Surface Energy Driven Cubic-to-Hexagonal Grain Growth of Ge2Sb2Te5 Thin Film

Abstract

Phase change memory (PCM) is a promising nonvolatile memory to reform current commercial computing system. Inhibiting face-centered cubic (f-) to hexagonal (h-) phase transition of Ge2Sb2Te5 (GST) thin film is essential for realizing high-density, high-speed, and low-power PCM. Although the atomic configurations of f- and h-lattices of GST alloy and the transition mechanisms have been extensively studied, the real transition process should be more complex than previous explanations, e.g. vacancy-ordering model for f-to-h transition. In this study, dynamic crystallization procedure of GST thin film was directly characterized by in situ heating transmission electron microscopy. We reveal that the equilibrium to h-phase is more like an abnormal grain growth process driven by surface energy anisotropy. More specifically, [0001]-oriented h-grains with the lowest surface energy grow much faster by consuming surrounding small grains, no matter what the crystallographic reconfigurations would be on the frontier grain-growth boundaries. We argue the widely accepted vacancy-ordering mechanism may not be indispensable for the large-scale f-to-h grain growth procedure. The real-time observations in this work contribute to a more comprehensive understanding of the crystallization behavior of GST thin film and can be essential for guiding its optimization to achieve high-performance PCM applications.

Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys

“…However, the Ge/Te and Sb/Te ratios we have measured, slightly larger than expected for a regular Ge2Sb2Te5, might indicate that this material is doped, with Ge and Sb atoms at sites normally vacant on the cationic lattice (20%). It is important to note that, despite the high temperature involved, the GST phase we evidence is still cubic, as demonstrated by the HRTEM images, and not hexagonal as expected after annealing above the cubic-to-hexagonal transition temperature of the Ge2Sb2Te5 i.e., 300-350°C [18]. It is thus suspected that this transition is hampered by the environment, the Ge phase and probable stress at the interfaces.…”

“…For this reason, countless studies have focused on the investigation of Ge2Sb2Te5 properties [9]. Ge2Sb2Te5 exists in two phases: a metastable face-centered-cubic (FCC) structure, which forms at a temperature T~150°C [16], and a stable hexagonal structure, which forms above T~300°C [18]. In the FCC phase, a FCC sub-lattice containing only Te atoms (anion sites) is intercalated with another FCC sub-lattice containing a solid solution of Ge and Sb atoms as well as vacancies (cation sites).…”

On Some Unique Specificities of Ge‐Rich GeSbTe Phase‐Change Material Alloys for Nonvolatile Embedded‐Memory Applications

“…It is interesting to note that the cubic to hexagonal phase transition of GST‐225 is observed to progressively occur at the “classical” temperature of 380–400 °C. [ 33,34 ]…”

“…Their sizes may exceed that of the initial thickness of the deposited film and this evolution gives rise to the spatial structuring of the surface. It is interesting to note that the cubic to hexagonal phase transition of GST-225 is observed to progressively occur at the "classical" temperature of 380-400 C. [33,34] 3. Effect of N Doping: Slow Down N doping is known to significantly enhance the thermal stability of GGST alloys.…”

Origin of the concentration-dependent effects of N on the stability and electrical resistivity in polycrystalline Ge1Sb2Te4

“…24 These concentration-sensitive doping effects so far lack the corresponding microscopic explanations. On the other hand, the grain boundary plays a vital role in the behavior of GST alloys, 25,26 for instance, the grain boundary provides favorable doping sites for small dopants as observed in the experiments. 27,28 In addition, the structural distortions brought in by the boundary have an influence on the electronic structures.…”