The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed. (c) 2005 Elsevier B.V. All rights reserved.

Fabrication of metallic air bridges using multiple-dose electron beam lithography

The techniques of fabricating metallic air bridges using different resists in a one-step electron beam lithography are presented. The exposure process employed a single-layer polymethyl methacrylate (PMMA) or photoresists with either different doses in the span and feet areas or with varying acceleration voltage of the electron beam. The process using photoresists with different doses has produced air bridges more stable than what the PMMA method using various acceleration voltages would achieve. Using this method, air bridges up to 12 mu m long have been fabricated. The length and height of these metallic air bridges vary with the photoresist thickness. (c) 2006 American Institute of Physics.

Spectral dependence of spin photocurrent and current-induced spin polarization in an InGaAs/InAlAs two-dimensional electron gas

The converse effects of spin photocurrent and current induced spin polarization are experimentally demonstrated in a two-dimensional electron gas system with Rashba spin splitting. Their consistency with the strength of the Rashba coupling as measured for the same system from beating of the Shubnikov-de Haas oscillations reveals a unified picture for the spin photocurrent, current-induced spin-polarization, and spin-orbit coupling. In addition, the observed spectral inversion of the spin photocurrent indicates a system with dominating structure inversion asymmetry.

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.

Quantum trajectory analysis for electrical detection of single-electron spin resonance

A Monte Carlo simulation on the basis of quantum trajectory approach is carried out for the measurement dynamics of a single-electron spin resonance. The measured electron, which is confined in either a quantum dot or a defect trap, is tunnel coupled to a side reservoir and continuously monitored by a mesoscopic detector. The simulation not only recovers the observed telegraphic signal of detector current, but also predicts unique features in the output power spectrum which are associated with electron dynamics in different regimes.

Room-temperature observation of electron resonant tunneling through InAs/AlAs quantum dots

Molecular beam epitaxy is employed to manufacture self-assembled InAs/AlAs quantum-dot resonant tunneling diodes. The resonant tunneling current is superimposed on the thermal current, and together they make up the total electron transport in devices. Steps in current-voltage characteristics and peaks in capacitance-voltage characteristics are explained as electron resonant tunneling via quantum dots at 77 or 300 K, and thus resonant tunneling is observed at room temperature in III-V quantum-dot materials. Hysteresis loops in the curves are attributed to hot electron injection/emission process of quantum dots, which indicates the concomitant charging/discharging effect. (c) 2006 The Electrochemical Society.

Resonant tunnelling diodes and high electron mobility transistors integrated on GaAs substrates

A1GaAs/1nGaAs high electron mobility transistors (HEMTs) and AlAs/GaAs resonant tunnelling diodes (RTDs) are integrated on GaAs substrates. Molecular beam epitaxy is used to grow the RTD on the HEMT structure. The current-voltage characteristics of the RTD and HEMT are obtained on a two-inch wafer. At room temperature, the peak-valley, current ratio and the peak voltage are about 4.8 and 0.44 V, respectivcly The HEMT is characterized by a, gate length of 1 mu m, a, maximum transconductance of 125 mS/mm, and a threshold voltage of -1.0 V. The current-voltage, characteristics of the series-connected RTDs are presented. Tire current-voltage curves of the parallel connection of one RTD and one HEMT are also presented.

Study of persistent photoconductivity and subband electronic properties of the two-dimensional electron gas in modulation doped GaAs/AlGaAs structure

We obtained the high mobility Of mu(2K) = 1.78 x 10(6) cm(2)/V . s in Si-doped GaAs/AlGaAs two-dimensional electron gas (2DEG) structures. After the sample was illuminated by a light-emitting diode in magnetic fields up to 6 T at T = 2K, we did observe the persistent photoconductivity effect and the electron density increased obviously. The electronic properties of 2DEG have been studied by Quantum-Hall-effect and Shubnikov-de Haas (SdH) oscillation measurements. We found that the electron concentrations of two subbands increase simultaneity with the increasing total electron concentration, and the electron mobility also increases obviously after being illuminated. At the same time, we also found that the electronic quantum lifetime becomes shorter, and a theoretical explunation is given through the widths of integral quantum Hall plateaus.

Spin-polarized transport in a lateral two-dimensional diluted magnetic semiconductor electron gas

The transport property of a lateral two-dimensional paramagnetic diluted magnetic semiconductor electron gas under a spatially periodic magnetic field is investigated theoretically. We find that the electron Fermi velocity along the modulation direction is highly spin dependent even if the spin polarization of the carrier population is negligibly small. It turns out that this spin-polarized Fermi velocity alone can lead to a strong spin polarization of the current, which is still robust against the energy broadening effect induced by the impurity scattering. (c) 2006 American Institute of Physics.

Quantum dissipation and broadening mechanisms due to electron-phonon interactions in self-formed InGaN quantum dots

Quantum dissipation and broadening mechanisms in Si-doped InGaN quantum dots are studied via the photoluminescence technique. It is found that the dissipative thermal bath that embeds the quantum dots plays an important role in the photon emission processes. Observed spontaneous emission spectra are modeled with the multimode Brownian oscillator model achieving an excellent agreement between experiment and theory for a wide temperature range. The dimensionless Huang-Rhys factor characterizing the strength of electron-LO-phonon coupling and damping constant accounting for the LO-phonon-bath interaction strength are found to be similar to 0.2 and 200 cm(-1), respectively, for the InGaN QDs. (c) 2006 American Institute of Physics.

Quantum measurement of an electron in a disordered potential: Delocalization versus measurement voltages

Quantum point contact (QPC), one of the typical mesoscopic transport devices, has been suggested to be an efficient detector for quantum measurement. In the context of two-state charge qubit, our previous studies showed that the QPC's measurement back-action cannot be described by the conventional Lindblad quantum master equation. In this work, we study the measurement problem of a multistate system, say, an electron in disordered potential, subject to the quantum measurement of the mesoscopic detector QPC. The effect of measurement back-action and the detector's readout current are analyzed, where particular attention is focused on some new features and the underlying physics associated with the measurement-induced delocalization versus the measurement voltages.

Magnetotransport in two-dimensional electron gas: The interplay between spin-orbit interaction and Zeeman splitting

We investigate theoretically the interplay between Zeeman splitting, Rashba spin-orbit interaction (RSOI), and Dresselhaus spin-orbit interaction (DSOI) and its influence on the magnetotransport property of two-dimensional electron gas (2DEG) at low temperature. Our theoretical results show that the nodes of the beating patterns of the magnetoresistivity rho(xx) for 2DEG with RSOI or DSOI alone depend sensitively on the total spin splitting induced by these three spin splitting mechanisms. It is interesting to find that the eigenstates in the presence of RSOI alone are connected with those in the presence of DSOI alone but with opposite Zeeman splitting by a time-reversal transformation. Consequently, the magnetoresistivities exhibit exactly the same oscillation patterns for these two cases. For strong RSOI or DSOI alone, the magneto-oscillation of rho(xx) shows two distinct periods. For 2DEG with both RSOI and DSOI, the beating patterns vanish for equal RSOI and DSOI strengths and vanishing Zeeman splitting. They will appear again, however, when Zeeman splitting or the difference between RSOI and DSOI strengths increases.

Theory of ultrafast optical manipulation of electron spins in quantum wells

Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta , Science 292, 2458 (2001)], we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed. (c) 2005 American Institute of Physics.

Electron transport properties of MM-HEMT with varied channel indium contents

Transport properties of two-dimensional electron gas (2DEG) are crucial to metamorphic high-electron-mobility transistors (MM-HEMT). We have investigated the variations of subband electron mobility and concentration versus temperature from Shubnikov-de Hass oscillations., and variable temperature Hall measurements. The results indicate that the electrical performance is the best when the In content is 0.65 in the channel for MM-HEMT. When the In content exceeds 0.65, a large lattice mismatch will cause dislocations and result in the decrease of mobility and the fall of performance in materials and devices.

Energy spectra of two-electron two-dimensional quantum dots confined by elliptical and bowl-like potentials

The laterally confining potential of quantum dots (QDs) fabricated in semiconductor heterostructures is approximated by an elliptical two-dimensional harmonic-oscillator well or a bowl-like circular well. The energy spectrum of two interacting electrons in these potentials is calculated in the effective-mass approximation as a function of dot size and characteristic frequency of the confining potential by the exact diagonalization method. Energy level crossover is displayed according to the ratio of the characteristic frequencies of the elliptical confinement potential along the y axis and that along the x axis. Investigating the rovibrational spectrum with pair-correlation function and conditional probability distribution, we could see the violation of circular symmetry. However, there are still some symmetries left in the elliptical QDs. When the QDs are confined by a "bowl-like" potential, the removal of the degeneracy in the energy levels of QDs is found. The distribution of energy levels is different for the different heights of the barriers. (C) 2003 American Institute of Physics.