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Materials and Methods: Porous Silicon’ production calibration
Today, the electrochemical etching is a standard method to fabricate nanostructured
porous silicon: a proper choice of the applied current density, the electrolyte
composition, and the silicon doping allow precise control over the morphology and,
consequently, on the physical and chemical properties of the porous silicon structure.
Computer controlled electrochemical etching processes are exploited for the
realization of porous silicon films of controlled thickness and porosity (defined as the
percentage of void in the silicon volume).
Nanoporous, mesoporous and macroporous structures can be achieved, with pore
size ranging from few nanometers up to microns. Moreover, since the etching
process is self-stopping, it is possible to fabricate with a single run process multilayer
stacks made of single layers of different porosity. The dielectric properties of each
PSi layer, and in particular its refractive index n, can be namely modulated between
those of crystalline silicon (n = 3.54, porosity = 0) and air (n= 1, porosity = 100 %); so
that alternating high and low porosity layers, lot of photonic structures, such as
Fabry-Perot interferometers, omni-directional Bragg reflectors, optical filters based on
microcavities, and even complicated quasi-periodic sequences (Thue-Morse) can be
simply realized, as it is shown in Figure 5.
Reflectivity (a.u)
Reflectivity (a.u)
1,0
0,8
0,6
0,4
0,2
2,0
1,5
1,0
0,5
0,0
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Wavelength (nm)
monolayer
optical microcavity
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Wavelength (nm)
Reflectivity (a.u.)
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Figure 5. Experimental reflectivity spectra of different PSi optical structures.
In this study Fabry-Perot single layer, Bragg mirrors, Thue Morse and optical
microcavities have been used as sensors.
The porous silicon layer was obtained by electrochemical etching in a HF-based
solution at room temperature. The substrate used was a highly doped p + -silicon,
oriented, 0.01 Ω cm resistivity, 400 μm thick. Before anodization the substrate
was placed in HF solution to remove the native oxide. With this substrate we can
obtain mesoporous layers, with a pore average dimension of about 50 nm. In figure 6
SEM images are reported.
1,0
0,8
0,6
0,4
0,2
0,0
Reflectivity (a.u)
1.0
0.8
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0.4
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0.0
Bragg
Wavelength (nm)
Thue Morse S6
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Wavelength (nm)