Silicon Based Nanostructures
Control of the effective bandgap of a silicon based
material is a very promising route towards third generation Si
photovoltaic devices such as silicon based tandem solar cells and
energy selective contacts for hot carrier solar cells. It has been
recognised that silicon nanostructures consisting of quantum well
(QW) or quantum dot (QD) superlattices can achieve such band gap
control.
Figure 1: Low resoltion TEM image of partly
organised silicon quantum dots in an oxide matrix
In the Centre of Excellence, Si QWs and Si/SiO2 QW
superlattices have been fabricated by the thermal oxidation of
silicon-on-insulator (SOI) wafers and by RF magnetron sputtering
respectively. Two-dimensional Si QWs (1D quantization) showed energy
confinement for well thicknesses of 1nm to 2.7nm from luminescence
and Raman data. However, it is difficult to achieve the ultra-thin
Si layer (<3nm) required for the Si/SiO2 QW superlattices
as good crystallographic quality material.
The additional quantization achievable using zero-dimensional
quantum dots (3D quantization) gives greater control over bandgap
with a less stringent size requirement for Si nanostructures. For
instance, the confinement energy in a 2 nm diameter QD will be
the same as in a 1 nm wide QW. We have already demonstrated 1.7
eV emission from a QW of a 1 nm width, suggesting such an effective
bandgap can be obtained in a silicon quantum dot superlattice in
which the QDs are about 2 nm in diameter. This is the ideal bandgap
for a 2-cell tandem PV cell with bulk silicon for the bottom cell.
A 1.4 nm diameter QD superlattice would, on the same basis, give
a 2.3eV bandgap, high enough for the top cell in a 5-cell tandem,
again on bulk silicon.
Figure 2: High resoltion TEM image of partly
organised silicon quantum dots in an oxide matrix (4 nm scale)
In the Centre, Si QD superlattices are fabricated
by alternate deposition of Silicon-rich-oxide (SRO) and SiO2 layers
by co-sputtering of Si and SiO2 (quartz) targets and
reactive plasma SiO2 deposition, respectively. SRO,
i.e., SiOx (x<2), is thermodynamically unstable precipitating
silicon on annealing at 1173°C. If the film thickness of the
SRO film is on the scale of a few nanometers and enclosed by an
insulator (SiO2, SiNx or SiC), the phase
separation in the SRO film creates self-organized nano-scale Si
QDs. If the packing density is high enough, overlap of the wavefunction
of the dots should allow a QD superlattice to form.
Surface energy minimisation would favour the formation
of near-spherical dots of as large a size as possible (i.e. the
thickness of the SRO layers). Their maximum size is therefore determined
by the thickness of the SRO layer as is the depth co-ordinate of
their position. This feature offers uniform size controllability
of Si QDs as shown in Fig. 1, with evidence that these are indeed
crystalline Si dots shown in Fig. 2. The spacing of quantum dots
in the SRO layer would be determined by the amount of silicon in
the layer, i.e. the stoichiometry of the sputtered SiOx.
This stoichiometry can be controlled by varying the Si area coverage
over the quartz target. The packing density of Si quantum dots
is thus controllable. |
