EXCITONIC EFFECTS IN FREESTANDING ULTRATHIN GAAS FILMS

We study theoretically the excitonic properties of vacuum/GaAs/vacuum quantum wells. The exciton binding energies are much larger and the ex citonic radial wave functions much more localized than those of AlxGa1 -xAs/GaAs/AlxGa1-xAs quantum wells. This is due to the enhancement of the electron-hole interaction by the image-charge effect. The absorpti on spectra of thin vacuum/GaAs/vacuum quantum wells clearly exhibit no t only the peaks due to the 1s excitons, but also those due to the 2s and 3s excitons. The energy of the continuum edge is enhanced since th e carriers interact with their own image charges.