Defect luminescence and its excitation spectra in As-doped Se single crystals

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Abstract
The influence of As doping (1-150 p.p.m.) on the luminescence of trigonal Se and on its excitation spectra has been studied in the temperature range between 2 and 200 K. Undoped crystals show, in addition to the richly structured narrow ‘edge’ luminescence from the resonant recombination of free excitons, a broad luminescence band near 0·72 eV which arises when free excitons recombine at defects. Weak As-doping quenches the ‘edge’ luminescence and alters both the mid-gap luminescence and its excitation spectra. Four new excitation levels appear close to the conduction band (1·61-1·82 eV) which all lead to two luminescence bands centred at 0·8 and at 0·35 eV. It is proposed that the four excitation levels belong to one defect, which consists of substitutional, threefold-coordinated As linked to a threefold-coordinated Se by an extra bond. When excited the extra bond destabilizes and the defect relaxes into one of two distinct configurations which give rise to two emission bands. After recombination the defect attains its original structure, consistent with the absence of any fatiguing effects. The excitation spectra show that only electron-hole pairs recombine radiatively, whereas free carriers relax non-radiatively to the ground state.