Title: Exciton in Absorption Spectrum

Absorption curve

The drawn through blue line shows the absorption spectrum of a direct gap semiconductor.

Exciton Peak

The sharp absorption excition peak on the lower energy side of the lowest energy edge of the continuous broad band is caused by the formation and recombination of excitons. An exciton forms when an excited electron (coming from the valence band) is interacting with the hole it formed.  The interaction is caused by the Coulomb attraction between the two charges. Due to this attractive energy, the absorption energy of the exciton is below the bottom of the conduction band.

Short movie about excitons and why they are important here.

Energy Difference

The energy lowering of the exciton is due to the attractive Coulomb force between the excited electron and the hole.

E(ex)=-e*e/(4πε(eff)r)

Typical values are:

Si: 14.7 meV

Ge: 4.15 meV

CdSe: 15 meV

InP: 4.0 meV

KCl: 400 meV