Photoexcited Carriers In GaAs Research Articles

P1-54 Ultrafast dynamics of photoexcited carriers in GaAs investigated by picosecond acoustics

P1-54 Ultrafast dynamics of photoexcited carriers in GaAs investigated by picosecond acoustics

Transient velocity overshoot dynamics in GaAs for electric fields ≤ 200 kV/cm

We have experimentally studied the transient velocity overshoot dynamics of photoexcited carriers in GaAs for electric fields as great as 200 kV/cm. Time domain waveforms proportional to the velocity and the acceleration of carriers have been acquired, respectively, from guided and free-space radiating signals which contain terahertz frequency components. The measurements demonstrated that the degree of overshoot was maximized for an electric field on the GaAs between 40 and 50 kV/cm when 1.44-eV photons in an 80-fs laser pulse excited the sample. For carriers excited with higher initial energy (1.55 eV), the degree of overshoot decreased and the maximum degree of overshoot occurred at a higher electric field.

Initial thermalization of photoexcited carriers in GaAs studied by femtosecond luminescence spectroscopy.

Relaxation of a nonequilibrium distribution of electrons and holes in GaAs following femtosecond photoexcitation is investigated via spectrally and time-resolved luminescence. A rapid onset of luminescence over a broad spectral range shows that both electrons and holes are redistributed over a wide energy range within 100 fs, even at excitation densities as low as ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The data demonstrate carrier-carrier scattering rates higher than predicted by calculations with a statically screened interaction potential. Monte Carlo simulations using dynamical screening account for the experimental results.

Effect of electron-electron scattering on intervalley transition rates of photoexcited carriers in GaAs.

Etude par methode de Monte Carlo couplee avec une approche de dynamique moleculaire. Mise en evidence de l'interaction de divers mecanismes de diffusion et de l'ecrantage dynamique des porteurs de charge chauds dans des semiconducteurs. A une densite pour laquelle GaAs est degenere, la diffusion du volume d'excitation est domine, dans les 10 femtasecondes initiales, par une diffusion electron-electron. Cette diffusion modifie les vitesses de transition et la densite des porteurs dans les vallees satellites

Electron-electron scattering modifications of intervalley transition rates and ultrafast relaxation of hot photoexcited carriers in GaAs

The femtosecond relaxation of photoexcited carriers in GaAs is investigated by the use of ensemble Monte Carlo calculations coupled with a molecular dynamics approach for the carrier-carrier interaction, to probe various scattering mechanisms and the dynamic screening of hot carriers in semiconductors. At a density for which the GaAs is degenerate (in equilibrium), scattering out of the excitation volume is dominated in the initial tens of femtoseconds by electron-electron scattering, and the lifetime decreases with increasing density. This decrease agrees both in lifetime magnitude and in density dependence with recent experimental measurements. The presence of electron-electron scattering modifies both the population transition rates and the carrier densities in the satellite valleys, primarily by reshaping the energy distribution of carriers in the central valley. Intervalley scattering rates slow by two orders of magnitude as the electronic system cools in the first picosecond. Thus, while intervalley processes do play a role in the initial decay, the same processes play a role in the picosecond-scale luminescence decay. We find that rates, for particular mechanisms of carrier transfer to the satellite valleys, must be estimated carefully since the Γ-L population shift contains a significant fraction of electrons that reach the L valleys by way of the X valleys.

Ultrafast relaxation of hot photoexcited carriers in GaAs

The roles of carrier-carrier interactions and non-equilibrium phonons on the ultrafast relaxation of photoexcited carriers in GaAs are examined. At low carrier concentrations, the e-ph interaction is the main energy loss channel for hot electrons, while at high carrier concentrations, the e-h interaction is the primary energy loss channel. This latter result follows from the high e-h scattering rate, the screening of the e-ph interaction, and the high efficiency of hole-phonon scattering through the unscreened deformation potential interaction. The electron energy-loss rates through the e-h interaction increases as the excitation energies and intensities are increased. In two-dimensional systems, the e-h interaction further complicates the problem since the transverse optical modes out are also driven out of equilibrium by their interaction with the holes.

Initial relaxation of photoexcited carriers in GaAs and GaAs quantum wells under subpicosecond excitation

After excitation by a subpicosecond pulse, we observe a very slow rise of the luminescence both in GaAs and in GaAs quantum wells. By comparing the results in GaAs and InP, we show that the slow rise in GaAs and GaAs quantum wells is due to the slow return of the electrons from the L to the Γ valley. Our results show the importance of electron-electron scattering and the inadequacy of a simple phonon cascade model at densities as low as 5×10 16 cm −3.

Ultrafast dynamics in photoexcited carriers in GaAs

Luminescence of GaAs under excitation by two femtosecond pulses with equal intensity separated by a time delay Δ T demonstrates a peak at Δ T=0, which has a correlation width no more than the autocorrelation width of the escitation pulses. This peak is caused by the bimolecular recombination of electrons and holes and by the absorption saturation effect due to state filling.

Monte Carlo investigation of the electron-hole-interaction effects on the ultrafast relaxation of hot photoexcited carriers in GaAs.

The roles of the electron-hole (e-h), electron-electron (e-e), hole-hole (h-h), and screened electron-phonon (e-ph) interactions on the ultrafast relaxation of photoexcited carriers in GaAs are examined. Theoretical expressions for the various scattering rates are obtained, and these are used in an ensemble Monte Carlo calculation. At low carrier concentrations the e-ph interaction is the main energy-loss channel for hot electrons, while at high carrier concentrations the e-h interaction is the primary energy-loss channel. This latter result follows from the high e-h scattering rate, the screening of the e-ph interaction, and the high efficiency of hole-phonon scattering through the unscreened deformation-potential interaction. The electron energy-loss rates through the e-h interaction increase as the excitation energies and intensities are increased. For excitation by an excess photon energy of 130 meV, for example, it is found that the e-e interaction slows the cooling rates at all excitation levels, while the h-h interaction enhances the cooling rate of the holes.

Ultrafast relaxation dynamics of photoexcited carriers in GaAs and related compounds

The femtosecond intraband relaxation of hot carriers in GaAs, Al0.32Ga0.68As, and the multiple-quantum-well structure is studied using the equal-pulse optical-correlation technique. An overview of the experimental application of this technique to semiconductors is presented. A detailed theoretical analysis of the coherent-artifact contribution to the transmission-correlation peak in the geometry of parallel copropagating beams and a calculation of the saturable-absorption symmetry coefficients for GaAs are given. The relaxation time of carriers from their initially excited states was measured to be in the range 50–100 fsec for the materials studied. The interpretation of the measured relaxation time in terms of electron and hole response functions is discussed. The relevant scattering processes and rates and the corresponding relaxation times calculated from these rates are given.

Femtosecond Orientational Relaxation of Photoexcited Carriers in GaAs

Mesure de l'anisotropie de la saturation de l'absorption optique par des impulsions intenses de lumiere polarisee lineairement, dans GaAs a 77 K, pour des energies de photons legerement superieures a la bande interdite. Pour des densites de paires electron-trou d'environ 6×10 17 cm −3 , le temps de relaxation d'orientation de la quantite de mouvement est 190 fs

Phonon transport in photoexcited GaAs

The thermalization of photoexcited carriers in GaAs at low temperatures can lead to the excitation of long-lived, high-frequency (\ensuremath{\sim} 1 THz) acoustic phonons. Experimental results are reported for liquid-encapsulated-Czochralski-grown GaAs which substantially clarify the effect of various experimental parameters on the time-resolved phonon transport. We report, for the first time, a significant broadening of the phonon pulse shape for laser excitation energies close to but below the direct band gap of GaAs (1.519 eV). In addition, we observe phonon pulse narrowing for a higher temperature (4.2 K) than previously reported (3.7 K). These results provide strong support for a significant contribution of high-frequency phonons to the observed phonon pulse shapes. Preliminary evidence for anisotropic defect scattering of high-frequency phonons is also presented.

Quantization of photo-excited carriers in GaAs doping superlattices

Photo-excited carriers in periodic doping multilayer structures of GaAs are studied using resonant inelastic light scattering techniques. Spin-flip single-particle intersubband excitations clearly demonstrate the quantization of electrons in purely space-charge induced potential wells. With increasing excitation intensity the electric subbands broaden to minibands with considerable dispersion with k perpendicular to the layers. A quasi-three-dimensional behavior is found in both self-consistent calculations and electronic Raman scattering experiments.

Quasi-two-dimensional photoexcited carriers in GaAs doping superlattices

Periodic multilayer structures of GaAs exhibit new semiconductor properties such as tunable effective band gap, long lifetime of photoexcited carriers, and quantization of carriers in space-charge-induced potential wells. We present a more careful treatment of the high excitation regime, where the subbands broaden into minibands, than earlier self-consistent calculations. Experimentally we have used photoluminescence and resonant in-elastic light scattering measurements to study the subband structure. The luminescence peak is found to shift with decreasing photoexcitation intensity far below the band gap of GaAs. By comparing the peak position with the theory we determine the carrier concentration. Resonant spin-flip single-particle excitations directly give the subband splittings which are in good agreement with the theoretical results. At high excitation intensities, the subbands merge and a quasi-three-dimensional behavior is found.

Relaxation of photoexcited carriers in GaAs

An experiment is described to measure directly the mean free lifetime of the photoexcited carriers in a semi-insulating crystal by monitoring its photoconductivity.