K-CENTER-DOT-P THEORY OF THE FRANZ-KELDYSH EFFECT

We use the k . p formalism to calculate Franz-Keldysh (FK) absorption spectra in direct-band-gap III-V semiconductors. This method allows us to include band anisotropy and nonparabolicity as well as band mixing . With k . p models of various complexity, we investigate how these ph enomena influence the main features of the electroabsorption using GaA s as an example. We show that the dependence of the FK absorption on t he polarization of the incoming light can be understood in analogy to quasi-two-dimensional systems by mostly decoupled heavy- and light-hol e bands involving the anisotropy of the bulk-momentum matrix elements. On the other hand, the FK absorption tail and the periods of the FK o scillations are mainly determined by the realistic energy dispersions of the bands. We discuss the applicability and the limits of simplifie d models and demonstrate that for most cases the field-induced interba nd mixing is negligible in comparison with the zero-field coupling of the bands. As an application, we show the consequences for the widely used method to determine the electric held from the FK oscillation per iods measured by absorption or photoreflectance experiments. Most of t he basic features have been observed in experiments. The qualitative d etails are of general validity.