Princeton University

School of Engineering & Applied Science

Relaxation in Phase Change Memory Cells

Dr. Daniel Krebs, Visiting Scientist at IBM Research-Zurich, IBM Research
Engineering Quadrangle, J401
Thursday, May 1, 2014 - 1:15pm

Even though phase change materials have found their way into commercial products for memory applications, some fundamental problems linked to the relaxation and crystallization kinetics remain to be solved. The inherent relaxation of the amorphous structure for example causes an increase of resistance over time, called resistance drift. This prompts a major challenge for the implementation of multilevel storage which is necessary to achieve high stoarage densities.
Previous studies have linked changes in the density of states with the resistance increase and found an empirical relation for the time evolution of the activation energy of conduction for a constant annealing temperature. The models that have been proposed to explain this power law dependence by structural relaxation are all based on a uniform distribution of activiation energies for structural relaxation.  This however seems unphysical since structural relaxation processes in glasses are typically attributed to a α-relaxations following a single activation.
In this work, we present temperature dependent resistance measurements and demonstrate how temperature and time dependence of drift can be decoupled. By describing the relaxation process with an order paramter and a single activation energy for structural relaxation we find a differential equation that can describe experimental data with arbitrary temperature and time evolution of the resistance. Furthermore we find that the empirical relation for the activation energy of conduction is a limiting case for constant temperature of this differential equation.
In order to find potential candidates to represent the order parameter in a microscopic picture, molecular dynamics simulations have been carried.  They show that there is a correlation between an increase of the optical band gap and a tendency towards a local structure that reassembles the local order in the crystal. This result suggests that the processes occurring during relaxation are related to the crystallization kinetics.