Magnetic properties for metastable negative-U defects in amorphous semiconductors

Optically induced metastable centers are studied in As2S3 chalcogenide glasses using the magnetic susceptibility technique for 3.5 K less than or equal to T less than or equal to 300 K. The temperature dependencies of the magnetic susceptibility (chi) exhibit spin instabilty of both a hole center localized at a nonbonding lone pair chalcogen orbital and an electron center formed by an As p orbital. The chi fatigue at low temperatures is shown to result from the negative-U dissociation of paramagnetic states 2D degrees --> D- + D+ that causes also the absence of an ESR signal before illumination. Irradiation with light whose energy corresponds to the Urbach tail of the absorption edge is found to result in a growing chi signal that is due to optically induced paramagnetic states revealed also by the EPR signals: D- + D+ + hv --> 2D degrees, whereas subsequent irradiation with infrared light in the midgap bleaches both the optically induced magnetic susceptibility and ESR signal to its cold dark efficiency. Metastable properties for paramagnetic states located in the gap of As2S3 are discussed in the framework of the negative-U reaction that is accompanied by the restoration of the PL and the fatigue of the ESR signals and the magnetic susceptibility value.