Characterization of Deep Impurities in Semiconductors by Terahertz Tunnel Ionization

The ionization of deep impurity centers by high intensity far-infrared radiation, with photon energies tens of times lower than the impurity binding energy, has been investigated. It is shown that the ionization is caused by phonon-assisted tunneling in the electric field of radiation, in which carrier emission is accompanied by electron tunneling and defect tunneling in the configuration space. The field and temperature dependencies of the ionization probability allow to obtain defect parameters like tunneling times and the Huang-Rhys factor as well as the basic structure of the defect adiabatic potentials. In very strong fields, the ionization changes to direct tunneling without assistance of phonons. It is shown that for the case of charged impurities and in a relatively low electric field strength the Poole-Frenkel effect plays a dominant role in ionization, which allows to identify the charge state of the center. Within a broad range of intensities, wavelengths and temperature, the terahertz electric field of the radiation acts as a static field. An enhancement of tunneling with increase of frequency has been observed for very high frequencies or low temperatures.