(a)

1. Amorphous States of Phase Change Materials: Effects of Cooling Rate and CompositionStephen G. Bishop, University of Illinois at Urbana-Champaign, DMR 0706267 • Phase change memory (PCM) materials thatcan be rapidly switched between amorphous and crystalline states are under development for next-generation non-volatile memories. We have demonstrated that Fluctuation Transmission Electron Microscopy (FTEM) can detect the nanoscale nuclei content in glassy films of chalcogenide PCM materials. Correlation of FTEM results with pulsed laser crystallization experiments allows us to determine the role of the nuclei in crystallization. A study of these phenomena as a function of sample composition, deposition conditions, capping materials, thermal annealing, and melt-quenching can probe the predictions of classical nucleation theory and guide the search for optimal PCM materials. • Results: Phase transformation of PCM materials from the crystalline to amorphous state requires rapid melt-quenching, and our recent FTEM studies detected higher subcritical nuclei content in melt-quenched amorphous Ge2Sb2Te5 (GST) samples than in as-deposited samples (figure (a)). For the case of nitrogen-alloyed GST, FTEM provides proof that the subcritical nuclei content is severely reduced as the nitrogen content is increased in as-deposited samples (figure (b)). • Significance: This study demonstrates that the amount of subcritical nuclei embedded in the melt-quenched amorphous state depends on the cooling rate. Nucleation rate in GST is fast so that subcritical nuclei may form and grow during melt quenching. In addition, the reduction of subcritical nuclei content detected by FTEM in as-deposited films of nitrogen-alloyed GST is consistent with the slower crystallization rates observed for these materials. These results suggest that FTEM might be a valuable tool in the search for better performance in PCM materials. • Future: Studies of heterogeneous vs. homogeneous nucleation in capped phase change samples are in progress. Comparison of subcritical nuclei content in various chalcogenide glass compositions is planned. (a) (b) (a) FTEM variance in melt-quenched GST film shows larger subcritical nuclei content than as-deposited GST. (b) Nitrogen alloying of GST reduces the subcritical nuclei content in as-deposited samples.

2. Amorphous States of Phase Change Materials: Effects of Cooling Rate and CompositionStephen G. Bishop, University of Illinois at Urbana-Champaign, DMR 0706267 Broader Impacts • Results published in “Fluctuation Transmission Electron Microscopy: Detecting Nanoscale Order in Disordered Structures,” Bong-Sub Lee, Stephen G. Bishop, and John R. Abelson, ChemPhysChem 2010, 11, 2311-2317 • Results presented in Materials Research Society Spring Meeting, San Francisco, USA, 2010 • Our paper “Observation of the Role of Subcritical Nuclei in Crystallization of a Glassy Solid,” Bong-Sub Lee, Geoffrey W. Burr, Robert M. Shelby, Simone Raoux, Charles T. Rettner, Stephanie N. Bogle, Kristof Darmawikarta, Stephen G. Bishop, and John R. Abelson, Science326, 980 (2009), has won the IBM Materials Research Community’s Sixth Annual Materials Research Best Paper Award. The competition was for materials science and engineering papers published in 2009 authored or co-authored by IBM researchers (our collaborators). • Established Fluctuation Transmission Electron Microscopy in the new state of the art Aberration Corrected TEM, JEOL 2200FS. • Close collaboration with IBM Almaden Research Center & IBM T. J. Watson Research Center (simulation, pulsed laser experiments, device sample preparation); with John Abelson (fluctuation electron microscopy to analyze nanoscale structural properties, supported by grant DMR 06-05890). • Graduate students trained on transmission electron microscopy (especially fluctuation electron microscopy), nanocalorimetry, scanning electron microscopy, pump-probe laser technique, x-ray diffraction, x-ray reflection, atomic force microscopy, profilometry, thin film deposition by rf/dc magnetron sputtering, and evaporation.