Enhanced photovoltaic properties of PbTiO3-based ferroelectric thin films prepared by a sol-gel process

PbTiO₃ (PTO), Pb(Mn_0.1Ti_0.9)O₃ (PMTO), Pb(Sr_0.1Ti_0.9)O₃ (PSTO), and Pb(Zr_0.1Ti_0.9)O₃ (PZTO) were prepared on an indium tin oxide (ITO)/glass substrate by a sol-gel method. PTO, PMTO, PSTO, and PZTO films exhibited energy band gaps of 3.55 eV, 3.63 eV, 3.59 eV, and 3.66 eV, respectively. All these films generated high photocurrents due to high shift currents, because carrier migration channels were successfully introduced by a lattice mismatch between the films and ITO substrates. The PMTO thin film exhibited the best ferroelectric and photovoltaic properties, with a photovoltage of 0.74 V, a photocurrent density of 70 μA/cm², and a fill factor of 43.34%, which confirms that shift current and ferroelectric polarization are two main factors that affect the ferroelectric photovoltaic properties. The PSTO, PZTO, and PTO thin films displayed space-charge-limited current (SCLC) when the electric field strength was below 10 kV/cm, and these three films broke down when the electric field strength was above 10 kV/cm. Analysis of the shift current mechanism confirmed that the breakdown of the PZTO and PSTO thin films resulted from Pool Frenkel emission current. The PMTO thin film displayed SCLC in the test range, which indicates that doping with Mn could inhibit defect formation in ferroelectric thin films.     

Significantly improved energy storage properties of sol-gel derived Mn-modified SrTiO3 thin films

In this paper, x mol% Mn-doped SrTiO₃ (STMx, x?=?0, 0.5, 1, 3 and 5) thin films were synthesized by a sol-gel method. The effect of Mn doping on the microstructure and electrical performance was investigated. STMx (x?≤?1) thin films shows a single cubic perovskite phase while impurity phase appears for STM3 and STM5 thin films confirmed by X-ray diffraction. X-ray photoelectron spectra reveals that STM1 thin film has the lowest concentration of oxygen vacancy. The dielectric constant and loss of STMx (x?≤?1) films display good frequency stability, while decrease with the frequency for STM3 and STM5 thin films. And all samples display excellent bias stability of dielectric constant; this is advantageous for applications in a high electric field. The ferroelectric test demonstrates that the electrical breakdown strength increases and leakage current decreases for Mn doped SrTiO₃ films. A great recoverable energy storage density of 23.8?J/cm³ with an efficiency of 69.8% at 2.286?MV/cm is obtained in STM1 thin film. Furthermore, STM1 thin film shows good frequency stability of energy storage properties. It indicates that Mn doping is a simple and effective method to improve the energy storage properties of dielectric capacitors.     

Enhanced ferroelectric properties of (Zn,Ti) equivalent co-doped BiFeO3 films prepared via the sol-gel method

High-quality BiFe_1-2xZn_xTi_xO₃ (BFZTO with x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) films were successfully prepared on fluorine-doped tin oxide (FTO)/glass substrates via the sol-gel method. The influence of (Zn, Ti) equivalent co-doping on the structure, surface morphology, and ferroelectric properties of BFZTO films was investigated systematically. X-ray diffraction (XRD) and Raman spectra analysis indicate that co-doping results in structural transformations. Scanning electron microscope (SEM) images show that BFZTO films with x = 0.02 exhibit uniform fine grains and higher density, which is instrumental for the development of ferroelectric properties. X-ray photoelectron spectroscopy (XPS) analysis reveals that BiFe_0.96Zn_0.02Ti_0.02O₃ film can inhibit the conversion of Fe³⁺ into Fe²⁺, thereby greatly reducing oxygen vacancy concentration. Therefore, under the electric field strength of 150 kV/cm, BiFe_0.96Zn_0.02Ti_0.02O₃ film was found to have the lowest leakage current density, J = 1.13 × 10^?6 A/cm², which is five orders of magnitude lower than that of pure BiFeO₃ (BFO) film. Furthermore, this film exhibits the largest remnant polarization at room temperature, P_r = 131.9 μC/cm², which is more than twice as large as that of pure BFO (P_r = 52.6 μC/cm²). Additionally, by comparing P-E hysteresis loops of different regions on the surface of BiFe_0.96Zn_0.02Ti_0.02O₃ film, it was found that the film has high uniformity and stable overall performance. Dielectric and magnetic properties were also enhanced via (Zn, Ti) co-doping.     

Microstructure and through-film electrical characteristics of vertically aligned amorphous carbon films

The microstructure and electrical properties of in-situ annealed carbon films is studied in this paper. In-situ annealing (150 °C to 600 °C) was done during the deposition of carbon films with −300 V substrate bias. Transmission electron microscopy and two points electrical probing studies were performed and the deduced transition for vertical orientated graphitic planes occurs at temperatures above 400 °C. The microstructure of the films strongly depends on the deposition temperature of the films (room temperature, 400 °C and 600 °C). Electrical conductivity of the film strongly depends on texturing due to the formation of preferred orientation in the vertical direction. The vertically orientated carbon (VOC) sheet provides effective nanochannels for electron transport, thus significantly improves the electrical properties of the annealed film.     

Structural, electrical and magnetic properties of carbon-nickel composite thin films

Carbon-nickel composite thin films (600 nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25-800 °C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni₃C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the aspect ratio as high as 1:60 in the sample prepared at 200 °C). The carbon matrix was amorphous, or graphite-like depending on deposition temperature. At low deposition temperatures T_S (25-400 °C) the Ni₃C nanoparticles were of hcp phase. Samples prepared at T_S ? 600 °C contained ferromagnetic fcc Ni nanoparticles. A correlation was found between the structural, electrical and magnetic properties of the composites. To characterise the films, dependences, such as resistivity vs. temperature, current vs. voltage, differential conductivity vs. bias voltage, and magnetoresistivity, were determined. For example, the tunneling effect was found in samples in which the metallic nanoparticles were separated by 2-3 nm thick amorphous carbon. When the metallic nanoparticles were connected by graphite-like carbon regions (having a metallic conductivity, in contrast to a-C), the temperature coefficient of the resistivity became slightly positive. An anisotropic magnetoresistivity of ∼0.1% was found in the sample that contained ferromagnetic columnar fcc Ni. Zero magnetoresistivity was found in the sample in which the metallic nanoparticles were of non-magnetic hcp phase.     

Tuning electrical properties of PZT film deposited by Pulsed Laser Deposition

The variation of the chemical composition and properties of PZT films as a function of oxygen pressure and laser fluence during pulsed laser deposition is used to tune the electrical properties of the PZT thin films. It is found that the deposition using a 248 nm laser fluence of 1.7 J/cm² and an oxygen pressure of 400 mtorr results the PZT films very similar to that of target material. Changing the laser fluences or oxygen pressure, affects the lead content of the deposited film. In the range of oxygen pressure 50–200 mtorr, the Zr/Zr+Ti and Ti/Zr+Ti ratio varies with oxygen pressure while the Pb/Zr+Ti ratio is almost uniform. Using oxygen pressure as a control parameter to tune the chemical compound and electrical properties of the deposited PZT films, the remnant polarization of the PZT films is tuned in the range of 6.6–42.2 µC/cm², the dielectric constant is controlled in the range of 29–130, and the piezoelectric constant d₃₃ is controlled in the range of 3.82–4.96 pm/V for a 40 nm thick PZT film.     

Modified electrical properties of chemical solution deposited epitaxial BiFeO3 thin films by Mn substitution

The effect of Mn substitution on microstructure and electrical properties of epitaxial BiFeO₃ (BFO) thin films grown by an all-solution approach was investigated. Raman analysis reveals that the Mn atoms substitution at Fe sites can result in Jahn-Teller distortion and thus lead to the weakness of long-range ferroelectric order. In addition, the break-down characteristics of BFO thin films are improved with the increase of Mn atoms content, although the leakage current is gradually increased. Meanwhile, the grain size, the dielectric constant and loss are also increased with the increase of Mn content. The P-E hysteresis loops and PUND results demonstrate that the intrinsic ferroelectric polarization is effectively improved with Mn atoms substitution as the grain size increased and Mn atoms play a role of nucleation sites. However, the ferroelectric properties are deteriorated with the excess substituted Mn content due to the higher leakage current.     

Effects of tungsten thickness and annealing temperature on the electrical properties of W-TiO2 thin films

TiO₂ thin films were prepared by RF magnetron sputtering onto glass substrates and tungsten was deposited onto these thin films (deposition time 15-60 s) to form W-TiO₂ bi-layer thin films. The crystal structure, morphology, and transmittance of these TiO₂ and W-TiO₂ bi-layer thin films were investigated. Amorphous, rutile, and anatase TiO₂ phases were observed in the TiO₂ and W-TiO₂ bi-layer thin films. Tungsten thickness and annealing temperature had large effects on the transmittance of the W-TiO₂ thin films. The W-TiO₂ bi-layer thin films with a tungsten deposition time of 60 s were annealed at 200 °C-400 °C. The band gap energies of the TiO₂ and the non-annealed and annealed W-TiO₂ bi-layer thin films were evaluated using (αhν)^1/2 versus energy plots, showing that tungsten thickness and annealing temperature had major effects on the transmittance and band gap energy of W-TiO₂ bi-layer thin films.     

Structural and electrical properties of the sol-gel derived multiferroic BiFeO3 monolayer and NiTiO3-BiFeO3 bilayer thin films

In this paper, the multiferroic BeFiO₃ monolayer and NiTiO₃–BiFeO₃ bilayer thin films were fabricated by spin-coating method on the SrRuO₃/n⁺-Si substrate. The structural and ferroelectric properties of multiferroic BeFiO₃ monolayer and NiTiO₃–BiFeO₃ bilayer thin films were investigated. Both multiferroic films showed the typical XRD patterns of the perovskite structure without presence of the second phase. The electrical properties, such as leakage current and remnant polarization, of the NiTiO₃–BiFeO₃ bilayer film were superior to those of BeFiO₃ monolayer film, which those values were 1.94 × 10⁻⁴ A/cm² at electric field of 0.75 MV/cm and 14.05 μC/cm², respectively. This outcome is due to the NiTiO₃–BiFeO₃ bilayer film with a high Schottky barrier height as well as a top NiTiO₃ layer on the BiFeO₃ film inducing the strain-induced polarization rotation and forming the strong domain-wall pinning.     

Structural and electrical properties of Na0.5Bi4.0RE0.5Ti4O15 (RE=Tm,Yb and Lu) thin films

In the current study, a series of lanthanide ions, Tm, Yb and Lu, were used for doping at the Bi-site of the Aurivillius phase Na_0.5Bi_4.5Ti₄O₁₅ (NaBTi) to investigate the structural, electrical and ferroelectric properties of the thin films. In this regard, Na_0.5Bi_4.5Ti₄O₁₅ and the rare earth metal ion-doped Na_0.5Bi_4.0RE_0.5i₄O₁₅ (RE=Tm, Yb and Lu, denoted by NaBTmTi, NaBYbTi, and NaBLuTi, respectively) thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. Formations of the Aurivillius phase orthorhombic structures for all the thin films were confirmed by X-ray diffraction and Raman spectroscopic studies. Based on the experimental results, the rare earth metal ion-doped Na_0.5Bi_4.0RE_0.5Ti₄O₁₅ thin films exhibited a low leakage current and the improved ferroelectric properties. Among the thin films, the NaBLuTi thin film exhibited a low leakage current density of 6.96×10⁻⁷ A/cm² at an applied electric field of 100 kV/cm and a large remnant polarization (2P_r) of 26.7 μC/cm² at an applied electric field of 475 kV/cm.     

Enhanced conductance properties of UV laser/RTA annealed Al-doped ZnO thin films

Thin films comprising 0.5 mol% aluminum-doped zinc oxide (AZO) were prepared on glass substrates by a spin-coating method for transparent conducting oxide (TCO) applications. UV laser was selected for the annealing of AZO thin films, due to the well matched energy bandgap between UV laser and AZO films. After the rapid thermal annealing (RTA) process, post UV laser annealing was carried out by varying the scan speed of the laser beam, and the effects of laser annealing on the structural, morphological, electrical, and optical properties were analyzed. The results indicated that UV laser annealing based on various scan speeds affects the microstructure, sheet resistance, and optical transmittance of the AZO thin films, compared with those of the only RTA processed thin films. X-ray diffraction (XRD) analysis showed that all films that preferentially grew normally on the substrate had a (002) peak. The optical transmittance spectra of the laser/RTA annealed AZO thin films exhibited greater than 83% transmittance in the visible region. Also, the sheet resistance (1.61 kΩ/sq) indicated that optimized UV laser annealing after the RTA process improves film conductance.     

Enhanced multiferroic properties in Pr-doped BiFe0.97Mn0.03O3 films

Bi_1−xPr_xFe_0.97Mn_0.03O₃ (x=0.00, 0.05, 0.10, 0.15, 0.20) thin films were deposited on FTO/glass substrate using chemical solution deposition. The influences of Pr doping on the crystalline structure and multiferroic properties were investigated. In the X-ray diffraction and Raman spectra results, the crystal structures of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films revealed a gradual transformation from the trigonal structure to the tetragonal structure. The leakage current densities of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films are one order of magnitude lower than that of BiFeO₃. Compared with unsaturated polarization-electric field hysteresis loop of BiFeO₃ film, the Pr and Mn co-doped BFO films have significantly improved ferroelectric properties. The improved remnant polarization (Pr=91.3 µC/cm²) and the positive switching current (I=0.028 A) have been observed in Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film. The improved electrical properties are attributed to the structure transformation, increasing grain boundaries, low oxygen vacancies ratio and increasing Fe³⁺ concentration. In addition, the saturation magnetization of Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film is 1.81 emu/cm³, which is approximately three times higher than pure BiFeO₃ (M_s=0.67 emu/cm³).     

Dielectric,ferroelectric properties and photoconductivity effect of sol-gel grown SrTiO3/BaTiO3 thin film heterostructure

SrTiO₃/BaTiO₃ (ST/BT) thin-film heterostructure was deposited on Pt/Ti/SiO₂/Si(100) substrate by spin-coating. X-ray diffraction pattern shows that the heterostructure is a perovskite structure composed of ST and BT without any impurity peaks. The dielectric constant and loss of the heterostructure at 10?kHz are 704 and 0.024, respectively. The electric-field dependence of dielectric response was investigated and the tunability of the sample under 200?kV/cm applied field is 21.5%. Compared with pure ST thin film, the polarization-electric field loops of ST/BT heterostructure display high polarization and good symmetry, which could be attributed to the introduction of BT with high dielectric constant and inhibition of the potential pitfalls movement for high barrier at ST/BT interface. Moreover, the heterostructure demonstrates photoconductivity behavior with 405?nm light illumination. The results indicate that ST/BT heterostructure is a potential material in application as multifunctional photoelectric devices.     

Mechanical failure dependence on the electrical history of lead zirconate titanate thin films

Piezoelectric micromechanical systems (piezoMEMS) are often subjected to harsh mechanical and electrical loads during operation. This study evaluates the effects of the electrical history of a lead zirconate titanate (PZT) layer on the electro-mechanical response and structural limits of multilayer stacks. Electro-mechanical characterization was performed under biaxial bending employing the Ball-on-three Balls (B3B) test on virgin, poled, and DC biased (80 kV/cm) samples. No significant effect on the characteristic strength or Weibull modulus of the stack was observed. However, the crack initiation stress was highest for the virgin samples (σ₀ ～ 485 ± 30 MPa); this decreased for both poled samples (σ₀ ～ 410 ± 30 MPa), and samples measured under 80 kV/cm (σ₀ ～ 433 ± 30 MPa). in situ ε_r and loss tangent measurements suggested electromechanical loading conditions can destabilize the domain structure. Overall, the electrical history and electromechanical loading conditions can reduce the PZT film’s fracture resistance.     

Microstructure,ferroelectric and piezoelectric properties of Bi4Ti3O12 platelet incorporated 0.36BiScO3-0.64PbTiO3 thick films for high temperature piezoelectric device applications

Bi₄Ti₃O₁₂ (BiT) platelet incorporated 0.36BiScO₃-0.64PbTiO₃ (BS-PT) thick films were successfully developed for high temperature piezoelectric device applications. Their microstructure and ferroelectric and piezoelectric properties were systematically investigated to demonstrate the effect of the BiT template. It was found that the incorporation of BiT template was not responsible for textured grain growth of the BS-PT thick films; however, it could result in grain growth and densification of the BS-PT thick films by optimizing the sintering temperature and amount of BiT template. In particular, a 4 wt% BiT-incorporated BS-PT thick film sintered at 1000°C exhibited a pure perovskite structure and excellent piezoelectric properties of d₃₃ (440 pC/N) and k_p (53%), despite the presence of micro-pores caused by molten BiT. The BS-PT thick film, exhibiting good ferroelectric characteristics of P_r of 39.4 μC/cm² and E_c of 3.0 kV/mm at 1 kHz, also had an extremely high T_c of approximately 402°C. This implies that the BiT platelet incorporated BS-PT thick film is applicable for high temperature piezoelectric devices.     

Mixed grains and orientation-dependent piezoelectricity of polycrystalline Nd-substituted Bi4Ti3O12 thin films

Polycrystalline Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films were prepared on Pt/Ta/glass substrates by a pulsed laser deposition method. X-ray diffraction measurements revealed that the BNT thin films were preferentially oriented along the (117) direction although they possessed a polycrystalline structure. Good ferroelectric properties of the BNT thin film were observed with a remnant polarization of 13 μC/cm² (2 P_r ~26 μC/cm²). The fatigue resistance test exhibited that the ferroelectric polarization of the BNT thin film degraded significantly after around 10⁹ switching cycles, which can be attributed to its crystal structure. We investigated the surface morphology and ferroelectric domain structure by atomic force microscopy (AFM) and piezoresponse force microscopy (PFM), respectively. Interestingly, mixed grains consisting of long and circular shapes were observed on the BNT film surface, which corresponded to a- and c-axes orientations of crystal growth, respectively. The PFM study revealed that the piezoelectric coefficient (d₃₃) of the long grains was much larger than that of the circular grains.     

Enhanced switchable ferroelectric photovoltaic in BiFeO3 based films through chemical-strain-tuned polarization

Ferroelectric photovoltaic (FE-PV) materials have generated widespread attention due to their unique switchable photovoltaic behavior, but suffering from low photocurrent and remanent polarization. Herein, enhanced ferroelectric polarization and switchable photovoltaic in BiFeO₃ based thin films were achieved by the optimization of Bi content. The compact and uniform films with few defects were obtained by the control of chemical composition. The remanent polarization increased from 3.4 to 73.9 μC cm^?2 showing a qualitative leap. Intriguingly, the control range of photovoltaic signal between two polarization directions of the short-circuit current density (J_SC) and open circuit (V_OC) in present films exhibited an increase of 99.2% and 278.9%, respectively. It is suggested that the ferroelectric polarization was the main driving force for enhancing switchable ferroelectric photovoltaic. Therefore, the present work outstands a simple idea to enhance switchable ferroelectric photovoltaic based on the chemical engineering, providing a promising pathway for the development of photovoltaic devices.     

Effects of Mn doping on dielectric properties and energy-storage performance of Na0.5Bi0.5TiO3 thick films

Lead-free Na_0.5Bi_0.5Ti_1−xMn_xO₃ (NBTMn_x, x=0, 0.01, 0.03 and 0.05) ferroelectric thick films have been fabricated on LaNiO₃/Si(100) substrate by using a polyvinylpyrrolidone-modified sol-gel method and the effects of Mn content on their microstructure, dielectric properties and energy-storage performance were investigated. Compared with the pure Na_0.5Bi_0.5TiO₃ (NBT) thick films, NBTMn_x thick films exhibited a large enhancement in dielectric properties and energy-storage performance. Particularly, a giant recoverable energy-storage density (W) of 30.2 J/cm³ and the corresponding efficiency (η) of 47.7% were obtained in NBTMn_0.01 thick film at 2310 kV/cm. Moreover, the NBTMn_0.01 thick film displayed good energy-storage stability over a large temperature range at different frequency.     

Recent advances in photocurrent spectroscopy of passive films

The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-metal alloys gives satisfactory agreement with the theoretical expectations. The experimental results evidence the role of the alloying metal in determining the crystalline or disordered structure of passive films grown on Al,d-metal alloys.     

Structural and thermal studies of silver nanoparticles and electrical transport study of their thin films

This work reports the preparation and characterization of silver nanoparticles synthesized through wet chemical solution method and of silver films deposited by dip-coating method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDX) have been used to characterize the prepared silver nanoparticles and thin film. The morphology and crystal structure of silver nanoparticles have been determined by FESEM, HRTEM, and FETEM. The average grain size of silver nanoparticles is found to be 17.5 nm. The peaks in XRD pattern are in good agreement with that of face-centered-cubic form of metallic silver. TGA/DTA results confirmed the weight loss and the exothermic reaction due to desorption of chemisorbed water. The temperature dependence of resistivity of silver thin film, determined in the temperature range of 100-300 K, exhibit semiconducting behavior of the sample. The sample shows the activated variable range hopping in the localized states near the Fermi level.