Optical properties of neutron-irradiation-induced defects in single-crystal tellurium near the fundamental energy gap

Abstract

Undoped tellurium single crystals have been irradiated at room temperature with thermal neutron doses between 2·1 × 10¹⁶ and 3·4 × 10¹⁷ n_th cm⁻² at a flux ratio ø_th/ø_fast of about 5. Photoconductivity measurements as a function of energy, temperature and modulation frequency provide evidence for radiation-induced defect states centred at an energy 10–13 meV above the valence band edge. These exhibit a distribution in energy with a full width at half maximum of 20 meV, depending on neutron dose and annealing treatment. The corresponding defect structures show an acceptor-like character similar to those observed in dislocation-rich crystals.

From the rise of the Fermi energy between 2·15 and 5 K after annealing the irradiated crystals, we conclude that the iodine atoms created by neutron transmutation doping have donor-like character. This proposal is confirmed by the results of simultaneous measurement of the optical absorption and the electrical resistivity.