Phase Transitions in Ge-Sb-Te Alloys Induced by Ion Irradiations

The variation of the electrical and optical properties under 150 keV Ar ion irradiation has been studied in Ge2Sb2Te5 polycrystalline films, either in the rocksalt or in the trigonal structure, by in situ reflectivity measurements and ex situ resistance measurements. As the irradiation dose increases, the disorder introduced in the crystalline films increases and the reflectivity decreases, down to a minimum value that corresponds to complete amorphization. Large differences are found by changing the irradiation temperature, for the two crystalline structures. Indeed, the measured amorphization threshold is the same for the two crystalline phases and equal to 1x10 cm under irradiation at 77K, whilst at room temperature the trigonal phase requires a dose almost double than the rocksalt phase to be amorphized. By structural analyses we found that, before amorphization, ion irradiation induces a transition from the trigonal to the rocksalt structure. The van der Waals gaps present in the trigonal phase might act as preferential sinks for the displaced and mobile atoms, thus promoting this transition. By further increasing the irradiation dose the formed disordered rocksalt phase converts into the amorphous phase. Ion irradiation also affects the electrical properties of the material: the disorder modifies the temperature dependence of resistance of the trigonal Ge2Sb2Te5 and induces a change of sign (from metallic to insulating behavior) at a dose of 2x10 cm, well below the amorphization threshold.