GaAs AlAs Short Period Superlattices Research Articles

Nature of the lowest electron states in short period GaAs-AlAs superlattices of type II

From an extensive study of time-resolved photoluminescence in the temperature range 1.8 – 50 K on a dozen of samples of type II GaAs-AlAs superlattices, we can identify the region where the states derived from X z valleys are lower than those derived from X xy valleys. This is in agreement with a simple model involving the competition between i) X valleys splitting caused by the residual uniaxial stress in AlAs layers related to the small lattice mismatch with GaAs and ii) the difference in the confinement energies of X type states in AlAs layers related to their effective mass anisotropy. Mixing due to interface disorder of X xy and X z components into the wavefunction of these localized excitons explains the evolution of luminescence with time-delay.

Optical, vibrational and electronic properties of short period GaAsAlAs superlattices

We have explored the effect of electronic coupling on the low temperature (∼ 6K) photoluminescence excitation spectrum observed from GaAsAlAs superlattices. In particular a decrease in the continuum contribution to the luminescence signal can be correlated with increased coupling between the ⌜ electron states. Oscillations due to the interaction of photoexcited electrons and optical phonons can be observed in the photoluminescence excitation spectrum of both type-I and type-II samples. The period of the oscillations allows a determination of the in-plane hole mass, which gets heavier as the AlAs thickness is reduced. For one sample, a 25Å-8Å GaAsAlAs superlattice, we estimate the heavy hole exciton binding energy to be ∼8.5 meV. A comparison of the observed direct ⌜-⌜ and pseudo-direct X z−⌜ transition with calculations using a rudimentary effective mass type of calculation shows surprisingly good agreement down to a period of about 12 monolayers.

A transition from quantum well to superlattice behaviour in GaAsAlAs short period superlattices

Interband photoconductivity and reflectivity measurements have been performed on thin barrier GaAsAlAs superlattices up to 15 Tesla. Oscillatory photoconductivity due to the resonant emission of LO phonons is observed. The period allows as to determine the in-plane hole mass. Excitonic Landau levels seen in reflectivity are fitted using the masses determined from the oscillatory photoconductivity. Applying the magnetic field parallel to the superlattice layers allows a determination of the perpendicular miniband mass and hence illustrates the mass anisotropy due to the miniband structure.

DX Center in GaAs-GaAlAs Superlattices Suppression and Identification

ABSTRACTThe DX center has been studied by Deep Level Transient Spectroscopy in series of GaAs-Ga1-xAlxAs (x = 0.3) and GaAs-AlAs short period superlattices. The existence or not of this center can be understood if its energy level is linked to the L miniband. This suggest a possible way to suppress them by using specific superlattices.

Structural features of MBE grown very short period GaAs-AlAs superlattices

Very short period GaAs-AlAs superlattices (period less than 25 Å) were grown by molecular beam epitaxy. Three techniques were carried out to study these samples: transmission electron microscopy, X-ray diffraction, and photoluminescence. Diffraction techniques have proved to be particularly suitable for the analysis of this type of samples. By combining electron and X-ray diffraction, the origin of the fine structure of diffraction peaks could be explained with respect to the growth process. Owing to the high sensitivity of diffraction techniques, very small period variations (less than one monolayer) could be evidenced. The period of the three described samples was shown to be a fractional number of monolayers, which is related to the growth conditions used in this MBE set-up. Photoluminescence results give evidence of the high quality of the superlattices, and are fully consistent with those given by the diffraction techniques.

Optical evidence of the direct-to-indirect-gap transition in GaAs-AlAs short-period superlattices.

We have studied electronic properties of GaAs-AlAs short-period superlattices grown by molecular-beam epitaxy. A direct-gap to ``indirect''-gap transition has been evidenced through optical experiments on a large number of samples. It also corresponds to a type-I to type-II superlattice transition. Our results are in good agreement with an envelope-function description applied to each extremum of the host-material band structure. As a result of spatial transfer of electrons in AlAs, we have obtained an accurate value of the offset parameter \ensuremath{\Delta}${E}_{c}^{\ensuremath{\Gamma}}$/\ensuremath{\Delta}${E}_{g}^{\ensuremath{\Gamma}}$=0.67. .AE

High-order resonant Raman scattering by combinations and overtones of interface phonons in GaAs-AlAs short-period superlattices.

We report the observation of high-order Raman scattering at the resonance with the lowest direct optical transition of GaAs-AlAs short-period superlattices. While the one-phonon spectrum involves both confined and interface modes of GaAs and of AlAs, the high-order scattering is dominated by combinations and overtones of a GaAs-like and an AlAs-like phonon of a new type, which appear to be interface modes with large in-plane wave vectors.