缺陷对β-Ga2O3薄膜的结构和光学特性的影响

采用射频磁控溅射技术和后期退火在蓝宝石衬底上成功制备了β-Ga₂O₃薄膜。借助于X射线衍射(XRD)、拉曼散射光谱(Raman)、X射线光电子能谱(XPS)、以及二次离子质谱(SIMS)研究了缺陷对β-Ga₂O₃薄膜的结构和光学特性的影响。结果表明,未退火的Ga₂O₃薄膜呈现非晶态,随高温退火时间逐渐增加,非晶Ga₂O₃薄膜逐步转变为沿(-201)方向择优生长的β-Ga₂O₃薄膜。所有Ga₂O₃薄膜在近紫外到可见光区的平均透过率都高达95%,β相Ga₂O₃薄膜的光学带隙比非晶态薄膜增加~0.3 eV,且随退火时间的增加,β-Ga₂O₃薄膜的光学带隙也随之变宽。此外,发现非晶Ga₂O₃薄膜富含氧空位缺陷,高温退火处理后,β-Ga₂O₃薄膜中的氧空位浓度明显降低,但蓝宝石衬底中的Al极易扩散至Ga₂O₃薄膜层,并随退火时间的增加Al浓度明显增加,氧空位的降低和Al杂质的增加是导致β-Ga₂O₃薄膜光学带隙变宽的主要原因。     

Hartree–Fock study of near-edge gap states in CaF2 with Na, Cl or Sr impurities

The energy of formation and electronic structure of Na⁺, Cl⁻ and Sr²⁺ impurity centers in CaF₂ have been computed using ab initio Hartree–Fock theory and the supercell approach. The work extends and complements recent results [Solid State Commun. 118 (2001) 569] for Mg²⁺ as a substitutional impurity in CaF₂. For Na⁺ substituting for Ca²⁺ [S(Na)], charge compensation by an F⁻ vacancy [V(F)] or by a second, interstitial Na⁺ [I(Na)] are both considered. In all cases, geometry optimization is done by relaxing the positions of nearest- and next-nearest-neighbors to minimize the total energy. After correction for electron correlation, the energies of formation increase in the order Mg²⁺2+−+ results are in agreement with previous Mott–Littleton formation energies. Ion charges, charge density maps and Mulliken bond populations are obtained to show the nature of bonding in the vicinity of the defect. Na⁺ leads to states just above the CaF₂ valence band maximum (VBM), and Na⁺ (and also Mg²⁺) produce states just below the conduction band minimum. The results are in qualitative agreement with available optical data for Na⁺ and Mg²⁺ impurity effects on CaF₂ near-edge absorption but show that gap states are important in addition to perturbed excitons. A Cl⁻ impurity yields a narrow band of states above the VBM which may significantly affect the deep-ultraviolet transmission of CaF₂. Sr²⁺ does not appear to produce states in the CaF₂ gap.     

双铁电复合材料的制备及其光电化学性能研究

通过模板法制备钒酸铋(BiVO₄)薄膜, 用溶胶-凝胶法制备铁电材料铁酸铋(BiFeO₃)并对BiVO₄进行修饰, 以半导体复合的方式提高BiVO₄的光电化学性能。电化学测试结果表明, 经BiFeO₃修饰后, BiVO₄薄膜的光电化学性能有所提高, 其中经BiFeO₃旋涂5次后的BiVO₄薄膜具有最优的光电化学性能, 光电流密度达到0.72 mA·cm^-2, 较未修饰样品提高了67.4%。利用外场极化调节能带弯曲可以显著地提高BiVO₄/nBiFeO₃铁电复合物的光电化学性能, 复合物经正极化20 V电压处理后的光电流密度最高为0.91 mA·cm^-2, 比BiVO₄薄膜提升了1倍以上, 具有良好的光电化学性能。BiFeO₃与BiVO₄复合后有利于形成异质结, 促进光生电子、光生空穴的产生与分离, 并且外场极化调节能带弯曲使光生电荷加速转移, 是铁电复合物光电化学性能提高的主要原因。     

Corrosion of tin in citric acid solution and the effect of some inorganic anions

The corrosion behaviour of tin in different concentrations of citric acid solutions (0.3–1.0 M, pH=1.8) was studied at 30 °C by potentiodynamic technique. The E/I profiles exhibit an active passive behaviour. The active dissolution involves one anodic peak A associated with a dissolution of the metals as Sn(II) species. The passivity is due to the formation of thin film of SnO₂ and or Sn(OH)₄ on the anode surface. The cathodic sweep shows a small peak C related to the reduction of the passive film. The peak current density I_p of peak A increases with increasing both acid concentration and sweep rate.

The effects of adding increasing concentrations of Na₂CrO₄, NaMoO₄, NaNO₃ and NaNO₂ on the corrosion of tin in 0.5 M citric acid at 30 °C were investigated. Both CrO₄²⁻ and MoO₄²⁻ ions inhibit the corrosion of tin and the extent of inhibition enhances with their concentrations. Addition of either NO₃⁻ or NO₂⁻ accelerates the corrosion of tin. NO₃⁻ ions are more aggressive than NO₂⁻ ions.     

Effect of Cl- on the Properties of the Passive Films Formed on 316L Stainless Steel in Acidic Solution

The corrosion behavior of 316L stainless steel （31 6L SS） has been investigated in solutions containing various concentrations of chloride ions by using potentiodynamic polarization, capacitance measurement and Mott- Schottky relationship analysis （M-S）. The result indicates that passive currents change slightly with the addition of chloride ions. The pitting potential （Epit） decreases linearly with Iog[CI-]. Correspondingly, the point defect diffusion coefficient （Do） of the passive film increases linearly with increasing Iog[CI-]. The results also indicate that the pitting corrosion of 316L SS follows the adsorption mechanism in NaCI solution.     

Electrical and optical properties of chemical solution deposited barium hafnate titanate thin films

We have investigated the electrical and optical properties of Ba(Hf_xTi_1 − x)O₃ (x = 0, 0.1, 0.2, 0.3, 0.4) (BHT) thin films deposited on platinized silicon and fused quartz substrates. Analyses of the X-ray diffraction patterns reveal that with the increase in Hf contents there is a systematic increase of the lattice constants of BHT films. Irrespective of the measurement frequencies the dielectric constants was found to be systematically decreased, whereas their frequency dispersion was found to be reduced with increasing Hf contents. The leakage current data measured using a metal-insulator–metal configuration reveal that the Schottky emission is the dominant leakage current mechanism in these films. BHT films, deposited on transparent fused quartz substrates, were also characterized in terms of their optical properties. For this purpose the transmittance of the undoped as well as Hf doped barium titanate thin films was measured as a function of wavelength in the range of 290 nm to 800 nm. The transmission spectra were analysed to estimate the wavelength dependence of the refractive indices/extinction coefficients as well as the variation of optical band gap of these films. With the increase of Hf contents, a systematic increase of the band gap [from 3.65 eV (undoped film) to 4.15 eV (40 at.% Hf doped barium titanate film)] was observed. The reduction of the leakage current with increasing hafnium substitution is discussed on the basis of an increasing Schottky barrier height and due to a simultaneous increase in the band gap of the material.     

Photoelectrochemical properties of sol–gel-derived Ti1−xVxO2 solid solution film photoelectrodes

Ti_1−xV_xO₂ solid solution film photoelectrodes were prepared by the dip-coating sol–gel method. X-ray diffraction and X-ray photoelectron spectroscopy were employed to ensure the formation of the solid solution and their composition. Obvious photoresponses were observed in the visible region for the solid solution film electrodes with x0.05 and the red shift of the photoresponse was enhanced with increasing x. Moreover, the solid solution film electrodes were found to be photoelectrochemically stable. However, the onset potential of photocurrent shifted positively with increasing x. Band model of the solid solution was suggested to explain the effects of the vanadium incorporation on the photoelectrochemical properties.     

Structure and photoconductive properties of dip-deposited SnS and SnS2 thin films and their conversion to tin dioxide by annealing in air

SnS and SnS₂ thin films have been prepared by the dip technique. In this technique, a substrate was dipped into an alcoholic solution of the corresponding chloride and thiourea and then withdrawn vertically at a controlled speed, and finally baked in a high temperature furnace at atmospheric condition. XRD and SEM data suggest that good quality SnS and SnS₂ films are obtained at a baking temperature of 300 and 360°C, respectively. Values of band gap for SnS and SnS₂ obtained from spectral response of photoconductivity are 1.4 and 2.4 eV, respectively. The indirect allowed band gap values for SnS₂ film obtained from optical absorption measurements are 1.95 and 2.05 eV. Open-air annealing of both SnS and SnS₂ films at 400°C converts them to transparent conducting SnO₂.     

Shallow defect states in hydrogenated amorphous silicon oxide

The theoretical cluster-Bethe-lattice method is used in this study to investigate the shallow defect states in hydrogenated amorphous silicon oxide. The electronic density of states (DOS) for the SiO₂ Bethe lattice of various Si–O–Si angles, non-bridging oxygen Si–O^√, peroxyl radical Si–O–O^√, threefold coordinated O₃ and Si–H bonds are calculated. The variation of the Si–O–Si bond angle causes the bandgap fluctuation and induces tail states near the conduction band minimum. The Si–O^√ and Si–O–O^√ bonds introduce shallow defect states in the energy gap near the top of the valence band. The Si–H bond induces a defect state, in the energy gap near the conduction band minimum, in a-SiO_x with high oxygen concentration, but not low oxygen concentration. The O₃ bond itself does not induce defect state in the energy gap. The O₃⁺D⁻ complex, formed by the O₃ and threefold coordinated silicon, induces shallow state in the energy gap near the conduction band minimum. This defect state can explain the energy shift of photoluminescence of a-SiO_x:H under annealing.     

IR-spectrophotoelectrochemical characterization of mesoporous semiconductor films

A combined IR-spectroscopic and electrochemical approach for the study of photo- and bias-induced reactions at the semiconductor/electrolyte interface is presented. Information on the electronic structure of a mesoporous semiconductor nanoparticle network, concretely the density and distribution of band gap states, as well as the nature of solution species are analyzed in situ. It has been shown that under appropriate conditions the electrode potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation, i.e., application of a bias voltage or electrode exposure to photons exceeding the semiconductor band gap at open circuit. Importantly, electronic properties of the semiconductor and vibrational properties of solution species can be addressed simultaneously by IR-spectroscopy. In addition, electrochemical methods provide a means for the active manipulation (in potentiostatic measurements) or the passive tracking (during open circuit potential decay) of the quasi-Fermi level in the mesoporous film together with the possibility of electron quantification (by charge extraction experiments).     

宽带隙、高折射率聚偏四氟乙烯/TiO2-ZrO2光学膜的制备和表征

以氧氯化锆(ZrOCl₂·8H₂O)为锆源, 钛酸丁酯(Ti(OBu)₄)为钛源, 聚偏氟乙烯(PVDF)为有机添加剂, 采用溶胶–凝胶法在K9玻璃基片上制备PVDF/TiO₂-ZrO₂光学膜, 提高了ZrO₂-TiO₂光学膜的综合性能。然后采用SEM、FT-IR、接触角以及紫外/可见/近红外透射光谱等手段对PVDF/TiO₂-ZrO₂光学膜的组成、光学性能和抗激光损伤阈值进行了研究。SEM测试表明, 在K9玻璃基片上制备了光学膜。PVDF的添加导致水与薄膜的接触角增大。ZrO₂-TiO₂光学膜的光学带隙随ZrO₂含量的增加而略微增大, PVDF/ZrO₂(50mol%)-TiO₂薄膜的光学带隙随PVDF质量分数的增加而增大。另外, ZrO₂-TiO₂光学膜的折射率随ZrO₂摩尔分数的减小而增大, PVDF/ZrO₂(50mol%)-TiO2膜的折射率随PVDF质量分数的增加而增大。     

Optical absorption of Nd, Sm and Dy in bismuth borate glasses with large radiative transition probabilities

This paper reports on the optical properties of Nd³⁺, Sm³⁺ and Dy³⁺ in bismuth borate glasses, with Bi₂O₃ content varying from 30 to 60 mol%. The variation of the optical properties with composition plays a dominant role in determining a good laser host material. The variation of the Judd–Ofelt intensity parameters Ω_t (t=2,4,6) and the radiative transition probabilities and the hypersensitive band positions, with composition, have been discussed in detail. The changes in position and intensity parameters of the transitions in the optical absorption spectra are correlated to the structural changes in the host glass matrix. The variation of Ω₂ with Bi₂O₃ content has been attributed to changes in the asymmetry of the ligand field at the rare earth ion site and to the changes in their rare earth–oxygen (R–O) covalency, whereas the variation of Ω₆ strongly depends on nephlauxetic effect. The shift of the hypersensitive band shows that the covalency of the R–O bond increases with increase of Bi₂O₃ content, due to increased interaction between the rare earth ions and the non-bridging oxygens. The radiative transition probabilities of the rare earth ions are large in bismuth borate glasses, suggesting their suitability for laser applications.     

平面应变对二维单层MoSi2N4能带结构和光电性质的影响

二维单层MoSi₂N₄具有优异的载流子输运能力与出色的化学稳定性, 受到了广泛关注, 但其光电性质与外加平面应变间的内在关系尚未展开深入探讨。本研究采用平面波超软赝势方法探索了平面应变对二维单层MoSi₂N₄能带结构和光电性质的影响, 发现单层MoSi₂N₄为间接带隙半导体, 其价带顶由Mo4d轨道和部分N2p轨道杂化而成, 导带底则均由Mo4d轨道组成。在拉应变作用下, 单层MoSi₂N₄的带隙逐渐变窄且光生载流子的有效质量不断减小; 在压应变作用下, 其带隙逐渐变宽, 光生载流子的有效质量缓慢增大。值得注意的是, 当压应变ε=-2.8%时, 体系由间接带隙转变为直接带隙。单层MoSi₂N₄的光学吸收表现出明显的各向异性, 且在平面应变作用下光吸收带边发生了不同程度的移动, 有效地拓展了体系的光谱响应范围, 有利于提升光电特性。这可为进一步研究二维单层MoSi₂N₄在新型可调谐纳米光电器件领域的应用提供理论指导。     

Thermally and optically induced irreversible changes in some Ge–As–S amorphous thin films

Thermally and optically induced irreversible changes in the optical gap and refractive index were studied for sulphur rich, nearly stoichiometric and sulphur poor Ge–As–S amorphous films prepared by thermal evaporation. For films studied the optical gap in the virgin state decreases from 2.55₉ (Ge_0.121As_0.172S_0.707) to 1.63₂ eV (Ge_0.254As_0.294S_0.452) and simultaneously the refractive index increases from 2.21 to 2.87, respectively. The most sensitive composition to illumination seems to be nearly stoichiometric film (Ge_0.153As_0.201S_0.646), where the blue shift of the gap is observed close to 150 meV. Sulphur poor film (Ge_0.254As_0.294S_0.452) was found insensitive to illumination. Highest thermally induced blue shift of the gap, close to 250 meV, we observed just for Ge_0.254As_0.294S_0.452 film and for this film it was observed also nearly “giant” decrease in refractive index from 2.85 to 2.42. The behaviour of Ge_0.254As_0.294S_0.452 film is qualitatively discussed using the concept of network rigidity (insensitivity to illumination) and assuming thermally induced changes in bonding arrangement (refractive index changes).     

Effect of Nb, Cr, Sn additions on gas sensing properties of TiO2 thin films

The effect of Nb and Cr dopants as well as Sn⁴⁺ additions on the electronic structure of rf-sputtered TiO₂ thin films and its subsequent influence on gas sensor performance is reported. The changes in the electrical conductivities of TiO₂ thin films doped with up to 10 at.% Nb, 4 at.% Cr and TiO₂-SnO₂ in the full range of compositions upon exposure to hydrogen and oxygen are demonstrated. The spectral dependence of the absorption coefficient in the vicinity of the band gap transition of TiO₂ is shown to be affected by doping.     

Electrical stability of polyimide siloxane films for interlayer dielectrics in multilevel interconnections

Electrical stability of a polyimide siloxane (PSI) film for ultra-large scale integrated circuit (ULSI) multilevel interconnections is studied. The PSI films, modified by p-aminophenyltrimethoxysilane (APTMS), are designed to have three-dimensional polymer structures through Si–O bonds. It has been revealed that the PSI films are more stable in electrical properties at higher temperatures than 150°C, as compared to the conventional polyimide (PI) films. The electrical conduction mechanism study for the PSI films has revealed that Schottky emission is dominant. Barrier height φ_B obtained from the electrical property for the PSI film was 0.460 eV in the temperatures ranging from 25–250°C. On the other hand, barrier height of 0.422 eV at lower temperatures than 150°C and activation energy of 1.09 eV at higher temperatures than 150°C were obtained for the conventional PI film. The difference in polymer structure is very sensitive to the electrical conduction at high temperature, due to sodium ion migration. The ideal band diagrams of metal-insulator-semiconductor (MIS) structures were also discussed. The optical band gaps for PSI and conventional PI films were 3.320 eV and 3.228 eV, respectively. This result suggests that the band gap of PI films can be enlarged by modification with Si–O components. The differential barrier height between the PSI and conventional PI films is 0.038 eV, and is close to the difference in half of optical band gaps (0.046 eV).     

Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique

Polycrystalline BaTiO₃ thin films having the perovskite structure were successfully produced on platinum coated silicon, bare silicon, and fused quartz substrate by the combination of the metallo-organic solution deposition technique and post-deposition rapid thermal annealing treatment. The films exhibited good structural, electrical, and optical properties. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 255 and 0.025, respectively, and the remanent polarization and coercive field were 2.2 μC cm⁻² and 25 kV cm⁻¹, respectively. The resistivity was found to be in the range 10¹⁰–10¹² Ω·cm, up to an applied electric field of 100 kV cm⁻¹, for films annealed in the temperature range 550–700 °C. The films deposited on bare silicon substrates exhibited good film/substrate interface characteristics. The films deposited on fused quartz were highly transparent. An optical band gap of 3.5 eV and a refractive index of 2.05 (measured at 550 nm) was obtained for polycrystalline BaTiO₃ thin film on fused quartz substrate. The optical dispersion behavior of BaTiO₃ thin films was found to fit the Sellmeir dispersion formula well.     

Preparation and photoelectrochemical properties of Ti1−xVxO2 solid solution thin film photoelectrodes with gradient bandgap

A design of a gradient bandgap Ti_1−xV_xO₂ thin film electrode for wet-type solar cells is provided. The gradient bandgap film electrodes were prepared by heating stacked layers of varying V/Ti ratios using the sol-gel method. A composition gradient was observed for some of the samples by X-ray photoelectron spectroscopy although it was not very large. For the Ti_1−xV_xO₂ film electrodes, conspicuous visible light photoresponse and photoelectrochemical stability were observed. The photocurrent increased with increasing bias potential. However, the photocurrent onset potentials of the Ti_1−xV_xO₂ film electrodes were more positive than those of TiO₂ film electrodes, probably owing to the high surface state density introduced by the diffusion of vanadium ions.     

压力对PbTiO3结构和热物性质影响的第一性原理研究

PbTiO₃是一种重要的铁电功能材料, 但压力对其结构、稳定性、力学和热力学性能的影响尚不明确, 从而限制了其在电子通讯领域的应用。本研究采用基于密度泛函理论的第一性原理方法, 研究了压力下四方铁电相钛酸铅(TP-PTO)、立方顺电相钛酸铅(CP-PTO)、四方前驱体相钛酸铅(PP-PTO)结构和热物性质。研究发现, 三种结构可压缩性由大到小依次为PP-PTO>TP-PTO>CP-PTO。能带结构和态密度均表明PTO在研究的压力范围内未发生相变。PTO带隙随着压力增大逐渐降低, TP-PTO在20 GPa由间接带隙半导体转变为直接带隙半导体, 其余两相在压力下恒为直接带隙半导体。PTO在0~30 Gpa范围力学稳定, 且具有各向异性, 其综合力学性能随着压力的增加而增强, 各向异性则是先降低后升高。通过准谐德拜近似理论研究了温度和压力对PTO德拜温度、熵、热容的影响, 结果表明德拜温度随着温度上升而下降, 随着压力增大而上升, 反映出共价键强度依次为CP-PTO>TP-PTO> PP-PTO。熵和热容随着温度上升而上升, 随着压力的增大而下降。     

铝酸盐体系下反应时间对ZrH1.8表面阻氢膜层制备的影响

采用微弧氧化技术在NaAlO₂电解液体系下制备ZrH_1.8表面阻氢膜层, 采用场发射扫描电子显微镜(FE-SEM), X射线衍射(XRD), 真空脱氢实验对膜层进行表征, 考察不同反应时间对ZrH_1.8表面阻氢膜层的厚度、形貌、相结构以及阻氢性能的影响规律。研究结果表明：当反应时间在7.5~15 min变化时, 膜层厚度由78.4 μm增大至152.8 μm。膜层由单斜相M-ZrO₂、四方相T-ZrO₂以及立方相C-ZrO₂组成, 反应时间对膜层的相组成并无明显影响。阻氢膜层由致密层和疏松层构成, 膜层外侧为疏松层, 靠近基体一侧为致密层。当反应时间为10 min时, 获得的膜层表面平整, 致密性较好, 厚度适中, 膜层的氢渗透降低因子PRF值为20。