The other explanation of observation two peaks is the formation of defects and exciton bands. Presence of lattice defects creates energy levels in the forbidden region and so absorption of photon causing an electron transition takes place at lower frequencies or higher wavelengths. An electron leaving filled band may not necessarily go to the conduction band instead it may be captured at energy level a little lower than conduction band called the exciton band. The electron in this case is still under the influence of Coulomb field from the hole it created. This bound e h pair is called exciton. The absorption peak at higher wavelength or lower frequencies is due to these exciton bands. These excitons move about in the lattice and interact with lattice defects giving of their energy.