Thermally- and photo-induced changes in the structure and optical properties of amorphous As40S30Se30 films

Annealing at a temperature near the glass transition temperature and exposu re with bandgap light, in aii of thermally-evaporated amorphous As40S30Se30 films, were found to be accompanied by structural effects, which, in turn, lead to changes in the refractive index and shifts in the optical absorpti on edge. Also, indications of photo-oxidation were found after light exposu re. An optical characterisation method, based only on the transmission spec tra at normal incidence of uniform thin films, has been used to obtain the thicknesses and optical constants corresponding to the virgin, annealed and exposed As40S30Se30 samples. The dispersion of the refractive index is dis cussed in terms of the single-oscillator Wemple-Di Domenico model. The abso rption edges are described using both the Urbach rule and the 'non-direct t ransition' model proposed by Tauc. Regarding the structural transformations that take place in As40S30Se30 chalcogenide films when annealed or exposed , the clear decrease and the small shift to higher angle of the intensity p rofile of the first sharp diffraction peak in the X-ray diffraction pattern , with both treatments, has been interpreted as a diminution of the interst itial volume around the AsS3-nSen pyramidal structural units, which form th e amorphous network. This significant result is certainly consistent with t he decrease of the thickness found for the annealed and illuminated samples . (C) 2001 Elsevier Science B.V. All rights reserved.