氧压对Ba0.6Sr0.4TiO3薄膜晶格常数的影响及BaTiO3/Ba0.6Sr0.4TiO3超晶格的制备

采用脉冲激光沉积(PLD)技术,经一系列的优化实验成功地制备了BaTiO₃(BT)和Ba_0.6Sr_0.4TiO₃(BST)单层膜.X射线衍射分析表明,在LaAlO₃(001)单晶平衬底上生长的BT和BST薄膜都是沿[001]取向的近外延生长.且随着氧压在10^-3—25 Pa范围内逐渐增大,BST薄膜的晶格常数与氧压之间近似满足Boltzmann函数关系.其次,在此优化条件下还 关键词： 超晶格 晶格常数 激光感生热电电压 脉冲激光沉积     

Nonlinear optical absorption and refraction of epitaxial Ba0.6Sr0.4TiO3 thin films on (001) MgO substrates

Highly epitaxial Ba_0.6Sr_0.4TiO₃ (BST) ferroelectric thin films were fabricated on (001) MgOsubstrates by pulsed laser deposition. The nonlinear optical absorption coefficients (β) and refraction indices (γ) of the BST thin films on (001) MgO substrates were investigated using the single beam Z-scan technique with femtosecond laser pulses at the wavelengths of 790 nm and 395 nm, respectively, at room temperature. The nonlinear absorption coefficients of BST thin films were measured to be ∼0.087 cm/GW and ∼0.77 cm/GW at 790 nm and 395 nm, respectively. The nonlinear refraction indices of BST thin films exhibit a strong dispersion from a positive value of 6.1×10^-5 cm²/GW at 790 nm to a negative value of -4.0×10^-5 cm²/GW at 395 nm near band gap. The dispersion of γ is roughly consistent with Sheik-Bahae’s theory for the bound electronic nonlinear refraction resulting from the two-photon resonance. These results show that the BST film is a promising material as a candidate for nonlinear optical applications. PACS 42.70.Mp; 78.20.-e; 81.05.-t     

Low-Voltage Depletion-Mode Indium-Tin-Oxide Thin-Film Transistors Gated by Ba0.4Sr0.6TiO3 Dielectric

We report on the fabrication and characterization of low-voltage indium-tin-oxide （ITO） thin-film transistors （TFTs） gated by a0.4Sr0.6TiO3 （BST） gate dielectric deposited at room temperature. The 400-nm-thick BST film shows a low leakage current density of 6 × 10^-8 A/cm^2 and a high specific capacitance of 83 nF/cm^2 （corresponding εr= 37）. The ITO TFTs gated by such BST dielectric operate in a depletion mode with an operation voltage of 5.0 V. The device exhibits a threshold voltage of -3.7 V, a subthreshold swing of 0.5 V/decade, a field effect mobility of 3.2cm^2/Vs and a current on/off ratio of 1.4× 10^4.     

Ba1-xSrxTiO3及Ba0.6-xPbxSr0.4TiO3陶瓷的制备和介温特性研究

用固相反应法制备了Ba_1-xSr_xTiO₃(BST)及Ba_0.6-xPb_xSr_0.4TiO₃(BPST)陶瓷,通过XRD,FESEM和拉曼谱分析了Pb掺杂对Ba_0.6Sr_0.4TiO₃样品的晶格、相变及表面形貌的影响.测试了BST及B 关键词： BST BPST 弥散相 介温特性     

Mechanisms of asymmetric leakage current in Pt/Ba0.6Sr0.4TiO3/Nb-SrTiO3 capacitor

Mechanisms of leakage current have been investigated in the capacitor consisting of a Ba_0.6Sr_0.4TiO₃ thin film, a Pt top electrode, and a Nb-doped SrTiO₃ (STON) bottom electrode. The leakage current shows asymmetric behavior for different bias voltage. For the Pt electrode negatively biased, the leakage current can be explained by modified Schottky emission mechanism, and the barrier height is obtained as 0.44 eV. For the Pt electrode positively biased, the leakage current shows a space-charge-limited current behavior. The trap in dielectric film is regarded as deep traps, and the density of trapped carrier is estimated as about 3.2×10²³/m³. PACS 77     

Optical properties of thin epitaxial Ba0.8Sr0.2TiO3 films

The properties of nanodimensional (Ba_0.8, Sr_0.2)TiO₃ films on single-crystal magnesia substrates are studied. The films are applied by rf sputtering and grow in the layer-by-layer mode. The lattice parameters are measured by the X-ray diffraction method. The transmission of the films with different thicknesses is studied in the wavelength range 190?C1100 nm. When analyzing experimental optical parameters, additional relaxation parameters depicting a final lifetime of the oscillator are used to characterize the refractive index and absorption factor in the dispersion relation. Such an approach allows a more accurate approximation of experimental data.     

Electrode reaction and microstructure of La0.6Sr0.4CoO3−δ thin films

Dense La_0.6Sr_0.4CoO_3−δ film electrodes were deposited by pulsed laser deposition (PLD) on Ce_0.9Gd_0.1O_1.95 electrolytes. The grain size of one film was 300–500 nm, and the other was 30–50 nm. DC polarization and AC impedance measurements were performed at 873 K–1073 K in O₂–Ar gas mixtures. From investigations of the electrochemical capacitances, the rate determining process for both electrodes were confirmed to be the surface reaction. The analyses in the electrochemical resistance revealed that the oxygen adsorption/desorption rate was faster on the electrode with smaller grain size. DC responses agreed with AC results, so the current density on the nano-grain electrode was larger by half an order than those of the sub-micron-grain electrode. Under a dilute oxygen atmosphere, the rate determining step transferred from a surface reaction to a gas phase diffusion.     

PbZr0.4Ti0.6O3 and Ba0.9Sr0.1TiO3 reflectors derived from chemical solutions containing polymers

Ba(0.9)Sr(0.1)TiO(3) (BST)-based and PbZr(0.4)Ti(0.6)O(3)-based quasi-periodic multilayers consisting of dense and porous ferroelectric layers have been fabricated by solgel technique using chemical solutions containing polyethylene glycol (PEG) or polyvinylpyrrolidone k30 (PVP). All multilayers exhibit good performance as dielectric mirrors. For each multilayer, the maximum peak reflectivity is over 90% and the photonic stopband width is no less than 30 nm at room temperature. The reflection-band position can be easily tuned by varying the thickness of the bilayer. With the same processing conditions and number of periods, the Bragg reflection performance is almost the same for quasi-periodic PZT multilayers derived from two precursors containing different polymers. The BST multilayers deposited by using a PVP-containing precursor are superior in optical properties, including peak reflectivities and stop-band width, to those deposited by using the PEG-containing solution.     

Ferroelectric switching behavior of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films

The effect of pulse amplitude on the ferroelectric and switching properties of pulsed laser deposited Ba_0.8Sr_0.2TiO₃ thin films has been studied. The structural and morphological analysis revealed that the films had a well crystallized perovskite phase and grain size of about 30–40 nm. A well saturated P–E hysteresis loop was observed with a remnant polarization (P_r) ≈ 4.8 μC/cm² and a coercive field ≈ 100 kV/cm at a frequency of 1 kHz. The P_r has been found to be decreased only 4.3% after passing 8.0 × 10⁸ cycles. The analysis of switching response with nucleation limited switching model reveals that characteristic switching time (t₀) variance is due to the random distribution of the local electric fields. The peak value of polarization current and t₀ exhibits exponential dependence on reciprocal of pulse amplitude.     

Investigation of the low-and infralow-frequency dielectric response of Ba0.7Sr0.3TiO3 thin films

The low-and infralow-frequency dielectric properties of Ba_0.7Sr_0.3TiO₃ thin films annealed at temperatures of 750 and 900°C are investigated over wide ranges of temperatures (from ?180 to +100°C), frequencies (from 0.1 Hz to 10.0 kHz), and amplitudes of the measuring electric field (from 15 to 255 kV/cm). The samples are found to undergo giant relaxation characteristic of layered heterogeneous structures. It is noted that, at a higher annealing temperature (900°C), the relaxation region is shifted toward lower temperatures (higher frequencies).     

The dependence of dielectric properties on the thickness of (Ba,Sr)TiO3 thin films

In the dielectric (Ba,Sr)TiO₃ thin films, the correlation between the film thickness and the dielectric properties was investigated. The dielectric properties such as the dielectric constant (ε) and dielectric loss (tan δ) were measured using the capacitor geometry. As the film thickness increased, the dielectric constant also increased due to the reduction of the interfacial dead-layer effect. However, the dielectric loss did not show a monotonous variation with the increasing film thickness. It was found that the dielectric loss correlated well with the non-uniform distribution of local strain, as analyzed by X-ray diffraction, according to the Curie–von Schweidler relaxation law.     

Impedance spectroscopy and conduction mechanism of multiferroic (Bi0.6K0.4)(Fe0.6Nb0.4)O3

Polycrystalline (Bi_0.6K_0.4) (Fe_0.6Nb_0.4)O₃ material has been prepared using a mixed-oxide route at 950 °C. It was shown by XRD that at room temperature structure of the compound is of single-phase with hexagonal symmetry. Some electrical characteristics (impedance, modulus, conductivity etc.) were studied over a wide frequency (1 kHz–1 MHz) and temperature (25–500 °C) ranges. The Nyquist plot (i.e., imaginary vs real component of complex impedance) of the material exhibit the existence and magnitude of grain interior and grain boundary contributions in the complex electrical parameters of the material depending on frequency, input energy and temperature. The nature of frequency dependence of ac conductivity follows Joncher׳s power law, and dc conductivity follows the Arrhenius behavior. The appearance of P–E hysteresis loop confirms the ferroelectric properties of the material with remnant polarization (2P_r) of 1.027 µC/cm² and coercive field (2E_c) of 16.633 kV/cm. The material shows very weak ferromagnetism at room temperature with remnant magnetization (2M_r) of 0.035 emu/gm and coercive field (2H_c) of 0.211 kOe.     

非晶钛酸锶钡薄膜的金属有机分解法制备及其光学性能

本文采用2-辛酸钡(Ba(C₈H₁₅O₂)₂)和3-甲基丁基醋酸盐(CH₃COOC₂H₄CH(CH₃)₂-)为基的特殊前驱体溶液,在硅和石英基片上低温制备Ba_0.7Sr_0.3TiO₃ (BST0.7)薄膜.性能测试结果表明,厚度约为214 nm的非晶BST0.7薄膜的光学带隙能和折射率分别为4.27 eV和n=1.94.薄膜在可见光和近红外区域的消光系数远远低于多晶BST薄膜,约为10^-3数量级.激发波长为450 nm时,在室温环境下非晶BST0.7薄膜在波长520—610 nm处发出强烈的可见光,峰值为540—570 nm,而结晶态的BST0.7薄膜则无发光现象. 关键词： 钛酸锶钡 非晶薄膜 金属有机分解法 光学性能     

Dielectric response of (Ba,Sr)TiO3 thin films in a terahertz and IR ranges

Dependences of the dielectric response spectra on the thickness of the films based on the (Ba,Sr)TiO₃ (BST) solid solutions, which were deposited on single-crystal MgO substrates, have been analyzed. Using the Lorentz and Lyddane-Sachs-Teller models, the mutual correspondence of longitudinal optical (LO) and transverse optical (TO) phonon frequencies has established. The giant LO-TO splitting of the lowest-frequency vibration and inversion of LO and TO frequencies of other phonons in the IR spectrum of BST films has been found for the first time.     

Anharmonicity and superconductivity in Ba0.6K0.4BiO3

The temperature dependence of the x-ray absorption spectra above the L ₃ absorption edge of bismuth in the superconducting oxide Ba_0.6K_0.4BiO₃ are investigated. It is found that the local structure is different from the simple cubic structure indicated by x-ray and neutron-diffraction data. It is shown that the oxygen atoms move in an anharmonic double-well potential arising as a result of the existence of two nonequivalent types of octahedral environments of bismuth. Vibrations in such potential modulate the Bi-O bond lengths at the low frequency of the rotational (“tilting” type) mode of the oxygen octahedra and thus give rise to a strong electron-phonon interaction, which explains the quite high superconducting transition temperatures T _c ∼30 K. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 977–982 (25 June 1998)     

Gradient-stress on polarization in Ba1−x Sr x TiO3 films

A group of position-thickness-dependent stresses are used to modified Landau-Devonshire theory to investigate the second-order phase transition in Ba_1?x Sr _x TiO₃ films. The result shows that the short-range interaction between the unit cells of the film and the substrate induces the phase transition dispersion and the rise of the transition temperature in the films. The dependence of the effective dielectric constant on the temperature and the average spontaneous polarization on the film thickness are computed, which qualitatively agree with the experiments.     

Structural,photoluminescent, and dielectric properties of Eu3+-doped Ba0.7Sr0.3TiO3 thin films

Eu³⁺-doped Ba_0.7Sr_0.3TiO₃ thin films were prepared by a chemical solution deposition method and characterized by X-ray diffraction, field emission scanning electron microscopy, photoluminescence and dielectric measurements. The thin films were well crystallized with a pure perovskite structure. A contraction of the unit cell was observed upon incorporation of Eu³⁺ ions below 2 mol%, while an expansion occurred as the Eu³⁺ concentration was further increased above 2 mol%, indicating that Eu³⁺ ions with different concentrations occupied different lattice sites. Photoluminescence spectra showed two prominent transitions of Eu³⁺ ions at 594 nm (⁵D₀ → ⁷F₁) and 618 nm (⁵D₀ → ⁷F₂) upon excitation at 395 nm (⁷F₀ → ⁵L₆). There existed two quenching concentrations at 2 mol% and 4 mol% due to different lattice sites of the Eu³⁺ ions. We also investigated the dielectric properties of the thin films. Our study suggests that Eu³⁺-doped Ba_0.7Sr_0.3TiO₃ thin films have potential applications in multifunctional optoelectronic devices.     

Critical neutron scattering in a uniaxial relaxor Sr0.6Ba0.4Nb2O6

This paper reports on the results of an investigation into the temperature evolution of the critical scattering of neutrons in the strontium barium niobate crystal SBN60 in a zero field and in applied external direct-current (dc) electric fields. It has been shown that the observed critical scattering consists of two components that are adequately described by a Lorentzian and a Lorentzian squared. These components differently depend on the temperature and the momentum transfer vector. The application of an external electric field significantly suppresses the second component.     

Linear and nonlinear optical properties of rare earth doped of Ba0.7Sr0.3TiO3 thin films

Ba_0.7Sr_0.3TiO₃:Eu ferroelectric films were deposited on quartz substrates by pulsed laser deposition. The linear absorption coefficient and the linear refractive index calculated from the transmission spectrum at 532 nm were found to be 1.67×10⁴ cm^?1 and 1.82 respectively. The room temperature photoluminescence shows the characteristic emission of Eu³⁺ ions. The nonlinear optical properties of the film were investigated by a single beam Z-scan setup. The negative nonlinear refractive index and two photon absorption coefficient was found to be ?1.508×10^?6 m²/GW and 240 m/GW respectively. The real and imaginary part of the third order susceptibility of the thin films is 2.58×10^?17 m²/V² and 1.16×10^?16 m²/V² respectively. The BST:Eu thin films show good optical limiting property.