Evolution of subcritical nuclei in nitrogen-alloyed Ge2Sb2Te5

Abstract

We analyze the evolution of subcritical nuclei as a function of nitrogen alloying and thermal annealing in the amorphous phase change material Ge2Sb2Te5. The existence of subcritical nuclei is inferred through measurement of the nucleation time in pulsed laser annealing, and is detected more directly using fluctuation transmission electron microscopy measurements that are sensitive to topological order on the nanoscale. In samples that are pre-annealed before crystallization experiments, the nanoscale order consistently increases and the nucleation times consistently decrease, in agreement with the interpretation that the nanoscale order corresponds to a population of subcritical nuclei that ripens upon annealing. However, this correlation is less obvious in as-deposited samples across a range of nitrogen contents: The quantity of nanoscale order diminishes only slightly with increased nitrogen alloying, whereas the nucleation times increase by nearly two orders of magnitude. We therefore interpret that nitrogen must affect either the critical size in order for a nucleus to be stable (the thermodynamics) or the rate at which nucleation and growth take place (the kinetics).

Microscopic Mechanism of Doping‐Induced Kinetically Constrained Crystallization in Phase‐Change Materials

“…Experimental observations at a macroscopic scale for N‐doped GST point to a significant alteration in crystallization kinetics for a low level of doping by nitrogen, notably resulting in a strong grain refinement, and a sizable increase in both crystallization temperature ( T c ) and the electrical resistance. In line with the increase in T c , the incubation time for crystal nucleation, seemingly along with the growth time, increased by up to an order of magnitude with N doping . It is noted that, contrary to most cases, an increase in crystallization time was reported when a nitride layer was inserted into a GST thin film .…”

“…The observed spatial extent of the (presumably transient) pinning of growth along the crystalline plane ( x–y plane) is found to reach to a length scale (at least >1 nm) much beyond the spatial extent of the N complexes themselves. Although a quantitative comparison with experiments is, unfortunately, not possible due to a seeming lack of experimental crystal‐growth data in NGST, as well as simulation limitations (such as the limited model size), the retarded crystal growth inferred from experiments is well reproduced here.…”

“…Significantly, though, the inclusion of only 1.6 at% N completely prevented the formation of stable crystalline clusters throughout the whole annealing time of ≈3 ns at 700 K (Figure b). This annealing time for the NGST simulation was much longer than the observed maximum incubation time for the GST model at 700 K, which can therefore be considered long enough for a meaningful comparison to be made between GST and NGST models; a similar behavior was found for a model with 2.7 at% N doping at 700 K. Although it is still not clear whether the simulated values represent actual incubation times due to possible finite model‐size effects, it becomes evident from a comparison between similarly sized GST and NGST models that, at least at 700 K, a minor level of N doping can effectively disturb the occurrence of a nucleation event, resulting in a substantial increase in the incubation time for nucleation, as has been similarly observed in experiment …”

“…Putting these simulation and theoretical observations together, we can conclude that one of the central consequences of N doping would be the formation of constrained nucleation, and growth, sites at the expense of normal sites, the former being more sluggish in speed, yet demanding a higher activation energy, for transformation than the latter. The formation (or coexistence) of such sites is supported by various experimental observations, including the elongated (average) nucleation times, accompanying the broadening in their distributions, and the change in the lattice constant …”

“…It is noted that, contrary to most cases, an increase in crystallization time was reported when a nitride layer was inserted into a GST thin film . At the same time, the N doping induced a substantial broadening in the distribution of nucleation times, indicative of the N‐induced formation of nucleation sites with very different nucleation behavior on the microscopic scale. Computational approaches, in particular ab initio molecular‐dynamics (AIMD) simulations, have been recently employed to understand the effect of N doping at the atomic scale.…”