A mechanism is proposed to account for the ability of Bi to change the electrical properties of certain chalcogenide glasses. EXAFS experiments have shown that there is a significant change in the local structural environment of the Bi at compositions close to the critical concentration of Bi(10 at.%) at which the p-n transition takes place. These structural changes are interpreted in terms of a model in which the type of bonding at the Bi sites changes from covalent to partially ionic. The presence of charged impurity centres upsets the equilibrium between native charged defects in the glass, and a p-n 'doping' transition subsequently occurs at a critical impurity concentration. © 1987 IOP Publishing Ltd.
