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The dynamics of carriers in GaAs/(A1GaAs) heterostructures is of great interest because of the technological applications of these materials and because of their novel electronic properties. The higher potential of the A1GaAs layers confines the electrons (and holes) to the GaAs layers. As a result, the electron gas shares all the properties of a two dimensional system, and the interactions of the electrons and holes with the lattice are expected to differ from the bulk GaAs. Using time-resolved Raman scattering of optical phonons in bulk GaAs, J. Kash et al. determined an average emission time of an optical phonon by a photo-excited electron and found it equal to 165 fms. Acoustic phonon scattering is a much weaker mechanism, but becomes the important cooling process for electronic temperatures below 35 K. J. Shah et al. studied the cooling rates of carriers in 2D-heterostructures and found that the electrons cool 25 times more slowly than the holes. So far, little attention has been paid to the relative rates of the cooling processes which involve carriers distributed on several electronic subbands. This is the point we discuss in this paper. There are two cases of interest: the first one occurs when the energy separation of the two lowest electronic subbands is greater than the energy of an optical phonon (ELO=36.7 meV), the second one occurs when the energy separation is less. In this latter case, one expects the scattering of an electron by an acoustic phonon to be the dominant cooling mechanism. Another scattering mechanism arises from the electron-electron interactions. For the large electronic densities generated by photoexcitation (n ~ 1011 electrons/cm2/well), electron-electron collisions effectively randomize the electron momentum and redistribute the energy of the photoexcited electrons. Carrier thermalization typically takes place in less than one or two picoseconds.
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