Experimental observation of two-stage crystallization of Ge2Sb2Te5 amorphous thin films under the influence of a pulsed laser

Abstract

The nature of crystallization in telluride based phase change memory materials is being actively discussed, and we present new experimental results on the effect of a pulsed laser treatment on the amorphous thin film crystallization of composition Ge2Sb2Te5 (GST225). It is shown that there are two ways of crystalline phase formation under femtosecond pulses excitation. At the moderate value of laser fluence, crystalline fraction forms inside the solid amorphous phase. This process is apparently two-stage, and this leads to formation of fine-grained polycrystalline material. The second way of crystallization proceeds at higher levels of the laser fluence, closer to the beam axis. In this case, larger grains are produced closer to the beam axis, and eventually no new nuclei can be formed due to high cooling velocity leading to inhibition of nucleation. The article also presents the results on the effect of the substrate type (metallic or oxide) and film thickness on crystallization, which is due to the light penetration depth and reflection from the film-substrate interface.