Thermal stability of 100 nm Ge2Sb2Te5 thin film during annealing from room temperature to 240 degrees C inside a UHV chamber was studied in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by Xray diffraction (XRD) and atomic force microscopy (AFM). Ge species are found to diffuse preferentially to the surface when GST film is annealed from 25 degrees C to 100 degrees C. This process is accompanied by a change of phase whereby the amorphous film completely becomes face-center-cubic (FCC) phase at 100 degrees C. From 100 degrees C to 200 degrees C, both Sb and Te species are observed to diffuse more to the surface. The FCC phase is partially changed into hexagonal-close-pack (HCP) phase at 200 degrees C. At 220 degrees C, FCC phase is completely transformed into HCP phase. Loss of Sb and Te are also detected from the surface and this is attributed to desorption due to their high vapor pressures. At 240 degrees C, Sb and Te species are found to have desorbed completely from the surface, and leave behind Ge-rich 3D droplets on the surface. The separation of Ge2Sb2Te5 into Sb, Te-rich phase and Ge-rich phase is thus the main mechanism to account for the failure of Ge2Sb2Te5-based phase change memory devices under thermal stress. (C) 2012 Elsevier B. V. All rights reserved.