RADIATIVE RECOMBINATION CENTER IN AS2SE3 AS STUDIED BY OPTICALLY DETECTED MAGNETIC-RESONANCE

The intrinsic luminescence of As2Se3 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 field depend strongly on the orientation of the magnetic field. The measurements confirm that a self-trapped triplet exciton is a 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 the 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 As2Se3 indicates that similar relaxation processes also occur in amorphous As2Se3, where they precede at least a fraction of the recombination processes.