(a)

1. Quantifying Nanoscale Order in Phase Change MaterialUsing Scattering CovarianceStephen G. Bishop, University of Illinois at Urbana-Champaign, DMR 1005929 (a) Chalcogenide materials for phase change data storage exhibit nanoscale order that is associated with subcritical nuclei. The ability to map out the size and volume density of ordered regions provides a fundamental understanding of differences observed in the transformation kinetics. To do this, we developed a new data analysis method within the technique of Fluctuation Transmission Electron Microscopy (FTEM). By computing the scattering covariance at two non-related Bragg conditions, we can probe the details of the order. We analyzed amorphous Ge2Sb2Te5 (GST) and took advantage off electron-induced heating to create different states of subcritical nucleation. As the material approaches the crystalline transition, a significant covariance signal emerges, i.e., the off-diagonal spots in (a). A mathematical interpretation was developed that takes account of the reciprocal lattice broadening for small crystal sizes and the probability for multiple crystals to exist within the volume probed by the electron beam. Monte-Carlo simulations revealed that the effects of size and of volume density interact to produce a covariance map of nanoscale order (b). The regime of interest for nucleation problems, upper-right corner, is rich in information. Neither the covariance data reduction nor the simulation require an atomistic model, hence, they are directly applicable to other materials. (b) (a) Covariance map of GST film under 60 minutes exposure of electron beam in TEM. (b) Simulated amorphous silicon covariance with contour line indicating average number of crystals hit by the electron beam.

2. FTEM Reveals Nanoscale Structural Differencesin Various Forms of Amorphous Silicon Stephen G. Bishop, University of Illinois at Urbana-Champaign, DMR 1005929 Our collaborators from Australia National University (ANU) have determined that amorphous silicon samples of different origins (ion-implanted vs. sputter deposited) have different transformation behavior under high pressure indentation. Two ANU Ph.D. students and a tenure-track professor visited our facility for two weeks to perform FTEM on their samples. This unique analysis revealed subtle structural differences between amorphous states that correlated with the property differences. The silicon samples were made electron transparent through wet etching, and had unavoidable thickness variations. Using the aberration-corrected JEOL 2200FS TEM, we devised methods to compensate for the thickness problem. This methodology can be applied other materials as well. Collaborators (left and middle) from Australia National University visiting UIUC to perform FTEM of ion implanted and sputtered a-Si on our JEOL 2200FS TEM.