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

The effect of a static uniform electric field on the photoionization p rocess of an isolated impurity in a semiconductor is studied. The impu rity potential is treated with the quantum-defect approach. The limiti ng hydrogenic and Lucovsky models are also considered. The photoioniza tion process is analyzed with the Fermi's golden rule in the dipolar a pproximation. We show that the main effects of the static electric fie ld are similar to the Franz-Keldysh effects well known for the photon- induced interband transition: the appearance of a redshift of the lowe r absorption edge and of an oscillation pattern superimposed on the ze ro-field photoionization cross section for higher energies. These osci llations are much stronger when the electric-field polarization of the photons is parallel to the static electric field than when it is perp endicular.