Films minces à base de Si nanostructuré pour des cellules ...

Experimental set-up and working
tel-00916300, version 1 - 10 Dec 2013
An electron beam, is produced by thermionic emission from a LaB 6 crystal or from a
tungsten lament on application of current, and a potential dierence generated extracts
the electrons. Electrons can also be produced using a eld emission gun with
a tungsten tip when subjected to intense electric eld. This electron beam propagates
in vacuum to illuminate the sample to be analyzed. In our laboratory JEOL
2010F set-up, the latter method is used. Instead of glass lenses focusing the light
in the light microscope, the TEM uses electromagnetic lenses to focus the electrons
into a very thin beam. The electron beam then interacts while travelling through
the sample. Depending on the density of the material present, some of the electrons
are scattered and disappear from the beam. At the bottom of the microscope the
unscattered electrons hit a uorescent screen, which gives rise to a 'shadow image'
of the sample with its dierent parts displayed in varied darkness according to their
density. The image is then captured with a CCD camera for analysis. It is possible
to observe either the image area exposed under the electron beam (image mode)
or its associated diraction pattern (diraction mode). The diraction pattern also
gives information on the orientation of the substrate during the observation. In
Image mode, two types of contrast can be viewed. A diaphragm placed in the focal
plane of the objective lens is used to select the electrons diracted in a particular
direction and this image is called dark eld. The image from transmitted beam, is
known as the bright eld image.
Informations extracted in this thesis
ˆ The crystallization and size of the Si-nps can be visualised.
ˆ The total thickness of the sample, and individual sublayer thickness in a multilayer
can be obtained.
Energy Filtered Transmission Electron Microscope
Principle
Imaging Si nanoparticles in a SiO 2 (or Si 3 N 4 ) matrix by "traditional" TEM is not
straightforward. Si nanocrystals exhibit only weak amplitude and phase contrast
because of the small dierences in atomic number and density between Si and SiO 2 .
For this reason, it is impossible to image individual Si-nps by conventional TEM (defocused
bright eld or Fresnel contrast) as it is usually done for example for Ge-nps
in SiO 2 . In the case of crystalline particles HR-TEM or dark eld (DF) imaging can
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