Biomedical Engineering Reference
In-Depth Information
Fig. 7.9.
Top
view and cross-sectional SEM images of mesoporous silicon with
an average pore diameter of approximately 20 nm, formed in a highly doped p-type
silicon substrate (0.01 ohm-cm), using an electrolyte with 15% HF in ethanol. (
c
,
d
):
SEM images of 60 nm macropores formed in a very highly doped n-type silicon
substrate (0.001 ohm-cm) using an electrolyte with 6% HF. (
e
,
f
): SEM images
of 120 nm macropores formed in highly doped n-type silicon substrate (0.01 ohm-
cm), using an electrolyte with 6% HF. (
g
,
h
): SEM images of 1.5
ยต
m macropores
etched from low doped p-type silicon (20 ohm-cm), using an HF/dimethylformamide
electrolyte
density for highly doped n-type (0.01 ohm-cm) silicon [8]. The pore morphol-
ogy is also affected by the choice of doping type and concentration, as illus-
trated in Fig. 7.9 [8]. The top view SEM images show PSi samples with differ-
ent pore diameters ranging from mesopores (pore size between 10 and 50 nm)
to macropores (pore size
>
50 nm). The bottom-row figures are cross-sectional
SEM images of the same samples. The mesopores formed in p+ silicon sub-
strates have very branchy pore walls (Fig. 7.9a, b). The macropores formed
in n-type wafers (Fig. 7.9c-h) have much smoother pore walls and larger pore
sizes. Note that most mesoporous silicon samples and certainly all microp-
orous silicon samples (pore sizes
<
10 nm) exhibit strong luminescence in the
visible to near infrared region [9]. This results from quantum confinement of
electrons and holes in nanometer-sized quantum structures, which increases
the bandgap [4, 10, 11] and enhances the radiative recombination rate [12].
7.2.2 Sensing Principle
PSi is a good host material for label-free optical biosensing applications be-
cause its optical properties (photoluminescence and reflectance) are highly
sensitive in the presence of chemical and biological species inside the pores [13].
PSi optical biosensors with a variety of configurations such as single layer
Fabry-Perot cavities [14], Bragg mirrors [15], rugate filters [16], and micro-
cavities [17-19] have been experimentally demonstrated for the detection of
toxins [20], DNA [17], bacteria [21], and proteins [8, 14, 22]. The capture of
