Optical Properties of Space-Charge Regions

This paper shows how the optical properties of solids, which depend upon the magnitude of an electric field, may be measured in the regions of space charge which may exist in these materials. The fundamental requirement is that the substance or interface have a uniform region of space charge which may be modulated by some external perturbation such as applied voltage. Although the magnitude of the electric field can vary over many orders of magnitude in a region of space charge, this experimental technique allows a determination of the absorption constant for a unique value of the electric field. Examples are presented to show how Stark shifts, Franz-Keldysh effects, carrier densities, cross sections, and surface states may be studied. In particular, the properties of a $p-n$ junction are worked out in detail.