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Research & Reviews: Journal of Material Sciences | ISSN: 2321-6212 | Volume 6
Theoretical, Materials and Condensed Matter Physics
5
th
International Conference on
November 26-28, 2018 | Los Angeles, USA
Chemical nanoanalyses of Si grain boundaries towards the fabrication of high- functional Si solar cells
P
olycrystalline materials with grain boundaries (GBs), involving excess free energy because of their structural imperfection,
can reduce their energy by the nanoscopic structural changes of the GBs via impurity segregation. Those local changes
at GBs can stabilize non-equilibrium nanostructures, resulting in the drastic change in the macroscopic properties of those
materials. The mechanism of GB segregation is, however, far from being understood due to difficulties in characterizing
both crystallographic and chemical properties of the same GB at atomistic levels. We have therefore developed an analytical
method to determine the impurity segregation ability on the same GB at the same nanoscopic location by a joint use of atom
probe tomography (APT) and scanning transmission electron microscopy (STEM) combined with
ab initio
calculations, and
discussed the segregation mechanism at atomistic levels. Three-dimensional distribution of impurity atoms was systematically
determined at the typical large-angle GBs, small-angle GBs, and dislocations on GBs in Si by APT with a high spatial resolution
(about 0.4nm), and it was correlated with the atomic stresses around the GBs estimated by
ab initio
calculations based on
atomic-resolution STEM data. It was shown that impurity atoms preferentially segregated at the atomic positions under specific
stresses so as to attain a more stable bonding network by reducing the local stresses. For example, the number of segregating
oxygen atoms per unit GB area
(N
GB
)
is proportional to both the number of the stressed positions per unit GB area
(n
bc
)
and the
average concentration of oxygen atoms around the GB ([O
i
]) with
N
GB
~ 50n
bc
[O
i
].
Biography
Yutaka Ohno (PhD: Physics) is working in the Institute for Materials Research, Tohoku University
(http://www-lab.imr.tohoku.ac.jp/~yutakaohno/). He is also working
in the CREST research project (Grant No. JPMJCR17J1 (2017-2023)) in Japan Science and Technology Agency. A focus is on quantitative analyses of the impurity
segregation ability of grain boundaries in Si and compounds by atom probe tomography (APT, with a spatial resolution less than 0.4nm) combined with scanning
transmission electron microscopy (STEM) and
ab initio
calculations, but also on the study of atomistic structures of semiconductor nanostructures by optical
measurements (cathodoluminescence, micro-photoluminescence, near-field optical measurements) under TEM.
yutakaohno@imr.tohoku.ac.jpYutaka Ohno
Tohoku University, Japan
Yutaka Ohno, Res. Rev. J Mat. Sci. 2018, Volume 6
DOI: 10.4172/2321-6212-C10-040