Effect of Charged Surfaces on the Optical Absorption Edge

Electric fields existing within the space-charge region near a charged surface of a solid are shown to be capable of significantly altering the fundamental optical absorption edge. Their effect is to produce below the gap energy an exponential tail whose slope and magnitude depend on the field strengths. Furthermore, at any given frequency in this tail, an apparent absorption coefficient should be observed having an inverse dependence on the sample thickness. Using representative values for the relevant quantities, it is found that this effect can produce an apparent absorption coefficient of 200 ${\mathrm{cm}}^{-1\mathrm{}}$ at the photon energy equal to the energy gap with the tail extending below this for ∼0.02 eV. Some data are cited to support the suggestion that such absorption effects occur very commonly although they have not been recognized.