TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO₂. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO₂. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 Å does not stem from either a-Si or a-SiO₂, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO₂. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.     

Growth and energy band gap of CaSeS thin films prepared by hot-wall epitaxy

This paper investigates preparation of CaSeS thin films using hot-wall epitaxy. These films can be grown epitaxially on cleaved BaF₂(1 1 1) at a substrate temperature of 873 K by tailoring the VI/II flux ratio vaporized from Ca and SeS resources. The optical absorption edge of these films thus tailored can be observed clearly, shifting toward higher photon energy with increasing S content. In particular, the energy band gap of CaSe_0.66S_0.34, capable of lattice-matching to InP was found to be 4.69 eV, producing considerably large band gap difference of 3.34 eV between the CaSe_0.66S_0.34 and InP.     

A comparison of the medium-range order in glassy GeS2 and Sb2S3

In amorphous (a-) GeS₂ there is only one medium-range order (MRO) dependent X-ray scattering feature, i.e., the ‘first sharp diffraction peak’ (FSDP), whereas in a-Sb₂S₃ all scattering features in the interval 1–2.4 Å⁻¹ are in fact MRO-dependent. The basic element of MRO in a-GeS₂ is a part of a chain of corner bound tetrahedra extracted from the high temperature form of c-GeS₂; in a-Sb₂S₃ it is a part of a crystalline-like (Sb₂S₃)_n band. Ordering of parts of these bands is characterized by a greater interband separation than in the crystal. By contrast with a-GeS₂, the MRO-dependent medium-angle scattering features in a-Sb₂S₃ are less sensitive to fluctuations of the interband (intercluster) ordering distance.     

Rotary target method to prepare thin films of CdS/SiO2 by pulsed laser deposition

Thin films of CdS-doped SiO₂ glass were prepared by using the conventional pulsed laser deposition (PLD) technique. The laser target consisted of a specially constructed rotary wheel which provided easy control of the exposure-area ratio to expose alternately the two materials to the laser beam. The physical target assembly avoided the potential complications inherent in chemically mixed targets such as in the sol–gel method. Time-of-flight (TOF) spectra confirmed the existence of the SiO₂ and CdS components in the thin-film samples so produced. X-ray diffraction (XRD) and atomic force microscopy(AFM) results showed the different sizes and structures of the as-deposited and annealed films. The wurtzite phase of CdS was found in the 600^oC-annealed sample, while the as-deposited film showed a cubic–hexagonal mixed structure. In the corresponding PL (photoluminescence) spectra, a red shift of the CdS band edge emission was found, which may be a result of the interaction between the CdS nanocrystallite and SiO₂ at their interface.     

Compositional dependence of the optical gap in Ge1−xSx, Ge40−xSbxS60 and (As2S3)x(Sb2S3)1−x non-crystalline systems

An attempt is made to calculate the compositional dependence of the optical gap (E_g) in Ge_1−xS_x, Ge_40−xSb_xS₆₀ and (As₂S₃)_x(Sb₂S₃)_1−x non-crystalline systems in an alloy-like approach. From the comparison of both the experimental dependence of E_g and the calculated ones using the Shimakawa relation [E_{g AB} = YE_{g A} + (1−Y)E_{g B}] it is assumed that this equation is useful for such systems or parts of the systems which behave like almost ideal solutions.     

Raman spectra of Ag- and Cu-photo-doped As40S60−xSex films

Raman spectra of the chalcogenide vitreous layers (As₄₀S₆₀, As₄₀S₄₀Se₂₀, As₄₀S₂₀Se₄₀, As₄₀Se₆₀) non-doped and photo-doped by Ag, Cu were measured. The spectra were analyzed in terms of a molecular model. It was ascertained that, for the spectra of photo-doped As₄₀S_60−xSe_x layers, the shift of the main bands to the high frequency side and the appearance of the additional scattering bands in the low frequency spectral range are characteristic features. Variations in the Raman spectra with photodoping by Ag or Cu are consistent with the supposition concerning normal covalent and coordinative bond formation between metal additives and chalcogen atoms, which results in the different activation energy values for the silver photo-stimulated diffusion into chalcogenide layers.     

A glass-forming system with compound-forming tendency: As4S6---P4S10

Glasses in the quasi-binary system (As₄S₆)_x(P₄S₁₀)_1−x x = 0.1, …, 1.0, are produced and studied by thermal analysis, X-ray, and Raman spectroscopy. The phase diagram of the system and the critical cooling rate for glass formation both have their maximum at x = 0.5, corresponding to the compound As₂P₂S₈; the X-ray structure of recrystallized samples can be described as a sum of the As₂P₂S₈ (x = 0.5)- and the P₄S₁₀-structure (As₄S₆ not visible); Raman spectra of the glasses are again sums of As₂P₂S₈- and As₄S₆/P₄S₁₀-spectra. All these observations support the assumption that a stable building block corresponding to the 1:1 compound As₂P₂S₈ and surplus As₄S₆ (or P₄₁₀) are the essential elements of the structure in the glasses.     

Low-temperature specific heats of porous silica xerogels of low densities

Low-temperature specific heat measurements have been performed in porous silica xerogels with densities varying from 670 to 1730 kg m⁻³ to study the low-energy vibrational dynamics. The specific heat, C_p, shows a bump in the temperature range above 4 K, when reported in a plot of C_p/T³ against the temperature, T. The bump is almost independent of the sample density and is close to the boson peak observed in melt-quenched amorphous silica (a-SiO₂). At temperatures <4 K, an additional contribution to that predicted by the Debye theory is observed. It follows an approximately linear temperature dependence (C_exc=aT^1+v, v being equal to about 0.25). In the xerogel with the largest density, specific heat of about a factor 5 larger than that of a-SiO₂ is measured, which increases with decreasing sample density. By comparison with the corresponding properties of a-SiO₂, we conclude that the disorder introduced by the presence of pores does not measurably affect the excess density of vibrational states in a frequency range of the boson peak (BP), but increases the density of the two-level systems (TLS).     

Photoluminescence dynamics in chalcogenide glasses and crystals

Time-resolved photoluminescence studies reveal distinct differences between the recombination processes in a chalcogenide glass and in its crystalline counterpart. Here the three luminescence bands of a-As₂S₃ are interpreted in terms of the recombination of an excition, a self-trapped exciton and a pair of electron- and hole-like small polarons. The two luminescence bands observed in the crystal are attributed to the recombination of two types of excitons composed respectively of a hole bound to a self-trapped electron, and a hole which is induced to self-trap in the presence of a self-trapped electron.     

Static B NMR studies of the short range order in alkali metal modified B2S3 glasses

The ¹¹B NMR spectra of xRb₂S+(1−x)B₂S₃ glasses in the range 0x0.75 and of xCs₂S+(1−x)B₂S₃ glasses in the range 0x0.60 are reported. The addition of Rb₂S to B₂S₃ creates on average approximately two and one-half tetrahedral borons for each added sulfur ion, whereas it is found that the addition of Cs₂S creates approximately 2 tetrahedral borons for each added sulfur ion. This behavior while more similar to that seen in the alkali borate glasses, contrasts that seen in the Na and K thioborate glasses, where six to eight and three, respectively, tetrahedral borons are formed for every sulfide anion added to the glass. These findings are supported by the IR and ¹¹B NMR spectra of the di-thioborate polycrystals (c-Rb₂S:2B₂S₃ and c-Cs₂S:2B₂S₃) whose structures appear to be comprised of two BS₄ tetrahedrals and two BS₃ trigonals (N₄0.5) like that in the alkali di-borate phases for both Rb and Cs. Unlike the ¹¹B NMR resonances of the sodium thioborate glasses where a single sharp line is observed for the tetrahedral boron site and a single quadrupolar broadened line is observed for all the trigonal sites, a third resonance line is observed at high alkali fractions for the rubidium and cesium thioborate glasses. This new structural feature may arise from asymmetric MBS₂ (meta-thioborate groups) or tetrahedral boron groups possessing a non-bridging sulfur.     

Preparation and characterization of Ni-incorporated FeS2 single crystals

Nickel-incorporated FeS₂ single crystals with various Ni compositions of Fe_0.99S₂:Ni_0.01, Fe_0.98S₂:Ni_0.02, Fe_0.96S₂:Ni_0.04, and Fe_0.9S₂:Ni_0.1 were grown by chemical vapor transport (CVT) method using ICl₃ as a transport agent. Physical properties of the Ni-incorporated FeS₂ single crystals were characterized using X-ray diffraction, Raman spectroscopy, electrical conductivity, and photoconductivity (PC) measurements. By means of the analyses of the X-ray diffraction patterns, the whole series of Ni-doped FeS₂ single crystals were determined to be single-phase and isostructural. Raman spectroscopy of the Ni-doped FeS₂ crystals was carried out at room temperature. Raman resonant peaks of the Ni-doped FeS₂ crystals demonstrate an energy red-shift behavior with respect to the increase of the dopant densities. Conductivity measurements show the resistivity of the Ni-doped FeS₂ decreased as the doping concentration of Ni is increased. Nickel is an n-type dopant, which behaves like a donor level existed near the conduction band edge of the synthetic FeS₂. On the other hand, dopant effect of nickel on the synthetic FeS₂ also destroys the photoconductive sensitivity in the photoconductivity measurements.     

Structure and Raman spectra of glasses containing several glass-forming oxides and no glass-modifying oxide

Binary glasses containing no modifying oxides, such as SiO₂---GeO₂, SiO₂---B₂O₃, SiO₂---P₂O₅, GeO₂---B₂O₃, Al₂O₃---P₂O₅ and ternary glasses SiO₂---GeO₂---P₂O₅, Al₂O₃---B₂O₃---P₂O₅, B₂O₃---SiO₂---P₂O₅, Al₂O₃---ZrO₂---P₂O₅ have been prepared by melting and CVD methods. The Raman spectra have also been measured. Structural characteristics of SiO₂, GeO₂, B₂O₃, P₂O₅ in different glass systems are analysed. There exist coordination number changes in B₂O₃- and GeO₂-containing glasses and linkage changes between tetrahedra (SiO₄) and (PO₄) in SiO₂ and P₂O₅ containing glasses. The structure of Al₂O₃ containing glasses is homogeneous and the structure of B₂O₃ containing glasses is inhomogeneous. These experimental results are in coincidence with the X-ray small angle scattering analysis.     

Effect of Asi on the optical properties of Ga1−xInxAs/InP grown by molecular beam epitaxy

This report focuses on the effect of the As species and the V/III ratio on the optical properties of Ga_1−xIn_xAs/InP grown by molecular beam epitaxy (MBE). The band gap energies of the layers were measured by low temperature photoluminescence (PL) while the indium contents x were determined by X-ray for samples in the investigated range of 0.50<x<0.56. For the analysis of these data we considered the model of Kuo et al. which we confined with a correction for the different measurement temperatures in PL (4.2 K) and X-ray (300 K). Using As₄ with an effective V/III ratio higher than 1.3, we find the best agreement of the band gap energies and predictions of the theory. A lower V/III ratio always implies a reduction of the band gap energy to values 5-15 meV lower than expected. In contrast, using As₂ the PL data fit quite well independent of the effective V/III ratio above unity.     

Wavelength selective materials modification of bulk As2S3 and As2Se3 by free electron laser irradiation

In this study we report first measurements of wavelength-selective infrared-induced materials modification of bulk As₂S₃ and As₂Se₃. These materials are currently being considered as candidate materials for infrared optical fiber transmission in the range of 1–10 μm. Our study is aimed at modifying oxygen, hydrogen and carbon impurities bound to chalcogenide constituent elements in the materials to reduce absorption. Tunable infrared radiation from the W.M. Keck Free Electron Laser (FEL) at Vanderbilt was used to excite specific vibrational modes, S–O–H and CH_x modes in bulk As₂S₃ and Se–H, CH_x and S–H₂ modes in bulk As₂Se₃. Changes in vibrational mode amplitudes are monitored by measuring the intensity of the Fourier transform infrared (FTIR) spectra before and after irradiation at appropriate wavelengths. By tuning wavelengths to hydrogen vibrational modes, we find evidence that hydrogen is released and/or redistributed athermally. In particular, following irradiation at specific resonant wavelengths, vibrational mode amplitudes as monitored by FTIR associated with CH_x are significantly reduced in bulk As₂S₃ and As₂Se₃ samples. In As₂S₃, the changes in CH_x modes are reversed by heat treatment at 115°C for 35 min in nitrogen atmosphere.     

Large-scale synthesis of ultrafine Si nanoparticles by ball milling

Si nanoparticles have been synthesized by ball-milling of graphite and SiO₂ powders. The solid-phase reaction, C(graphite)+SiO₂→Si+CO₂, was found to be a novel technique for fabrication of ultrafine Si particles. The reduced Si in the powder assembled as nanoparticles with a wide range of diameters. Multiple-peak structures were observed in the photoluminescence spectra of the Si nanoparticles at room temperature. The peak energies were found to coincide with the PL peak pinning energies previously reported in porous Si. Thus, the fine structures of luminescence spectra could be attributed to the size quantization of the Si nanoparticles formed via solid-phase reaction during ball-milling.     

Optical third-harmonic generation from some high-index glasses

Optical third-harmonic generation from some high-index glasses was investigated. The highest χ⁽³⁾ was obtained from As₂S₃ glass 2.2 × 10⁻¹² esu/ This value was 100 times higher than that of pure SiO₂ glass and comparable with that of the polymer with monomer-doping, which are known as organic materials with quite high χ⁽³⁾. From the relationship between χ⁽³⁾ and composition, sulfide glasses were found to have higher χ⁽³⁾ than oxide or semiconductor-doped oxide glasses with similar refractive indices.     

Surface structure and chemistry of high surface area silica gels

Combined Raman and ²⁹Si NMR investigations of high surface area silica gels indicate that dehydroxylation of the a-SiO₂ surface results preferentially in the formation of cyclic trisiloxanes (3-membered rings). Estimates of the maximum experimentally observed concentration of 3-membered rings correspond to a surface coverage of about 28–58%. Two consequences of 3-membered rings on the a-SiO₂ surface are enhanced hydrolysis rates and increased skeletal densities.     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Structure and thermal stability of MOCVD ZrO2 films on Si (1 0 0)

The structure and thermal stability of ZrO₂ films grown on Si (1 0 0) substrates by metalorganic chemical vapor deposition have been studied by high-resolution transmission electron microscopy, selected area electron diffraction and X-ray energy dispersive spectroscopy. As-deposited films consist of tetragonal ZrO₂ nanocrystallites and an amorphous Zr silicate interfacial layer. After annealing at 850°C, some monoclinic phase is formed, and the grain size is increased. Annealing a 6 nm thick film at 850°C in O₂ revealed that the growth of the interfacial layer is at the expense of the ZrO₂ layer. In a 3.0 nm thick Zr silicate interfacial layer, there is a 0.9 nm Zr-free SiO₂ region right above the Si substrate. These observations suggest that oxygen reacted with the Si substrate to grow SiO₂, and SiO₂ reacted with ZrO₂ to form a Zr silicate interfacial layer during the deposition and annealing. Oxygen diffusion through the tetragonal ZrO₂ phase was found to be relatively easier than through the monoclinic phase.     

具有n-n型异质结的复合材料Bi2S3/MIL-125(Ti)光电性能研究

以硫代硫酸钠·五水合物(Na₂S₂O₃·5H₂O)、硝酸铋·五水合物(BiN₃O₉·5H₂O)为硫源和铋源,尿素(CON₂H₄)为结构导向剂,制备了纳米棒状结构的硫化铋(Bi₂S₃),使其原位生长在MIL-125(Ti)的笼状结构表面。PEC性能测试显示,在0.5 mol·L^-1的硫酸钠电解液(pH=6.0)中,Bi₂S₃/MIL-125(Ti)_0.07(MIL-125(Ti)加入量为0.07 g)的复合材料表现出最高的光电性能。光电性能的显著增强主要取决于Bi₂S₃/MIL-125复合材料的带隙重整效应,对紫外光以及可见光的吸收能力显著提高。但由于Bi₂S₃/MIL-125...