Showing papers by "Paulo V. Santos published in 2001"

Transport and lifetime enhancement of photoexcited spins in GaAs by surface acoustic waves.

Abstract: We demonstrate spin transport and spin lifetime enhancement in GaAs quantum wells induced by the traveling piezoelectric field of a surface acoustic wave (SAW). Spin transport lengths of about $3\ensuremath{\mu}\mathrm{m}$ corresponding to spin relaxation times during transport over 1 ns are observed, which are considerably longer than the exciton spin diffusion lengths in the absence of a SAW. The slow spin relaxation is attributed to a reduced electron-hole exchange interaction, when the carriers are spatially separated by the lateral potential modulation induced by the SAW.

Dynamic band-structure modulation of quantum wells by surface acoustic waves

Abstract: T. Sogawa, P. V. Santos,* S. K. Zhang, S. Eshlaghi, A. D. Wieck, and K. H. Ploog Paul-Drude-Institut fur Festkorperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany NTT Basic Research Laboratories, 3-1, Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan Lehrstuhl fur Angewandte Festkorperphysik, Ruhr-Universitat Bochum, 44780 Bochum, Germany ~Received 13 November 2000; published 7 March 2001!

Dynamic carrier distribution in quantum wells modulated by surface acoustic waves

Abstract: We have investigated the dynamics of photogenerated carriers under surface acoustic wave (SAW) fields in GaAs quantum wells using spatially and time-resolved photoluminescence (PL). The frequency and phase of the PL oscillations under a SAW yield information about the carrier distribution and the band-gap modulation induced by the SAW. We directly prove that the transport properties of the carriers ultimately control their distribution, storage and, subsequent recombination in the modulated potential.

Optical anisotropy of (001)-GaAs surface quantum wells

Abstract: We report a reflectance difference spectroscopy (RDS) study of the optical anisotropy of GaAs:(001) surface quantum wells consisting of a thin GaAs layer (3--30 nm thick) embedded between an arsenic reconstructed surface and an AlAs barrier. The RDS spectra display anisotropic contributions from the free surface and from the GaAs/AlAs interface. By comparing RDS spectra for the $c(4\ifmmode\times\else\texttimes\fi{}4)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ surface reconstructions, we separate these two contributions, and demonstrate that the anisotropy around the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions comprises a component originating from modifications of bulk states near the surface. The latter is attributed to anisotropic strains induced by the surface reconstruction. The experimental data are well described by a model for the RDS response of the multilayer structures, which also takes into account the blue energy shifts and the changes in oscillator strength of the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ transitions induced by quantum-well confinement.

Collinear light modulation by surface acoustic waves in laterally structured semiconductors

Abstract: The modulation of light beams by surface acoustic waves (SAW) in periodically modulated photonic materials is investigated. The periodic modulation allows for new scattering processes which can improve the efficiency of SAW-based modulators, as proposed by P. St. J. Russell [Phys. Rev. Lett. 56, 596 (1986)]. In particular, scattering geometries with collinear propagation paths for the SAW and the light beam become possible using SAW wavelengths much larger than the light wavelength. We analyze the performance of such modulators based on semiconductor materials and examine the role of random fluctuations in structure dimensions on their operation.

Epitaxial pulsed laser crystallization of amorphous germanium on GaAs

Abstract: We have investigated the crystallization of amorphous germanium films on GaAs crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial films are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a (GaAs)1−xGe2x epitaxial alloy with a graded composition profile at the interface with the substrate. Since Ge and GaAs are thermodynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling process. Lower laser fluencies lead to polycrystalline layers with a patterned surface structure. The latter is attributed to the freeze-in of instabilities in the melt during the fast solidification process.

Photoluminescence modulation by high-frequency lateral electric fields in quantum wells

Abstract: We investigate the modulation of the excitonic photoluminescence (PL) of GaAs quantum wells by high-frequency (frf) lateral electric fields. Under these fields, the PL becomes modulated in the form of pulses with repetition frequency of 2 frf. The periodic PL modulation is attributed to the time-dependent ionization of photogenerated excitons under the lateral electric field. The exciton ionization mechanism is proposed to be the impact ionization with electrons accelerated by the electric fields with a threshold field for ionization of about 15 V/cm. The different transport properties of electrons and holes are found to play a role in the exciton ionization process.