Excitonic effects in free-standing ultrathin GaAs films

We study theoretically the excitonic properties of vacuum/GaAs/vacuum quantum wells. The exciton binding energies are much larger and the excitonic radial wave functions much more localized than those of ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum wells. This is due to the enhancement of the electron-hole interaction by the image-charge effect. The absorption spectra of thin vacuum/GaAs/vacuum quantum wells clearly exhibit not 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 the carriers interact with their own image charges.