Storage of electrons in shallow donor excited states of GaP:Te

Tellurium donors in GaP have been ionized by phonon-assisted tunneling in the electric field of pulsed far-infrared laser radiation. In response to the laser pulse a photoconductive signal has been detected with a fast component that follows in time the laser pulse and a slow component that rises after the irradiation has ceased and finally exponentially decays with a strongly temperature-dependent time constant varying from several microseconds to milliseconds. It is shown that this temporal structure of the signal is due to a storage of carriers in the valley-orbit split 1s(E) shallow donor state. Observation of far-infrared to mid-infrared up-conversion demonstrates that the final step of cascade recombination is achieved by radiative transitions

Ionization of deep impurities by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E)}exp(E2/Ec 2) (where E is the wave field, and Ec is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole–Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant