Understanding phase change materials on a fundamental level is crucial to enable advancements in the key technological challenges of phase change memories.
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Haris Pozidis
IBM Research scientist
Fig. 1: Density of states of amorphous phase change materials. Besides a strongly temperature-dependent band gap, amorphous phase change materials exhibit pronounced band tails as well as deep and shallow defect states that dominate the electrical transport properties.
Fig. 2: Temperature dependence of the conductivity of GeTe before (black) and after (red) annealing. The conduction is dominated by p-type trap limited band transport above 180 K. Below, variable range hopping is dominant. Upon annealing, the band gap opens, leading to an overall reduced conductivity.
One of the biggest challenges for storage-like applications is resistance drift and drift variability in amorphous phase change materials. For memory-like applications, a precise understanding of the transient, field dependent conductivity is of crucial importance.
Both resistance drift as well as transient conductivity are closely related to the density of states [2012-2] of the amorphous phase (Fig. 1). In particular, defects in the band gap dominate the electrical transport behavior [2013-2].
Our group is investigating the correlation between density of states and electrical transport via experimental and theoretical means [2012-3, 2012-4, 2013-5]. On thin films we utilize infrared spectroscopy and temperature dependent conductivity and photo-conductivity measurements [2011-9] down to liquid nitrogen temperature (Fig. 2). On nanofabricated devices we perform transient switching experiments on the nanosecond timescale [2009-3].
In combination with computer aided simulations, these measurements give insights about the influence of specific features of the density of states on transport and on the nature of its evolution upon relaxation [2011-7] (Fig. 3). Finally, using molecular dynamics simulations in collaboration with the computational sciences department, the features in the density of states are linked to the structural arrangement in the amorphous phase.
In order to study the impact of electrical transport and other material characteristics on memory cells, finite element model (FEM) simulations are applied. They include physics based models for the phase change material parameters and their temperature and field dependence and solve Poisson and heat transport equations including thermoelectric effects [2009-1, 2009-2, 2010-1]. Combined with experimental data, they give insight into the thermal and electrical efficiency of cells and allow for scaling studies and the design of novel cell structures (Fig. 4).
Fig. 3: Resistance at room temperature and activation energy increase upon annealing. The simulation reproduces this behavior with an increase of the temperature-dependent band gap.
Fig. 4: FEM simulation of the RESET process of a phase change memory cell. The heat flux towards the neighboring cell shows the potential for thermal disturbance.