Transition of reversible photodarkening to photobleaching in chalcogenide films

Institute of Solid State Physics, Bulgarian Academy of Sciences - 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria, EU

Corresponding author: vateva@pronto.phys.bas.bg

Received: 16 December 2009
Accepted: 6 March 2010

Abstract

Photodarkening (PD) of chalcogenide glasses and annealed films (i.e., photoinduced shift of the optical absorption edge to lower energies) is accompanied with changes in refractive index and thickness. The current studies of thin Ge-As-S films show that with decreasing the film thickness (d) to ~ 700 nm PD has an unusual dependence on illumination time. In particular, we detected that after reaching a maximal value PD begins to decrease and at long enough time it fully disappears and photobleaching (PB, shift to higher energies) may appear. This new “light annealing effect” is related to the dual action of light, which induces an ordering process predominantly in the near-surface region leading to PB in competition with the disordering process which attends the PD. The PB prevails in thinner films and at d < 100 nm only PB appears. The transition of PD to PB is a nano-sized effect due to the increase of the surface/volume thickness ratio.