Reversible Resistance Switching Effect in Amorphous Ge1Sb4Te7 Thin Films without Phase Transformation

We demonstrate a reversible resistance switching effect that does not rely on amorphous-crystalline phase transformation in a nanoscale capacitor-like cell using Ge1Sb4Te7 films as the working material. The polarity and amplitude of the applied electric voltage switches the cell resistance between low- and high-resistance states, as revealed in the current-voltage characteristics of the film by conductive atomic force microscopy (CAFM). This reversible SET/RESET switching effect is induced by voltage pulses and their polarity. The change of electrical resistance due to the switching effect is approximately two orders of magnitude.

Numerical and experimental analysis on green laser crystallization of amorphous silicon thin films

The effect of laser fluence on the crystallization of amorphous silicon irradiated by a frequency-doubled Nd:YAG laser is studied both theoretically and experimentally. An effective numerical model is set up to predict the melting threshold and the optimized laser fluence for the crystallization of 200-nm-thick amorphous silicon. The variation of the temperature distribution with time and the melt depth is analyzed. Besides the model, the Raman spectra of thin films treated with different fluences are measured to confirm the phase transition and to determine the optimized fluence. The calculating results accord well with those obtained from the experimental data in this research. (C) 2008 Elsevier Ltd. All rights reserved.

Investigation of annealing effects on the laser-induced damage threshold of amorphous Ta2O5 films

Ta2O5 films were deposited using the conventional electron beam evaporation method and then annealed at temperatures in the range 373-673 K. Chemical composition, scattering and absorption were examined by X-ray photoelectron spectroscopy (XPS), total integrated scattering (TIS) measurement and the surface thermal lensing (m) technique, respectively. The laser-induced damage threshold (LIDT) was assessed using the output from an Nd:YAG laser with a pulse length of 12 ns. The results showed that the improvement of the LIDT after annealing was due to the reduced substoichiometric and structural defects present in the film. The LIDT increased slightly below 573K and then increased significantly with increase in annealing temperature, which could be attributed to different dominant defects. Moreover, the root mean square (RMS) roughness and scattering had little effect on the LIDT, while the absorption and the LIDT were in accord with a general relation. (c) 2008 Elsevier Ltd. All rights reserved.

Reducing organic substances from anaerobic decomposition of hydrophytes

Oxidation-reduction properties of surface sediments are tightly associated with the geochemistry of substances, and reducing organic substances (ROS) from hydrophytes residues may play an important role in these processes. In this study, composition, dynamics, and properties of ROS from anaerobic decomposition of Eichhornia crassipes (Mart.) Solms, Potamogenton crispus Linn, Vallisneria natans (Lour.) Hara, Lemna trisulca Linn and Microcystis flos-aquae (Wittr) Kirch were investigated using differential pulse voltammetry (DPV). The type of hydrophytes determined both the reducibility and composition of ROS. At the peak time of ROS production, the anaerobic decomposition of M. flos-aquae produced 6 types of ROS, among which 3 belonged to strongly reducing organic substance (SROS), whereas there were only 3-4 types of ROS from the other hydrophytes, 2 of them exhibiting strong reducibility. The order of potential of hydrophytes to produce ROS was estimated to be: M. flos-aquae > E. crassipes > L. trisulca > P. crispus approximate to V. natans, based on the summation of SROS and weakly reducing organic substances (WROS). The dynamic pattern of SROS production was greatly different from WROS. The total SROS appeared periodic fluctuation with reducibility gradually weakening with incubation time, whereas the total WROS increased with incubation time. Reducibility of ROS from hydrophytes was readily affected by acid, base and ligands, suggesting that their properties were related to these aspects. In addition to the reducibility, we believe that more attention should be paid to the other behaviors of ROS in surface sediments.

Sorptive interaction between goethite and strongly reducing organic substances from anaerobic decomposition of green manures

Strongly reducing organic substances (SROS) and iron oxides exist widely in soils and sediments and have been implicated in many soil and sediment processes. In the present work, the sorptive interaction between goethite and SROS derived from anaerobic decomposition of green manures was investigated by differential pulse voltammetry (DPV). Both green manures, Astragaltus sinicus (Astragalus) and Vicia varia (Vicia) were chosen to be anaerobically decomposed by the mixed microorganisms isolated from paddy soils for 30 d to prepare different SROS. Goethite used in experiments was synthesized in laboratory. The anaerobic incubation solutions from green manures at different incubation time were arranged to react with goethite, in which SROS concentration and Fe(II) species were analyzed. The anaerobic decomposition of Astragalus generally produced SROS more in amount but weaker in reducibility than that of Vicia in the same incubation time. The available SROS from Astragalus that could interact with goethite was 0.69 +/- 0.04, 0.84 +/- 0.04 and 1.09 +/- 0.03 cmol kg(-1) as incubated for 10, 15 and 30 d, respectively, for Vicia, it was 0.12 +/- 0.03, 0.46 +/- 0.02 and 0.70 +/- 0.02 cmol kg(-1). One of the fates of SROS as they interacted with goethite was oxidation. The amounts of oxidizable SROS from Astragalus decreased over increasing incubation time from 0.51 +/- 0.05 cmol kg(-1) at day 10 to 0.39 +/- 0.04 cmol kg(-1) at day 30, but for Vicia, it increased with the highest reaching to 0.58 +/- 0.04 cmol kg(-1) at day 30. Another fate of these substances was sorption by goethite. The SROS from Astragalus were sorbed more readily than those from Vicia, and closely depended upon the incubation time, whereas for those from Vicia, the corresponding values were remarkably less and apparently unchangeable with incubation time. The extent of goethite dissolution induced by the anaerobic solution from Vicia was greater than that from Astragalus, showing its higher reactivity. (c) 2008 Published by Elsevier Ltd.

Environmental signals: Synthetic humic substances act as xeno-estrogen and affect the thyroid system of Xenopus laevis

According to outdated paradigms humic substances (HS) are considered to be refractory or inert that do not directly interact with aquatic organisms. However, they are taken up and induce biotransformation activities and may act as hormone-like substances. In the present study, we tested whether HS can interfere with endocrine regulation in the amphibian Xenopus laevis. In order to exclude contamination with phyto-hormones, which may occur in environmental isolates, the artificial HS 1500 was applied. The in vivo results showed that HS 1500 causes significant estrogenic effects on X. laevis during its larval development and results of semi-quantitative RT-PCR revealed a marked increase of the estrogenic biomarker estrogen receptor mRNA (ER-mRNA). Furthermore, preliminary RT-PCR results showed that the thyroid-stimulating hormone (TSH beta-mRNA) is enhanced after exposure to HS1500, indicating a weak adverse effect on T3/T4 availability. Hence, HS may have estrogenic and anti-thyroidal effects on aquatic animals, and therefore may influence the structure of aquatic communities and they may be considered environmental signaling chemicals. (c) 2005 Elsevier Ltd. All rights reserved.

Peculiarity of constant photocurrent method for silicon films with mixed amorphous-nanocrystalline structure

The results of conductivity, photoconductivity and constant photocurrent method absorption measurements by DC and AC methods in hydrogenated silicon films with mixed amorphous-nanocrystalline structure are presented. A series of diphasic silicon films was deposited by very high frequency plasma enhanced chemical vapor deposition technique, using different hydrogen dilution ratios of silane. The increase of hydrogen dilution ratio results in five orders of magnitude increase of conductivity and a sharp increase of grain volume fraction. The comparison of the absorption spectra obtained by DC and AC methods showed that they are similar for silicon films with the predominantly amorphous structure and films with high grain volume fraction. However we found a dramatic discrepancy between the absorption spectra obtained by DC and AC constant photocurrent methods in silicon films deposited in the regime of the structure transition from amorphous to nanocrystalline state. AC constant photocurrent method gives higher absorption coefficient than DC constant photocurrent method in the photon energy range of 1.2-1.7 eV. This result indicates the possibility of crystalline grains contribution to absorption spectra measured by AC constant photocurrent method in silicon films with intermediate crystalline grain volume fraction. (c) 2008 Published by Elsevier B.V.

Oxygen pressure dependences of structure and properties of ZnO films deposited on amorphous glass substrates by pulsed laser deposition

ZnO films are prepared on glass substrates by pulsed laser deposition (PLD) at different oxygen pressures, and the effects of oxygen pressure on the structure and optoelectrical properties of as-grown ZnO films are investigated. The results show that the crystallite size and surface roughness of the films increase, but the carrier concentration and optical energy gap E-g decrease with increasing oxygen pressure. Only UV emission is found in the photoluminescence (PL) spectra of all the samples, and its intensity increases with oxygen pressure. Furthermore, there are marked differences in structure and properties between the films grown at low oxygen pressures (0.003 and 0.2 Pa) and the films grown at high oxygen pressures (24 and 150 Pa), which is confirmed by the fact that the crystallite size and UV emission intensity markedly increase, but the carrier concentration markedly decreases as oxygen pressure increases from 0.2 to 24 Pa. These results show that the crystal quality, including the microstructural quality and stoichiometry proportion, of the prepared ZnO films improves as oxygen pressure increases, particularly from 0.2 to 24 Pa.

Design of plasmonic back structures for efficiency enhancement of thin-film amorphous Si solar cells

Metallic back structures with one-dimensional periodic nanoridges attached to a thin-film amorphous Si (a-Si) solar cell are numerically studied. At the interfaces between a-Si and metal materials, the excitation of surface-plasmon polaritons leads to obvious absorption enhancements in a wide near-IR range for different ridge shapes and periods. The highest enhancement factor of the cell external quantum efficiency is estimated to be 3.32. The optimized structure can achieve an increase of 17.12% in the cell efficiency. (C) 2009 Optical Society of America

Crystallization of amorphous Si films by pulsed laser annealing and their structural characteristics

Nanocrystalline silicon (nc-Si) films were prepared by pulsed laser annealed crystallization of amorphous silicon (alpha-Si) films on SiO2-coated quartz or glass substrates. The effect of laser energy density on structural characteristics of nc-Si films was investigated. The Ni-induced crystallization of the a-Si films was also discussed. The surface morphology and microstructure of these films were characterized by scanning electron microscopy, high-resolution electron microscopy, atomic force microscopy and Raman scattering spectroscopy. The results show that not only can the alpha-Si films be crystallized by the laser annealing technique, but also the size of Si nanocrystallites can be controlled by varying the laser energy density. Their average size is about 4-6 nm. We present a surface tension and interface strain model used for describing the laser annealed crystallization of the alpha-Si films. The doping of Ni atoms may effectively reduce the threshold value of laser energy density to crystallize the alpha-Si films, and the flocculent-like Si nanostructures could be formed by Ni-induced crystallization of the alpha-Si films.

Photoluminescence of Er-doped hydrogenated amorphous silicon nitride

Er photoluminescence (Er PL) and dangling bonds (DBs) of annealed Er-doped hydrogenated amorphous silicon nitride (a-SiN:H(Er)) with various concentrations of nitrogen are studied in the temperature range 62-300 K. Post-annealing process is employed to change the DBs density of a-SiN:H(Er). PL spectra, DBs density and H, N concentrations are measured. The intensity of Er PL displays complicated relation with Si DBs density within the annealing temperature range 200-500 degreesC. The intensity of Er PL first increases with decreasing density of Si dangling bonds owing to the structural relaxation up to 250 degreesC, and continues to increase up to 350 degreesC even though the density of Si DBs increases due to the improvement of symmetry environment of Er3+. (C) 2003 Elsevier B.V. All rights reserved.

Amorphous silicon carbide films prepared by H-2 diluted silane-methane plasma

This paper reports on the preparation and characterization of hydrogenated amorphous silicon carbide films prepared by H-2 diluted silane-methane plasma. Carbon-rich a-SiC:H film with band gap of up to 3.3 eV has been achieved. IR and UV Vis spectra were employed to characterize the chemical bonding and optical properties of as-prepared films. It is shown that hydrogen dilution is crucial in obtaining these wide band gap carbon-rich films. Raman and PL measurements were performed to probe the microstructure and photoelectronic properties of these films before and after annealing. Films with intermediate carbon concentration seem more defective and exhibit stronger photoluminescence and subband absorption than others. Films with different compositions exhibit different annealing behaviours. For silicon rich and carbon rich films, high temperature annealing results in the formation of silicon crystallites and graphite clusters, respectively. (C) 2003 Elsevier B.V. All rights reserved.