The theory of interband transitions given in Chapter 3 neglects the interaction between an electron which has made a transition to the conduction band and the hole which it leaves in the valence band. It is clear that there will be an electrostatic interaction between the electron and hole, screened by other electrons and ions. When this interaction is included in the theory it is found that excited states occur with energies inside the forbidden energy gap of band theory. Transitions to these states can therefore cause absorption at energies below the minimum energy for interband transitions. The excited states are associated with bound electron-hole pairs, called excitons, which can propagate through the crystal.
KeywordsInterband Transition Optical Conductivity Spatial Dispersion Exciton State Exciton Energy
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