J. F. Palmier

Optical evidences of assisted tunneling in a biased double quantum well structure

Experimental evidences of electron and hole resonant tunnelings are obtained in a biased double quantum well system using optical methods. Calculations of the defect‐induced tunneling rates are performed and compared with experiments.

Gunn oscillations up to 20 GHz optically induced in GaAs/AlAs superlattice

Direct observation of Gunn oscillations up to 20 GHz, induced by picosecond light pulses in an undoped GaAs/AlAs superlattice, is reported. They are obtained in the superlattice growth direction and from 7 K up to room temperature. The frequency is strongly dependent on the applied bias voltage and on the photoexcited carrier density. The oscillation frequency and the mode of operation are modeled by a classical numerical simulation.

Coexistence of Wannier–Stark localization and negative differential velocity in superlattices

We show the coexistence of Wannier–Stark localization as monitored by photoconduction experiments, and of negative differential velocity (NDV) for electrons in a perpendicularly biased GaAs/AlAs superlattice. The critical electric field for the onset of NDV, and that for the appearance of localization as monitored optically are nearly the same, which exemplifies the fundamental link between Wannier–Stark quantization [Phys. Rev. 117, 432 (1960)], and Esaki–Tsu nonlinear transport [IBM J. Res. Dev. 14, 61 (1970)].

Semi-classical magnetotransport in a superlattice miniband

We experimentally investigate the influence of a high magnetic field parallel to the layers on the Electron. perpendicular transport in GaAs/AlAs superlattices. The key magnetoeffects are satisfactorily accounted for by a simple modified Esaki model of Electronic conduction. These results provide an excellent confirmation of semi-classical miniband transport at room temperature in semiconductor superlattices.

Hole spin relaxation in a n-doped quantum well structure

Abstract The dependence of the hole spin relaxation on the electron density is studied in a n-modulation doped 75 A GaAs/ AlGaAs quantum well by means of cw and time-resolved photoluminescence techniques. A slow hole spin relaxation time has been measured (~ 1 ns) and the polarization has been found to be strongly dependent on the in-plane wavevector of the photocreated holes. Calculations are presented which support the experimental findings.

Time of flight of electrons and holes at 77 and 300 K in GaAs/AlAs superlattices

Abstract The carrier drift velocity versus applied electric field in GaAs/AlAs superlattices is directly determined from a photocurrent time of flight technique. The electron negative differential velocity regime is clearly evidenced and in good agreement with static current-voltage data in the case of a relatively thick barrier superlattice (7 AlAs layers). For a thinner barrier superlattice (4 AlAs layers) the low field electron mobility seems temperature dependent, and also a drastic hole mobility decrease is observed at low temperature and interpreted in terms of light hole freeze out.

Excitonic photoluminescence in a shallow quantum well under electric field

We report a study of electrophotoluminescence in a biased shallow GaAs/AlxGa1−xAs (x=0.04) quantum well. It is shown that photocarriers escape from the well via direct tunneling, resulting in a drastic quenching of the photoluminescence at remarkably low fields (F<10 kV/cm). We develop a simple method to measure the field-induced escape time from a set of cw photoluminescence, photocurrent, and time-resolved photoluminescence experiments. Comparison with a semiclassical model shows that, in this regime, Coulomb interaction affects significantly the single electron direct tunneling scheme.

Relaxation of polarized excitons in asymmetric double quantum wells

SummaryWe consider the intrawell and interwell relaxation of circularly polarized excitons in an asymmetric double-quantum-well structure. For the intrawell case (photocreated light excitons coupled to the detected heavy excitons by an optical-phonon emission) the polarization changes sign after a fast relaxation. For the interwell case (transfer of polarization from the ground heavy exciton of the narrow well towards the ground heavy exciton of the wide well) we observe that the initial polarization is recovered after energy relaxation only if a fast-relaxation path is available which involves only intermediate heavy exciton states.

Time resolved dynamics of the excitonic transfer in double quantum well structures

Abstract In biased asymmetric double quantum wells a photocreated excitation can tunnel from one well to the other before recombination. By tuning the external electric field, the fundamental conduction level and the fundamental valence level become localized in different wells and a crossed exciton appears. The excitonic transfer from a direct to a crossed exciton is studied by means of time resolved photoluminescence. The formation of the crossed exciton also induces a dipolar electric field that leads to a non linear behaviour of the transfer and recombination mechanisms.

Excitonic transitions in GaAsGaAlAs superlattices under a weak electric field

Abstract We report a study of the photoluminescence spectra of GaAsGa0.65Al0.35As superlattice under an electric field of about 10 kVcm−1, from 9 K up to 80 K; the ratio ϱ = I −1 I 0 of the intensities of the −1 and 0 peaks of the Wannier-Stark ladder varies with the temperature T. To fit these variations, we assume for both transitions, a trapped exciton density of states with a gaussian distribution of eigenenergies, and a two dimensional density of states for free excitons. We show that above about 50 K there is a thermal equilibrium between direct and crossed free excitons whereas at low temperatures, the equilibrium statistics are not respected: the excitons trap into the direct state rather than to the crossed one.