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Title: Fluctuation microscopy analysis of amorphous silicon models

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USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
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Journal Article: Publisher's Accepted Manuscript
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Journal Volume: 176; Journal Issue: C; Related Information: CHORUS Timestamp: 2018-01-11 18:21:01; Journal ID: ISSN 0304-3991
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Gibson, J. M., and Treacy, M. M. J. Fluctuation microscopy analysis of amorphous silicon models. Netherlands: N. p., 2017. Web. doi:10.1016/j.ultramic.2017.01.013.
Gibson, J. M., & Treacy, M. M. J. Fluctuation microscopy analysis of amorphous silicon models. Netherlands. doi:10.1016/j.ultramic.2017.01.013.
Gibson, J. M., and Treacy, M. M. J. Mon . "Fluctuation microscopy analysis of amorphous silicon models". Netherlands. doi:10.1016/j.ultramic.2017.01.013.
title = {Fluctuation microscopy analysis of amorphous silicon models},
author = {Gibson, J. M. and Treacy, M. M. J.},
abstractNote = {},
doi = {10.1016/j.ultramic.2017.01.013},
journal = {Ultramicroscopy},
number = C,
volume = 176,
place = {Netherlands},
year = {Mon May 01 00:00:00 EDT 2017},
month = {Mon May 01 00:00:00 EDT 2017}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1016/j.ultramic.2017.01.013

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  • In this letter, we report fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films and the effect of annealing on this ordering. Annealed and unannealed diamond-like carbon films have almost identical short-range order. Our fluctuation microscopy results, however, indicate the presence of medium range order or clustering in the films on a lateral length scale that exceeds 1 nm. Within the clustered regions, the dominant local ordering appears to be diamond-like, and graphite-like ordering is not observed. Thermal annealing up to 600 {sup o}C leads to an increase in diamond-like clustering with no onset of graphite-like clustering. However,more » after high temperature annealing up to 1000 {sup o}C, graphite-like clustering becomes apparent as a result of the conversion of diamond-like carbon to graphite-like carbon. The results on the as-deposited films and films annealed up to 600 {sup o}C suggest that a spontaneous medium range ordering process occurs in diamond-like carbon films during and subsequent to film growth, and this may play an important role in stress relaxation.« less
  • Fluctuation electron microscopy (FEM) measurements and simulations have identified nanoscale aluminum-like medium-range order in rapidly quenched amorphous Al{sub 92}Sm{sub 8} which devitrifies by primary Al crystallization. Al{sub 92}Sm{sub 8} amorphized by plastic deformation shows neither Al nanoscale order, nor primary crystallization. Annealing the rapidly quenched material below the primary crystallization temperature reduces the degree of nanoscale Al order measured by FEM. The FEM measurements suggest that 10-20 A diameter regions with Al crystal-like order are associated with primary crystallization in amorphous Al{sub 92}Sm{sub 8}, which is consistent with the quenched-in cluster model of primary crystallization.
  • Silicon oxycarbide is a metastable material that has generated interest because of the great flexibility in properties that is attainable with a mixture of divalent and tetravalent anions within the network structure. In addition to the network bonding, however, these materials have also exhibited a strong propensity to include carbon-carbon bonding-so-called ‘free carbon’-within the structure regardless of synthesis method. While evidence for the presence of free carbon is overwhelming, traditional diffraction characterization methods have been unable to definitively identify ordering or segregation in the material. Fluctuation electron microscopy (FEM) is a relatively new transmission electron microscopy technique that is specificallymore » sensitive to medium-range order, which is ordered bonding on the length scale of roughly 8-50 A° . We utilize this method to identify semi-ordered bonding present in silicon oxycarbide thin films deposited by reactive rf sputtering over a wide composition range. These results indicate that the use of FEM can be extended to materials which are compositionally heterogeneous at the nano-scale. We show evidence of clusters approximately 1.8 nm in diameter that exhibit correlations similar to the bonding in turbostratic carbon.« less
  • The nanometer-scale (medium range) structural order in hydrogenated polymorphous silicon films is analyzed using fluctuation electron microscopy. The polymorphous growth regime occurs under relatively high gas pressure during plasma-enhanced chemical vapor deposition, such that small aggregates and nanocrystals form in the gas phase and impinge on the film surface. All polymorphous samples appear completely amorphous in diffraction or Raman scattering analyses. In fluctuation microscopy, carried out in the transmission electron microscope, the statistical variance V in the dark field image intensity is acquired as a function of the scattering vector k at a chosen resolution Q. Theory shows that Vmore » is quantitatively related to the three- and four-body atomic correlation functions, and thus to the nanometer scale order, in the material. Unlike typical hydrogenated amorphous silicon, the variance V is a strong function of growth conditions and displays a maximum at a silane pressure of 1.4-1.8 Torr. The images also reveal the presence of a small number of unusually bright spots, roughly 5 nm in diameter, only in samples grown at 0.8 and 1.4 Torr; we interpret that these correspond to nanocrystallites. The observation of enhanced structural order as revealed by the variance V is consistent with previous, but less conclusive, analyses of hydrogenated polymorphous silicon.« less
  • The authors have characterized with fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H{sub 2} dilution of the SiH{sub 4} precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition methods and H{sub 2} content. The PECVD film grown with high H{sub 2} dilution contains Si crystals {approximately} 5 nm inmore » diameter at a density of {approximately} 10{sup 9} cm{sup 2}. The amorphous matrix surrounding these crystals shows no difference in medium-range order from the standard PECVD film.« less