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The key-technique for 2D-photonic
crystals
(Thesis of S. Ottow in the Group of H. Föll, 1996) |
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The experimental setup for macropore growth is shown in the
left figure and the principle of macropore etching is shown in the right
figure, respectively. Photogenerated minority carriers (in the case of n-type
Si this means "holes") diffuse from the back side of the sample to
the pore (etch) pits and promote dissolution there, because of the enhanced
electrical field in the space charge layer (SCL). The photographs show some
examples of pore arrays obtained with this technique. We use two arrays, which
differ in diameter of the pores, the pore spacing and the geometry (othogonal
and hexagonal, the last with a closer package of pores). |
Step 2 |
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The next process step is to close the pores. This is necessary
for for the definition of the microstructures with a photolithographic process.
But before that a CVD Si3N4 passivation layer is deposited. We will see in
paragraph 4 why this is important. The pores are closed with aluminum, which is
evaporated on the top of the sample. This material was choosen because of its
good stability for instance against plasma etching (see paragraph). The
photograph shows that the pores are completely closed. Now it is possible to
perform a photolithography to define the microstructures. After the
photolithography the uncovered part of the aluminum mask is etched with
phosphoric acid based etch. |
Step 3 |
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In addition one can see in the next two photographs on the
right why the Si3N4 passivation layer is necessary. In the top picture the
etching is performed without any passivation and the typical isotropic etch
profile is obtained. By using the passivation the etch profile in nearly
anisotropic and, in addition, the overall mask undercut is reduced twice. |
 
Step 4 |
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2D-Photonic crystals |
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To fabricate photonic crystals suitable for near-IR and
visible spectral ranges leads to the limits of today's technology due to a
demanded regularity in the range of 10 nm. Electrochemical pore-formation of
n-type silicon in hydrofluoric acid can lead to regularly patterned uniform
pores with a diameter in the µm-range and a depth of several hundreds of
micrometers [V. Lehmann, J. Electrochem. Soc. 140 (1993) 2836]. Based on this
technique and the technique for mircostructuring from S. Ottow, an IR-photonic
crystal was fabricated recently [U. Grüning, et al. Appl. Phys. Lett. 68
(1996) 747]. Optical measurements shows an photonic band gap. |
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Point-defects can be studied with these structures. In the
left picture a cavity for ligth is formed. |
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see: F. Müller, A. Birnerm, U. Gösele, V. Lehmann,
S. Ottow, H. Föll, Journal of Porous Materials, Vol 7, 1/2/3, S. 201
(2000) |
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With a periodic variation of the diameter of the macropores a
quasi-3D-photonic crystal can be build. |
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