Transition of reversible photodarkening to photobleaching in chalcogenide films

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.

Dual Effects of Photo-Darkening and Photo-Bleaching in Ge-Se Films

“…2 suggests that the PB effect is much slower than the PD. One of the reasons of the PB effect can be further homopolar bond breaking, occurring in a quite limited fashion, compared to the homopolar bonds reduction related to the PD process, similarly to the results reported in [22,24]. The ratio of Raman peak areas of the building blocks is quite close to that observed for relaxed bulk material with the same composition [29].…”

“…The dual action of light was shown earlier in Ge-As-S [22,23] and in Ge-As-Se [24] films. In essence, this shows the reversibility of the processes for which Elliott has created a unified model [25].…”

Dual action of light in photodarkened Ge–As–S films

“…It has been accepted as 'giant' in accordance with [4]. Moreover, at decreasing the film thickness to about 600-700 nm a new effect has been found -after reaching a saturated value the PD begins to decrease and completely disappears upon prolonged enough illumination time [1]. It has been supposed that this is a light annealing effect (LAE) where the PD is in competition with another photobleaching (PB) process [1].…”

“…Moreover, at decreasing the film thickness to about 600-700 nm a new effect has been found -after reaching a saturated value the PD begins to decrease and completely disappears upon prolonged enough illumination time [1]. It has been supposed that this is a light annealing effect (LAE) where the PD is in competition with another photobleaching (PB) process [1]. Usually, PD is explained by bond changes in chalcogenide atoms, accompanied with changes in the interactions of their LP electrons, which lead to increase of network disorder.…”

“…In recent years, we have studied intensively the photostuctural changes in thin, nearly nanosized films, in which surface effects are essential. It has been established that new effects appear with decreasing the film thicknesses down to nanosizes, when the surface changes became dominant over those in volume [1,2]. These changes could be related to adsorption of some gases and/or to the changes of the near-surface disorder.…”