Strain development and damage accumulation under ion irradiation of polycrystalline Ge-Sb-Te alloys

The atomic displacement produced by ion irradiation with 150 keV Ar ions has been studied in GeSbTe and GeSbTe. Electrical, optical and structural measurements have been employed to characterize the induced electrical and structural modifications. At low temperature the amorphization threshold, evaluated by in situ reflectivity measurements, is independent of the composition and the crystalline structure, and it is equal to 1 x 10 cm. At room temperature, at which dynamic annealing can take place, GeSbTe and GeSbTe in the rocksalt phase exhibit the same amorphization threshold (3 x 10 cm). In the trigonal structure, instead, a higher fluence is required to amorphize the GeSbTe, compared to GeSbTe. The observed differences between the two compositions can be explained considering the effect of dynamic annealing during ion irradiation of the trigonal phase, which is characterized by the presence of van der Waals gaps. These may act as a preferential sink for the diffusion of the displaced atoms and the filling of these gaps tunes the electronic and structural properties. Filling of about 30% of the gaps produces an electronic transition from metallic to insulating behavior. By further increasing the disorder and filling more than 70% of the gaps the films convert into the rocksalt phase.