Study of memory effect in amorphous chalcogenide semiconductor GST225 thin films
We explored phase change memory phenomena in thin film of chalcogenide-glass semiconductors GST225 by passing through the film a sequence of triangular current impulses with increasing amplitude. Initially the film was in amorphous state. We took simultaneous oscilloscope pictures of the voltage across the thin-film “sandwich” type sample and the current passing through the above unit as a function of time. Then we got the current-voltage characteristics of the unit done. Distinctive features observed on some of the oscilloscope pictures and current-voltage characteristics are places with rapid decrease in voltage and negative differential resistance. After passing current impulse with the biggest amplitude the current-voltage characteristic of the unit is becoming almost ohmic. Analysis of all the current-voltage characteristics obtained allows us to suggest that the memory state formation apparently occurs in stages with gradual crystallization of amorphous substance of the film on thickness depending on amplitude of the triangular current impulses passing through the unit. This possible scenario is discussed here only from qualitative point of view. The rapid decreases in voltage apparently can under certain conditions be explained by peculiarities of crystallization process (“explosive” crystallization) while crystallization itself is initiated by electrical field. Taking also into account that under injection of current, when the film is in poorly conducting state, a space charge appears to form and as consequence a nonuniform distribution of field on thickness of the film can arise we can assume that crystallization starts at one of electrodes where the field is the biggest. “Explosive” crystallization of amorphous substance dies down when the field near the front of crystallization turns out to be not enough for holding it. After a part of the film is crystallized such a way and conditions for “explosive” crystallization are not more fulfilled the further crystallization can run by heating. As to a part of current-voltage characteristic with negative differential resistance they apparently are connected with switching phenomena that is characteristic of amorphous chalcogenide semiconductors.