Silicon onion-layer periodic threedimensional nanostructures
Two crucial facts urge to explore new
strategies. On the one hand the undeniable superiority of organic artificial
opals in terms of quality and range of availability and, on the other, the
impossibility of these compounds to withstand the high temperatures required for
the CVD synthesis (around 350-400ºC for silicon). In general, thin artificial
opals are more easily produced from polymer beads [polymethyl-methacrilate (PMMA)
or polystyrene (PS)]
and their optical quality is far superior but they are thermolabile above ca.
This was initially dealt with by infiltrating the
air cavities with silica through a sol-gel process at room temperature to
produce resilient silica replica for further processing. This technique allows
growing silicon in the inner surface of the silica inverse opals (MISO
technique). This sol-gel infiltration, however, lends little or no control over
infiltration. A different strategy was recently demonstrated that allows not
only silicon infiltration in polymer 3D templates but a fine tuning of their
photonic crystal properties.
The use of silica CVD permits a fine control over the degree of infiltration in
polymer opals which can be used to generate a novel topology consisting of two
silicon concentric leaves. These 3D macroporous silicon structures may be of
importance due to their versatile photonic properties tailored through design.
Since the photonic ban gap functionality was the centre
issue, optical transmission and specular reflectance were taken after each
fabrication stage. However, since the optical response is extremely sensitive to
changes in morphology and refractive index of the structures it served also to
monitor the process with great accuracy.
Fabrication process from left to right: starting from a polymer opal (1) and
after silica CVD (2) and thermal calcination (3) silicon CVD can now be done in
the silica backbone (4) which can finally be etched leaving the interpenetrated
double silicon network (5).
Using a polymer opal as template (Fig. 1) we grow
a silica layer around the spheres by room temperature CVD
with the desired thickness and, therefore, to the desired filling fraction. Here
silica infiltration was done by room temperature CVD in a home made reactor as
explained elsewhere. When the desired silica shell thickness was accomplished,
samples were calcined in air at 450ºC for 3 hours after a slow ramp (1 ºC per
minute) that ensured gentle removal of the polymer. After this treatment, a 3D
porous structure of silica shells is obtained which can now be used for silicon
infiltration at higher temperatures. This structure is bonded to the substrate
and is comprised of thin shells of silica welded together and having an inner
surface (initially in contact with the original organic spheres) and an outer
surface that corresponded to the growth surface in the silica CVD process.
Silicon will grow on both of these surfaces. It is important to point out that
this first silica infiltration step is critical in the whole fabrication process
as many of the properties of the final structure will depend on it. In addition,
these structures can present more that 97% air and average refractive indices as
low as 1.03 with a controlled overall thickness and can be seen as 3D ordered
silica aerogels. Once the silica scaffold has been prepared from the initial
polymer structure, silicon CVD can be performed with all guaranties.
Silicon CVD was performed at 360 ºC using disilane
as a precursor and allowed a precision in the layer growth of a few nanometres.
The fabrication process is carefully monitored by optical characterization and
morphology checked by SEM. Here, due to silica partial infiltration, silicon can
grow both in the inner and the outer surfaces of silica layer. The internal
spherical air cavities are connected by circular windows of diameter d
(Fig. 1) which originate from the contact points between spheres. To reach the,
in principle inaccessible, inner surface all is needed is an entry point opened
in the silica crust. This may be provided by naturally occurring cracks or can
be prepared, for instance, by selective reactive ion etching
or other means. Disilane can thus reach the whole interior of the structure. The
connecting windows make the silicon growth possible on the inner surface to a
layer thickness of d/2, then the windows close preventing further growth.
Silicon was seen to grow equally outside and inside the silica shell producing
two layers of the same thickness. The final structure consists of onion-like
multilayered Si/SiO2/Si spherical strata enclosing air cavities.
Their thicknesses can be easily controlled through experimental parameters. A
new silica growth is possible at this point that would produce two new layers
both on the inner and the outer surfaces of the present composite. This process
can be repeated while connecting windows remain open: outer shells can
conformally grow up to 86% of initial opal pore volume (layer thickness around
16% of the sphere radius for a perfect fcc). This is only limited by the size of
the channels between initial spheres. The inner layers can grow up to a
thickness of d/2.
Up to this point a superstructure is produced
consisting of onion-like Si/SiO2/Si layered shells surrounding empty
pores arranged in fcc symmetry. These architectures hold promise in Si/SiO2
light emitting structures.
This property combined with the photonic crystal nature of the 3D superstructure
might be exploited in terms of spontaneous emission inhibition and, eventually,
Silica can be selectively etched with diluted HF,
obtaining a final all-silicon structure consisting of concentric air-separated
spherical shells ordered in 3D. The power of the method lies in the fact that
the geometrical magnitudes (sphere size, layers thicknesses, etc) of this kind
of structures can be easily and independently controlled by experimental
parameters, which will finally determine their photonic response. Even the final
inner silicon layer thickness can be controlled by adjusting the initial filling
fraction of the polymer opal modifying the connecting windows size d
(which can be done by sintering).
Figure 2 shows an SEM picture of an opal (sphere diameter, D
= 1100 nm) after etching the silica layer and a computer generated model
(inset). At this final stage it is possible to observe the internal
nanostructured shells enclosing the cavities once occupied by the polymer
spheres. A closer look at the structure reveals the multilayered spherical
shells. Outer and inner silicon layers separated by an air gap can be easily
seen. It is possible to estimate these layers thicknesses in ca. 30 nm.
Both in the SEM picture and the computer generated 3D model one can appreciate
the line where the (111) and (100) planes meet. Upon careful inspection one can
observe the accurate match between model and reality.
Fig. 2 SEM
picture where the double silicon network separated by an air gap can be
appreciated. The inset shows a computer simulation of the same structure.
Due to the fact that the first step of the
process, the silica infiltration, is done at room temperature, the present
strategy can be applied to virtually any kind of polymers. In particular, as
mentioned in the introduction, it could be applied to photosensitive resins,
extensively used in microelectronics as well as in DLW
and 3D holographic lithography. The two latter techniques have shown to be very
promising for the fabrication of 3D photonic crystals templates. In fact, very
recently, silicon infiltration has been successfully accomplished in 3D ordered
photonic crystal polymeric templates by similar techniques.