Aging mechanisms in amorphous phase-change materials (Invited)

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram.

The direct melt-and-quench DFT based Molecular Dynamics approach leads to models with a few hundred atoms, and, hence a small number of atomic environments. Here we sample a large number of local atomic environments, and bonding schemes, by chemically substituting different alloys to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging.

GST alloys are known to display complex bonding that does not follow the chemist’s “octet-rule”. This lead to many controversies, especially concerning the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed. We can then identify their fingerprints in the available structural experimental data and assess their stability to find the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments.

Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous [1].

[1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nature Comm. To appear.