Changes in volume in Ge2Sb2Te5 (GST) films due to the density change during phase transformations between amorphous and crystallized states and consequent void formations were examined. Transmission electron microscopy (TEM) was used to measure changes in volume during laser-induced crystallization for both as-deposited and melt-quenched GST amorphous films, and changes in density were estimated. We found the formation of voids and their coalescence during repeated heat cycling by laser. A variation in chemical composition around the voids was confirmed using energy-dispersive spectroscopy analysis. The void formation and compositional changes can lead to degradation of reliability in phase-change random access memory devices. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3439647] All rights reserved.