current density, anodization time, and illumination of the etching reaction, upon
the morphology of porous silicon have also been examined [9]. A range of pore
sizes, total porosities, and surface areas are readily accessible through different
etching conditions, demonstrating the ability to fabricate porous silicon matrices
with particular dimensions [23]. The anisotropic nature of the anodic etch tends to
result in the unidirectional formation of pore structures, although some lateral
etching does occur (Figure 16.2); this is attributed to the different kinetic reactivities of the silicon crystal faces, which exhibit unequal rates of dissolution under
these reaction conditions. Control over the formation of these pore channels, along
with regulation of the substrate thickness and the size distribution of the silicon
nanocrystallites, is imperative for the fabrication and optimization of devices and
physical applications based upon porous silicon.
The dimensions of the silicon nanocrystallites are also believed to play an integral role in the luminescent properties of porous silicon [6]. The origin of photoluminescence, still under investigation, is hypothesized to manifest from the
nanocrystalline domains where quantum confinement of charge carriers occurs
[4]. Recombination of these confined electron–hole pairs (excitons) releases energy
in the visible spectrum; the color of the emission depends upon the sizes of these
domains and shifts from red to blue as the diameters of the nanocrystallites
decrease. In contrast to flat single-crystal silicon, which emits light only weakly
(< 0.001%) in the infrared owing to its indirect bandgap [24], nanocrystalline porous silicon luminesceses with much greater efficiency, exhibiting emission quantum yields up to several percent and functioning more like a direct bandgap semiconductor (Figure 16.4) [25]. The large fraction of silicon surface atoms, however,
suggests that surface states may greatly affect the luminescence behavior of this
material. Oxidation or conversion of the native hydride surface to other groups
often results in changes to the photoluminescence intensity and wavelength, thus
supporting this hypothesis [26, 27]. Dependence upon interfacial parameters has
also been reported for the electroluminescence of porous silicon induced by formic
acid/sodium formate [28]. Just as for physical applications, effective and efficient
optoelectronic devices based upon porous silicon require strict control over the
substrate properties.
16.3
Surface Chemistry of Porous Silicon Surfaces
Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect
the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only
metastable under ambient conditions and oxidation of freshly prepared porous
silicon commences within minutes when exposed to air. While surface oxide can
suitably passivate the nanocrystalline silicon and stabilize its photoluminescence,
the electrically insulating and structurally defective character of this oxide layer
16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
522
the morphology of porous silicon have also been examined [9]. A range of pore
sizes, total porosities, and surface areas are readily accessible through different
etching conditions, demonstrating the ability to fabricate porous silicon matrices
with particular dimensions [23]. The anisotropic nature of the anodic etch tends to
result in the unidirectional formation of pore structures, although some lateral
etching does occur (Figure 16.2); this is attributed to the different kinetic reactivities of the silicon crystal faces, which exhibit unequal rates of dissolution under
these reaction conditions. Control over the formation of these pore channels, along
with regulation of the substrate thickness and the size distribution of the silicon
nanocrystallites, is imperative for the fabrication and optimization of devices and
physical applications based upon porous silicon.
The dimensions of the silicon nanocrystallites are also believed to play an integral role in the luminescent properties of porous silicon [6]. The origin of photoluminescence, still under investigation, is hypothesized to manifest from the
nanocrystalline domains where quantum confinement of charge carriers occurs
[4]. Recombination of these confined electron–hole pairs (excitons) releases energy
in the visible spectrum; the color of the emission depends upon the sizes of these
domains and shifts from red to blue as the diameters of the nanocrystallites
decrease. In contrast to flat single-crystal silicon, which emits light only weakly
(< 0.001%) in the infrared owing to its indirect bandgap [24], nanocrystalline porous silicon luminesceses with much greater efficiency, exhibiting emission quantum yields up to several percent and functioning more like a direct bandgap semiconductor (Figure 16.4) [25]. The large fraction of silicon surface atoms, however,
suggests that surface states may greatly affect the luminescence behavior of this
material. Oxidation or conversion of the native hydride surface to other groups
often results in changes to the photoluminescence intensity and wavelength, thus
supporting this hypothesis [26, 27]. Dependence upon interfacial parameters has
also been reported for the electroluminescence of porous silicon induced by formic
acid/sodium formate [28]. Just as for physical applications, effective and efficient
optoelectronic devices based upon porous silicon require strict control over the
substrate properties.
16.3
Surface Chemistry of Porous Silicon Surfaces
Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect
the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only
metastable under ambient conditions and oxidation of freshly prepared porous
silicon commences within minutes when exposed to air. While surface oxide can
suitably passivate the nanocrystalline silicon and stabilize its photoluminescence,
the electrically insulating and structurally defective character of this oxide layer
16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
522