[en] Fluctuation Transmission Electron Microscopy (FTEM) has a unique ability to probe topological order on the 1–3 nm length scale in diffraction amorphous materials. However, extracting a quantitative description of the order has been challenging. We report that the FTEM covariance, computed at two non-degenerate Bragg reflections, is able to distinguish different regimes of size vs. volume fraction of order. The covariance analysis is general and does not require a material-specific atomistic model. We use a Monte-Carlo approach to compute different regimes of covariance, based on the probability of exciting multiple Bragg reflections when a STEM nanobeam interacts with a volume containing ordered regions in an amorphous matrix. We perform experimental analysis on several sputtered amorphous thin films including a-Si, nitrogen-alloyed GeTe and Ge₂Sb₂Te₅. The samples contain a wide variety of ordered states. Comparison of experimental data with the covariance simulation reveals different regimes of nanoscale topological order. - Highlights: • A statistical analysis that reveals quantitative information of nanoscale order in amorphous material. • Extends upon standard nanobeam diffraction mode of Fluctuation Transmission Electron Microscopy. • General and non-model specific; does not require atomistic model to interpret the information. • The study combines theoretical development, TEM measurements, and Monte Carlo simulation results

[en] The nanoscale order in amorphous GeTe thin films is measured using fluctuation transmission electron microscopy (FTEM). The order increases upon annealing at 145 °C, which indicates a coarsening of subcritical nuclei. This correlates with a reduction in the nucleation delay time in laser crystallization. A shift in the FTEM peak positions may indicate a transformation in local bonding. In samples alloyed with 12 at. % nitrogen, the order does not change upon annealing, the peak does not shift, and the nucleation time is longer. The FTEM data indicate that nitrogen suppresses the structural evolution necessary for the nucleation process and increases the thermal stability of the material

[en] The nanoscale crystal nuclei in an amorphous Ge₂Sb₂Te₅ bit in a phase change memory device were evaluated by fluctuation transmission electron microscopy. The quench time in the device (∼10 ns) afforded more and larger nuclei in the melt-quenched state than in the as-deposited state. However, nuclei were even more numerous and larger in a test structure with a longer quench time (∼100 ns), verifying the prediction of nucleation theory that slower cooling produces more nuclei. It also demonstrates that the thermal design of devices will strongly influence the population of nuclei, and thus the speed and data retention characteristics

[en] Phase change memory devices are based on the rapid and reversible amorphous-to-crystalline transformations of phase change materials, such as Ge₂Sb₂Te₅ and AgInSbTe. Since the maximum switching speed of these devices is typically limited by crystallization speed, understanding the crystallization process is of crucial importance. While Ge₂Sb₂Te₅ and AgInSbTe show very different crystallization mechanisms from their melt-quenched states, the nanostructural origin of this difference has not been clearly demonstrated. Here, we show that an amorphous state includes different sizes and number of nanoscale nuclei, after thermal treatment such as melt-quenching or furnace annealing is performed. We employ fluctuation transmission electron microscopy to detect nanoscale nuclei embedded in amorphous materials, and use a pump-probe laser technique and atomic force microscopy to study the kinetics of nucleation and growth. We confirm that melt-quenched amorphous Ge₂Sb₂Te₅ includes considerably larger and more quenched-in nuclei than its as-deposited state, while melt-quenched AgInSbTe does not, and explain this contrast by the different ratio between quenching time and nucleation time in these materials. In addition to providing insights to the crystallization process in these technologically important devices, this study presents experimental illustrations of temperature-dependence of nucleation rate and growth speed, which was predicted by theory of phase transformation but rarely demonstrated