Excitons

Abstract

Optical band-to-band absorption can produce an electron and a hole in close proximity which attract each other via Coulomb interaction and can form a hydrogen-like bond state, the exciton. The spectrum of free Wannier--Mott excitonsWannier--Mott excitonsin bulk crystals is described by a Rydberg seriesRydberg serieswith an effective Rydberg constantRydberg constantgiven by the reduced effective massReduced Effective Massand the dielectric constant. A small dielectric constant and large effective mass yield a localized Frenkel excitonFrenkel excitonresembling an excited atomic state. Excitons increase the absorption slightly below the band edge significantly. The interaction of photons and excitons creates a mixed state, the exciton--polaritonExciton--polariton, with photon-like and exciton-like dispersion branches. An exciton can bind another exciton or carriers to form molecules or higher associates of excitons. Free charged excitons (trionsTrions) and biexcitonsBiexcitonshave a small binding energy with respect to the exciton state. The binding energy of all excitonic quasiparticles is significantly enhanced in low-dimensional semiconductors. Basic features of confined excitons with strongest transitions between electron and hole states of equal principal quantum numbers remain similar. The analysis of exciton spectra provides valuable information about the electronic structure of the semiconductor.