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

In annealed chalcogenide films, thinner than 1?mu m, highly expressed photodarkening (PD) can be fully restored after prolonged illumination. It was supposed that this light annealing effect (LAE) results from the dual action of light, which provokes not only increase of network disorder at PD but also decrease of disorder leading to photobleaching (PB), competing with PD. To confirm it, the changes of two parameters of disorder are studied: the Tauc slope B1/2, dependent on network bonding character, and the Urbach energy Eu, associated with tailing of the band edges. It is found that B1/2 decreases during PD and increases when PB overcomes the PD, which is direct evidence that the basis of the LAE is the dual action of light. The changes of disorder defined by Eu are found to be mostly inversely related to the changes of B1/2. The peculiarities of this relationship after various treatments are presented and the problem of the correspondence of both parameters of disorder is discussed.