Lead-free ferroelectric BaTiO3 doped-(Na0.5Bi0.5)TiO3 thin films processed by pulsed laser deposition technique

The difficulties in synthesizing phase pure BaTiO₃ doped-(Na_0.5Bi_0.5)TiO₃ are known. In this work, we reporting the optimized pulsed laser deposition (PLD) conditions for obtaining pure phase 0.92(Na_0.5Bi_0.5)TiO₃-0.08BaTiO₃, (BNT-BT_0.08), thin films. Dielectric, ferroelectric and piezoelectric properties of BNT-BT_0.08, thin films deposited by PLD on Pt/TiO₂/SiO₂/Si substrates are investigated in this paper. Perovskite structure of BNT-BT_0.08 thin films with random orientation of nanocrystallites has been obtained by deposition at 600 °C. The relative dielectric constant and loss tangent at 100 kHz, of BNT-BT_0.08 thin film with 530 nm thickness, were 820 and 0.13, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 22 μC/cm² and a coercive field of 120 kV/cm. The piezoresponse force microscopy (PFM) data showed that most of the grains seem to be constituted of single ferroelectric domain. The as-deposited BNT-BT_0.08 thin film is ferroelectric at the nanoscale level and piezoelectric.     

Asymmetric electrical properties in Pt/Ba0.5Sr0.5Ti0.99Co0.01O3/Nb-doped SrTiO3 capacitors

Ba_0.5Sr_0.5Ti_0.99Co_0.01O₃ (BSTC) thin films have been fabricated with pulsed laser deposition on Nb-doped SrTiO₃ (STN) substrate. In Pt/BSTC/STN capacitor, we systematically investigated the capacitance, leakage current and polarization versus bias voltage characteristics, and found that curves of capacitance versus voltage and leakage current versus voltage were not symmetric, and polarization hysteresis loop exhibited large relaxation of the remnant polarization at negatively poled state. A detailed analysis of capacitance data demonstrated a difference of the built-in voltage between top Pt/BSTC interface (V_b,t=2.5 V) and bottom BSTC/STN interface (V_b,b=1.1 V). Such different built-in voltages lead to the presence of an internal electric field, which results in asymmetric electric characteristics in Pt/BSTC/STN capacitor.     

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.     

Effect of deposition temperature on orientation and electrical properties of (K0.5Na0.5)NbO3 thin films by pulsed laser deposition

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) thin films have been prepared on Pt/TiO₂/SiO₂/Si substrates by pulsed laser deposition process. The structures, crystal orientations and electrical properties of thin films have been investigated as a function of deposition temperature from 680 °C to 760 °C. It is found that the deposition temperature plays an important role in the structures, crystal orientations and electrical properties of thin films. The crystallization of thin films improves with increasing deposition temperature. The thin film deposited at 760 °C exhibits strong (0 0 1) preferential orientation, large dielectric constant of 930 and the remnant polarization of 8.54 μC/cm².     

Electronic properties of nanoporous TiO2 films investigated in real space by means of scanning tunnelling spectroscopy

Nanoporous TiO₂ films with a thickness between 100 nm and 8 μm were studied by scanning tunnelling spectroscopy. The bias voltage of significantly increased differential conductivity, indicating the conduction and valence bands, was found to be strongly dependent on layer thickness and the underlying substrate material. This effect is traced back to the high resistivity of the oxide films and the formation of Schottky barriers at the TiO₂-substrate contact. All films showed a strong hysteresis as a function of sweep direction of the bias voltage pointing towards the existence of a high number of localized electronic trap states. This effect is getting even more pronounced upon sample ageing. Laterally resolved measurements show that the major part of the surface exhibits similar I(V) characteristics with minor deviations, while smaller areas with significantly different response are identified. These areas are comparable in size to the individual crystals the material is composed of.     

Growth and structure of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 thin films prepared by pulsed laser deposition technique

Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ (BNKLT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition (PLD) technique. The films prepared were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of the processing parameters, such as oxygen pressure, substrate temperature and laser power, on the crystal structure, surface morphology, roughness and deposition rates of the thin films were investigated. It was found that the substrate temperature of 600 °C and oxygen pressure of 30 Pa are the optimized technical parameters for the growth of textured film, and all the thin films prepared have granular structure, homogeneous grain size and smooth surfaces.     

C-V studies on metal-ferroelectric bismuth vanadate (Bi2VO5.5)-semiconductor structure

Ferroelectric bismuth vanadate Bi₂VO_5.5 (BVO) thin films have been successfully grown on p-type Si(100) substrate by using chemical solution decomposition (CSD) technique followed by rapid thermal annealing (RTA). The crystalline nature of the films has been studied by X-ray diffraction (XRD). Atomic force microscopy (AFM) was used to study the microstructure of the films. The dielectric properties of the films were studied. The capacitance-voltage characteristics have been studied in metal-ferroelectric-insulator-semiconductor (MFIS) configuration. The dielectric constant of BVO thin films formed on Si(100) is about 146 measured at a frequency of 100 kHz at room temperature. The capacitance-voltage plot of a Bi₂VO_5.5 MFIS capacitor subjected to a dc polarizing voltages shows a memory window of 1.42 V during a sweep of ±5 V gate bias. The flatband voltage (V_f) shifts towards the positive direction rather than negative direction. This leads to the asymmetric behavior of the C-V curve and decrease in memory window. The oxide trap density at a ramp rate of 0.2 V/s was estimated to be as high as 1.45×10¹² cm⁻².     

Piezoelectric non-linearity in PbSc0.5Ta0.5O3 thin films

Epitaxial (001)-oriented PbSc_0.5Ta_0.5O₃ (PST) thin films were deposited by pulsed laser deposition. Local piezoelectric investigations performed by piezoelectric force microscopy show a dual slope for the piezoelectric coefficient. A piezoelectric coefficient of 3 pm/V was observed at voltages up to 0.8 V. However, at voltages above 0.8 V, there is a steep increase in piezoelectric coefficient mounting to 23.2 pm/V. This nonlinear piezoelectric response was observed to be irreversible in nature. In order to better understand this nonlinear behavior, voltage dependent dielectric constant measurements were performed. These confirmed that the piezoelectric non-linearity is indeed a manifestation of a dielectric non-linearity. In contrast to classical ferroelectric systems, the observed dielectric non-linearity in this relaxor material cannot be explained by the Rayleigh model. Thus the dielectric non-linearity in the PST films is tentatively explained as a manifestation of a percolation of the polar nano regions.     

Dielectric and leakage current properties of Li doped (Ba,Sr)TiO3 thick film interdigital capacitors on the alumina substrates

Li doped (Ba,Sr)TiO₃ thick films were fabricated by employing the screen printing method on the alumina (Al₂O₃) substrates. Interdigital capacitor patterns with seven fingers of 200 μm gap, 250 μm length were designed and screen printed on the alumina substrates. Ba_0.5Sr_0.5TiO₃ materials, paraelectric state at the room temperature, have been chosen for the microwave devices due to high dielectric permittivity and low loss tangent, however, the sintering temperature of (Ba,Sr)TiO₃ is over 1350 °C. In order to lower the sintering temperature, Li (3 wt%) was added to the (Ba,Sr)TiO₃ materials. Li doped (Ba,Sr)TiO₃ thick films screen printed on the alumina (Al₂O₃) substrates were sintered at 900 °C for 1.5 h. The structural feature was analyzed with X-ray diffraction method. Temperature dependent dielectric properties were characterized from 303 to 403 K at 1 MHz. Within the ±100 V of bias voltage, current-voltage characteristics of Li doped (Ba,Sr)TiO₃ films were investigated from 303 to 403 K. Through the current-voltage characteristics, the resistivity of Li doped (Ba,Sr)TiO₃ films were calculated.In this paper, the significant negative temperature coefficient of resistance (NTCR) of Li doped (Ba,Sr)TiO₃ films will be presented through the activation energy fitting. Measured activation energy is approximately 0.366 eV.     

Multiferroic properties of Bi0.8La0.2FeO3/CoFe2O4 multilayer thin films

Bi_0.8La_0.2FeO₃/CoFe₂O₄ (BLFO/CFO) multilayer thin films (totally 20 layers BLFO and 19 layers CFO) were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. X-ray diffraction and transmission electron microscope measurements show that the films are polycrystalline and consisted of multilayered structure. Ferroelectric hysteresis loops with remnant polarization and saturated polarization of 4.2 and 13.3 μC/cm², respectively, were observed. On the other hand, the films show well-shaped magnetization hysteresis loops with saturated and remnant magnetization of 34.7 and 11.4 emu/cm³, respectively, which are significantly larger than pure BLFO thin films deposited under the same conditions. These results indicate that constructing epitaxial superlattice might be a promising way to fabricate multiferroics with improved properties.     

Improvement of electrical properties of Sr-modified (Pb,Ca)TiO3 thin films grown by soft chemistry route

Strontium and calcium-modified lead titanate (Pb_0.70Ca_0.15Sr_0.15)TiO₃ soft chemistry-derived thin films were prepared on platinum-coated silicon substrate by spin-coating method. Investigations were made on the structure, surface morphology and electrical properties of the film. The results by XRD and FE-SEM showed that the film exhibits a pure tetragonal perovskite phase and an average grain size of about 50-60 nm, respectively. Electrical measurements of a metal-ferroelectric-metal type capacitor exhibited a stable and switchable electrical polarization in the film. The structure of the Au/PCST/Pt capacitor showed well-saturated hysteresis loops at an applied voltage of 300 kV/cm with remanent polarization and coercive field values of 22 μC/cm² and 100 kV/cm, respectively. At 100 kHz, the dielectric constant and the dielectric loss of the (Pb_0.70Ca_0.15Sr_0.15)TiO₃ thin film with thickness 240 nm were 528 and 0.05, respectively.     

Large piezoresponse of lead-free Bi0.5(Na0.85K0.15)0.5TiO3 thin film

0.85Bi_0.5Na_0.5TiO₃-0.15Bi_0.5K_0.5TiO₃ (BNKT15) lead-free thin films were prepared on Pt(111)/TiO₂/SiO₂/Si(100) substrates by the chemical solution deposition method. BNKT15 are MPB composition in the Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃ (BNT-BKT) system. The maximum piezoelectric coefficient (d_33,f) value of BNKT15 thin film is approximately 75 pm/V, which is comparable to that of polycrystalline PZT thin films. These results suggest that BNKT15 thin film can be used as an alternative for PZT films in piezoelectric micro-electromechanical systems.     

Enhanced dielectric and ferroelectric properties of Pb1−3x/2Lax(Zr0.5Ti0.5)O3 thin films with low lanthanum substitution

Effects of lanthanum (La) substitution (0.003 ≤ x ≤ 0.015) on the dielectric and ferroelectric properties of Pb(Zr_0.5Ti_0.5)O₃ thin films have been investigated. The films were synthesized on the Pt (1 1 1)/Ti/SiO₂/Si (1 0 0) substrates by a sol-gel method. Large dielectric constants of the films are obtained within range of 800-1600 which are almost comparable to those observed in bulk ceramics. The films also show improved remnant polarization values and reduced coercive field values with the increasing addition of La substitution. Our results suggest that low La substitution contributes to enhance film electric properties due to the improvement of non-180° domain wall mobility as well as the stabilization of tetragonal phase.     

Third-order optical nonlinearities of lead-free (Na1−xKx)0.5Bi0.5TiO3 thin films

(Na_1−xK_x)_0.5Bi_0.5TiO₃ (NKBT) (x = 0.1, 0.2, and 0.3) thin films with good surface morphology and rhombohedral perovskite structure were fabricated on quartz substrates by a sol-gel process. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties were investigated by Z-scan technique performed at 532 nm with a picosecond laser. A two-photon absorption effect closely related with potassium-doping content was found in thin films, and the nonlinear refractive index n₂ increases evidently with potassium-doping. The real part of the third-order nonlinear susceptibility χ⁽³⁾ is much larger than its imaginary part, indicating that the third-order optical nonlinear response of the NKBT films is dominated by the optical nonlinear refractive behavior. These results show that NKBT thin films have potential applications in nonlinear optics.     

Spin polarization measurements of Co2Mn (Ga0.5Sn0.5) thin films

Co₂MnGa_0.5Sn_0.5 (CMGS) thin films were epitaxially grown on MgO (0 0 1) substrates by magnetron sputtering and the current spin polarizations of the films with different post annealing conditions were measured by the point contact Andreev reflection method. The film deposited at a substrate temperature of 150 °C had a B2 structure and its spin polarization was estimated to be 59%. The film was ordered to the L2₁ structure by annealing at 600 °C, and the spin polarization was enhanced to 66%. The spin polarization and the intensity of the L2₁ diffraction showed clear correlation, suggesting L2₁ ordering is essential to achieve higher spin polarization of this quaternary Heusler alloy.     

The effect of potential of impedance in the pulsed-laser-deposited SrBi2Ta2O9 thin film

Ferroelectrics SrBi₂Ta₂O₉ (SBTO) thin films were grown on a highly oriented Pt/Ti/SiO₂/Si substrates using the pulsed laser ablation. The ac impedance of SBTO thin films have been measured at room temperature both in the frequency range from 10⁻¹ to 10⁶ Hz and bias voltage range from −6 to 6 V. The ac impedance dispersion was observed at low frequency with increasing bias voltage, which was interpreted based on a blocked charge. We can explain that the blocking interface gives rise to constant phase element (CPE) response, and we give an impedance model function that can fit data along the low frequency range when such a CPE is found. The low frequency dispersion phenomena of SBTO thin film are related to a charge diffusion process at the surface of thin film.     

Dependence of excess bismuth content in precursor sols on ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films fabricated by chemical solution deposition

Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were fabricated on Pt/Ti/SiO₂/Si(1 0 0) substrates by chemical solution deposition (CSD), and the dependence of ferroelectric and dielectric properties of the as-deposited BLT thin films on excess Bi content in precursor sols was studied. It is found that the prepared BLT thin film shows the best polarization-electric field, capacitance-voltage and dielectric constant (?_r)-frequency characteristics, when the value of excess Bi content in precursor sols is 10%. In detail, its remnant polarization (2P_r) value is 40 μC/cm², the capacitance tunability is 21% measured at room temperature under conditions of an applied voltage of 8 V and measurement frequency of 10 kHz, and the ?_r is 696 at 100 kHz frequency.     

Spray-pyrolysis deposition of LaMnO3 and La1−xCaxMnO3 thin films

Ethylene glycol solutions of La-Mn(II) and La-Ca-Mn(II) citric complexes has been used as a starting material for spray-pyrolysis deposition of LaMnO₃ and La_1−xCa_xMnO₃ thin films on β-quartz, fused quartz, Si(0 0 1) and SrTiO₃(1 0 0) substrates heated during the deposition at 380 °C. At suitable post-deposition heating conditions highly uniform films, 0.1-1 μm in thickness, with good crystal structure were obtained. Highly textured LaMnO₃ films are obtained on SrTiO₃(1 0 0) substrate. Interaction between the layer and Si-containing substrates is observed during the post-deposition heating in static air.     

The effect of Si surface nitridation on the interfacial structure and electrical properties of (La2O3)0.5(SiO2)0.5 high-k gate dielectric films

Thermal stability, interfacial structures and electrical properties of amorphous (La₂O₃)_0.5(SiO₂)_0.5 (LSO) films deposited by using pulsed laser deposition (PLD) on Si (1 0 0) and NH₃ nitrided Si (1 0 0) substrates were comparatively investigated. The LSO films keep the amorphous state up to a high annealing temperature of 900 °C. HRTEM observations and XPS analyses showed that the surface nitridation of silicon wafer using NH₃ can result in the formation of the passivation layer, which effectively suppresses the excessive growth of the interfacial layer between LSO film and silicon wafer after high-temperature annealing process. The Pt/LSO/nitrided Si capacitors annealed at high temperature exhibit smaller CET and EOT, a less flatband voltage shift, a negligible hysteresis loop, a smaller equivalent dielectric charge density, and a much lower gate leakage current density as compared with that of the Pt/LSO/Si capacitors without Si surface nitridation.     

Effect of different annealing methods on ferroelectric properties of 0.95Pb(Sc0.5Ta0.5)O3-0.05PbTiO3 thin films

0.95Pb(Sc_0.5Ta_0.5)O₃-0.05PbTiO₃ thin films were prepared on LaNiO₃/SiO₂/Si substrate by radio frequency magnetron sputtering, and the films were annealed subsequently with repeated many times by two approaches: normal one-step rapid thermal annealing and innovative two-steps rapid thermal annealing. X-ray diffraction demonstrates that all the films were preferred (1 0 0) oriented and an appropriate repeat of annealing process can enhance perovskite phase of the films. Scanning electron microscopy suggests that the films treated by two-steps rapid thermal annealing show crack-free, uniform size grains and dense microstructure. Measurement of remnant polarization and leakage current dependence of electric field confirms that the films treated by two-steps rapid thermal annealing exhibit better ferroelectric properties than the films treated by one-steps rapid thermal annealing. The results reveal that microstructure plays an important role in enhanced ferroelectric properties of the 0.95Pb(Sc_0.5Ta_0.5)O₃-0.05PbTiO₃ thin films.