Crystallization accompanied by local distortion behavior of Sn-doped amorphous Ge2Sb2Te5 induced by a picosecond pulsed laser

Phase change materials (PCMs) are highly promising for the digital-storage technology based on the fast phase transition between the crystal and amorphous states. In order to optimize the properties of PCMs, the doping of PCMs has become research hot points nowadays. In this paper, effects of Sn doping (0%, 10%, 30%) on the crystallization behavior of Ge2Sb2Te5 (GST) films induced by a picosecond pulsed laser with Gaussian energy profile were investigated experimentally. As compared to the previous study irradiated by a nanosecond pulsed laser, such factors as melting, ablation and crystallization thresholds dropped remarkably by a picosecond laser irradiation, and with the increase of Sn content those factors were further decreased. Due to the replacement of Ge atoms by Sn atoms, the Sn incorporation expanded the lattice constant so as to introduce the mechanical stress which, together with the thermal stress generated after laser irradiation, significantly affected the atomic movement during the crystallization process. High-resolution transmission electron microscopy (HRTEM) observations showed that GST with a lower Sn content of 10% tended to form sub-grain boundaries by local atomic shear to ensure long-range ordering of crystals, but with the increase of the Sn doping to 30%, the long-range ordering structure was broken, instead of some atoms arranged randomly, which were mainly Ge atoms, separating out from the lattice because of the replacement by Sn atoms. These extra Ge atoms were not involved in the crystallization process, proved by the Raman spectroscopy. The present study is fundamental for the design of advanced storage device based on PCMs.