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We have measured the critical phase change conditions induced by electricalpulses in Ge2Sb2Te5 nanopillar phase change memory devices by constructing acomprehensive resistance map as a function of pulse parameters (width,amplitude and trailing edge). Our measurements reveal that the heating schemeand the details of the contact geometry play the dominant role in determiningthe final phase composition of the device such that a non-uniform heatingscheme promotes partial amorphization/crystallization for a wide range of pulseparameters enabling multiple resistance levels for data storage applications.Furthermore we find that fluctuations in the snap-back voltage and set/resetresistances in repeated switching experiments are related to the details of thecurrent distribution such that a uniform current injection geometry (i.e.circular contact) favors more reproducible switching parameters. This showsthat possible geometrical defects in nanoscale phase change memory devices mayplay an essential role in the performance of the smallest possible devicesthrough modification of the exact current distribution in the activechalcogenide layer. We present a three-dimensional finite element model of theelectro-thermal physics to provide insights into the underlying physicalmechanisms of the switching dynamics as well as to quantitatively account forthe scaling behaviour of the switching currents in both circular andrectangular contact geometries. The calculated temporal evolution of the heatdistribution within the pulse duration shows distinct features in rectangularcontacts providing evidence for locally hot spots at the sharp corners of thecurrent injection site due to current crowding effects leading to the observedbehaviour.

Electrical Switching Dynamics in Circular and Rectangular Ge2Sb2Te5 Nanopillar Phase Change Memory Devices