Macroporous p-Type Silicon Fabry-Perot Layers
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 12109
silicon thin films with large surface areas were prepared to give,
depending on the fabrication conditions employed, a wide range
of pore sizes. The pore size and the structure of the porous
silicon layer were characterized using scanning force microscopy
(SFM) and scanning electron microscopy (SEM). Reflectance
interference spectroscopy and effective medium approximations
were used to determine the porosity and the thickness of the
porous layer. Appropriately functionalized porous silicon
matrixes displayed the desired stability in aqueous solutions and
were used to study specific protein-ligand binding interactions.
the hillocks are due to collapsed nanopores formed in the early
stages of the etching process caused by disturbing the equilib-
rium lattice forces in the crystal, introducing a lateral strain in
the surface layer. As the current densities are increased larger
pore sizes can be obtained (Figure 1). The root mean square
of the surface roughness is in the range 0.3-0.5 nm for porous
silicon layers etched at current densities <200 mA/cm2, which
is in good agreement with a recent study by Yau et al.15 The
surface roughness was only determined for samples etched at
current densities less than 200 mA/cm2 because the macropores
generated at higher current densities prevent an accurate
measurement of this parameter with SFM.16
Results and Discussion
The pore radius is approximately exponentially dependent
on the current density (Figure 2). The three smallest pore radii
obtained at low current densities in the plot presented in Figure
2 were obtained from the BJH (Barett-Joyner-Halenda)
calculation17 while the rest of the values were derived from SFM
measurements. The pore radii vary from 4-12 nm for samples
etched at 220 mA/cm2 up to 300-800 nm for samples etched
at 600 mA/cm2. The surface porosity of silicon layers etched
at current densities larger than 300 mA/cm2 was calculated from
the SFM images by integrating the number of pixels. The
surface porosity increases slightly from 27% with etching at
330 mA/cm2 to 30% with etching at 410 mA/cm2 and finally
up to 40% by applying current densities >440 mA/cm2. The
artifact arising from the tip convoluting with the shape of the
sample structure prevents accurate measurements of the surface
porosity at higher current densities.
Fabrication and Characterization of p-Type Porous Sili-
con Interference Layers. The pore shape, pore size, and
orientation of porous silicon layers depend on the surface
orientation and the dopant level of the crystalline silicon
substrate, the current density, the temperature, and the composi-
tion of the HF etching solution. Recently He´rino10 reported
the fabrication of macropores with diameters in the range 25-
100 nm by anodizing heavily doped porous p-type silicon with
a resistivity of 10-3 Ω cm in 25% ethanolic HF solution. Rieger
and Kohl11 have also shown that etching p-type silicon in dry
solvents such as acetonitrile or DMF in a moisture-free
environment leads to the formation of macroporous p-type
silicon layers. We have extensively studied various parameters
in the fabrication of porous silicon (vide infra). p-Type silicon
with resistivities of 0.1-10 Ω cm etched in aqueous or ethanolic
HF solutions generally displays a network of micropores
(diameter d < 2 nm), rather than the desired well-defined
cylindrical meso- (d ) 2-50 nm) or macropores (d > 50 nm).
However, the pore size of p-type porous silicon can be increased
by increasing the concentration of the dopant and decreasing
the aqueous HF concentration, but low current densities result
in a random orientation of highly interconnected filament-like
micropores. Large and cylindrically shaped pores can be
obtained when higher current densities are applied near the
electropolishing region. Our studies detailed below indicate that
porous silicon layers can be fabricated to give cylindrically
shaped structures with pore diameters in the tunable range of 5
to 1200 nm by anodizing heavily doped (10-3 Ω cm) p-type
silicon (100) in ethanolic HF solution at ambient temperatures.
Scanning Force Microscopy. Scanning force microscopy
was used to determine topographical parameters of the porous
surface, such as surface porosity, pore size, and roughness.12
TappingMode was used in this study since the surfaces of the
porous layers were too fragile to be imaged in contact mode
SFM.13 The SFM images of samples obtained by etching silicon
at different current densities are shown in Figure 1. Using
relatively low current densities (150 mA/cm2), pores are scarcely
visible and the nearly flat surface is dominated by a distinct
hillock structure (Figure 1A). According to Campbell et al.,14
It is difficult to obtain reliable information about the pore
size from samples with pores smaller than 5 nm because in this
size regime the apparent pore size strongly depends on the shape
of the SFM probe and the threshold height chosen. Imaging
the large macropores (d > 800 nm) can result in unstable scans
because the tip loses contact with the surface over a relatively
long distance. In general, the pore sizes obtained from SFM
images are slightly smaller than they are in reality. In particular,
the walls of the large macropores have a rounded appearance
due to the predominant interaction of the side of the tip with
the pore walls.
Scanning Electron Microscopy. Scanning electron micro-
graphs also corroborate the results obtained by TappingMode-
SFM (see Supporting Information). In general, the images show
similar meso/macroporous structures and pore sizes compared
to the SFM images. Small surface features such as the hillock
structures observed in Figure 1A could not be resolved by
scanning electron microscopy (SEM). The SEM images of the
samples etched at high current densities (>500 mA/cm2) exhibit
larger pore dimensions than the SFM images due to the
geometric convolution between the tip and the walls of the large
pores.
Generally the material investigated in this study follows the
trend proposed by Smith and Collins2 and Beale et al.18
Anodization of the more highly doped p-type silicon leads to
larger and more cylindrically shaped pores compared to the
filament-like interconnected pores obtained using lightly doped
p-type silicon. Furthermore, as suggested by the study of
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Canham: London, 1997; pp 89-96.
(11) Rieger, M. M.; Kohl, P. A. J. Electrochem. Soc. 1995, 142, 1490.
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He, Z. J.; Li, A. Z.; Tang, T. A. J. Appl. Phys. 1997, 81, 492. (d) Parkhuitik,
V. P.; Albella, J. M.; Martinez-Duart, J. M.; Go´mez-Rodr´ıguez, J. M.; Baro´,
A. M.; Shershulsky, V. I. Appl. Phys. Lett. 1993, 62, 366. (e) Amisola, G.
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(16) Since the shape of the tip determines the penetration depth in the
pores, the parameter would be tip dependent.
(17) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area and porosity,
2nd ed.; Academic Press: New York, 1982.
(13) Aktsipetrov, O. A.; Melnikov, A. V.; Moiseev, Y. N.; Murzina, T.
V.; van Hasselt, C. W.; Rasing, T.; Rikken, G. Appl. Phys. Lett. 1995, 67,
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Zajchowski, L. D.; Thomas, D. F. J. Vac. Sci. Technol., B 1995, 13, 1184.
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