Dielectric relaxation of poled polymer DCNP/PMMA

We have prepared the polymer thin films of a 3-(1,1-dicyanothenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DChTP)/poly (methyl methacrylate) (PMMA) guest-host system by spin coating. In order to investigate their temporal and temperature stability, we have measured their dielectric relaxation spectra including the frequency dependence of the real and imaginary parts of dielectric constants. The investigated frequency ranged from 50 Hz to 10 MHz. The measured temperature range above the glass transition temperature T-g (95 degrees C) of the DCNP/PMMA system was from 95 degrees C to 1250C. By using the Adam-Gibbs model, the temperature dependence of the characteristic time tau above T-g was fitted, and the values of the characteristic times tau below T-g were estimated. The lifetimes of the polymer were evaluated by the Kohlrausch-Williams Watts (KWW) empirical decay model. (C) 2000 Elsevier Science B.V. All rights reserved.

Optical characterization of high-purity cubic GaN grown on GaAs (001) substrate by metalorganic chemical vapor deposition

The optical properties of cubic GaN films have been investigated in the temperature range of 10-300 K. Five peaks were observed at 10 K. From the dependence of photoluminescence emissions on the temperature and excitation intensity, we have assigned two of the five peaks (2.926 and 2.821 eV) to donor-acceptor pair (DAP) transitions. Furthermore, these two peaks were found to be related to a common shallow donor involved in the peak position previously reported at 3.150 eV. The intensities of DAP transitions were much weaker than that of excitonic emission even at low temperature, indicating a relatively high purity of our samples. (C) 2000 American Institute of Physics. [S0003-6951(00)00921-9].

Anomaly of the current self-oscillation frequency in the sequential tunneling of a doped GaAs/AlAs superlattice

An anomalous behavior of the current self-oscillation frequency is observed in the dynamic de voltage bands, emerging from each sawtoothlike branch of the current-voltage characteristic of a doped GaAs/A1As superlattice in the transition process from static to dynamic electric field domain formations. Varying the applied de voltage at a fixed temperature, we find that the frequency increases while the averaged current decreases. Inside each voltage band, the frequency has a strong voltage dependence in the temperature range where the averaged current changes with the applied de voltage. This dependence can be understood in terms of motion of the system along a limit cycle.

Self-aligned current aperture in native oxidized AlInAs buried heterostructure InGaAsP/InP distributed feedback laser

A InGaAsP/InP self-aligned, native oxidized buried heterostructure (BH) distributed feedback (DFB) laser is proposed. It is as easy to process as the ridge waveguide DFB laser and has superior performance. The current aperture can be easily controlled without selective regrowth. The laser exhibits a low threshold of 5.0 mA with 36 dB side mode suppression ratio at the emission wavelength of 1.562 mu m. It emits in a single lobe with full width at half maximum angles of 33.6 degrees and 42.6 degrees for the lateral and vertical fields, respectively. Its beam is more circular than that of the as-grown BH laser because the lower refractive index of oxide compared to the as-grown layer and results in a larger lateral optical confinement. Its characteristic temperature (T-0) is 50 K at room temperature but increases in value at the higher temperature range. (C) 2000 American Institute of Physics. [S0003-6951(00)00812-3].

A comparison of photoluminescence properties of InGaAs GaAs quantum dots with a single quantum well

Variable temperature photoluminescence (PL) measurements for In0.3Ga0.7As(6 nm)/GaAs(34 nm) quantum dot superlattices with a period of 20 and an In0.3Ga0.7As(6 nm)/GaAs(34 nm) reference single quantum well have been conducted. It is found that the temperature dependence is different between the quantum dots and the reference single quantum well. The PL peak energy of the single quantum well decreases faster than that of the quantum dots with increasing temperature. The PL peak energy for the InGaAs/GaAs quantum dots closely follows the InAs band gap in the temperature range from 11 to 170 K, while the PL peak energy for the InGaAs/GaAs quantum well closely follows the GaAs band gap. In comparison with InAs/GaAs quantum dots, the InGaAs/GaAs quantum dots are more typical as a zero-dimensional system since the unusual PL results, which appear in the former, are not obvious for the latter. (C) 1999 American Institute of Physics. [S0021-8979(99)08615-6].

Hetero-epitaxial growth of ZnO films on silicon by low-pressure metal organic chemical vapor deposition

Quality ZnO films were successfully grown on Si(100) substrate by low-pressure metal organic chemical vapor deposition method in temperature range of 300-500 degrees C using DEZn and N2O as precursor and oxygen source respectively. The crystal structure, optical properties and surface morphology of ZnO films were characterized by X-ray diffraction, optical refection and atomic force microscopy technologies. It was demonstrated that the crystalline structure and surface morphology of ZnO films strongly depend on the growth temperature.

Small-divergence singlemode emitting tapered distributed-feedback quantum cascade lasers

A tapered distributed feedback quantum cascade laser emitting at lambda similar to 8.1 mu m is reported. Utilising a tapered waveguide structure with a surface metal grating, the device exhibited singlemode operation over the temperature range of 100 to 214 K, with sidemode suppression ratio > 20 dB and a nearly diffraction limited far-field beam divergence angle of 5.4 degrees.

Magnetoresistance in a nominally undoped InGaN thin film

The magnetotransport properties of a nominally undoped InGaN thin film grown by metal-organic chemical vapor deposition were investigated. Resistivity was measured under a magnetic field up to 5 T over the temperature range of 3 to 298 K. The film exhibits a negative magnetoresistance at low temperatures. Its magnitude decreases with increasing temperature, and turns to be positive for temperatures above 100 K. The negative component was described by a model proposed by Khosla and Fischer for spin scattering of carriers in an impurity band. The positive part was attributed to the effect of Lorentz force on the carrier motion. Agreement between the model and the data is presented.

The design and operation of a new clapboard-type internal circulating fluidized-bed gasifier

The design and operation of a new clapboard-type internal circulating fluidized-bed gasifier is proposed in this article. By arranging the clapboard in the bed, the gasifier is thus divided into two regions, which are characterized by different fluidization velocities. The bed structure is designed so that it can guide the circulating flow passing through the two regions, and therefore the feedstock particles entrained in the flow experience longer residence time. The experimental results based on the present new design, operating in the temperature range of 790 degrees C-850 degrees C, indicate that the gas yield is from 1.6-1.9 Nm(3)/kg feedstock, the gas enthalpies are 5,345 kJ/Nm(3) for wood chip and 4,875 kJ/m(3) for rice husk, and a gasification efficiency up to 75% can be obtained.

Investigation of Interfacial Tensions for Carbon Dioxide Aqueous Solutions by Perturbed-Chain Statistical Associating Fluid Theory Combined with Density-Gradient Theory

The perturbed-chain statistical associating fluid theory and density-gradient theory are used to construct an equation of state (EOS) applicable for the phase behaviors of carbon dioxide aqueous solutions. With the molecular parameters and influence parameters respectively regressed from bulk properties and surface tensions of pure fluids as input, both the bulk and interfacial properties of carbon dioxide aqueous solutions are satisfactorily correlated by adjusting the binary interaction parameter (k(ij)). Our results show that the constructed EOS is able to describe the interfacial properties of carbon dioxide aqueous solutions in a wide temperature range, and illustrate the influences of temperature, pressure, and densities in each phase on the interfacial properties.

Clathrate Dissociation Conditions for Methane + Tetra-n-butyl Ammonium Bromide (TBAB) + Water

Hydrate equilibrium data of the CH4 + tetra-n-butyl ammonium bromide (TBAB) + water have been measured by using the isothermal pressure search method for four components of TBAB aqueous solutions. The three-phase equilibrium lines obtained in the present study are shifted to the low-temperature or high-pressure side from that of the stoichiometric TBAB solution. Moreover, methane uptake into semi-clathrates is confirmed by a shift in the clathrate regions when methane is present. The experiments are carried out in the pressure range of (0.5 to 11) MPa and in the temperature range of (281.15 to 295.15) K.

INVESTIGATION OF NEGATIVE TRANSIENT CURRENT OF ARGON-IMPLANTED GAAS USING PHOTOINDUCED TRANSIENT-CURRENT SPECTROSCOPY

The investigation of deep levels of argon-implanted LEC-grown semi-insulating GaAs with implantation dosages ranging from 1 x 10(11) to 1 x 10(15) cm-2 has been performed. Using a photoinduced transient-current spectroscopy (PITCS) it was demonstrated that, for implantation dosages below 1 X 10(13) cm-2, a negative peak or negative transient current (NTC) was observed in the temperature range from 330 to 350 K. The magnitude of this negative peak increased with dosage up to a level of 1 X 10(12) cm-2, beyond which it decreased with dosage. The dosage dependence of the EL3 peak height and the resistance of the specimen have also been investigated. It was observed that the variation of the EL3 peak height with dosage was similar to the variation of the magnitude of the negative peak, that is the EL3 peak height likewise increased with dosage up to 1 X 10(12) cm-2, and then decreased. The resistance of the original high-resistivity specimen dropped abruptly when the dosage reached 1 X 10(12) cm-2. This critical dosage (1 X 10(12) cm-2) was found to be a threshold for the generation of a highly disordered state.

Donor acceptor pair in molecular beam epitaxy grown GaN

We have recently found evidence of new donor acceptor pair (DAP) luminescence in molecular beam epitaxy (MBE) grown films. A variety of nominally undoped samples have been studied by photoluminescence (PL) over a temperature range of 5-300 K. The samples show intensive luminescence al energies of 3.404-3.413 eV varying with different sample at 5 K, as well as a fairly strong (DX)-X-0 line at low temperature. We attribute the Line at 3.404-3.413 eV to DAP recombination which is over 0.1 eV different from the well known DAP caused by ME-doping in GaN. The DAP line shows fine structure. it even predominates in one particular sample. The peak position shifts to higher energy with temperature increasing from 5 up to 70 K, and as the excitation laser intensity increases. The data are consistent with DAP luminescence involving an acceptor level of about 90 meV (presumably carbon) above the valence band edge in GaN. It is much shallower than the acceptor level of 250 meV produced by the p-type dopant Mg which is commonly used at present. (C) 1997 Elsevier Science S.A.

DEVELOPMENT OF CURRENT-BASED MICROSCOPIC DEFECT ANALYSIS-METHODS AND ASSOCIATED OPTICAL FILLING TECHNIQUES FOR THE INVESTIGATION ON HIGHLY IRRADIATED HIGH-RESISTIVITY SILICON DETECTORS

Current based microscopic defect analysis methods such as current deep level transient spectroscopy (I-DLTS) and thermally stimulated current (TSC) have been further developed in accordance with the need for the defect analysis of highly irradiated (Phi(n) > 10(13) n/cm(2)) high resistivity silicon detectors. The new I-DLTS/TSC system has a temperature range of 8 K less than or equal to T less than or equal to 450 K and a high sensitivity that can detect a defect concentration of less than 10(10)/cm(3) (background noise as low as 10 fA). A new filling method using different wavelength laser illumination has been applied, which is more efficient and suitable than the traditional voltage pulse filling. It has been found that the filling of a defect level depends on such factors as the total concentration of free carriers generated or injected, the penetration length of the laser (laser wavelength), the temperature at which the filling is taking place, as well as the decay time after the filling (but before the measurement). The mechanism of the defect filling can be explained by the competition between trapping and detrapping of defect levels, possible capture cross section temperature dependence, and interaction among various defect levels in terms of charge transferring. Optimum defect filling conditions have been suggested for highly irradiated high resistivity silicon detectors.

PHOTOLUMINESCENCE OF ORDERED GA0.5IN0.5P GROWN BY METALORGANIC VAPOR-PHASE EPITAXY

Optical properties of ordered Ga0.5In0.5P epitaxial layers grown by metalorganic vapor phase epitaxy are investigated by photoluminescence (PL) in a temperature range of 10-200 K using excitation power densities between 0.35 W/cm(2) and 20 W/cm(2). It is found that the intensity of the highest-energy PL peak of the ordered Ga0.5In0.5P epilayer decreases first, then increases and finally goes down again with increasing temperature. A model of ordered Ga0.5In0.5P epitaxial layers is proposed, in which the ordered Ga0.5In0.5P epilayer is regarded as a type-II quantum well structure with band-tail states, and the dependence of PL spectra on the temperature and excitation intensity is reasonably explained. (C) 1995 American Institute of Physics.