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Materials that are transparent to
visible or - more important - infra red
light (IR) may be investigated
in transmission. This usually requires that the sample is optically polished on
both sides. Especially semiconductors are transparent in IR light and
IR microscopy is often used to investigate defects; particularly in
III-V compounds. Defects may be rendered visible by: |
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Polarization
microscopy. Elastic strain fields may rotate the polarization angle of
polarized light to some (small) degree. The strain fields around defects can
thus be made visible; an example is shown in the link. |
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Absorption contrast. Precipitates, for example, consist
of some other material with different optical properties - it may not be
transparent to IR light. In this case they would be directly visible as
dark spots. |
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If the primary defects are not
precipitates but e.g. small dislocation loops resulting from vacancy
agglomeration, they may be turned into a precipitate by a technique called
defect decoration. This is usually done
as follows: |
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Diffuse a fast moving element into the sample (e.g.
Li or Cu
for Si) at relatively high temperatures (however, without changing
the primary defect configuration). |
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Cool down sufficiently fast to nucleate the precipitation of
the decorating element only at defects, but
not so fast that not enough diffusion jumps are possible and you do not get any
precipitation. If you cool too slowly, homogeneous nucleation may produce
precipitates everywhere and the technique is useless. |
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The primary defects are now heavily decorated with impurity
precipitates and visible in IR microscopy (or other techniques).
However, the dimensions have been enlarged, the primary defect structure may
have changed, and you must keep in mind that you are now looking at a different
defect from what you wanted to study in the first place! |
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Nevertheless, IR-microscopy
with or without decoration, has made important contributions to the study of
defects in crystals. Its weaknesses and strengths can be summarized as
follows. |
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| Strength |
Weaknesses |
- Relatively cheap
- Partially quantitative (strain fields)
- Large and small areas can be investigated at medium resolution (ca. 1
µm).
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- Well polished surfaces on both sides required
- Involved specimen preparation if decoration is used
- Often not very specific as to the nature of defects
- Only applicable to "medium" defect densities
- Not overly sensitive
- Interpretation uncertain if decoration techniques are used.
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© H. Föll (Defects - Script)
