Evidence of Crystallization–Induced Segregation in the Phase Change Material Te-Rich GST

When the phase change material Ge2Sb2Te5 melts during the RESET switching of a phase change memory (PCM) device, it is known that electromigration (elemental segregation under electric field or current) can cause the region near the positively-biased electrode to become rich in Tellurium. We show that the laser-induced crystallization of deposited films of similarly Tellurium-rich GeSbTe (GST) material is 1000 x slower than the crystallization of conventional Ge2Sb2Te5 material, and that this material can readily flow and create voids at temperatures as low as 350 degrees C. AFM and Auger analysis of Te-rich GST films reveals significant thermal-induced motion and elemental segregation, occurring in the absence of an electrical field and at similarly low temperatures. This phenomenon, termed crystallization-induced segregation, occurs when material rearrangement within the poorly-crystallizing, under-cooled-liquid Te-rich GST matrix forms a local stoichiometry which is capable of much more rapid crystallization. The process of crystallization itself then reinforces this stoichiometry, at the expense of the elements not needed, with a growth-rate limited by material diffusion. This crystallization-induced segregation - which can readily occur in GST-based phase change memory (PCM) devices, especially after the onset of electromigration - is an important additional component to understanding both bias-polarity-based phenomena and endurance failure in GST-based PCM devices. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3614508] All rights reserved.