Photoelectrochemical behavior of a novel composite In0.15Ga0.85As/GaAs vertical bar GaAs/Al0.3Ga0.7As multiple quantum well electrode

A novel composite InxGa1-xAs/GaAs/GaAs/AlxGa1-xAs multiple quantum well material with different well widths was studied as a new kind of photoelectrode in a photoelectrochemical cell. The photocurrent spectrum and photocurrent-electrode potential curve were measured in ferrocene nonaqueous solution. Pronounced quantization effects and strong exciton absorption were observed in the photocurrent spectrum. The effects of surface states and interfacial states on the photocurrent-electrode potential curve are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

Annealing behavior of hexagonal phase content in cubic GaN thin films grown on GaAs (001) by MOCVD

The annealing behavior of the hexagonal phase content in cubic GaN (c-GaN) thin films grown on GaAs (001) by MOCVD is reported. C-GaN thin films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition (MOCVD). High temperature annealing is employed to treat the as-grown c-GaN thin films. The characterization of the c-GaN films is investigated by photoluminescence (PL) and Raman scattering spectroscopy. The change conditions of the hexagonal phase content in the metastable c-GaN are reported. There is a boundary layer existing in the c-GaN/GaAs film. When being annealed at high temperature, the intensity of the TOB and LOB phonon modes from the boundary layer weakens while that of the E-2 phonon mode from the hexagonal phase increases. The content change of hexagonal phase has closer relationship with annealing temperature than with annealing time period.

Pressure behavior of deep centers in ZnSxTe1-x alloys

We have measured photoluminescence of ZnSxTe1-x alloys (x > 0.7) at 300 K and under hydrostatic pressure up to 7 GPa. The spectra contain only a broad emission band under excitation of the 406.7 nm line. Its pressure coefficients are 47, 62 and 45 meV/GPa for x = 0.98, 0.92 and 0.79 samples, which are about 26%, 7% and 38% smaller than that of the band gap in the corresponding alloys. The Stokes shifts between emission and absorption of the bands were calculated by fitting the pressure dependence of the emission intensity, being 0.29, 0.48 and 0.13 eV for the three samples, respectively. The small pressure coefficient and large Stokes shift indicate that the emission band observed in our samples may correspond to the Te isoelectronic center in the ZnSxTe1-x alloy.

Thermomagnetic behavior and first order magnetization processes of Sm3Fe29-xTx and Sm3Fe29-xTxN4 (T = V and Cr)

A systematic investigation of structure and intrinsic magnetic properties of the compounds Sm3Fe29-xTx (T = V and Cr) and their nitrides has been performed. Nitrogenation resulted in remarkable improvements in the saturation magnetization and anisotropy fields at 4.2 K and room temperature. First order magnetization processes are observed at around 5.7 T for Sm3Fe26.7V2.3 and around 2.8 T for Sm3Fe24.0Cr5.0 and Sm3Fe24.0Cr5.0N4, respectively. The spin reorientation of the easy magnetization direction of Sm3Fe26.7V2.3 is observed at around 230 K. As a preliminary result, the maximum remanence B-r of 0.94 T, the coercivity mu(0)H(C) of 0.75 T, and the maximum energy product (BH) of 108.5 kJ/m(3) for the nitride magnet Sm3Fe26.7V2.3N4 are achieved by ball-milling at 293 K.

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.

Growth behavior of AlInGaN films

The structural and surface properties of AlInGaN quaternary films grown at different temperatures on GaN templates by metalorganic chemical vapor deposition are investigated. Formation of two quaternary layers is confirmed and the difference between them is pronounced when the growth temperature is increased. The surface is featured with V-shaped pits and cracks, whose characteristics are further found to be strongly dependent on the growth temperature of AlInGaN layers. The two-layer structure is interpreted by taking into account of the strain status in AlInGaN layers. (C) 2008 Elsevier B.V. All rights reserved.

Behavior model of a CMOS process compatible photo-diode

A behavior model of a photo-diode is presented. The model describes the relationship between photocurrent and incident optical power, and it also illustrates the impact of the reverse bias to the variation of the junction capacitance. With this model, the photo-diode and a CMOS receiver circuit were simulated and designed simultaneously under a universal circuit simulation environment.

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15 +/- 0.15 eV and conduction band offset is -0.90 +/- 0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.

Tunable Ag surface-plasmon-resonance wavelength and its application on the photochromic behavior of TiO2-Ag films

Surface and bulk plasmon resonance of noble metal particles play an essential role in the multicolor photochromism of semiconductor systems containing noble metal particles, Here we examined several key parameters affecting surface plasmon resonance wavelength (SPRW) of Ag particles and investigated the relation between surface plasmon and photochromic reaction wavelength. From the transmission spectra of sandwiched (TiO2/Ag/TiO2) and overcoated (Ag/TiO2) films deposited on quartz substrates at room temperature by rf helicon magnetron sputtering, we demonstrated that the SPRW can be made tunable by changing the surrounding media and thickness of the metal layer. The coloration and bleaching in visible light region due to photochromism were clearly observed for the films inserted with a 0.55 nm Ag layer.

Study on the hydrolysis/precipitation behavior of Keggin Al-13 and Al-30 polymers in polyaluminum solutions

The hydrolysis/precipitation behaviors of Al3+, Al-13 and Al-30 under conditions typical for flocculation in water treatment were investigated by studying the particulates' size development, charge characteristics, chemical species and speciation transformation of coagulant hydrolysis precipitates. The optimal pH conditions for hydrolysis precipitates formation for AlCl3, PAC(A113) and PAC(A130) were 6.5-7.5, 8.5-9.5, and 7.5-9.5, respectively. The precipitates' formation rate increased with the increase in dosage, and the relative rates were AlCl3 >> PAC(A130) > PACA113. The precipitates' size increased when the dosage increased from 50 mu M to 200 mu M, but it decreased when the dosage increased to 800 AM. The Zeta potential of coagulant hydrolysis precipitates decreased with the increase in pH for the three coagulants. The isoelectric points of the freshly formed precipitates for AlCl3, PAC(A113) and PAC(A130) were 7.3, 9.6 and 9.2, respectively. The Zeta potentials of AlCl3 hydrolysis precipitates were lower than those of PAC(A113) and PAC(A130) when pH > 5.0. The Zeta potential of PAC(A130) hydrolysis precipitates was higher than that of PACA113 at the acidic side, but lower at the alkaline side. The dosage had no obvious effect on the Zeta potential of hydrolysis precipitates under fixed pH conditions. The increase in Zeta potential with the increase in dosage under uncontrolled pH conditions was due to the pH depression caused by coagulant addition. Al-Ferron research indicated that the hydrolysis precipitates of AlCl3 were composed of amorphous AI(OH)3 precipitates, but those of PACA113 and PACA130 were composed of aggregates of Al-13 and Al-30, respectively. Al3+ was the most un-stable species in coagulants, and its hydrolysis was remarkably influenced by solution pH. Al-13 and Al-30 species were very stable, and solution pH and aging had little effect on the chemical species of their hydrolysis products. The research method involving coagulant hydrolysis precipitates based on Al-Ferron reaction kinetics was studied in detail. The Al species classification based on complex reaction kinetic of hydrolysis precipitates and Ferron reagent was different from that measured in a conventional coagulant assay using the Al--Ferron method. The chemical composition of Al-a, Al-b and Al-c depended on coagulant and solution pH. The Al-b measured in the current case was different from Keggin Al-13, and the high Alb content in the AlCl3 hydrolysis precipitates could not used as testimony that most of the Al3+ Was converted to highly charged Al-13 species during AlCl3 coagulation.

Chaotic behavior of pulsating heat pipes

Chaotic behavior of closed loop pulsating heat pipes (PHPs) was studied. The PHPs were fabricated by capillary tubes with outer and inner diameters of 2.0 and 1.20 mm. FC-72 and deionized water were used as the working fluids. Experiments cover the following data ranges: number of turns of 4, 6, and 9, inclination angles from 5 degrees (near horizontal) to 90, (vertical), charge ratios from 50% to 80%, heating powers from 7.5 to 60.0 W. The nonlinear analysis is based on the recorded time series of temperatures on the evaporation, adiabatic, and condensation sections. The present study confirms that PHPs are deterministic chaotic systems. Autocorrelation functions (ACF) are decreased versus time, indicating prediction ability of the system is finite. Three typical attractor patterns are identified. Hurst exponents are very high, i.e., from 0.85 to 0.95, indicating very strong persistent properties of PHPs. Curves of correlation integral versus radius of hypersphere indicate two linear sections for water PHPs, corresponding to both high frequency, low amplitude, and low frequency, large amplitude oscillations. At small inclination angles near horizontal, correlation dimensions are not uniform at different turns of PHPs. The non-uniformity of correlation dimensions is significantly improved with increases in inclination angles. Effect of inclination angles on the chaotic parameters is complex for FC-72 PHPs, but it is certain that correlation dimensions and Kolmogorov entropies are increased with increases in inclination angles. The optimal charge ratios are about 60-70%, at which correlation dimensions and Kolmogorov entropies are high. The higher the heating power, the larger the correlation dimensions and Kolmogorov entropies are. For most runs, large correlation dimensions and Kolmogorov entropies correspond to small thermal resistances, i.e., better thermal performance, except for FC-72 PHPs at small inclination angles of theta < 15 degrees.

Study on the Dissociation Behavior of Gas Hydrate in Porous Sediment Based on Fractal Theory

The dissociation process of gas hydrate was regarded as a gas-solid reaction without solid production layer when the temperature was above the zero centigrade. Based on the shrinking core model and the fractal theory, a fractional dimension dynamical model for gas hydrate dissociation in porous sediment was established. The new approach of evaluating the fractal dimension of the porous media was also presented. The fractional dimension dynamical model for gas hydrate dissociation was examined with the previous experimental data of methane hydrate and carbon dioxide hydrate dissociations, respectively. The calculated results indicate that the fractal dimensions of porous media acquired with this method agree well with the previous study. With the absolute average deviation (AAD) below 10%, the present model provided satisfactory predictions for the dissociation process of methane hydrate and carbon dioxide hydrate.