Structural and electronic transitions in Ge2Sb2Te5 induced by ion irradiation damage

Ge2Sb2Te5 polycrystalline films either in the trigonal stable phase or in the metastable rock-salt structure have been irradiated with 150 keV Ar+ ions. The effects of disorder are studied by electrical, optical, and structural measurements and density functional theory (DFT) simulations. In the metastable structure, the main effect of ion irradiation is a progressive amorphization, with an optical threshold at a fluence of 3 × 1013 cm-2. For the trigonal structure, a metal-insulator transition and a crystalline transition to rock-salt structure occur prior to amorphization, which requires a fluence of 8 × 1013 cm-2. The bonds of Te atoms close to the van der Waals gaps, present in the trigonal phase and identified by Raman spectroscopy, change as a function of the disorder induced by the irradiation. Comparison with DFT simulations shows that ion irradiation leads to the gradual filling of the van der Waals gaps with displaced Ge and Sb lattice atoms, giving rise first to a metal-insulator transition (9% of displaced atoms) correlated to the modification of the Te bonds and then induces a structural transition to the metastable rock-salt phase (15% of displaced atoms). The data presented here not only show the possibility to tune the degree of order, and therefore the electrical properties and the structure of phase change materials by ion irradiation, but also underline the importance of the van der Waals gaps in determining the transport mechanisms and the stability of the crystalline structure.