D. Gammon

Low-temperature spin relaxation in n-type GaAs

on GaAs. Basic facts and a considerable body of ex-perimental and theoretical results related to bulk Group III-Vsemiconductors are collected in Ref. 5. Later on, much newinformation concerning mainly low-dimensional structureshas been obtained. Nevertheless, there remain gaps in thisknowledge that have become visible with the emerging of anapplication-directed angle on spin-related phenomena~spintronics!.

Wavelength modulation spectroscopy of single quantum dots

We demonstrate that external cavity diode lasers with large mode-hop-free tuning ranges (up to 80 GHz) together with wavelength modulation spectroscopy can be used to study excitonic transitions in semiconductor nanostructures. Such transitions are characterized by homogeneous linewidths typically on the order of a few GHz. Wavelength modulation spectroscopy offers a high signal-to-noise method for the determination of resonance line shapes. We have used this technique to accurately measure dipole moments and dephasing rates of single semiconductor quantum dot eigenstates. These measurements are important for the use of quantum dots in semiconductor cavities and quantum logic gates, and for an improved understanding of the physics of exciton confinement.

Optically induced variability of the strain-induced electric fields in (111) GaSb/AlSb quantum wells

Abstract Electronic Raman scattering and x-ray diffraction measurements on GaSb/AlSb quantum wells grown on (111) and (100) oriented substrates have been performed. A kinematic simulation of the x-ray diffraction intensity profiles has been used to determine the strain in the layers of the multiple quantum wells. Using the piezoelectric tensor of GaSb and the strain estimated from the x-ray measurements, one expects that an electric field of ∼ 60 kV/cm should exist in the GaSb layers of the (111) oriented sample. Intersubband transitions in the conduction band in the (111) and (100) oriented samples exhibit very different energies. A blue shift of the intersubband transition of the (111) oriented sample from that of (100) sample occurs because of the piezoelectrically-generated electric field in the former. The energy of the intersubband transition of the (111) oriented sample is sensitive to optical excitation density. This sensitivity arises because the electron-hole pairs screen the piezoelectrically generated electric field.

Coherent optical spectroscopy of a strongly driven quantum dot

We report the demonstration of the Autler-Townes splitting and Mollow absorption spectrum by means of coherent optical spectroscopy of a strongly driven quantum dot.

A spectroscopic study of screening effects in a quasi-two-dimensional system☆

Abstract We report resonant Raman scattering and far-infrared magneto-absorption studies of GaAs/AlxGa1−xAs quantum-well structures selectively doped with donors both in the wells and in the barriers. The resulting quasi-2D system, consisting of neutral donors and a degenerate electron gas, shows evidence of substantial static and dynamical screening effects.

Enhanced and quenched Raman scattering by interface phonons in semiconductor superlattices: What are the defects?

Abstract We report on the magnetic field and power density dependences of resonant Raman scattering by interface phonons in GaAsAl x Ga 1−x As superlattices. Strong photoexcitation leads to quenching of the nominally forbidden (and sample-dependent) scattering while a dramatic enhancement of the intensity is observed in the presence of a magnetic field. Alternative mechanisms that partially account for the experimental findings are discussed.

Intersubband transitions in piezoelectric superlattices

Abstract Charge-density and spin-density intersubband transitions of electrons in [100] and [111] GaSb/AlSb multiple quantum wells have been measured with inelastic light scattering. These measurements indicate that large built-in electric fields exist in the [111] oriented samples because of the piezoelectric effect. Furthermore, these built-in fields can be screened by the optical generation of an electron-hole plasma.

Donor impact ionization and electron heating in a GaAsAlxGa1−xAs superlattice

Abstract Impact ionization of donors in a GaAs/Al x Ga 1−x As superlattice is observed when an electric field above a threshold value is applied parallel to the growth direction. Impact ionization arises from the electric field-induced heating of electrons in the superlattice to energies above the donor binding energy. The threshold electric field for impact ionization is a strong function of magnetic field which is applied parallel to the current. The effect of the magnetic field is to increase the donor binding energy which then requires higher energy electrons and, correspondingly, higher electric fields for impact ionization. The electric field dependence of the mean electron energy is gleaned from the impact ionization data. We find that the electrons are heated well above the predicted miniband width of the superlattice which indicates that the transport proceeds by resonant tunneling between lifetime-broadened quantum levels.

Resonant Raman scattering study of localized collective excitations

Abstract The presence of a neutral donor in a quasi-two-dimensional electron gas of a GaAs AlGaAs quantum well causes the coupled intersubband plasmon-LO phonon excitations to split into doublets in the Raman spectrum. The extra peaks are identified as modes which are localized at the neutral donor through the resonant dipole field associated with the intersubband donor transition. The localized excitations are treated quantitatively in terms of a dielectric model.

Acceptor Raman scattering in GaAs-AlxGa1-xAs quantum-well structures

Abstract We report resonant Raman scattering from Be acceptors in GaAs-Al x Ga 1− x As quantum well structures, grown by molecular beam epitaxy. Center- and edge-doped samples with well widths in the range 70–165 A were investigated as a function of temperature and uniaxial stress. The data show confinement-induced shifts and splittings of the lowest-lying acceptor levels in good agreement with recent calculations, and also excitations that may involve impurity states derived from higher subbands. The stress dependence of the spectra reveals coupling of the lowest acceptor transition to transverse acoustic phonons. Confinement-split lines exhibit a not fully understood intensity exchange with increasing temperature.