The electric fields of charged defects in a solid broaden its fundamental optical absorption edge by a mechanism which is an internal analog of the Franz-Keldysh effect. Using this analogy, a semiquantitative calculation is made of such broadening due to impurities in semiconductors having direct absorption edges. Without the use of any adjustable parameters, reasonable agreement is found with published absorption tails of InAs, for both the magnitude of the effect and its dependence on impurity concentration. It is proposed that such effects account for the common Urbach's rule observations, and a qualitative discussion is given for the corresponding effects in insulators.

• Received 17 December 1962

DOI:https://doi.org/10.1103/PhysRev.130.916