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Degradation of phase change memories

They must be encapsulated !

par PREVOTS Evelyne - publié le

CEMES researchers have shown that the natural oxidation of Ge-Sb-Te alloys is highly selective, it generates a redistribution of the elements below the surface and the preferential heterogeneous crystallization of Ge from the surface, at a temperature 50-60°C lower than when the surface is protected from ambient air. This study explains the reasons for failures observed when these layers are not encapsulated immediately after their deposition.

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Phase change memories (PCMs) appear today as the memories of the future, fast, energy efficient, and integrable in matrices located immediately close to processors, as required for big data processing (1). A PCM cell most often consists of a thin layer of Ge-Sb-Te alloy (GST) sandwiched between two electrodes. Electric current pulses and Joule-induced heating are used to thermally switch the PCM from the RESET state (low electrical conductivity in the amorphous phase) to the SET state (highly conductive in the crystalline phase) and vice versa, via crystallization or quenching after melting, respectively.

It has been repeatedly reported that the voluntary or accidental oxidation of layers of alloys of the Ge-Te or Ge-Sb-Te type greatly degrades their physical characteristics and, consequently, the performance of the devices using them. The crystallization temperature of the layers is greatly reduced, thereby reducing the stability of the RESET state. We have studied in detail the impact of prolonged exposure to air of layers of Ge-rich GST alloys, on the characteristics of the crystallization of such layers deposited homogeneously in the amorphous phase.

When these layers are encapsulated after deposition, their crystallization takes place from 380 °C via the separation of the chemical Ge and GST phases, the homogeneous crystallization of Ge, and finally the crystallization of the Ge2Sb2Te5 phase (2, 3). When the surface comes into contact with air, it oxidizes to a few nanometers thick. This oxidation is highly selective, mainly affecting Ge and, to a lesser extent, Sb. The “in-situ” TEM shows that Germanium crystallizes, not homogeneously in the layer, but heterogeneously, from the surface to the depth of the layer, and at a lower temperature, around 330 °C. These observations, as well as the evidence of the strong chemical redistributions which occur during crystallization, show that the oxidation has the effect of providing seeds for crystallization, probably in the form of Sb2O3, which then allow the subsequent heterogeneous crystallization of Ge.

1) https://fr.wikipedia.org/wiki/3D_XPoint

2) M. Agati et al., MRS Communications (2018),doi:10.1557/mrc.2018.168

3) M. Agati et al., J. Mat. Chem. (2019), doi : 10.1039/c9tc02302

 

Reference

M. Agati et al., Applied Surface Science 518 (2020) 146227, doi:10.1016/j.apsusc.2020.146227

 

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