Amorphous-Crystal Phase Transitions in GexTe1-x Alloys

The crystallization of amorphous GexTe1-x (x = 0.36,0.51, 0.63) films (50 nm) has been investigated by time resolved reflectivity, transmission electron microscopy, Raman spectroscopy and X-ray diffraction. The Ge-rich film has the highest crystallization temperature (354 degrees C) with respect to both GeTe (180 degrees C) and Te-rich samples(244 degrees C). In non-stoichiometric films, the precipitation of the excess atomic species is the first step during the crystallization process: amorphous Ge and crystalline Te precipitates were detected in Ge and Te rich alloys, respectively. The atomic interdiffusivity was estimated to be similar to 5x10(-15) cm(2)/s at 220 degrees C in the Te-rich alloy and similar to 4x(10-14) cm(2)/s in the Ge-rich film at 330 degrees C. In both cases Tellurium is likely to be the diffusing species. This description accounts for the presence of amorphous Ge precipitates at the initial stage of the crystallization of the Ge rich alloy. The GeTe crystalline grains subsequently act as a seed for Ge crystallization. Non stoichiometric crystalline alloys have been reamorphized by laser or ion irradiation. In laser irradiated samples, the crystallization is similar to that of as deposited film since the precipitates mix with GeTe during the melt duration. In ion implanted samples this mixing doesn't occur and GeTe crystallization temperature is close to that of stoichiometric film.