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Title: Medium-range order in silicon oxycarbide glass by fluctuation electron microscopy

Abstract

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 specifically 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.

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1035009
Report Number(s):
PNNL-SA-85357
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physics. Condensed Matter, 19(45):Article No. 455205
Country of Publication:
United States
Language:
English

Citation Formats

Ryan, Joseph V., and Pantano, C. G. Medium-range order in silicon oxycarbide glass by fluctuation electron microscopy. United States: N. p., 2007. Web. doi:10.1088/0953-8984/19/45/455205.
Ryan, Joseph V., & Pantano, C. G. Medium-range order in silicon oxycarbide glass by fluctuation electron microscopy. United States. doi:10.1088/0953-8984/19/45/455205.
Ryan, Joseph V., and Pantano, C. G. Mon . "Medium-range order in silicon oxycarbide glass by fluctuation electron microscopy". United States. doi:10.1088/0953-8984/19/45/455205.
@article{osti_1035009,
title = {Medium-range order in silicon oxycarbide glass by fluctuation electron microscopy},
author = {Ryan, Joseph V. and Pantano, C. G.},
abstractNote = {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 specifically 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.},
doi = {10.1088/0953-8984/19/45/455205},
journal = {Journal of Physics. Condensed Matter, 19(45):Article No. 455205},
number = ,
volume = ,
place = {United States},
year = {Mon Mar 19 00:00:00 EDT 2007},
month = {Mon Mar 19 00:00:00 EDT 2007}
}
  • Despite occasional experimental hints, medium-range structural order in covalently bonded amorphous semiconductors had largely escaped detection until the advent of fluctuation electron microscopy (FEM) in 1996. Using FEM, we find that every sample of amorphous silicon and germanium we have investigated, regardless of deposition method or hydrogen content, is rich in medium-range order. The paracrystalline structural model, which consists of small, topologically ordered grains in an amorphous matrix, is consistent with the FEM data; but due to strain effects, materials with a paracrystalline structure appear to be amorphous in diffraction measurements. We present measurements on hydrogenated amorphous silicon deposited bymore » different methods, some of which are reported to have greater stability against the Staebler-Wronski effect. FEM reveals that the matrix material of these samples is relatively similar, but the order changes in different ways upon both light soaking and thermal annealing. Some materials are inhomogeneous, with either nanocrystalline inclusions or large area-to-area variation in the medium-range order. We cite recent calculations that electronic states in the conduction band tail are preferentially located around the boundaries of the nm-scale paracrystalline regions that we have identified. This is new evidence in support of spatially inhomogeneous conduction mechanisms in a-Si. The key discovery in our work is that all samples of amorphous silicon must be described as having nm-scale topological crystalline order. This strongly modifies the long-standing model of a covalent random network. Our new understanding of medium-range order must be considered in all future models of electronic properties and the Staebler-Wronski effect.« less
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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
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