SUB-BANDGAP OPTICAL ELECTROABSORPTION IN THE FIELD OF A P-N-JUNCTION

A sub-bandgap light absorption coefficient a has been calculated as a function of light frequency omega for a semiconductor with a built-in non-homogeneous electric field of arbitrary shape. The electroabsorpti on spectrum a(omega) is shown to possess the following features: a) as the energy of absorbed quanta h omega approaches a bandgap E-g, the a bsorption coefficient a(omega) is close to that described by the Franz -Keldysh law for the homogeneous effective field equal to the maximum field within the p-n junction; b) for smaller omega, the coefficient a drops more abruptly than in the case of a homogeneous held because it is determined by the lateral sides of the junctions where the local f ields are weaker; and c) it is terminated at the point omega(t) = (E-g -Delta)/h, where Delta is the total potential barrier across the p-n junction. Possible applications for delta-doped semiconductor superstr uctures are discussed. In particular, for materials with asymmetrical effective masses of the carriers (m(c) < m(lh)), the long-wavelength t ail of the absorption spectrum is more sensitive to the shape of the s patial distribution of donors than to that of accepters.