Microfabries of lacustrine sediments to pollutant adsorption from a polluted Eutrophic lake, the Yangtze delta region

Two types of microfabrics relating to pollutant adsorption were studied in the scanning electronic microscope (SEM) in a polluted, eutrophic lake, the Yangtze delta region. Agglutinational texture or the aggregates of small particles are composed of clay minerals and fine organic fragments among the silty grains and the coatings with a thickness about 1 mu m were on the surfaces of the silty grains in the sediments. The chemical constituents of the aggregates and the coatings are K, Na, Ca, Mg, Si, Al, O, Fe, Ti, C, N and P determined in X-ray energy spectrometry connected with the SEM. In some cases, Pb was detected in the aggregates in the top sediment. It is suggested that nutrients and metals are adsorbed to the aggregates, which were formed by electrostatic attraction of physicochemical floes. The coatings on the surface of quartz grains were formed by the interaction of dissociated Al, Si, Fe, etc from silicates with dissolved N, P and C nutrients in interstitial water, which was aroused by human pollution to the lake in recent two decades.

Sedimentary records of water environment in lake Chaohu in the Yangtze delta region, China

Three lacustrine core samples were collected from Chaohu lake in December 2002 in the Yangtze delta region. The grain sizes were analyzed using a Laser Analyzer to obtain grain-size parameters. Sediment geochronology was determined in radioisotopes Cs-137 and the average sedimentary rates are 0.29cm.a(-1), 0.35 cm.a(-1) and 0.24cm-a(-1) in Cores C 1, C2 and C3, respectively. The grain-size parameters of the deposits vary regularly with the fluctuation of hydrodynamics. From 1950s to the beginning of 20th century, coarse-grained sediment was deposited, suggesting strong hydraulic conditions and high water-level periods with much precipitation; from the start of 20(th) century to latter half of 18(th) century, fine-grained sediment was deposited, indicating that weak hydraulic conditions and low water-level periods with less precipitation; before the first half of 18(th) century, coarse-grained sediment was deposited, suggesting great velocity of flow and high water-level periods of more precipitation.

Sigma-Delta模数转换器的设计研究

Submitted by zhangdi (zhangdi@red.semi.ac.cn) on 2009-04-13T11:45:31Z

高性能开关电流存储单元及其Sigma-Delta 调制器的研究

Submitted by zhangdi (zhangdi@red.semi.ac.cn) on 2009-04-13T11:45:31Z

Polarization effects simulation of AlGaN/GaN heterojunction by using a symbolistic delta-doping layer

Polarization effects in AlGaN/GaN heterojunction are simulated based on a traditional semiconductor device simulator. A delta doping layer is purposely inserted at the interface of the heterojunction in the simulation, so the ionized donors or acceptors can represent polarization-induced positive or negative fixed charges. The free electron distribution of single AlGaN/GaN heterostructures with Ga-face and N-face growth is compared, and the results of the simulation show that carrier confinement takes place only in the former structure. The dependence of sheet density of free electrons at the interface of Ga-face growth AlGaN/GaN on Al composition and the thickness of AlGaN is also investigated. The consistency of simulation results with the experiments and calculations reported by other researchers shows that this method can be effectively used to deal with the polarization effects in the simulation of GaN-based heterojunction devices. (C) 2004 Elsevier Ltd. All rights reserved.

Effect of interaction between periodic delta-doping in both well and barrier layers on modulation of superlattice band structure

The modulation of superlattice band structure via periodic delta-doping in both well and barrier layers have been theoretically investigated, and the importance of interaction between the delta-function potentials in the well layers and those in the barrier layers on SL band structure have been revealed. It is pointed out that the energy dispersion relation Eq. (3) given in [G. Ihm, S.K. Noh, J.I. Lee, J.-S. Hwang, T.W. Kim, Phys. Rev. B 44 (1991) 6266] is an incomplete one, as the interaction between periodic delta-doping in both well and barrier layers had been overlooked. Finally, we have shown numerically that the electron states of a GaAs/Ga0.7Al0.3As superlattice can be altered more efficiently by intelligent tuning the two delta-doping's positions and heights. (c) 2007 Elsevier B.V. All rights reserved.

Observations on subband electron properties in In0.65Ga0.35As/In0.52Al0.48As MM-HEMT with Si delta-doped on the barriers

Magneto-transport measurements have been carried out on a Si delta-doped In0.65Ga0.35As/In0.52Al0.48As metamorphic high-electron-mobility transistor with InP substrate in a temperature range between 1.5 and 60 K under magnetic field up to 13 T. We studied the Shubnikov-de Haas (SdH) effect and the Hall effect for the In0.65Ga0.35As/In0.52Al0.48As single quantum well occupied by two subbands and obtained the electron concentration and energy levels respectively. We solve the Schrodinger-Kohn-Sham equation in conjunction with the Poisson equation self-consistently and obtain the configuration of conduction band, the distribution of carriers concentration, the energy level of every subband and the Fermi energy. The calculational results are well consistent with the results of experiments. Both experimental and calculational results indicate that almost all of the delta-doped electrons transfer into the quantum well in the temperature range between 1.5 and 60 K.

Optical properties of the E-0+Delta(0) energy level higher than the bandgap of GaAs studied by micro-photoluminescence technique

Using micro-photoluminescence technique, we observed a new photoluminescence peak about 0.348 eV above the bandgap of GaAs (E-0). By analyzing its optical characteristics, we assigned this peak to the nonequilibrium luminescence emission from the E-0 + Delta(0) bandgap in semi-insulated GaAs, which was further verified by Raman results. The observed polarization, excitation power dependence and temperature dependence of the photoluminescence spectra from the E-0 + Delta(0) energy level were very similar to those from the E-0 of GaAs. This mainly resulted from the common conduction band around Gamma(6) that was involved in the two optical transition processes, and indicated that the optical properties of bulk GaAs were mainly determined by the intrinsic properties of the conduction band. Our results demonstrated that the micro-photoluminescence technique is a powerful tool to investigate the high energy states above the fundamental bandgap in semiconductor materials.

Enhanced performance of p-GaN by Mg delta doping

The electrical and structural properties of Mg delta-doped GaN epilayers grown by MOCVD were investigated. Compared to uniform Mg-doping GaN layers, it has been shown that the delta-doping (delta-doping) process could suppress the dislocation density and enhance the p-type performance. The influence of pre-purge step on the structural properties of GaN was also investigated. The hole concentration of p-GaN decreases when using a pre-purge step. These results can be explained convincingly using a simple model of impurity incorporation under Ga-free growth condition. (C) 2007 Elsevier B.V. All rights reserved.

Pseudospin in Si delta-doped InAlAs/InGaAs/InAlAs single quantum well

Magneto-transport measurements have been carried out on double/single-barrier-doped In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well samples from 1.5 to 60 K in an applied magnetic field up to 13 T. Beating Shubnikov-de Haas oscillation is observed for the symmetrically double-barrier-doped sample and demonstrated due to a symmetric state and an antisymmetric state confined in two coupled self-consistent potential wells in the single quantum well. The energy separation between the symmetric and the antisymmetric states for the double-barrier-doped sample is extracted from experimental data, which is consistent with calculation. For the single-barrier-doped sample, only beating related to magneto-intersubband scattering shows up. The pesudospin property of the symmetrically double-barrier-doped single quantum well shows that it is a good candidate for fabricating quantum transistors. (c) 2007 Elsevier Ltd. All rights reserved.

Self-consistent analysis of double-delta-doped InAlAs/InGaAs/InP HEMTs

We have carried out a theoretical study of double-delta-doped InAlAs/InGaAs/InP high electron mobility transistor (HEMT) by means of the finite differential method. The electronic states in the quantum well of the HEMT are calculated self-consistently. Instead of boundary conditions, initial conditions are used to solve the Poisson equation. The concentration of two-dimensional electron gas (2DEG) and its distribution in the HEMT have been obtained. By changing the doping density of upper and lower impurity layers we find that the 2DEG concentration confined in the channel is greatly affected by these two doping layers. But the electrons depleted by the Schottky contact are hardly affected by the lower impurity layer. It is only related to the doping density of upper impurity layer. This means that we can deal with the doping concentrations of the two impurity layers and optimize them separately. Considering the sheet concentration and the mobility of the electrons in the channel, the optimized doping densities are found to be 5 x 10(12) and 3 x 10(12) cm(-2) for the upper and lower impurity layers, respectively, in the double-delta-doped InAlAs/InGaAs/InP HEMTs.

A direct digital frequency synthesizer with fourth-order phase domain Delta Sigma noise shaper and 12-bit current-steering DAC

This paper presents a direct digital frequency synthesizer (DDFS) with a 16-bit accumulator, a fourth-order phase domain single-stage Delta Sigma interpolator, and a 300-MS/s 12-bit current-steering DAC based on the Q(2) Random Walk switching scheme. The Delta Sigma interpolator is used to reduce the phase truncation error and the ROM size. The implemented fourth-order single-stage Delta Sigma noise shaper reduces the effective phase bits by four and reduces the ROM size by 16 times. The DDFS prototype is fabricated in a 0.35-mu m CMOS technology with active area of 1.11 mm(2) including a 12-bit DAC. The measured DDFS spurious-free dynamic range (SFDR) is greater than 78 dB using a reduced ROM with 8-bit phase, 12-bit amplitude resolution and a size of 0.09 mm(2). The total power consumption of the DDFS is 200)mW with a 3.3-V power supply.

Magneto-transport characteristics of two-dimensional electron gas for Si delta-doped InAlAs/InGaAs single quantum well

Magneto-transport measurements have been carried out on a Si heavily delta-doped In0.52Al0.48As/In(0.53)G(0.47)As single quantum well in the temperature range between 1.5 and 60 K under magnetic field up to 10 T. We studied the Shubnikov-de Haas(SdH) effect and the Hall effect for the In0.52Al0.48As/In(0.53)G(0.47)As single quantum well occupied by two subbands, and have obtained the electron concentration, mobility, effective mass and energy levels respectively. The electron concentrations of the two subbands derived from mobility spectrum combined with multi-carrier fitting analysis are well consistent with the result from the SdH oscillation. From fast Fourier transform analysis for d(2)rho/dB(2)-1/B, it is observed that there is a frequency of f(1)-f(2) insensitive to the temperature, besides the frequencies f(1), f(2) for the two subbands and the frequency doubling 2f(1), both dependent on the temperature. This is because That the electrons occupying the two different subbands almost have the same effective mass in the quantum well and the magneto-intersubband scattering between the two subbands is strong.

Longitudinal optic phonon-plasmon coupling in delta-doped metamorphic InAlAs/InGaAs high-electron-mobility transistor structures on GaAs substrates

InAlAs/InGaAs metamorphic high-electron-mobility transistor structures with different spacer layers on GaAs substrates are characterized by Raman measurements. The influence of In0.52Al0.48As spacer thickness on longitudinal optic phonon-plasmon coupling is investigated. It is found that the intensity of GaAs-like longitudinal optic phonon, which couples with collective intersubband transitions of two-dimensional electron gas, is strongly affected by the different subband energy spacings, subband electron concentrations, and wave function distributions, which are determined by different spacer thicknesses. (C) 2001 American Institute of Physics.