Silicon/Germanium multilayers shells growth

Silicon/Germanium multilayers shells growth

Both amorphous semiconductors were synthesized by Chemical
Vapour Deposition (CVD) performing slight modifications over already published
methods. In the case of Si the precursor gas was disilane (Si2H6)
and the selected decomposition temperature was 375ºC. For Ge, the precursor was
germane (GeH4) and decomposition temperature 270ºC. Lower
temperatures resulted in very slow synthesis rates while higher ones allowed
little degree of control. It was observed that high temperatures induce the
growth of undesired particles.

To calculate the percentage of semiconductor present in the
pores, reflectance spectra of the samples were performed. The spectra are then
compared to the band structures. Theoretical calculations were done assuming
that semiconductor growth is layered, meaning that silica spheres are surrounded
by semiconductor shells. The refractive index values used for SiO2,
Si and Ge were: 1.43, 3.80 and 4.10 respectively.

Reflectance
spectra of the samples loaded with different amounts of Ge for opals of 660 nm
diameter spheres.

An
example of control of degree of infiltration of amorphous Ge is shown in
the preceding figure. In these spectra we can observe the
diffraction peak caused by the first pseudogap, the Fabry-Perot fringes due to
the finite thickness of the sample and other reflection peaks associated with
various other photonic bands. Apart from the percentage of loaded semiconductor
some other information can be deduced from the spectra. From the Fabry-Perot
fringes thickness of the sample can be extracted. Our thin opals were typically
made of 14-16 layers; this is a final thickness of 13-15 microns. The right
panel in Fig. 2 summarizes positions and peak widths of the first pseudogap
extracted from reflectance spectra as in the left panels. For low semiconductor
filling fraction Full Width at Half Maximum (FWHM) decreases with increasing
infiltration and then increases again for further loading. The pseudogap almost
closes for a 15 % of Ge in the pores and 20 % in the case of Si (not shown).
This is the result of matching the effective refractive index of the pore
(air-semiconductor) and the silica spheres.

The key parameter to control the amount of semiconductor
loaded is reaction time. Next Figure summarizes
the percentage of pore loaded with Si or Ge as a function of synthesis time. In
our experimental conditions, for a single reaction, Si reaches a maximum at
approximately the 53% of the pore infilling (24 nm of layer thickness for opals
made of 660 nm silica spheres). In the case of Ge, the pore can be loaded
without saturation. Pressure build up by hydrogen production is a limiting
factor. The only limitation for complete infillings
seems to be a geometrical one related to the closure of the pores in the {111}
close packed planes.

Infilling as a function of
single reaction times. Si (full circles) is formed at 375ºC and Ge (open
circles) at 270ºC. Silica spheres of 660 nm of diameter are used. The inset
shows the layer thickness dependence of filling fraction for an fcc. Layer
thickness is normalized to lattice parameter.

With this degree of control
over the Si and Ge growth we can grow multilayer structures. The method allows
not only growing both materials on silica but either on the other. A further
degree of freedom will be provided by the selectivity of different solvents that
can be
used to remove some of the materials. Aqua regia can selectively
remove Ge damaging neither the silica spheres nor the Si layer. Ge can be
oxidized at 500ºC without changing the properties of the rest of materials.
To test this ability, a sample
with a 30% of the pore loaded with Ge was re-grown with Si up to the 80%. Then,
Ge was etched with aqua regia. Next Figure
shows an electron microscope picture of the resulting
structure after all the process. The Si network is interconnected and fixed to
the substrate and remains separate from the spheres by the air layer.

Cleaved edge SEM image of a
doubly connected structure. The sample shows the continuous Si layer separated
from the continuous silica structure by air shell. The dark lines in the cleaved
edge are the air gaps between Si shell and the silica spheres. Scale bar is 1
micron.

A more complicated structure was fabricated and optically
characterized. First a sample was loaded with Si up to 20% of the pore, then a
layer of Ge to complete 45% and finally Si to a 60%. At this point the composite
presents the following composition: SiO2(74%)-Si(5.2%)-Ge(6.5%)-Si(3.9%);
percentages representing the total volume fraction of each material and a 10.4%
air remaining. This sample was then dipped in aqua regia for 60 minutes.
The outcome is two homogeneous Si shells separated by air gap and grown on
silica spherical cores.

Reflectance spectra of a
sample after successive semiconductors infilling and etching. a) Bare opal; b) a
Si layer is grown; c) a second layer of Ge and d) a third layer of Si; e)
partial etching of the Ge; f) etching is completed and regions with a pseudogap
at 2.22 μm disappear.

Another
tool is available as hydrofluoric acid (HF) can selectively remove both oxides.
A particular case where these techniques may be applied to tailor the photonic
bands is illustrated by the following four-step process. First, an opal is
loaded with Ge to a 25% of the available volume, followed by its oxidation. A
further Ge load completes 86% of the initially available volume. Finally the
oxides (GeO2 and SiO2) are removed with HF. The
corresponding photonic band structure is shown in the next Figure.
Two PBG open now: a larger one (12.6%) between the 8th and 9th
bands and a narrower one (1.3%) between the 5thth
bands. The later is an interesting case that only very recently has been
reported for fcc-based structures.
and the 6

Photonic band diagram of
interpenetrating air spheres coated with amorphous Ge (n=4.1) in a fcc lattice.
Being 0.3645a the radius of the air spheres and 0.4087a the external radius of
the semiconductor shell. Two gaps are developed: one between the 5th and 6th and
another between the 8th and the 9th. The inset shows the corresponding real
space structure