A new interpretation of the photodoping effect in amorphous As- and Ge-chalcogenides

Photodoping is considered as a radiation-enhanced solid state reaction. The reaction kinetics is governed by two physical processes: (i) The generation of electrons and silver (copper) ions at the Schottky-barrier originated at the metal-chalcogenide interface and (ii) a transport of the electrons and the metal ions through the amorphous reaction product layer silver-(copper-) chalcogenide. The formation of that compound is a three-dimensional intercalation reaction. The intercalation product possesses a high ionic conductivity after reaching a percolation threshold concentration. The percolation mechanism is responsible for the occurrance of a step-like doping profile. Both processes (i) and (ii) are stimulated by absorption of light. Light absorption at the Schottky-barrier generates electrons and metal ions. The light absorption within the bulk of the doped chalcogenide supplies free photoelectrons that have to maintain charge neutrality during cation migration and for insertion of the cations into the amorphous network. The insertion is based on the capability of the chalcogens to form C centres and the appearance of CM+ (M: Ag, Cu) ionic bond units within the amorphous chalcogenide (C + e− + M+ → CM+). A quadrupolar deformibility of Ag+ and Cu+ causes the high ionic mobility of these species.