Excitons in Thin Films

Abstract

Excitons in quantum wells (QWs) have been the object of extensive investigation, in view of their properties (large binding energy and oscillator strength, strong nonlinearities, etc.) which are interesting for both basic and applied research. Their understanding is based on variational wave functions1,2 whose common feature is to disregard the effect of the electron-hole (e-h) interaction along the direction of confinement, namely the z-direction. The exciton behavior along z is determined by the separate e and h wave functions. This assumption is reasonable as far as the kinetic energy due to the confinement is larger than the e-h interaction, that is if the well depth L is smaller than the exciton radius aB. How to describe excitons in thicker QWs, i.e. with L>aB (called thin films throughout this paper) is not clear. The study of such thick QWs is motivated by several reasons. One reason is to understand optical experiments3 that have lead to the hypothesis that excitons follow the separate e-h quantization in GaAs films as thick as 50 exciton radii, at variance with the more reasonable assumption that the motion of the exciton as a whole, namely the center-of-mass motion, is quantized for these thicknesses. Another reason is the recent development of heterostructures where the well material has so narrow excitons (e.g. CuCI)4 that L<aB can hardly be attained. Finally, the fact that the oscillator strength of excitons in quantum dots and wells increases with the volume available to the exciton motion5 suggests that excitons in thicker QWs may have larger oscillator strengths than those in thinner QWs. Therefore thin films, in spite of the little interest they have attracted so far, might result good materials for construction of optical switching devices, since large nonlinearities and short decay times are usually associated to giant oscillator strengths.