Photoinduced changes

Figure 12: Influence of illumination and annealing on the optical absorbtion of amorphous chalcogenides films [30]

Photoinduced structural changes are phenomena unique to vitreous chalcogenides and are not observed in amorphous group IV. semiconductors or a-As [31] nor in crystalline chalcogenides [32]. Possible explanations of this uniqueness will be given below and we shall start with phenomenological description of the effect. In the early 1970s, reversible photoinduced changes in the optical properties of a vitreous chalcogenide thin films were reported by Keneman [33], Berkes [34] and de Neufville [30]. Later the similar effect was observed also in melt-quenched bulk glasses [32]. The effect is demonstrated in fig. 12. Solid curves in the figures correspond to the optical transmission of as-evaporated or so-called virgin As$_2$S$_3$ and As$_2$Se$_3$ films. On illumination the absorbtion edge shifts to lower energies (longer wavelengths) (dotted curves). Subsequent annealing near to glass transition temperature leads to a recovery of the initial parameters of film (broken curves) but the recovery is never complete; the curve occupies the immediate position between the curves describing the as-evaporated (virgin) and illuminated states. The annealed film should be now illuminated, its absorbtion edge will move again to the position described by the dotted curve and annealing will return it to the position depicted by broken curve. One has therefore a completely reversible behavior during "illumination-annealing" cycle of previously annealed film. On the figure 12 is as example the As$_2$S$_3$ and As$_2$Se$_3$ film but similar changes can be observed in the most Ge- and As- based chalcogenides. The first irreversible component, which can be annealed out by heating the film at temperature close to glass transition temperature, is believed to be caused by thermally induced or photoinduced polymerization of as-evaporated film which is formed from numerous vapor species present during the evaporation procedure. Mass spectroscopy studies [35] have shown that large number of low-mass fragments such as As, As$_4$, S$_2$, S$_8$, AsS, As$_4$S$_4$, As$_4$S$_5$ and others are present in the vapor phase during the evaporation of As$_2$S$_3$. As result we can observe even in stoichiometric film such as As$_2$S$_3$, where stoichiometry would normally only allow As-S bonds, large number of so-called wrong bonds (As-As, S-S).

Figure 13: Photoinduced reversible and irreversible change in Raman spectrum of As$_{40}$S$_{60}$ and As$_{60}$S$_{40}$ (respectively from left) [27].

This can be clearly proven by using the Raman spectroscopy [27,36](see fig. 13). These homopolar bonds are broken during annealing or illumination and thermodynamically more stable heteropolar bond are formed instead, making a glass network continuous [37,38,39,40]. This photoinduced changes in the optical absorbtion of as-evaporated films can lead to either decrease or an increase in the absorbtion coefficient, depending on the material composition and details of preparation. Thus, in As-based chalcogenides it is usually photodarkening [30] although in certain cases, for example in As-S films prepared by very fast evaporation (800 Ås$^{-1}$) of As$_2$s$_3$ film glass powder as the starting material, the irreversible photobleaching was observed under illumination at room temperature by Tanaka and Kikuchi(1974) which is attributed to the decomposition of the films, namely the formation of As$_2$O$_3$ crystals. As the evidence of this interesting fact the appearance of the corresponding peak in the X-ray diffraction (XRD) pattern of illuminated film was shown. Unusual behavior of these films is explained by the fact that, under such high evaporation rates, films enriched with As were formed and the excess As was supposed to be photo-oxidized [41]. Photoinduced processes in as-evaporated are very important for practical applications of chalcogenides such as submicron lithography