Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Plasmonic phase shifts and light-trapping in SOI photodetectors and nc-Si solar cells
Mumtaz Murat Arik * , Birol Ozturk, Hui Zhao, Eric Schiff
Department of Physics, Syracuse University, Syracuse, New York
Abstract- We report our work on the measurement of photoconductances in SOI devices with and without silver nanoparticle layers. The
silver nanoparticles were fabricated by thermal annealing of evaporated silver thin films and by nanosphere lithography. Since these devices
are not deposited onto textured substrates, they exhibit prominent interference fringes in their quantum efficiencies. An important effect that
we have found in both nc-Si:H solar cells and SOI is a shift of the interference fringes that is induced by the nanoparticle layer. We present
experiments and calculations indicating that the fringe-shift is a consequence of optical phase shifts by surface plasmon resonance of the metal
nanoparticles.
An interesting alternative to texturing in thin film solar cells
is "plasmonic" light-trapping based on specular cells and
using an overlayer of metallic nanoparticles to produce lighttrapping.
While this type of light-trapping has not yet been
demonstrated for nc-Si:H solar cells, significant photocurrent
enhancements have been reported on silicon-on-insulator
devices with similar optical properties to nc-Si:H [1,2].
Here, we report our work on plasmonic light-trapping on
silicon-on-insulator (SOI) photodetectors and nc-Si:H solar
cells. We observed that the photocurrent ratios in SOI
photodetectors are affected by interference fringes, which are
substantially shifted by the metal nanoparticle monolayers.
The measurements of the normalized photoconductance
spectra for SOI samples are shown in the upper panel and
inset of Fig.1. The gray curves show the corresponding
spectra when there was a Ag-np monolayer on top of the LiF.
panel of the figure expands on this spectral region. In the
lower panel we have plotted the ratio of the
photoconductances with and without the Ag-np film. The
value of 11 at 1025 nm is consistent with previous reports
suggesting that the Ag-np film leads to a pronounced
enhancement of photocarrier generation. It is important to
note that the fringes for the sample with the Ag-np layer are
"red shifted" from the fringes seen without the Ag np film;
we have indicated this shift as in the figure. This red-shift
modifies the interpretation of the photocurrent ratio. The
smooth lines through the photoconductance measurements
Photoconductance
G/(eF) (10 -3 cm 2 /V)
Photoconductance ratio
0.4
0.3
0.2
0.1
0.0
10
8
6
4
2
X 0.5
SOI
600 8001000
Unprocessed
Fringe-averaged
+Ag
0
700 800 900 1000
Wavelength (nm)
Figure 1. (upper) Normalized photoconductance spectra
G p eF for LiF-capped SOI structures with and without a Ag
nanoparticle film. The inset shows the spectra over a wider range.
Solid lines (without symbols) are averaged to remove interference
fringes. (lower) Ratios of photoconductances with and without the
Ag film; the solid line is the ratio of the fringe-averaged
photoconductances.
are "processed" to remove the fringes; the smooth line in the
lower panel indicates the ratio of these "fringe-removed"
curves. The enhancement now reaches a reduced value of
about 5 [3].
We speculate that the ratios of unprocessed photocurrent
spectra reported in previous SOI work [1,2], which also
exhibit the very strong oscillations seen in the lower panel of
Fig. 1 and even larger ratios, are due to similar effects.
In Figure 2, we also plot phase shifts for Ag-np films
deposited on the top ITO layer of nc-Si:H solar cells. The
films were prepared by thermal annealing and by nanosphere
lithography [4]. The associated phase shift is negative,
corresponding to a blue shift of the interference fringes.
Phase-shift (radians)
-1
-2
700 800 900 1000
Wavelength (nm)
Figure 2. Optical phase shifts by silver nanoparticle films as inferred
from interference fringe shifts in photocurrent spectra. Results are
shown for films on LiF-capped SOI and on nc-Si:H solar cells with a
top ITO layer. The silver nanoparticle films were created by
annealing (“ann”) and by nanosphere lithography (“nsl”) of
evaporated silver. Note the differing signs of the phase shift.
The difference in signs for the Ag-np films on Si and on
ITO is striking. This result is expected from the smaller
surface plasmon resonance frequency when a Ag-np is
proximate to silicon (index of refraction n ~ 3.5) than when
it is proximate to ITO (n ~ 1.9) [5].
This research has been partially supported by the U. S.
Department of Energy through the Solar America Initiative
(DE-FC36-07 GO 17053). Additional support was received
from the Empire State Development Corporation through the
Syracuse Center of Excellence in Environmental and Energy
Systems.
*mumtazmurat@yahoo.com
3
2
1
0
SOI (ann)
nc-Si (ann)
nc-Si (nsl)
[1] Stuart H.R. and Hall D.G., Appl. Phys. Lett 69, 2327 (1996)
[2] Pillai S. et.al., J. of Appl. Phys., 101 093105 (2007)
[3] Ozturk B., Zhao H., Schiff E.A., Guha s., Yan B., Yang J,
To be submitted for publication.
[4] Ozturk B., et.al., Mater. Res. Soc. Symp. Proc. Vol. 1153,
1153-A07-14 (2009).
[5] Ozturk B., Zhao H., Schiff E.A., Damkaci F., Guha s., Yan
B., Yang J, Submitted to Mater. Res. Soc. Symp. Proc. (2010)
6th Nanoscience and Nanotechnology Conference, zmir, 2010 758