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