How does temperature affect excitonic absorption?

[hokhani]

As far as I know, free electron-hole or excitons can be created by absorbing a photon of relevant energy. So dissociation of excitons shouldn't make any effects on absorption because dissociation happens after the absorption. Nevertheless I saw in some texts that in the limit of the temperatures $K_B T>>(exciton's binding energy)$, that excitons would dissociate, exciton absorption lines are not observed.
please guide me.

 

[Cthugha]

Excitonic absorption will still occur at elevated temperatures, but it will be significantly broadened. Due to acoustic phonons, absorption will not only occur at the exciton energy, but also at the exciton energy +/- one phonon energy. As acoustic phonons have a rather broad energy distribution, this will also cause the absorption spectrum to be very broad if the phonon density is high - which is what happens if you go to elevated temperatures. As soon as the broadened excitonic and band gap absorption overlap significantly, it will become complicated to distinguish them. As you already noted, the point where this happens, is determined by the ratio between the thermal energy present and the exciton binding energy.