Photoexcited Electrons In GaAs Research Articles

On the mobility-lifetime products in photorefractive GaAs-AlGaAs quantum wells structures determined by moving grating technique measurements

The method to determine mobility-lifetime products of photoexcited electrons and holes in semi-insulating GaAs-AlGaAs quantum wells structures is proposed. The method is based on measurements of a photoconductivity and optical investigations of a photorefractive material response in wave-mixing setup using the running grating technique. Full Text: PDF References L. Solymar, D.J. Webb, A. Grunnet-Jepsen, The physics and applications of photorefractive materials (Clarendon Press: Oxford 1996). D.D. Nolte, M.R. Melloch, Photorefractive effects and Materials, ed. by D.D.Nolte (Kluwer, Dordrecht, 1995). S. Balasubramanian, I. Lahiri, Y. Ding, M.R. Melloch, D.D. Nolte, "Two-wave-mixing dynamics and nonlinear hot-electron transport in transverse-geometry photorefractive quantum wells studied by moving gratings", Appl. Phys. B 68, 863 (1999). CrossRef M Wichtowski, "Wave mixing analysis in photorefractive quantum wells in the Franz–Keldysh geometry under a moving grating", Appl. Phys. B 115, 505 (2014). CrossRef Q. Wang, R.M. Brubaker, D.D. Nolte, "Photorefractive phase shift induced by hot-electron transport: multiple-quantum-well structures", J. Opt. Soc. Am. B 9, 1773 (1994). CrossRef M. Shur GaAs devices and circuits (Springer, New York, 1989). D.D. Nolte, "Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications", J. Appl. Phys. 85, 6259 (1999). CrossRef G.C. Valley, H. Rajbenbach, H.J. von Bardeleben, "Mobility?lifetime product of photoexcited electrons in GaAs", Appl. Phys. Lett. 56, 364 (1989). CrossRef A. Ziółkowski, "Temporal analysis of solitons in photorefractive semiconductors", J. Opt. 14, 035202 (2012). CrossRef

The dual role of nitrogen as alloying and confining element in GaAs-based dilute nitride semiconductors

We examine transport and relaxation dynamics of optically excited electrons in GaAs-based heterostructure layers, involving dilute nitrides in percent-level concentration range. Such heterostructures contain materials with very different mobilities. Drift instead of Hall mobility is determined using a special technique. The value of the mobility of photogenerated electrons in a freestanding, 3% N InGaAsN layer, as part of a two-layer structure of 100 nm GaAs/2 μm nitride, is found to be of the order of 2 cm2/Vs, much lower than other values reported in the literature. The concomitant presence of carriers in the GaAs and nitride layers leads to formation of a barrier at the interface region that hinders electrons to enter in the nitride material. The dwell time of photoexcited electrons in GaAs interfacing the nitride layer is of the order of milliseconds, as seen by photoconductivity transients after pulsed optical excitation, much longer than the resulted time from optical experiments. Comparison of optical with transport properties reveals that the same centers involved in luminescence, that appear to be extended, are responsible also for hopping transport, where they appear as deep states. A theoretical explanation is given.

Electrical determination of the spin relaxation time of photoexcited electrons in GaAs

Spin-dependent transport for photoexcited electrons in an epitaxial Fe/GaAs interface was characterized from 5 to 300 K. The presence of spin-dependent transport was confirmed at all the measured temperatures and the spin polarization across the interface is found to increase with decreasing temperature. A time-of-flight-type model based on the Dyakonov–Perel (DP) spin relaxation mechanism was employed to explain the temperature dependence, providing that the estimated spin relaxation time in GaAs is 62 ps at 5 K. This short spin relaxation time can be explained by the stronger efficiency of the DP mechanism for hot-electrons.

Spin-polarized electron transport in ferromagnet/semiconductor hybrid structures induced by photon excitation

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in $3{\mathrm{nm}\mathrm{}\mathrm{A}\mathrm{u}/5\mathrm{}\mathrm{nm}\mathrm{}\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}(100)$ structures with doping density in the range ${10}^{23}$ to ${10}^{25}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}.$ At negative bias a helicity-dependent photocurrent dependent upon the magnetization configuration of the film and the Schottky barrier height was detected. The helicity-dependent photocurrent polarization decreases with increasing doping density and has the same variation with photon energy as found for the polarization of photoexcited electrons in GaAs. The results provide unambiguous evidence of spin-dependent electron transport through the NiFe/GaAs interface and show that the Schottky barrier height controls the spin-dependent electron current passing from the semiconductor to the ferromagnet.

Spin-dependent electron transport in ferromagnet/semiconductor Schottky barrier structures

ABSTRACTClear evidence for high efficiency spin-polarized electron transport across ferromagnet/semiconductor Schottky barrier interfaces was observed in Ni80Fe20/GaAs structures. Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane. At negative bias, an almost constant difference between the helicity-dependent photocurrent obtained for the magnetization parallel and perpendicular to the photon helicity was detected. An effective asymmetry, A, was also estimated from the helicity-dependent photocurrent difference, attributed to spin-polarized electron tunneling from GaAs to NiFe (spin filtering). A decreases with increasing photon energy, which is consistent with the energy-dependence of the asymmetry of photoexcited electrons in GaAs. Weak spin injection from NiFe to GaAs was seen at a bias corresponding to the Schottky barrier height, which is likely to occur via a ballistic process.

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.

Quantum Kinetic Influences on Electron–Phonon Scattering in Semiconductors

The dynamics of photoexcited electrons in GaAs due to LO phonon emission is investigated. On an ultrafast timescale energy conservation is seen to be violated and non-Markovian memory effects are observed. Initially, the phonon replica are substantially broadened with respect to the unrelaxed excitation peak. As time proceeds, quantum interference becomes operative resulting in a narrowing of the maxima to the shape expected in the semiclassical limit. The timescale for these phenomena is given by the LO oscillation period. Quantum kinetic simulations based on a simplified model that allows an exact treatment of the memory effects agree with our experimental data.

Intervalley scattering in GaAs and InP probed by pulsed far-infrared transmission spectroscopy

The dynamics of photoexcited electrons in GaAs and InP were studied using the transmission of 200-fs pulses of far-infrared radiation in the spectral range 15–100 cm−1. Kinetic traces of the infrared transmission as a function of delay between optical excitation and infrared probe show a probe-limited decrease in transmission followed by a more gradual (0.7–2 ps) drop to a steady value, consistent with the slow return of electrons from high-mass satellite valleys. Infrared transmission spectra, analyzed in the context of a Drude model, reveal density-dependent electron mobilities 3–4 times below equilibrium n-doped values. Electron-hole collisions likely account for the lower mobility.

Mobility-lifetime product of photoexcited electrons in GaAs

We show that photorefractive beam coupling gain as a function of grating period measured in semi-insulating GaAs with an external ac electric field or with a dc field and moving fringes can be explained theoretically by a mobility-lifetime product about four orders of magnitude smaller than that obtained in low-field conditions. This reduction is caused by enhanced occupation of the L band at high fields coupled with the low L band mobility and by the increase in recombination due to the cascade capture process that occurs for electric fields above a few kV/cm.

Effect of valence band anisotropy on the Ultrafast relaxation of photoexcited electrons in GaAs

The effect of valence band anisotropy on the Ultrafast relaxation of photoexcited e-h plasma in GaAs is investigated using an ensemble Monte Carlo approach. For excitation by a 1.80 eV laser pulse and excitation level of 5×10 16 cm −3, the time evolution of the energy distribution are influenced by the warping only during times shorter than 0.2 ps. At longer times, the shapes of energy distribution are independent of the valence band model. The initial energy spread of the electrons does not affect the form of the hot phonon distribution.

Energy exchange via electron-electron scattering in many-valley semiconductors

The theoretical treatment of electron-electron (e-e) scattering in semiconductor transport has always posed formidable problems, due to the long range and to the non-linearity of the interaction. Here we present a Monte Carlo study of the e-e interaction based on a molecular-dynamics approach. The full long range of the Coulomb interaction is accounted for, and no assumptions are needed on the screening process. This is particularly important when different valleys are involved, and a multi-component plasma has to be simulated. The Monte Carlo algorithm is applied to the study of the relaxation of photo-excited electrons in GaAs looking in particular at the energy exchange between electrons in different valleys. Such type of interaction has never been considered up to now in Monte Carlo simulations. The dependence of the e-e interaction on the electron density is discussed in detail. A comparison with a different approach, that treats the e-e scattering in k-space using a screened potential formulation, is also presented.

Effect of carrier-carrier interaction on intervalley transfer rates of photoexcited electrons in GaAs.

The intervalley transfer of carriers photoexcited by a 2.04-eV laser pulse and their response to a 500-V/cm field in GaAs is examined for two excitation levels of 5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ and ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. It is found that the transfer rates are not affected by electron-hole or electron-electron interactions at low excitation levels. At high excitation levels the electron-hole interaction accelerates the return rates to the central valley and provides an important energy-loss channel for the electrons. In response to a 500-V/cm field, the electrons exhibit very small velocities during the first 2 ps. At times beyond 4 ps, the velocities are smaller for higher electron densities.

Quantum theory of transient transport in semiconductors: A Monte Carlo approach.

A new Monte Carlo method is presented for the evaluation of the density matrix from the solution of the Liouville--von Neumann equation for an ensemble of noninteracting electrons in a semiconductor crystal. The method is applied to the study of the electron transient response to a high external electric field in Si and to the relaxation of photoexcited electrons in GaAs in absence of external electric fields. The phonon population is always assumed at equilibrium, but no assumptions are made about the strength of the electron-phonon interaction. Results show that typical quantum features such as energy-nonconserving transitions, intracollisional field effect, and multiple collisions change the very first transient of the system with respect to a semiclassical description.

Carrier-carrier interaction and intervalley transfer effects on the ultrafast relaxation of photoexcited electrons in GaAs

The roles of the carrier-carrier (c-c) and intervalley deformation potential interactions on the ultrafast cooling of electrons in GaAs are examined for excitation energy of 2.0 eV and in the presence of uniform 500 V/cm applied electric field, using an Ensemble Monte Carlo (EMC) approach. It is found that immediately after excitation, a significant portion of the electrons transfer to the upper valleys where they relax primarily through the emission of LO, equivalent and nonequivalent intervalley phonons. Subsequently, the electrons return to the central valley. When the electron-hole interaction is included, 70% of the electron population to return to the central valley after 2.3 ps, compared to 3.1 ps when the e-h interaction is ignored.

Electron-hole interaction and high-field transport of photoexcited electrons in GaAs

We use a theoretical ensemble Monte Carlo method to study the response of carriers photoexcited by a 1.55-eV laser pulse to applied electric fields (less than 5 kV/cm) for excited carrier densities between 1017 cm−3 and 1018 cm−3. It is found that the electron-hole interaction reduces the fraction of electrons that transfer to the upper valleys and reduces the velocity of the electrons. These effects are more significant at low electric fields and higher excitation levels. The energy of the holes rises initially due to the energy transfer from the hot electrons through the electron-hole interaction. This is also reflected in a higher velocity for the holes during the first picosecond.

A theoretical study of hot photoexcited electrons in GaAs

A theoretical study of hot photoexcited electrons in GaAs

A theoretical study of hot photoexcited electrons in GaAs

The energy distribution function for hot photoexcited electrons in GaAs is studied by means of numerical calculations. The calculations are appropriate to conditions of continuous monochromatic photoexcitation and lattice temperatures in the liquid helium range. A Boltzmann equation approach is used to take account of the following effects: injection of electrons into the conduction band (at an energy below the threshold for LO phonon emission), collisions between electrons of unlike spin, collisions between electrons of like spin, scattering of electrons by acoustic phonons via the piezoelectric and deformation potential interactions and departure of electrons from the band via recombination. The Boltzmann equation is solved numerically by using an iterative technique. The numerical results are used to show how the distribution functions are characteristic of the scattering events undergone by the photoexcited electrons. The results also lead to quantitative estimates of the range of electron densities in which the electrons have a quasi-equilibrium distribution and the range of electron densities in which the electron distribution does not have a quasi-equilibrium form.

THE EFFECT OF ELECTRON-ELECTRON SCATTERING ON HOT PHOTOEXCITED ELECTRONS IN GaAs

The effect of electron-electron scattering on hot photoexcited electrons in GaAs is studied by performing numerical calculations of the electron distribution function. The calculations apply to conditions of continuous monochromatic photoexcitation and take account of the following effects : injection of electrons into the conduction band (the injection energy is taken to be less than the L.O. phonon energy), collisions between electrons of unlike spin, collisions between electrons of like spin, electron-phonon collisions (via the piezoelectric and deformation potential interactions) and recombination. It is shown that various departures from quasi-equilibrium occur if the electron density is sufficiently low (≤1012 cm-3) or sufficiently high (≥1018 cm-3).

Electron-acceptor luminescence from a non-Maxwellian distribution of photoexcited electrons in GaAs

Distribution functions are calculated for photoexcited electrons in GaAs, under conditions of continuous, monochromatic excitation. The lattice temperature is taken to be 1.2 K and the excitation intensity such that the density of photoexcited carriers is insufficient for the distribution to be affected by intercarrier scattering. A Boltzmann equation approach is used to take account of the effects of, injection of electrons into the conduction band, at an energy below the threshold for longitudinal optical phonon emission, scattering by acoustic phonons, via the deformation potential and piezoelectric interactions, and recombination. The equation is solved numerically using an iterative technique and the distribution functions are found to differ significantly from a Maxwellian form. Emission spectra due to conduction band to neutral acceptor transitions are derived from the computed distribution functions and are compared with recent experimental results.