Spin relaxation in submonolayer and monolayer InAs structures grown in a GaAs matrix

Electron-spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3-1 monolayer) embedded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin-relaxation time of the submonolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. The electron-spin-relaxation time and the effective g factor in submonolayer samples were found to be strongly dependent on the photogenerated carrier density. The contribution from both the D'yakonov-Perel' mechanism and Bir-Aronov-Pikus mechanism are discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

Role of interface dipole in metal gate/high-k effective work function modulation by aluminum incorporation

The interface dipole and its role in the effective work function (EWF) modulation by Al incorporation are investigated. Our study shows that the interface dipole located at the high-k/SiO2 interface causes an electrostatic potential difference across the metal/high-k interface, which significantly shifts the band alignment between the metal and high-k, consequently modulating the EWF. The electrochemical potential equalization and electrostatic potential methods are used to evaluate the interface dipole and its contribution. The calculated EWF modulation agrees with experimental data and can provide insight to the control of EWF in future pMOS technology.

The Hartman effect in graphene

We investigate theoretically the Hartman effect in quantum tunneling through single and double barriers in a single graphene layer. The numerical results indicate that the Hartman effect in graphene depends heavily on the incident angle and the energy of the carrier in the tunneling process through single and double barriers. We find that the Hartman effect disappears for normal incidence and appears when the incident angle and energy are larger than some critical values.

Anisotropic transport in quantum waveguides due to the spin-orbit interactions

We theoretically investigate the charge transport in the quantum waveguides in the presence of the Rashba spin-orbit interaction and the Dresselhaus spin-orbit interaction. We find that the interplay between the Rashba spin-orbit interaction and Dresselhaus spin-orbit interaction can induce a symmetry breaking and consequently leads to the anisotropic charge transport in the quantum waveguides, the conductance through the quantum waveguides depends sensitively on the crystallographic orientations of the quantum waveguides. The anisotropy of the charge transport can even survive in the presence of disorder effect in realistic systems.

Charge and spin currents in a three-terminal mesoscopic ring

We theoretically investigate the charge and spin currents in a three-terminal mesoscopic ring in the presence of a uniform and nonuniform Rashba spin-orbit interaction (SOI). It is shown that a fully spin-polarized charge current and a pure spin current can be generated by tuning the probe voltages and/or the strength of the Rashba SOI. The charge and spin currents oscillate as the strength of the Rashba SOI increases induced by the spin quantum interference. The ratio of probe voltages oscillates synchronously with the pure spin current as the strength of the Rashba SOI increases in a nonuniform Rashba ring, while it remains constant in a uniform Rashba ring. We demonstrate theoretically that a three-terminal uniform Rashba ring can be used as a spin polarizer and/or spin flipper for different spin injections, and a nonuniform Rashba ring could allow us to detect the pure spin current electrically. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3054322]

Computer-aided design of PV/wind hybrid system

A complete set of match calculation methods for optimum sizing of PV/wind hybrid system is presented. In this method, the more accurate and practical mathematic models for characterizing PV module, wind generator and battery are adopted; combining with hourly measured meteorologic data and load data, the performance of a PV/wind hybrid system is determined on a hourly basis; by fixing the capacity of wind generators, the whole year's LPSP (loss of power supply probability) values of PV/wind hybrid systems with different capacity of PV array and battery bank are calculated, then the trade-off curve between battery bank and PV array capacity is drawn for the given LPSP value; the optimum configuration which can meet the energy demand with the minimum cost can be found by drawing a tangent to the trade-off curve with the slope representing the relationship between cost of PV module and that of the battery. According to this match calculation method, a set of match calculation programs for optimum sizing of PV/wind hybrid systems have been developed. Applying these match calculation programs to an assumed PV/wind hybrid system to be installed at Waglan island of Hong Kong, the optimum configuration and its hourly, daily, monthly and yearly performances are given. (C) 2003 Elsevier Science Ltd. All rights reserved.

Performing Fast Addition and Multiplication by Transferring Single Electrons

This paper proposes novel fast addition and multiplication circuits that are based on non-binary redundant number systems and single electron (SE) devices. The circuits consist of MOSFET-based single-electron (SE) turnstiles. We use the number of electrons to represent discrete multiple-valued logic states and we finish arithmetic operations by controlling the number of electrons transferred. We construct a compact PD2,3 adder and a 12x12bit multiplier using the PD2,3 adder. The speed of the adder can be as high as 600MHz with 400nW power dissipation. The speed of the adder is regardless of its operand length. The proposed circuits have much smaller transistors than conventional circuits.

Wavelet denoising and feature extraction of seismic signal for footstep detection

Seismic sensors are widely used to detect moving target in ground sensor networks. Footstep detection is very important for security surveillance and other applications. Because of non-stationary characteristic of seismic signal and complex environment conditions, footstep detection is a very challenging problem. A novel wavelet denoising method based on singular value decomposition is used to solve these problems. The signal-to-noise ratio (SNR) of raw footstep signal is greatly improved using this strategy. The feature extraction method is also discussed after denosing procedure. Comparing, with kurtosis statistic feature, the wavelet energy feature is more promising for seismic footstep detection, especially in a long distance surveillance.

Fabrication of improved FD SOIMOSFETs for suppressing edge effect

FD SOI MOSFETs with MESA and Irradiated FD SOI MOSFETs with LOCOS isolation usually show the edge effect, that is, the leakage current called hump is generated in the subthreshold region. According to different reasons for generating the edge effect, rounded corner process and BTS structure are applied to improve device performance. The results indicate that the above two methods are effective to reduce the edge effect and qualified devices are fabricated successfully.

Monostable-Bistable Transition Logic Element (MOBILE) Model for Single-Electron Transistors

The traditional monostable-bistable transition logic element (MOBILE) structure is usually composed of resonant tunneling diodes (RTD). This letter describes a new type MOBILE structure consisting of single-electron transistors (i.e. SET-MOBILE). The analytical model of single-electron transistors ( SET) has been considered three states (including an excited state) of the discrete quantum energy levels. The simulation results show negative differential conductance (NDC) characteristics in I-DS-V-DS curve. The SET-MOBILE utilizing NDC characteristics can successfully realize the basic logic functions as the RTD-MOBILE.

Evolution of wetting layers in InAs/GaAs quantum-dot system studied by reflectance difference spectroscopy

The wetting layers (WL) in InAs/GaAs quantum-dot system have been studied by reflectance difference spectroscopy (RDS), in which two structures related to the heavy-hole (HH) and light-hole (LH) transitions in the WL have been observed. The evolution and segregation behaviors of WL during Stranski-Krastanow (SK) growth mode have been studied from the analysis of the WL-related optical transition energies. It has been found that the segregation coefficient of Indium atoms varies linearly with the InAs amount in WL. In addition, the effect of the growth temperature on the critical thickness for InAs island formation has also been studied. The critical thickness defined by the appearance of InAs dots, which is determined by AFM, shows a complex variation with the growth temperature. However, the critical thickness determined by RDS is almost constant in the range of 510-540 degrees C.

High epitaxial growth rate of 4H-SiC using TCS as silicon precursor

High homoepitaxial growth of 4H-SiC has been performed in a home-made horizontal hot wall CVD reactor on n-type 4H-SiC 8 degrees off-oriented substrates in the size of 10 mm x 10 mm, using trichlorosilane (TCS) as silicon precursor source together with ethylene as carbon precursor source. Cross-section Scanning Electron Microscopy (SEM), Raman scattering spectroscopy and Atomic Force Microscopy (AFM) were used to determine the growth rate, structural property and surface morphology, respectively. The growth rate reached to 23 mu m/h and the optimal epilayer was obtained at 1600 degrees C with TCS flow rate of 12 seem in C/Si of 0.42, which has a good surface morphology with a low Rms of 0.64 nm in 10 mu mx10 mu m area.

Operational Optimization of GaN Thin Film Growth Employing Numerical Simulation in a Showerhead MOCVD Reactor

A detailed reaction-tran sport model was studied in a showerhead reactor for metal organic chemical vapor deposition of GaN film by using computational fluid dynamics simulation. It was found that flat flow lines without swirl are crucial to improve the uniformity of the film growth, and thin temperature gradient above the suscptor can increase the film deposition rate. By above-mentioned research, we can employ higher h (the distance from the susceptor to the inlet), P (operational pressure) and the rate of susceptor rotation to improve the film growth.

High Quality AlGaN Grown on GaN Template with HT-AlN Interlayer

When AlGaN is grown on GaN template, crack networks invariably generate when the thickness of the AlGaN layers over GaN exceeds the critical value. We used thin high temperature deposited AlN layer (HT-AlN) as the interlayer between GaN template and AlGaN epilayer which was very effective in eliminating the cracks in AlGaN epilayer. AlGaN layers with high Al mole fractions were also grown. Characterization showed that the crystalline quality of AlGaN epilayer was fairly good even when the At mole fraction was high.

Monolithic Integration of Light Emitting Diodes, Photodetector and Receiver Circuit in Standard CMOS Technology

A monolithic silicon CMOS optoelectronic integrated circuit (OEIC) is designed and fabricated with standard 0.35 mu m CMOS technology. This OEIC circuit consists of light emitting diodes (LED), silicon dioxide waveguide, photodiodes and receiver circuit. The silicon LED operates in reverse breakdown mode and can be turned on at 8.5V 10mA. The silicon dioxide waveguide is composed of multiple layers of silicon dioxide between different metals layers. A two PN-junctions photodetector composed of n-well/p-substrate junction and p(+) active implantation/n-well junction maximizes the depletion region width. The readout circuitry in pixels is exploited to handle as small as 0.1nA photocurrent. Simulation and testing results show that the optical emissions powers are about two orders higher than the low frequency detectivity of silicon CMOS photodetcctor and receiver circuit.