Low-temperature electron transport in GaAs

For temperatures between 5 and 80 K, electron mobilities in n-type GaAs have been calculated using ionized impurity cross sections derived from the partial wave phase shift method. The mobilities obtained differ significantly from those calculated previously using Brooks-Herring theory. Although agreement with data from a number of experimental samples is good above 30 K, the theoretical mobilities tend to overestimate experiment at lower temperatures. In order to improve the agreement it may be necessary to incorporate the effects of multi-ion scattering and collision-broadening, and to generalize the screened Coulomb scattering potential.     

Quantum transport in periodically δ-doped GaAs

The Shubnikov-de Haas spectra of GaAs samples with a periodical doping with Si were measured at 4.2 K using fields of 0–14 T. From the Shubnikov-de Haas spectrum the quantum mobilities associated with individual minibands were estimated for each sample. The sheet density of Si atoms in each doping plane was fixed at approximately 2.5 × 10¹² cm^?2 in all samples and the doping period was varied in the range 40–1000 Å. The quantum mobilities obtained experimentally are compared with theoretical calculations using the random phase approximation to describe the screened interaction between electrons and charged impurities. The theory describes qualitatively the results of the experiment, i.e. an increase of the quantum mobility with the index of the miniband, and a decrease in the mobility in all minibands when the doping period is made shorter (with a weak maximum at intermediate values of the doping period). Various possible improvements of the model are suggested.     

Electron transport in modulation-doped GaAs v-groove quantum wires

We report the growth of modulation-doped GaAs/Al_xGa_1−xAs v-groove quantum wires and structural, electrical and optical investigations of their electronic states and transport properties. By using alternative group III precursors on partially SiO₂ masked pre-patterned GaAs substrates, samples have been fabricated which permit electrical measurements of single isolated wire structures without the need for additional electron-beam lithography. Magneto-transport was measured as a function of tilt angle of the incident magnetic field to identify the formation of low-dimensional electron gases in different parts of the structure. Photoluminescence investigations reveal 1D and 2D confined states which show different carrier heating when electric fields are applied along the wire structure.     

Electron transport in sub-micron GaAs channels at 300 K

Transient velocity-field characteristics have been computed for GaAs channels having lengths of 0.1, 0.2, 0.5, 1, and 20 μm for electric fields between 1 and 50 kV/cm at 300 K. The results are compared with earlier calculations and the significant features of the computed results are discussed. It is found that the electron motion for all channel lengths and for all fields is significantly affected by collisions. The threshold field for negative differential mobility increases, and the magnitude of the differential mobility decreases with decrease in the length of the sample. The maximum steady-state velocity increases with decrease in the length and may be as high as 5.4×10⁷ cm/s for 0.1 μm samples.     

Tight-binding study of quantum transport in nanoscale GaAs Schottky MOSFET

This paper explores the band structure effect to elucidate the feasibility of an ultra-scaled GaAs Schottky MOSFET (SBFET) in a nanoscale regime. We have employed a 20-band sp3d5s* tight-binding (TB) approach to compute E K dispersion. The considerable difference between the extracted effective masses from the TB approach and bulk values implies that quantum confinement affects the device performance. Beside high injection velocity, the ultra-scaled GaAs SBFET suffers from a low conduction band DOS in the Γ valley that results in serious degradation of the gate capacitance. Quantum confinement also results in an increment of the effective Schottky barrier height (SBH). Enhanced Schottky barriers form a double barrier potential well along the channel that leads to resonant tunneling and alters the normal operation of the SBFET. Major factors that may lead to resonant tunneling are investigated. Resonant tunneling occurs at low temperatures and low drain voltages, and gradually diminishes as the channel thickness and the gate length scale down. Accordingly, the GaAs (100) SBFET has poor ballistic performance in nanoscale regime.     

Anisotropic spin transport in (110) GaAs quantum wells

Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.     

Exciton transport by moving strain dots in GaAs quantum wells

Spatially resolved photoluminescence spectroscopy was employed to investigate the dynamic control of excitons in GaAs/AlGaAs double quantum wells within the confined and moving potentials formed by the interference of orthogonal surface acoustic waves. We demonstrate the unique ability of these dynamic strain dots to transport photogenerated indirect excitons over macroscopic distances.     

Investigation of transient transport and recombination phenomena in semiinsulating GaAs

Photoconductivity and Hall voltage kinetics were measured simultaneously in SI GaAs monocrystals, using the pulsed neodimium laser excitation. The scattering and recombination centres were found to have a different influence at different time intervals of the transients (from 10 ns to some seconds). It is shown that in GaAs the photoconductivity relaxation in some time intervals can be interpreted correctly only by taking into account the mobility changes. The obtained resuls are explained in terms of recharging of the scattering centres and variations of the capture cross-section of charge carries on the local centres.     

Influence of impurities on short range electron transport in GaAs

The influence of Cr impurities on muonium atom formation in GaAs has been studied using muon spin relaxation techniques with alternating electric fields. The results suggest that electron transport to and capture by the muon is suppressed by capture/scattering on intervening Cr centers. The length scale involved is estimated to be about 3x10(-6) cm. This offers an opportunity to study electron transport to positive centers in semiconductors on a microscopic scale.     

Electrical spin transport in a GaAs (110) channel

A homoepitaxial GaAs (110) channel gives a great interest in the field of semiconductor spintronics due to the longer spin diffusion. By utilizing optimal temperature process and V/III flux ratio control, the GaAs layer is grown without a serious defect. In a ferromagnet/semiconductor hybrid device, Tb₂₀Fe₆₂Co₁₈/Ru/Co₄₀Fe₄₀B₂₀ films are deposited on the GaAs (110) channel as a spin source to investigate the spin transport in (110)-oriented channel. To measure the Hanle signal, an in-plane magnetic field is applied to the perpendicularly polarized spins which are injected from the Tb₂₀Fe₆₂Co₁₈ layer. From the experimental results, the spin diffusion length in a GaAs (110) is longer than that in a GaAs (100) by up to 25%. The proper selection of crystalline growth direction for the spin transport channel is a viable solution for an efficient spin transport.     

Picosecond luminescence approach to vertical transport in GaAs/GaAlAs superlattices

Picosecond luminescence of GaAs/GaAlAs superlattices has been measured at 5 K. Asymetrical structures where one larger well is introduced at 9000 Å from the surface are studied. It is then possible to estimate the mean transfer time of photoexcited carriers through 9000 Å of superlattice. This time is found to be about 4 nsec in a 40/40 Å superlattice and 800 psec in a 30/30 Å one. This evidences the rather high mobility of small period superlattices in the growth direction.     

Optical and transport studies in coupled InAs quantum dots embedded in GaAs

We studied optical and electron transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. Photoluminescence (PL) from the high dot density samples indicated asymmetry in the PL spectra when the ambient temperature is lower than about 50 K. Comparing this result with theoretical calculations, it is shown that this phenomenon is explained by the inter-dot electronic coupling effect. In the photo-conductance measurement, resonance peaks in the current–voltage characteristics were observed in the low-temperature region. The dependence of the resonance voltage on the magnetic field intensity was studied to extract the g-factor. It is also shown that the resonances are attributed to the current corresponding to the electron transport through QDs. According to these results, it is concluded that the inter-dot electronic coupling in the self-assembled InAs/GaAs QD systems occurs when the inter-dot spacing is as low as several nanometers and the ambient temperature is less than about 50 K.     

Electrical transport in superlattices containing InAs quantum dots in GaAs and InP

Selectively-doped heterostructures based on both GaAs and InP containing several atomic layers coverage of InAs as both strained 2D and partially relaxed 3D (quantum dot) have been grown by gas source molecular beam epitaxy and the transport properties have been investigated. We show that while coherently strained InAs in 2D layers results in increased electron mobilities, the formation of 3D quantum dots appear to trap electrons and decrease significantly the mobility of those remaining. The degree of trapping is dependent on the size and density of the dots.