GaAs Conduction Band Research Articles

First-principles study of the origin and nature of ferromagnetism inGa1−xMnxAs

The properties of diluted Ga$_{1-x}$Mn$_x$As are calculated for a wide range of Mn concentrations within the local spin density approximation of density functional theory. M\"ulliken population analyses and orbital-resolved densities of states show that the configuration of Mn in GaAs is compatible with either 3d$^5$ or 3d$^6$, however the occupation is not integer due to the large $p$-$d$ hybridization between the Mn $d$ states and the valence band of GaAs. The spin splitting of the conduction band of GaAs has a mean field-like linear variation with the Mn concentration and indicates ferromagnetic coupling with the Mn ions. In contrast the valence band is antiferromagnetically coupled with the Mn impurities and the spin splitting is not linearly dependent on the Mn concentration. This suggests that the mean field approximation breaks down in the case of Mn-doped GaAs and corrections due to multiple scattering must be considered. We calculate these corrections within a simple free electron model and find good agreement with our {\it ab initio} results if a large exchange constant ($N\beta=-4.5$eV) is assumed.

Anti-crossing of Landau levels of different two-dimensional subbands in GaAs in normal magnetic field

Tunnel current measurements between strongly disordered two-dimensional electron systems in a perpendicular magnetic field are presented. Two-dimensional electron accumulation layers are formed by Si delta doping of GaAs on either sides of an AlGaAs tunnel barrier. Strong interaction between Landau levels of the two-dimensional subbands in each accumulation layers are observed as an anti-crossing of the related peak positions in the tunnel current versus voltage curves as a function of magnetic field. The splitting of the interacting Landau levels is about 10meV, which cannot be explained by non-parabolicity of the conduction band in GaAs. Possible reasons for the observed interaction are discussed.

Observation of the interaction between Landau levels of different two-dimensional subbands in GaAs in a normal magnetic field

Tunnel current measurements between strongly disordered two-dimensional electron systems in a perpendicular magnetic field are presented. Two-dimensional electron accumulation layers are formed by an extremely narrow layer of Si donors (Si delta doping) in GaAs on either side of an AlGaAs tunnel barrier. Strong interaction between Landau levels of the two-dimensional subbands in each accumulation layer is observed as an anticrossing of the related peak positions in the tunnel current vs. voltage curves as a function of magnetic field. The splitting of the interacting Landau levels is about 10 meV, which cannot be explained by nonparabolicity of the conduction band in GaAs. A possible reason for the observed interaction connected with the collective excitations in the 2DES is discussed.

Fast processes at semiconductor–liquid interfaces: reactions at GaAs electrodes

The kinetics of charge transfer processes between GaAs and various metallocenes in acetonitrile (ACN) have been studied. The oxidized as well as the reduced species are adsorbed at the GaAs surface. A model for the overall electron transfer process was derived that incorporates the adsorption of the redox system. The analysis of impedance spectra showed that the electron transfer from the conduction band of GaAs to the acceptor is extremely fast and that the overall rate of electron transfer is limited by the rate of thermionic emission from the GaAs bulk region to the surface.

Effect of the Conduction Band Non-Parabolicity on D0 Binding Energy in a GaAs/Ga0.7Al0.3As Spherical Quantum Dot

The ground state binding energy of a donor centered in a GaAs/Ga0.7Al0.3As spherical quantum dot is calculated, by using a numerical procedure based on trigonometric sweep method. It is considered a model that takes into account the effects produced by (i) a graded variation of Al concentration at the interface, (ii) the dependence of the material parameters on the Al concentration and (iii) the non-parabolicity of the GaAs conduction band. Comparing to previous results, obtained for a model with a spherical rectangular potential, a considerable binding energy enhancement, ranging from 20 to 50% for different quantum dot radii, has been found.

Hilbert-space structure of a solid-state quantum computer: Two-electron states of a double-quantum-dot artificial molecule

We study theoretically a double quantum dot hydrogen molecule in the GaAs conduction band as the basic elementary gate for a quantum computer with the electron spins in the dots serving as qubits. Such a two-dot system provides the necessary two-qubit entanglement required for quantum computation. We determine the excitation spectrum of two horizontally coupled quantum dots with two confined electrons, and study its dependence on an external magnetic field. In particular, we focus on the splitting of the lowest singlet and triplet states, the double occupation probability of the lowest states, and the relative energy scales of these states. We point out that at zero magnetic field it is difficult to have both a vanishing double occupation probability for a small error rate and a sizable exchange coupling for fast gating. On the other hand, finite magnetic fields may provide finite exchange coupling for quantum computer operations with small errors. We critically discuss the applicability of the envelope function approach in the current scheme and also the merits of various quantum chemical approaches in dealing with few-electron problems in quantum dots, such as the Hartree-Fock self-consistent field method, the molecular orbital method, the Heisenberg model, and the Hubbard model. We also discuss a number of relevant issues in quantum dot quantum computing in the context of our calculations, such as the required design tolerance, spin decoherence, adiabatic transitions, magnetic field control, and error correction.

Observation of excitons formed by the holes confined at the Al 0.5Ga 0.5As/GaAs interface

The radiative recombination in the Be-doped single heterojunction Al 0.5Ga 0.5As/GaAs, so-called H-band, has been studied by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Magnetophotoluminescence measurements were also performed. The H-band was interpreted as the recombination of holes confined on the ground level in the quantum well at the interface and electrons from GaAs conduction band. The carriers involved in the recombination originate from 3D excitons excited in the flat band region of GaAs, which diffuse towards the interface. In the PLE spectra two new lines were observed. We interpret them in means of short life-time excitons composed of a confined 2D hole and a free electron, which are excited near the interface.

Electronic characteristics of InAs self-assembled quantum dots

Deep-level transient spectroscopy and photoluminescence studies have been carried out on structures containing self-assembled InAs quantum dots formed in GaAs matrices. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the deep-level transient spectroscopy technique. From analysis of deep-level transient spectroscopy measurements it follows that the quantum dots have electron levels 130 meV below the bottom of the GaAs conduction band and heavy-hole levels at 90 meV above the top of the GaAs valence band. Combining with the photoluminescence results, the band structures of InAs and GaAs have been determined.

Spin-polarized electron transport processes at the ferromagnet/semiconductor interface

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in ferromagnet/semiconductor hybrid structures. The Schottky characteristics at the interface were varied by using NiFe, Co and Fe as the ferromagnet. The Schottky characteristics were clearly observed with NiFe and Co/GaAs, while almost ohmic I-V characteristics were seen with Fe/GaAs. At negative bias a helicity-dependent photocurrent, dependent upon the magnetization configuration of the film and the Schottky barrier height, was detected upon modulating the polarization from right to left circular, For the magnetization along or perpendicular to the surface normal, the helicity-dependent photocurrent I/sup n/ or I/sup 0/, respectively, was measured. The asymmetry P=(I/sup n/-I/sup 0/)/(I/sup n/+I/sup 0/) of the helicity-dependent photocurrent decreases upon increasing the doping density of the GaAs substrates. P also decreases with photon energy h/spl nu/ as found for the polarization of photoexcited electrons in GaAs. In NiFe/GaAs samples for h/spl nu/=1.59 eV, P=16% for n/sup +/=10/sup 23/ m/sup -3/ and P=-23% for p/sup -/=10/sup 25/ m/sup -3/ doped substrates, i.e. P is comparable in magnitude to the theoretically predicted spin polarization of 50% for the optically pumped conduction band in GaAs. The results provide unambiguous evidence of spin-polarized electron transport through the ferromagnet/semiconductor interface and show that the Schottky barrier height controls the spin-polarized electron current passing from the semiconductor to the ferromagnet. The asymmetry data indicates that spin-polarized electrons are transmitted from the semiconductor to the ferromagnet with a high efficiency.

Emission of electrons from the ground and first excited states of self-organized InAs/GaAs quantum dot structures

Capacitance- and conductance-voltage studies have been carried out on Schottky barrier structures containing a sheet of self-organized InAs quantum dots. The dots are formed in GaAs n-type matrices after the deposition of four monolayers of InAs. Quasi-static analysis of capacitance-voltage measurements indicates that there are at least two filled electron levels in the quantum dots, located 60 and 140 meV below the GaAs conduction band edge. The conductance of the structure depends on the balance between measurement frequency and the thermionic emission rate of carriers from the quantum dots. An investigation of the temperature-dependent conductance at different frequencies as a function of the reverse bias allows us to study separately the electron emission rates from the ground and first excited levels in the quantum dots. We estimate that the electron escape times from both levels of the quantum dots become comparable at room temperature and equal to about 100 ps.

Ballistic transport and Andreev resonances in Nb/In superconducting contacts to InAs and LTG-GaAs

We have formed superconducting contacts in which Cooper pairs incident from a thick In layer must move through a thin Nb layer to reach a semiconductor, either InAs or low temperature grown (LTG) GaAs. The effect of pair tunneling through the Nb layer can be seen by varying the temperature through the critical temperature of In. Several of the In/Nb–InAs devices display a peak in the differential conductance near zero-bias voltage, which is strong evidence of ballistic transport across the NS interface. The differential conductance of the In/Nb–LTG-GaAs materials system displays conductance resonances of McMillan–Rowell type. These resonant levels exist within a band of conducting states inside the energy gap, formed from excess As incorporation into the LTG-GaAs during growth. Electrons propagating in this band of states multiply reflect between the superconductor and a potential barrier in the GaAs conduction band to form the conductance resonances. A scattering state theory of the differential conductance, including Andreev reflections from the composite In/Nb contact, accounts for most qualitative features in the data.

Electron and hole levels of InAs quantum dots in a GaAs matrix

Photoluminescence, capacitance-voltage and transmission electron microscopy studies have been carried out on structures containing a sheet of a self-assembled InAs quantum dots formed in GaAs matrices after the deposition of a 1.7 ML of InAs at 480°C. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the capacitance-voltage technique. From analysis of photoluminescence and capacitance-voltage measurements it follows that the quantum dots have electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 meV and 170 meV above the top of the GaAs valence band.

Deep and shallow electronic states at ultrathin InAs insertions in GaAs investigated by capacitance spectroscopy

Single, ultrathin InAs insertions in GaAs are investigated by deep-level transient Fourier spectroscopy and capacitance–voltage measurements near the transition from layer-by-layer to three-dimensional growth. The formation of a broad band of deep levels between 0.60 and 0.80 eV below the GaAs conduction band edge is shown to be related to the incorporation of the strained InAs layer. The defect density can be as high as 5×1010 cm−2. In addition, distinct interfacial levels, the formation of which is correlated to the GaAs growth conditions, are found at the position of the ultrathin InAs sheet. Due to their short-range potential, these interfacial deep-level defects are suitable to probe the local properties of the ultrathin InAs insertion. It is experimentally verified that the small InAs islands which are formed at the onset of three-dimensional growth give rise to laterally confined quantum states in the InAs insertion.

Simulation of a gallium arsenide running wave amplifier with a schottky barrier by the monte carlo method

Process of transfer of electrons in a gallium arsenide running-wave amplifier with a Schottky barrier has been investigated. Simulation of the process has been implemented by the Monte Carlo method with the use of the three-valley (ΓLX) model of the GaAs conduction band with account for the nonparabolicity of the bands, scattering on phonons, and transport between valleys. The increment of spatial-charge wave (SCW) has been calculated; it has been shown that the amplification in such a device can be substantially greater than in a beam-instability amplifier.

Electronic structure of self-assembled InAs quantum dots in GaAs matrix

Capacitance–voltage characteristics have been measured at various frequencies and temperatures for structures containing a sheet of self-assembled InAs quantum dots in both n-GaAs and p-GaAs matrices. Analysis of the capacitance–voltage characteristics shows that the deposition of 1.7 ML of InAs forms quantum dots with electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 and 170 meV above the top of the GaAs valence band. The carrier energy levels agree very well with the recombination energies obtained from photoluminescence spectra.

Electron escape from self-assembled InAs/GaAs quantum dot stacks

Capacitance–voltage characteristics have been measured at various frequencies and temperatures for a Schottky barrier structure containing three sheets of self-assembled InAs quantum dots in an n-GaAs matrix. By changing the frequency of the measuring signal at fixed temperature, it is possible to control the ratio between the thermionic and the tunnel contributions to the electron escape from the quantum dots. An applied magnetic field reduces the thermionic emission rate and increases the importance of the tunnel part of escape of electrons from the dots due to the deepening of electron level in the dots by Landau quantization in the GaAs conduction band.

Energy resolved noise measurements in quantum well infrared photodetectors

In quantum well infrared photodetectors, the detector dark current usually is composed of a wide range of energies originated from thermionic emission as well as thermally assisted tunneling. Yet, it is a common practice to assign a single noise gain to all electrons irrespective of their energies. This assigned value can only represent the mean since both the hot-electron lifetime and the transit time, whose ratio determines the gain, are energy dependent. In this work, we have resolved the energy dependence of the noise gain using an electron energy filter. We find that although the noise gain increases initially with energy as expected, it reaches a maximum at 0.27 eV above the GaAs conduction band edge, and then decreases and forms a minimum at 0.31 eV. We attribute this decrease to the Γ-L intervalley scattering, which increases the transit time of the electrons.

A New Concept for Solving the Boltzmann TransportEquation in Ultra‐fast Transient Situations

A concept based on relaxation of the hydrodynamic parameters is introduced to arrive at a computational model for the extreme non‐equilibrium distribution function of carriers in multi‐valley bandstructure. The relaxation times are taken to describe the evolution scale of the distribution function. The developed model is able to account for transport phenomena at the momentum relaxation scale. The model, together with the Monte Carlo simulation, is applied to obtain the electron distribution function in each valley of the lower conduction band in GaAs, and to study the evolution of the distribution function in GaAs subjected to rapid changes in field.

Interface Defects and Rectification Properties of Heterojunctions Between Solid C60 and GaAs

AbstractSolid C60/GaAs contacts were fabricated by growing solid C60 films on both n-type and p-type GaAs(100) substrates through vacuum deposition. The electronic states at C60/GaAs interfaces were studied. Both C60/n-GaAs and C60/p-GaAs contacts were found to be strong rectification junctions with a rectifying ratio higher than 106 at a bias of ± 1.OV. Two distinct traps were also observed at the C60/GaAs interfaces with deep level transient spectroscopy (DLTS); one is the electron trap at 0.35 eV below the GaAs conduction band and the other is a hole trap at 0.45 eV above the GaAs valence band.

Electric Field Effects on Hot Electron Luminescence from p-GaAs

Photoelectrons in the conduction band of GaAs have energy, momentum and spin distributions determined by the anisotropic valence band structure and the optical matrix elements. In p-type GaAs, a small fraction recombine with acceptor states, producing a Hot Electron Luminescence (HEL) spectrum with a series of peaks due to discrete energy losses through LO-phonon emission. The highest peak has an energy structure due to warping of the initial heavy hole (HH) band. We report measurements of its lineshape and polarisation, identifying emission from electrons along particular k-directions. An electric field of 1 kV cm—1 distorts the momentum distribution, and this is reflected in the polarisation profiles. The lineshapes and profiles are modelled using a k · p band structure and optical matrix elements, the effect of the field being included using a k-broadening model.