Radiative recombination center in As2Se3 as studied by optically detected magnetic resonance.

Abstract

The intrinsic luminescence of ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ single crystals and glass has been studied by optically detected magnetic resonance (ODMR) using 16-GHz microwaves and magnetic fields up to 3 T. From these measurements we extract detailed structural information about a photoexcited center in a chalcogenide semiconductor. In the crystals a strong ODMR response is observed with the same spectral dependence as the luminescence spectrum. Resonant magnetic fields depend strongly on the orientation of the magnetic field. The measurements confirm that a self-trapped triplet exciton is the radiative state for midgap luminescence. Hyperfine splittings of the microwave transition and large zero-field splittings reveal a triplet state that is highly localized and anisotropic (almost uniaxial) with its symmetry axis oriented along a particular As-Se bond. Self-trapping of the exciton occurs at a center where lone-pair electronic states of Se interact strongly. The self-trapping leads to a redistribution of electronic charge that strengthens locally a weak intermolecular (intralayer) bond at the expense of covalent intralayer bonds. Magnetic fields enhance the luminescence efficiency, a fact that suggests the presence of a competing nonradiative recombination process, even at low temperatures, which is assumed to be related to diffusion of the self-trapped exciton. Comparison of the ODMR powder spectrum of the crystal with the ODMR spectrum of glassy ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ indicates that similar relaxation processes also occur in amorphous ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$, where they precede at least a fraction of the recombination processes.