PHOTOIONIZATION OF SEMICONDUCTOR IMPURITIES IN THE PRESENCE OF A STATIC ELECTRIC-FIELD

The effect of a static uniform electric field on the photoionization process of an isolated impurity in a semiconductor is studied. The impurity potential is treated with the quantum-defect approach. The limiting hydrogenic and Lucovsky models are also considered. The photoionization process is analyzed with the Fermi's golden rule in the dipolar approximation. We show that the main effects of the static electric field are similar to the Franz-Keldysh effects well known for the photon-induced interband transition: the appearance of a redshift of the lower absorption edge and of an oscillation pattern superimposed on the zero-field photoionization cross section for higher energies. These oscillations are much stronger when the electric-field polarization of the photons is parallel to the static electric field than when it is perpendicular.