G. Fasol

Effect of spatial localization of dopant atoms on the spacing of electron subbands in δ‐doped GaAs:Si

Using Raman spectroscopy we have investigated the spacing of the electron subbands in nominally δ‐doped GaAs structures which show a considerable spread of the silicon dopant atoms along the growth direction. For optical excitation in resonance with the E0+Δ0band gap, spin‐density intersubband excitations are observed. For excitation in resonance with the E1 band gap we find a strong enhancement of scattering by collective intersubband plasmon‐phonon modes. The measured energy spacings between the electron subbands deviate significantly from what is expected for ideal δ doping. Self‐consistent electronic subband calculations taking into account the spread of the dopant atoms along the growth direction, in contrast, yield a good quantitative agreement between calculated and measured subband spacings. This demonstrates the potential of intersubband Raman spectroscopy for the analysis of the spatial localization of dopant atoms in δ‐doped structures.

Raman scattering verification of nonpersistent optical control of electron density in a heterojunction

We report electronic Raman scattering measurements of the plasmon mode in a single GaAs/AlGaAs heterojunction, with a δ layer of acceptors in the GaAs buffer a well‐defined distance from the interface. Under illumination above the band gap of the AlGaAs barrier, a dynamic charge‐transfer effect occurs in which the quasi‐two‐dimensional electron concentration of the hetrojunction decreases. From Raman measurements of the plasmon mode we directly determine the change in carrier concentration with excess illumination. We obtain a time of τ=120 ps for the transfer of electrons from the AlGaAs barrier into the two‐dimensional channel.

Spectral and temporal features of photoluminescence of gallium arsenide quantum-wire crystals

Abstract Optical properties of GaAs quantum-wire crystals are investigated by means of time-resolved photoluminescence measurements. Recombination dynamics of carriers in the quantum-wire crystals are characterized on the basis of a rate equation which includes surface recombination and effect of depletion layer. Surface treatment with sulphur solution decreases significantly the surface charge density and surface recombination. Reduction in radiative life time is observed after the surface treatment. This implies that the modification of spatial profile of carrier wave functions is caused by a change in band bending at the surface.

Light scattering determination of subband structure and population of modulation‐doped multiple quantum wells

We show that electronic Raman scattering measurements of the plasmon dispersion in combination with calculations of the random phase approximation dielectric response and self‐consistent electronic subband calculations can determine the subband structure and populations of modulation‐doped GaAs/AlxGa1−xAs multiple quantum wells with multiple subband occupancy. Thus we present a contactless optical alternative to measurements of Shubnikov–de Haas oscillations for the determination of carrier concentrations. The same technique can also be applied for the characterization of parallel conduction from doped AlGaAs layers in modulation‐doped heterostructures.

Hot electron scattering with cold plasma in GaAs from CW hot electron luminescence spectroscopy

Continuous wave (CW) hot electron luminescence (HEL) spectra indicate that hot electrons scatter predominantly with cold electrons rather than with hot electrons or hot holes. Two series of experiments are compared. In the first series the sample is GaAs:Be. Increasing the concentration of optically injected carriers results in a widening distribution of hot electrons due to scattering and to an increasing band-gap-related background. In the second series, the authors keep the injection of hot electrons low, but study different samples GaAs:Be,Si which contain a rising background of donor electrons. In this case the hot electron population widens in a comparable way, indicating that hot electrons scatter with electrons low in kinetic energy.

Optical detection of biatomic sheets of silicon in Si/Ge superlattices

A distinctive Raman spectrum associated with biatomic sheets of silicon in Si/Ge superlattices has been found in the energy range 370–410 cm−1. This double‐peaked structure was obtained over an order of magnitude of germanium layer thickness, but was not found in the alloy control layers or structures with thicker Si layers. It is proposed that the signal is due to modes that are normally forbidden in this scattering configuration. Strong direct optical transitions have been predicted for certain Si/Ge superlattices incorporating biatomic sheets of Si and this distinctive Raman signal could be used to characterize the biatomic sheets of silicon in these structures.

Raman spectroscopy of low-dimensional structures

The authors demonstrate two applications of Raman scattering for the characterization of low-dimensional structures: they measure the plasmon properties of a modulation-doped multiple quantum well with multiple subband occupation by Raman scattering and hence determine the electronic subband structure and the subband occupation. Secondly, they report work on the phonon properties of silicon-germanium strained layer superlattices. Using a fully three-dimensional model they calculate the 3D phonon dispersion of confined and extended superlattice phonons and measure the phonon spectra using Raman spectroscopy.

Raman scattering determination of nonpersistent optical control of electron density in a heterojunction

We show that electronic Raman scattering measurements of the intrasubband plasmon dispersion in a GaAs/A1GaAs heterojunction is a viable contactless optical method for the determination of sheet carrier density of the two-dimensional electron gas. We demonstrate non-persistent optical control of the carrier density by a dynamic charge transfer effect: from Raman measurements of the plasmon we directly determine the change in carrier concentration with excess illumination.

Raman scattering characterization of direct gap Si/Ge superlattices

STRACT We use Raman scattering to investigate direct gap Si/Ge superlattices for which the Si layers are in the form of biatomic sheets. We find a characteristic signal arising from the presence of the biatomic Si sheets. A range of samples have been investigated and 3-dimensional lattice dynamical calculations have been performed which pinpoint the Si layers as the origin of the signal. We illustrate how the signal may be used to characterise the quality of for example (Si)2/(Ge)6 superlattices grown at various substrate temperatures.

Gamma -> L intervalley and polar optic scattering times in GaAs from cw hot electron luminescence spectroscopy

We deduce the F-L intervalley and poiar optic phonon scattering times of hot electrons in bulk GaAs from cw hot (e, A°) luminescence spectra as a function of electron kinetic energy at low excitation densities. We obtain the lifetime broadening due to these two processes from comparison with lineshape calculations using a 16x16 k.p Hamiltonian, a full integration over k-space and a dipole model for the optical matrix elements. We find for the LO-phonon emission time tLO=(l32±lO)fS. The threshold for IT-*L scattering is determined as (330±1O)meV, above which a distinct decrease in total lifetime is observed. Minimum F÷L scattering times are l5Ofs to 200fs. We discuss an estimation for the deformation potential DTL.