Voltage polarity effects in Ge2Sb2Te5-based phase change memory devices

We assess voltage polarity effects in phase-change memory (PCM) devices that contain Ge2Sb2Te5 (GST) as the active material through the study of vertically asymmetric pore-cell and laterally symmetric bridge-cell structures. We show that bias polarity can greatly accelerate device failure in such GST-based PCM devices and, through extensive transmission electron microscopy-based failure analysis, trace these effects to a two-stage elemental segregation process. Segregation is initially driven by bias across the molten region of the cell and is then greatly enhanced during the crystallization process at lower temperatures. These results have implications for the design of pulses and PCM cells for maximum endurance, the use of reverse polarity for extending endurance, the requirements for uni- or bi-polar access devices, the need for materials science on active rather than initial stoichiometries, and the need to evaluate new PCM materials under both bias polarities. (C) 2011 American Institute of Physics. [doi:10.1063/1.3626047]