Trapping and detrapping effects in lithium-drifted germanium and silicon detectors

Abstract

The trapping-detrapping behaviour of Ge(Li) and Si(Li) detectors at low temperatures ( T<77K) has been quantitatively analyzed by means of the phenomenological theory published by J.W. Mayer 1). The conclusion has been reached that the trapping centres are the primary dopants: Li and Ga in Ge(Li) detectors, Li and B in Si(Li) detectors. Normal spectrometer operation is not affected by these shallow traps because their small activation energies result in very fast detrapping at T⩾77 K. A field assisted detrapping effect has been identified for the first time in Ge. This effect allows good spectrometer performance down to at least 8 K at high electric fields. No such field detrapping effect was observed in Si(Li) detectors, so the low temperature limit of operation is established at ≈ 50 K by the thermal re-emission of carriers from the shallow traps. Deep trapping effects at 77 K in Ge(Li) detectors have also been briefly investigated. A probable explanation is offered for the observed improvement in performance of many detectors at high electric fields. The values of the measured capture cross-sections for trapping of the carriers by the ionized dopants in Ge and Si fall in the same range as those in the literature. This demonstrates the potential of lithium-drifted devices for the study of these processes.