XRD) studies of these films reflect that the Mn³⁺ ions have substituted Sn⁴⁺ ions without changing the tetragonal rutile structure of pure SnO₂. A linear increase in c-axis lattice constant has been observed with corresponding increase in Mn concentration. No impurity phase was detected in XRD patterns even after doping 5 at% of Mn. A systematic change in magnetic behavior from ferromagnetic to paramagnetic was observed with increase in substrate temperature from 500 to 700 °C for Sn_1−xMn_xO₂ (x=0.01) films. Magnetic studies reveal room-temperature ferromagnetism (RTFM) with 3.61×10⁻⁴ emu saturation magnetization and 92 Oe coercivity in case of Sn_1−xMn_xO₂ (x=0.01) films deposited at 500 °C. However, paramagnetic behavior was observed for the films deposited at a higher substrate temperature of 700 °C. The presence of room-temperature ferromagnetism in these films was observed to have an intrinsic origin and could be obtained by controlling the substrate temperature and Mn doping concentration.     

Structural, optical and magnetic properties of (ZnO)1−x(MnO2)x thin films deposited at room temperature

The structural, magnetic and optical properties of (ZnO)_1−x(MnO₂)_x (with x = 0.03 and 0.05) thin films deposited by pulsed laser deposition (PLD) were studied. The pellets used as target, sintered at different temperatures ranging from 500 °C to 900 °C, were prepared by conventional solid state method using ZnO and MnO₂ powders. The observation of non-monotonic shift in peak position of most preferred (1 0 1) ZnO diffraction plane in XRD spectra of pellets confirmed the substitution of Mn ions in ZnO lattice of the sintered targets. The as-deposited thin film samples are found to be polycrystalline with the preferred orientation mostly along (1 1 0) diffraction plane. The UV-vis spectroscopy of the thin films revealed that the energy band gap exhibit blue shift with increasing Mn content which could be attributed to Burstein-Moss shift caused by Mn doping of the ZnO. The deposited thin films exhibit room temperature ferromagnetism having effective magnetic moment per Mn atom in the range of 0.9-1.4μ_B for both compositions.     

Microstructure and electrical properties of Mn-doped barium strontium titanate thin films prepared on copper foils

Ba_0.7−xSr_0.3Mn_xTiO₃ (x = 0, 0.025, 0.05) thin films have been prepared on copper foils using sol-gel method. The films were processed in an atmosphere with low oxygen pressure so that the substrate oxidation is avoided and the formation of the perovskite phase is allowed. XRD and SEM results showed that Mn doping enhanced the crystallization of the perovskite phase in the films. The Mn substitution prevents the reduction of Ti⁴⁺ to Ti³⁺, which is supported by XPS analysis. The Ba_0.7−xSr_0.3Mn_xTiO₃ film with x = 0.025 (BSMT25) exhibits preferred dielectric behavior and a lower leakage current density among the three thin films. The dielectric constant and loss of the BSMT25 film are 1213.5 and 0.065 at 1 MHz and around zero field, which are mostly desired for embedded capacitor applications. The mechanism of Mn doping on improving the electrical properties of barium strontium titanate (BST) thin films was investigated.     

Leakage current, ferroelectric and structural properties in Pb1−xBaxTiO3 thin films prepared by chemical route

Single-phase perovskite structure Pb_1−xBa_xTiO₃ thin films (x=0.30, 0.50 and 0.70) were deposited on Pt/Ti/SiO₂/Si substrates by the spin-coating technique. The dielectric study reveals that the thin films undergo a diffuse type ferroelectric phase transition, which shows a broad peak. An increase of the diffusivity degree with the increasing Barium contents was observed, and it was associated to a grain decrease in the studied composition range. The temperature dependence of the phonon frequencies was used to characterize the phase transition temperatures. Raman modes persist above tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of breakdown of the local cubic symmetry by chemical disorder. The absence of a well-defined transition temperature and the presence of broad bands in some interval temperature above FE-PE phase transition temperature suggested a diffuse type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. The leakage current density of the PBT thin films was studied at different temperatures and the data follow the Schottky emission model. Through this analysis the Schottky barrier height values 0.75, 0.53 and 0.34 eV were obtained to the PBT70, PBT50 and PBT30 thin films, respectively.     

Preparation, structure and ferromagnetic properties of the nanocrystalline Ti1-xMnxO2 thin films grown by radio frequency magnetron co-sputtering

This paper reported that the Mn-doped TiO2 films were prepared by radio frequency （RF） magnetron cosputtering. X-ray diffraction measurements indicate that the samples are easy to form the futile structure, and the sizes of the crystal grains grow big and big as the Mn concentration increases. X-ray photoemlssion spectroscopy measurements and high resolution transmission electron microscope photographs confirm that the manganese ions have been effectively doped into the TiO2 crystal when the Mn concentration is lower than 21%. The magnetic property measurements show that the Ti1-xMnxO2 （x = 0.21） films are ferromagnetic at room temperature, and the saturation magnetization, coercivity, and saturation field are 16.0 emu/cm^3, 167.5 × 80 A/m and 3740 × 80 A/m at room temperature, respectively. The room-temperature ferromagnetism of the films can be attributed to the new futile Ti1-xMnxO2 structure formed by the substitution of Mn^4＋ for Ti^4＋ into the TiO2 crystal .lattice, and could be explained by O vacancy （Vo）-enhanced ferromagnetism model.     

The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films

Due to the fault of the first author, this article entitled “The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films”, published in “Chinese Physics B”, 2011,Vol.20, Issue 12, Article No. 127701, has been found to copy from the article entitled“Decisive role of oxygen vacancy in ferroelectric versus ferromagnetic Mn-doped BaTiO3 thin films”, published in “Journal of Applied Physics”, 2011,Vol.109, Issue 8, article No. 084105. So the above article in “Chinese Physics B” has been withdrawn from the publication.<     

Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films

Co-doped TiO₂ films were fabricated under different conditions using reactive facing-target magnetron sputtering. Co doping improves the transformation of TiO₂ from anatase phase to rutile phase. The chemical valence of doped Co in the films is +2. All the films are ferromagnetic with a Curie temperature above 340 K. The average room-temperature moment per Co of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases from 0.74 μ_B at x=0.03 to 0.02 μ_B at x=0.312, and decreases from 0.54 to 0.04 μ_B as x increases from 0.026 to 0.169 for the Co-doped TiO₂ films fabricated at 0.27 Pa. The ferromagnetism originates from the oxygen vacancies created by Co²⁺ dopants at Ti⁴⁺ cations. The optical band gaps value (E_g) of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases linearly from 3.35 to 2.62 eV with the increasing x from 0 to 0.312. For the Co-doped TiO₂ films fabricated at 1.86 Pa, the E_g decreases linearly from 3.26 to 2.53 eV with increasing x from 0 to 0.350.     

Effect of Bi substitution on the magnetic and dielectric properties of epitaxially grown BaFe0.3Zr0.7O3−δ thin films on SrTiO3 substrates

The structural, dielectric and magnetic properties of single crystalline Ba_1−xBi_xFe_0.3Zr_0.7O_3−δ (x=0.0-0.29) thin films have been studied. The pseudotetragonal epitaxial thin films were obtained by pulsed laser-beam deposition (PLD) on (0 0 1) SrTiO₃ (STO) single-crystal substrates. The Bi substitution for the Ba ions up to an extent of x=0.18 caused a slight improvement in the leakage current properties, as well as an enhancement of the apparent dielectric constant. The saturation magnetization of the films was significantly decreased following Bi substitution. These changes were thought to be related to the increase in oxygen deficiencies in the films. The effect of the Bi substitution on the dielectric and magnetic properties was analyzed in conjunction with the change in valence value of the Fe ions.     

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Electrical property of HfOxNy-HfO2-HfOxNy sandwich-stack films

The effects of HfO_xN_y on the electrical property of HfO_xN_y-HfO₂-HfO_xN_y sandwich-stack (signed as SS) films were investigated. Excellent electrical performances were achieved in SS films, with a high dielectric constant of 16 and a low leakage current of ∼2 × 10⁻⁸ A/cm² at 1 MV/cm. Schottky (SK) emission and Frenkel-Poole (PF) emission are found to be the dominant mechanisms for the current conduction behavior. After a long time stress, the flat-band voltage shift in the SS film is much smaller than that in a pure HfO_xN_y film indicating fewer charge traps existed in the SS film. Based on the experiments, the new SS structure is more favorable for the improvement of electrical performances than a pure HfO_xN_y or HfO₂ structure.     

Sputter deposition of high transparent TiO2−xNx/TiO2/ZnO layers on glass for development of photocatalytic self-cleaning application

In this study, TiO_2−xN_x/TiO₂ double layers thin film was deposited on ZnO (80 nm thickness)/soda-lime glass substrate by a dc reactive magnetron sputtering. The TiO₂ film was deposited under different total gas pressures of 1 Pa, 2 Pa, and 4 Pa with constant oxygen flow rate of 0.8 sccm. Then, the deposition was continued with various nitrogen flow rates of 0.4, 0.8, and 1.2 sccm in constant total gas pressure of 4 Pa. Post annealing was performed on as-deposited films at various annealing temperatures of 400, 500, and 600 °C in air atmosphere to achieve films crystallinity. The structure and morphology of deposited films were evaluated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The chemical composition of top layer doped by nitrogen was evaluated by X-ray photoelectron spectroscopy (XPS). Photocatalytic activity of samples was measured by degradation of Methylene Blue (MB) dye. The optical transmittance of the multilayer film was also measured using ultraviolet-visible light (UV-vis) spectrophotometer. The results showed that by nitrogen doping of a fraction (∼1/5) of TiO₂ film thickness, the optical transmittance of TiO_2−xN_x/TiO₂ film was compared with TiO₂ thin film. Deposited films showed also good photocatalytic and hydrophilicity activity at visible light.     

A study of room-temperature ferromagnetism in transition metal and fluorine-doped spray-pyrolyzed SnO2 thin films

Room-temperature ferromagnetism has been observed in Co- or Mn-doped SnO₂ and Co- and F-co-doped SnO₂ thin films. A maximum magnetic moment of 0.80μ_B/Co ion has been observed for Sn_0.90Co_0.10O_1.925−δF_0.075 thin films, whereas in the case of Sn_1−xMn_xO_2−δ it was 0.18μ_B/Mn ion for x=0.10. The magnetization of both Sn_1−xCo_xO_2−δ and Sn_1−xCo_xO_2−y−δF_y thin films depends on the free carrier concentration. An anomalous Hall effect has been observed in the case of Co-doped SnO₂ films. However, the same was not observed in the case of Mn-doped SnO₂ thin films. Carrier-mediated interaction is convincingly proved to be the cause of ferromagnetism in the case of Co:SnO₂. It is, however, proposed that no carrier-mediated interaction exists in the case of Mn:SnO₂. Present studies indicate that dopants and hence electronic cloud-lattice interaction plays an important role in inducing ferromagnetism.     

Ferromagnetism in MnxGe1−x films prepared by magnetron sputtering

Mn_xGe_1−x thin films were prepared by magnetron sputtering with a substrate temperature of 673 K and subsequently annealed at 873 K. The X-ray diffraction (XRD) measurements showed that all samples had a single Ge cubic structure. No films showed clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements showed that the films had an uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicated that the valences of both Mn and Ge atoms increase with the Mn concentration. The resistance decreased with increasing temperature, suggesting that the films were typical semiconductors. Magnetic measurements carried out using a Physical Property Measurement System (PPMS) showed that all samples exhibited ferromagnetism at room temperature. There was a small concentration of Mn₁₁Ge₈ in the films, but the ferromagnetism was mainly induced by Mn substitution for Ge site.     

Pure and Nb-doped TiO1.5 films grown by pulsed-laser deposition for transparent p-n homojunctions

Pure and Nb-doped titanium oxide thin films were grown on sapphire substrates by pulsed-laser deposition in vacuum (10⁻⁷ mbar). The PLD growth leads to titanium oxide thin films displaying a high oxygen deficiency (TiO_1.5) compared with the stoichiometric TiO₂ compound. The structural and electrical properties (phase, crystalline orientation, nature and concentration of charge carriers, etc.) of these titanium oxide films were studied by XRD measurements and Hall effect experiments, respectively. The undoped TiO_1.5 phase displayed a p-type semiconductivity. Doping this titanium oxide phase with Nb⁵⁺ leads to an n-type behaviour as is generally observed for titanium oxide films with oxygen deficiency (TiO_x with 1.7 < x < 2). Multilayer homojunctions were obtained by the stacking of TiO_1.5 (p-type) and Nb-TiO_1.5 (n-type) thin films deposited onto sapphire substrates. Each layer is 75 nm thick and the resulting heterostructure shows a good transparency in the visible range. Finally, the I-V curves obtained for such systems exhibit a rectifying response and demonstrate that it is possible to fabricate p-n homojunctions based only on transparent conductive oxide thin films and on a single chemical compound (TiO_x).     

Effect of sulfur additive on density of localized states in nanostructures chalcogenide Se95−xSxZn5 thin films

This paper reports the measurement of space charge limited conduction (SCLC) on the fabricated thin films of Se_95−xS_xZn₅ (0.2≤x≤10) in temperature range 313–353 K for the first time. At high electric fields (E∼10⁴ V/cm), the current could be fitted into the theory of space charge limited conduction, in case of uniform distribution of localized states in mobility gap. The homogeneity and surface morphology of thin films were assessed by scanning electron microscopy. The crystalline nature of the thin films was confirmed by powder XRD and the crystallite size was calculated using Scherer's formula. The crystallite size and density of localized states were found to increase with the increase of sulfur concentration. DC conductivity and activation energy were calculated and found to decrease and increase respectively, with the increase of sulfur concentration.     

Preparation and characterization of patterned copper sulfide thin films on n-type TiO2 film surfaces

Micro-arrayed patterns of p-type copper sulfide (Cu_xS) thin films with positive and negative features were deposited onto the surfaces of n-type TiO₂ semiconductor films via a selective nucleation and growth process from aqueous solution. The surface functional molecules of the UV photo-oxidised patterned SAMs were utilized to direct the nucleation and growth of Cu_xS crystallites. The resultant Cu_xS/TiO₂ composite films with negative and positive Cu_xS patterns on the TiO₂ film surface were investigated using SEM, XRD, XPS and a 3D Surface Profiler. It is demonstrated that regular and compact patterned films of Cu₂S crystallites had been deposited onto the n-type TiO₂ surface, with sharp edges demarcating the boundaries between the patterned Cu₂S region and the TiO₂ film region. The UV-vis spectra for three Cu₂S/TiO₂ films exhibit a wide absorption between 300 nm and 450 nm. The maximum wavelength differences in the spectra of Cu₂S/TiO₂ films and TiO₂ film were attributed to the added absorption of Cu₂S films at 302 nm and the unchanged adsorption of TiO₂ films. The absorption intensities of the Cu₂S/TiO₂ films could be varied in the UV-vis range using the Cu₂S patterned features (positive, negative).     

Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition

Nitrogen-doped titanium dioxide (TiO_2−xN_x) thin films have been prepared by pulse laser deposition on quartz glass substrates by ablated titanium dioxide (rutile) target in nitrogen atmosphere. The x value (nitrogen concentration) is 0.567 as determined by X-ray photoelectron spectroscopy measurements. UV-vis spectroscopy measurements revealed two characteristic deep levels located at 1.0 and 2.5 eV below the conduction band. The 1.0 eV level is attributable to the O vacancy state and the 2.5 eV level is introduced by N doping, which contributes to narrowing the band-gap by mixing with the O2p valence band. The enhanced degradation efficiency in a broad visible-light range was observed from the degradation of methylene blue and methylene orange by the TiO_2−xN_x film.