Ferroelectric thin films prepared by sol-gel processing

Ferroelectric thin films are playing a growing role as key elements in variety of devices. For various techniques of ferroelectric thin film preparation, the sol-gel processing is one of the most promising. In comparison with different deposition techniques, we survey the intrinsic advantages and the recent improvements of the sol-gel processing. In this review paper, several interesting topics, including epitaxial growth and grain-orientation, symmetric and asymmetric P-E hysteresis loops, heterojunction effect of the interface between film and substrate, to attest pyroelectricity of the films, electrooptic coefficients in poled and unpoled films, possibility of amorphous ferroelectrics, etc., are introduced.     

Electrical properties of TiO2 thin films prepared by the sol-gel method

A TiO₂ thin film was prepared by the sol-gel method using a metal alkoxide [Ti(O-i-Pr)₄ tetrasopropoxy titanium: TIPT]. The crystalline form of the film depends on the preparation conditions and the heat treatment temperature. When the ratio (γ) of H₂O to TIPT is 7, the crystalline phase is formed at 400°C; when γ = 2 it is formed at 500°C. The anatase crystal structure was obtained for both γ = 2 and γ = 7. The electrical conductivity of the TiO₂ film increased with the heat treatment temperature.     

Structural and ferroelectric properties of textured KNN thick films prepared by sol-gel methods

Based on the templates of sheet-like KNN powders synthesized by a hydrothermal method, lead-free textured KNN films with a thickness of 5 μm were prepared by sol-gel methods. The composition, microstructure, crystal structure, ferroelectric properties and grain growth mechanisms of the textured KNN thick films were studied. It was found that the KNN thick films possessed large grain sizes, dense morphology and increased ratio of Na⁺. After annealed at 650 °C, the perovskite structure was observed in the KNN thick films textured along the <00l> crystallography direction with an orientation factor of 0.393, reflecting a preferential growth characteristics. Furthermore, the residual polarization value of 12.3 µc/cm² was achieved under the same condition.     

Dielectric properties for Tantalum-doped strontium barium titanate thin films prepared by sol-gel method

In this work, Sr_0.5Ba_0.5Ti_1-zTa_zO₃ (SBT) thin films were prepared on a Pt/SiO₂/Si substrate by sol-gel process. The microstructures of SBT thin films were examined by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The influences of Ta on the microstructure and dielectric properties of SBT thin films were studied. It is found that tetragonal perovskite crystal grains existed in SBT thin films. Ta⁵⁺ doping fines the grain of SBT thin films. It is found that Ta⁵⁺ doping decreases dielectric loss for SBT thin films.     

Piezoelectricity in cellular electret films

Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high piezoelectric constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene (pp). In particular, measurements of the direct and inverse piezoelectric constants in the frequency range 0 to 10 kHz and of the variation of these constants across the surface of the films are discussed. These measurements, performed by direct application of stress or by the use of a profilometer, an accelerometer and an interferometer yield a frequency-independent piezoelectric d₃₃ constant of ≲ 220 pC/N. Assuming reasonable charge distributions and charge densities, the calculated piezoelectric constants are in good agreement with the measured values. The theoretical model shows the reciprocity of the piezoelectric constants     

溶胶-凝胶自蔓延燃烧法制备的Mn-Zn铁氧体粉体

以硝酸锰、硝酸锌、硝酸铁为原料,以柠檬酸为凝胶剂,采用溶胶-凝胶自蔓延燃烧法合成了Mn-Zn铁氧体粉体.借助XRD、SEM和FTIR等方法对所制备产物的磁性能和结构进行分析和表征,研究了溶胶pH值和退火温度对产物的影响.确定出溶胶的pH值为7、退火温度500℃时为最适宜的合成条件,从而制备出粒径尺寸均一、单相、形貌较好的锰锌铁氧体粉体.     

Formation and properties of lead Titanate thick films prepared by nanopowder-metal-organic decomposition process

Abstract

Crystallization behavior and ferroelectric properties of the PbTiO₃, PT, thin films prepared by spin-coating of nano-powder incorporated carboxylates were systematically examined. Comparing to the direct synthesis of PT films by metal-organic decomposition process, the incorporation of nano-sized PT powders into the carboxylates markedly modifies the crystallization kinetics of the PT films. Pure perovskite phase can be obtained by post-annealing these films at 600°C for 1 h and the remanent polarization for the films can be optimized only by post-annealing at high enough temperature (650°C). However, the nano-powder incorporated PT films possess pronouncedly lower remanent polarization (P_r) and smaller coercive field (E_c) than the non-powdered PT films. Such a phenomenon is ascribed to the formation of multi-grain structure in each column of the films.     

Influence of platinum-based electrodes on the microstructure of sol-gel and MOD prepared lead zirconate titanate films

Abstract

The microstructure and preferred orientation of PZT films deposited by spin-coating techniques are shown to depend on the morphology of the Ti/Pt bottom electrode. Annealing of the as-deposited Ti/Pt electrodes results in the formation of hillocks. These hillocks serve as nucleation sites for perovskite formation and thus determine the microstructure and preferred orientation of the crystallized PZT films. The microstructure features are investigated by X-ray diffraction analyses and scanning and transmission electron microscopies.     

Characterization of Ba(Zr0.05 Ti0.95)O3 thin film prepared by sol-gel process

Ba(Zr_0.05Ti_0.95)O₃ (BZT) thin film (∼330 nm) was grown on Pt/Ti/SiO₂/Si(100) substrate by a simple sol-gel process. The microstructure and the surface morphology of BZT thin film were studied by X-ray diffraction and atomic force microscopy. The optical properties of BZT thin film were obtained by spectroscopic ellipsometry. The optical bandgap was found to be 3.74 eV of direct-transition type. Ferroelectric and dielectric properties of BZT thin film were also discussed. The electrical measurements were conducted on BZT films in metal-ferroelectric-metal (MFM) capacitor configuration. The results showed the film exhibited good ferroelectrity with remanent polarization and coercive electric field of 3.54 μC/cm² and 95.5 kV/cm, respectively. At 10 kHz, the dielectric constant and dielectric loss of the film are 201 and 0.029, respectively.     

LiFePO薄膜的制备及其性质研究

以醋酸锂、硝酸铁、磷酸二氢铵为原料,采用溶胶-凝胶法制备锂离子薄膜电池正极材料LiFePO薄膜。用X射线衍射、红外光谱、扫描电镜等分析了薄膜的物相和表面形貌。用循环伏安法扫描、恒电流充放电和交流电阻抗技术研究LiFePO薄膜的电化学性能。结果表明该法于700℃制备的LiFePO4薄膜比容量为145mAh·g-1,经20次循环后的每次循环容量损失为0.07%,薄膜具有良好的循环性能。交流阻抗表明锂离子在薄膜中的扩散系数为4.2×10-14cm·2s-1,说明锂离子在薄膜中具有较快的扩散速度。     

尖晶石型LiMn2O4的电化学性质

利用循环伏安法(CV)、恒流充放电、电化学阻抗谱(EIS)等电化学手段研究了溶胶凝胶法制备的尖晶石型LiMn2O4的电化学性质.结果表明用溶胶凝胶法制备的样品放电容量大,可逆性好,其中550℃煅烧的样品电化学性能最好;XRD结果表明,样品经过充放电,尖晶石结构仍保留,但出现深度放电产物Li2Mn2O4.     

Photosensors based on Ni-doped ZnO/p-Si junction prepared by sol-gel method

A high quality and adherent transparent layer of ZnO thin films were grown on p-type Si substrates to fabricate undoped and Ni-doped ZnO/p-Si heterojunction. The C _Adj ?V and G _Adj ?V plots of the diodes are significantly affected with the frequency and Ni-dopant contents. The interface state density (D _it ) is changed with the frequency and Ni contents. The high value of ideality factor (n) is due to the interface states at the interface of the diode and the effect of barrier inhomogeneities or to the high value of R _s . The transient photocurrent measurements were used to analyze the photoresponse of the diodes. The obtained results indicate that the diodes could be used as an optical sensor especially for the Ni-doped at 0.5 % ZnO/p-Si diode.     

化学水浴法制备ZnS薄膜

采用了化学水浴法制备异质结薄膜太阳电池中的窗口层ZnS薄膜.尝试了酸性溶液制备ZnS薄膜,讨论了溶液成分、水浴时间和温度对薄膜成分和形貌的影响.研究结果表明在75℃,6 h条件下沉积得到的薄膜最平整,成分最符合化学计量比,退火后薄膜主要为纤锌矿结构的ZnS相.     

Ferroelectric properties of Al and Nb doped PbTiO3 thin films prepared by chemical solution deposition process

Abstract

Thin films of pure and acceptor (Al) as well as donor (Nb) doped PbTiO₃ were prepared using a chemical solution deposition method. The nominal compositions of the solutions were equivalent to those of PbTiO₃, Pb(Ti_0.95Al_0.95)O₃ and Pb(Ti_0.95Nb_0.05)O₃, respectively. In the case of PbTiO₃, the remanent polarization amounted to 43 μC/cm², but the shape of the hysteresis curve was round. On the other hand, Al and Nb doped PbTiO₃ thin films exhibited well-saturated P-Ehysteresis curves. The shape of these curves was slim compared to that of PbTiO₃. The remanent polarization of the Pb(Ti_0.95Al_0.05)O₃ and Pb(Ti_0.95Al_0.05)O₃ thin films amounted to 18 and 15 μC/cm², respectively, and the coercive field decreased in comparison with that of PbTiO₃. The leakage currents of the PbTiO₃, Pb(Ti_0.95Al_0.05)O₃ and Pb(Ti_0.95Al_0.05)O₃ thin films were 7.68x10^?3, 3.21x10^?6 and 6.58x10^?4 A/cm², respectively, at+10V.     

The influence of elevated humidity on the stability of the electret state in polyimide films

The influence of environmental humidity on the stability of spatial charge storage in polyimide films has been studied. The decay of the electret potential difference under isothermal conditions and the spectra of thermally stimulated depolarization discharge (TSD) currents have been measured. The experimental results obtained show that as the environmental humidity grows, the stored spatial charge stability drops significantly. It has been shown that the spectra TSD currents in films with increased moisture absorption are shifted towards the lower temperature region. These changes are related to growth in the film’s electric conductivity     

溶胶凝胶法制备高性能LiMn2O4薄膜

用溶胶凝胶法在Pt片上采用慢速旋转涂布工艺制备LiMn2O4薄膜.该膜表面呈非致密态,厚度在200 nm左右,晶体颗粒大小约为100nm.实验表明:该膜具有较大的放电容量,电流密度在0.01 mA/cm2时,容量可达到75μAh/cm2·μm;可承受1 mA/cm2的电流密度放电,容量仍有49μAh/cm2·μm.该膜循环性能良好,400次循环后容量没有衰减.     

Effect of preheating process on crystallization and optical properties of sol-gel derived ZnO semiconductor thin films

Highly transparent ZnO semiconductor thin films were deposited onto alkali-free glass substrates by a sol-gel spin coating process. This research investigated the effects of different preheating rates (4 or 10°C/min) on the various surface morphologies, crystallization characteristics, and optical properties of these thin films. The ZnO sol was synthesized by dissolving the zinc acetate dihydrate in isopropanol (IPA), and then adding monoethanolamine (MEA). These as-coated films were preheated at 300°C for 10 min and annealed in an air ambiance at 500°C for 1 h. Experimental results revealed that the as-prepared films had a hexagonal wurtzite structure, and that the heating rates of the preheating process obviously affected the surface morphologies, crystallization qualities, and transparency levels of the thin films. A ZnO thin film preheated at 10°C/min exhibited preferential orientation along the (002) plane, a flat surface, and also achieved a high transmittance value, 92.5%, for light with a wavelength of 550 nm.     

Dielectric behavior of hexagonal and orthorhombic YFeO3 prepared by modified sol-gel method

Polycrystalline YFeO₃ ceramic powders were synthesized by modified polyacrylamide sol-gel method. X-ray diffraction analysis demonstrated that the crystal structures of YFeO₃ could be controlled from hexagonal phase (h-YFeO₃) to orthorhombic phase (o-YFeO₃) through the calcined temperature. Temperature-dependent dielectric constants (ε _r) and losses (tanδ) show that strong frequency dispersion phenomena occur in whole measuring temperature. Both the h-YFeO₃ and o-YFeO₃ exhibit dielectric relaxation behavior at T?<?250 K and at T?>?340 K, which could be attributed to thermal activation. Whereas, only h-YFeO₃ shows an anomaly dielectric peak at T _m ?=?297 K fitted with Curie-Weiss law and modified Curie-Weiss law, indicating a possible P6 ₃ mc-P6 ₃ cm transition with slightly diffuse characteristics.     

溶胶-凝胶法制备的Li_(1.15)V_3O_8的电化学性能

以Li2CO3和NH4VO3为原料,分别采用柠檬酸和酒石酸为螯合剂,通过溶胶-凝胶法制备了正极材料Li1.15V3O8。用红外光谱、TG-DTG、XRD和SEM等方法对前驱体和产物进行分析,并进行了电化学性能测试。用酒石酸制备的Li1.15V3O8,首次放电比容量比用柠檬酸制备的高,且循环性能较好,以0.14 mA在1.8～3.9 V充放电的首次放电比容量为190.17 mAh/g,在第32次和第47次循环时达到210.99 mAh/g,循环过程中容量折线上升。     

Electrical and optical properties of PZT ferroelectric films fabricated by the PVP-assisted sol-gel method

Lead zirconate titanate (PZT) films were deposited on platinized silicon substrates by spin coating using organic macromolecule polyvinylpyrrolidone (PVP) as an additive, which has the hybrid effects, was dispersed uniformly into the sol-gel precursor solution of PZT. The films were coated by spin-coating and then annealed at a proper temperature by repaid thermal processing. PZT ferroelectric films were polycrystalline and perovskite phase structure. The Coercive Field and remnant polarization of the PZT films annealed at 600 °C were 91 kV/cm and 19 μC/cm² respectively. The fatigue resistance properties were improved and the polarization value decreased only 8% of the initial polarization at 10⁸ cycles. The optical properties were measured by spectrophotometer and the band energy was calculated about 3.59eV.