Influence of an underlayer on the crystallization of thin Ge-rich Ge–Sb–Te films

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Ge-rich Ge–Sb–Te alloys exhibit a high (>350 °C) crystallization temperature, which is compatible with applications requiring a high thermal stability such as the use of non-volatile memories in cars. As the composition of these alloys does not correspond to any stable crystalline phase, crystallization from the amorphous phase is accompanied by phase separation (Ge2Sb2Te5 + Ge). The complexity of the elemental processes associated with this multiphase amorphous-to-crystal transition calls for in-depth materials studies. In this work, we focus on the influence of a thin (5 nm) underlayer (Ge2Sb2Te5 or Sb2Te3) on the crystallization of a Ge-rich Ge–Sb–Te alloy. Using in situ synchrotron x-ray diffraction, we demonstrate that the underlayers facilitate crystallization by decreasing crystallization temperature and suppressing incubation time. These results are interpreted in the framework of classical nucleation theory and are supported by crystallographic texture measurements, which evidence ⟨111⟩ fiber texture in the crystallized cubic Ge2Sb2Te5, particularly marked in the case of an Sb2Te3 underlayer. These results bear important consequences for the cycling of memory cells based on these materials.