APPLIED PHYSICS LETTERS 89, 101907 共2006兲

Surface plasmon polariton mediated photoluminescence from excitonsin silicon nanocrystals
Eiji Takeda, Toshihiro Nakamura, Minoru Fujii,a兲 Satoru Miura, and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,Nada, Kobe 657-8501, Japan

共Received 13 April 2006; accepted 11 July 2006; published online 6 September 2006兲
Surface plasmon polaritons 共SPPs兲 of a metal film can efficiently be excited when a light emitter isplaced nearby. The excited SPPs are converted to photons by compensating for the momentum
mismatch. The authors study SPP-mediated emission from excitons in Si nanocrystals 共Si-nc’s兲 byplacing an organic grating on a thin Au film placed near Si-nc’s. The dispersion relation is obtained
from angle-resolved photoluminescence measurements, and all the observed modes are well
explained by model calculation. The results indicate that excitons in Si-nc’s can efficiently excite
SPPs in thin metal films and directed photoluminescence can be realized. © 2006 American Instituteof Physics. 关DOI: 10.1063/1.2345261兴
Silicon nanocrystals 共Si-nc’s兲 exhibit strong visible andnear infrared luminescence at room temperature due to therecombination of quantum confined excitons.1–4 The zerodimensional quantum confinement effect increases the overlap of electron and hole wave functions in the momentum
space resulting in significant enhancement of the radiativerecombination rate of excitons. However, even for very small
Si-nc’s as small as a few nanometers in diameter, the excitonradiative lifetime at room temperature is of the order of
10 ␮s, which is still longer than that of direct band gap semiconductors. Furthermore, the luminescence quantum efficiency is usually limited to several percent.5–7 Because of
these limitations, Si-nc based light emitting devices are still
far from practical applications.
The luminescence intensity of an emitter is known to bestrongly enhanced by the proper coupling of the emitter fieldwith surface plasmon polaritons 共SPPs兲 supported by thinmetal films.8–10 SPP is a coupled oscillation of free electronsand electromagnetic field induced by the fluctuation of theelectrons. The electromagnetic field decays exponentiallyaway from the surface. When a light emitter is placed very
close to a metal surface, SPPs can be excited via efficientnonradiative energy transfer from the emitter. If excited SPPs
can be converted to photons with high efficiency, the effective emission rate of the emitter is expected to be enhanced.11
One of the standard methods to allow SPPs to couple withphotons is to introduce a corrugated surface, which allows
the compensation of the momentum mismatch between SPPs
and photons.12 The coupling of SPPs with photons by a corrugated surface results in the well-defined directional emission with a characteristic polarization as well as the enhancement of emission rate.
The purpose of this work is to demonstrate the enhancement of photoluminescence 共PL兲 from Si-nc’s by the coupling with SPPs. In this work, we employ a one-dimensional
grating to couple excited SPPs to photons. The advantage ofusing a grating is that the modes can be identified unambiguously by analyzing the dispersion relation. We use a simple
method to realize Bragg scattering of SPPs, i.e., forming acorrugated organic film on a flat Au film deposited on ana兲Electronic mail: fujii@eedept.kobe-u.ac.jp

active layer containing Si-nc’s. We will demonstrate that thecorrugated organic layer can scatter SPPs to light efficiently
and directed PL is obtained.
We employed a cosputtering method to prepare
Si-nc’s.13–15 Si and SiO2 were simultaneously sputter deposited on a fused-quartz substrate and the deposited film
共200 nm in thickness兲 was annealed in a nitrogen gas atmosphere for 30 min at 1200 ° C. By the annealing Si-nc’s weregrown in thin SiO2 films 共Si-nc: SiO2兲. A Au film was thendeposited by vacuum evaporation. The thicknesses were 50and 100 nm for samples A and B, respectively. The lighttransmittances of Au films with 50 and 100 nm thicknesses
are about 3% and 0.3% at 850 nm, respectively. The corrugated surface was obtained by exposing a photoresist
共TDMR-AR80 HP, Tokyo Ohka Kogyo兲 film spin coated ona Au film to an interference pattern produced with a 325 nm
line of a He–Cd laser. An atomic force microscope 共AFM兲
image of the sample surface and the schematic illustration of
the sample structure are shown in the insets of Figs.1共b兲 and
1共c兲, respectively.
In order to measure the angular dependence of PL spectra from the corrugated surface, samples were mounted on arotating stage and illuminated through the transparent fusedquartz substrate. The excitation source was a 488 nm line of
an Ar ion laser. The excitation light was incident normal to
the surface, while the collection angle ␪ was changed from0° 共normal to the surface兲 to 50°. PL was collected throughan aperture 共about 1 mm in diameter兲 from the corrugatedside, limiting the angular acceptance to about 1°. PL spectra
were measured by using a single grating monochromatorwith a liquid nitrogen cooled Si charge coupled device. The
spectral response of the detection system was calibrated withthe aid of a reference spectrum of a standard tungsten lamp.
Figure 1共a兲 shows the PL spectra of samples A and B
collected at ␪ = 20°. The spectra of the samples without organic gratings measured with the same setup are also shown.
By the presence of the grating, the shape of PL spectra isstrongly modified and several peaks are observed.
Polarization-resolved measurements revealed that the strong
peaks at 900 and 960 nm in sample A and those at 740, 830,
and 970 nm in sample B are transverse magnetic 共TM兲 polarized. At the wavelengths of the highest PL intensity, the0003-6951/2006/89共10兲/101907/3/$23.00
89, 101907-1© 2006 American Institute of Physics
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Appl. Phys. Lett. 89, 101907 共2006兲

FIG. 2. 共Color online兲 Dispersion relations for sample A 共a兲 and sample B共b兲. Circles correspond to experimentally obtained data, while color map
and curves to those obtained by calculation. The region deeper in color
represents larger dissipated power. The modes of which the power dissipation is too small to show by color gradation are represented by curves. Solidcurves represent Au/PR SPP modes and dashed ones the lowest orderwaveguided modes in photoresist layer.

FIG. 1. 共Color online兲 共a兲 PL spectra of samples A and B collected at a polaremission angle of 20° 共solid line兲 compared with those of the uncorrugatedsamples 共dashed line兲. Angular dependence of PL spectra of sample A 共b兲
and sample B 共c兲. Au/ Si-nc: SiO2 and Au/PR SPP represent SPP modes
associated with the interface between Au and optically active layer andbetween Au and photoresist layer, respectively. Inset: 共b兲 AFM image of thesample surface; 共c兲 schematic illustration of a sample structure.

intensities are enhanced by factors of 6.5 for sample A and
10 for sample B. To compare the integrated PL intensities
emitted through a thin Au film between the samples with andwithout a grating, we measured the angle-integrated PL spectra without the aperture. For both samples, the integrated PL
intensities are enhanced in the full range of emission wavelength by the presence of a grating. As will be discussed
later, the enhancement is due to a better extraction efficiencyof the emission for the samples with the corrugated surface.
Figures 1共b兲 and 1共c兲 show angular dependence of PL spectrafor samples A and B, respectively. We can see the shift of thepeaks with increasing the collection angle. The shift is due toBragg scattering of SPPs or the guided modes. From theangular dependence of PL peak wavelengths, the dispersion
relations are obtained. The experimentally obtained dispersion relations are shown in Figs. 2共a兲 and 2共b兲 for samples Aand B, respectively, by open circles.
In order to assign the modes, we calculate the dispersion
relations for the five-layer system, i.e., air/photoresist/Au/Si-nc:SiO2 / SiO2.16–19 In the calculation, an emitter is regarded
as an oscillating electric dipole and the power dissipatedfrom an isotropic dipole positioned at the center of an optically active Si-nc: SiO2 layer is calculated as a function ofin-plane wave vector. For the calculation, the refractive indices of Au, Si, and SiO2 are taken from literatures20–22 andthat of Si-nc: SiO2 layer is estimated from the Bruggeman
effective medium theory.23 The wavelength dispersions of
the refractive indices of Au, Si, and SiO2 are taken into account, while that of the photoresist layer is set to 1.68. By thepresence of Bragg grating the wave vector of SPPs or theguided modes is augmented or reduced, as k储共␻兲= k0共␻兲sin ␪ = km共␻兲 ± nG, where k储共␻兲 and km共␻兲 denote thein-plane wave vector of the emitted light and the guided
mode, respectively, k0共␻兲 the absolute value of the wave vector of the emitted light, ␻ the angular frequency, G = 2␲ / ⌳the grating vector with the grating period ⌳, and n an integer
meaning the order of the scattering process. In order to reproduce experimentally obtained dispersion relations, we adjusted the values of the grating period and the thickness ofthe photoresist layer. The best result is obtained when thegrating periods and photoresist layer thicknesses are 765 and189 nm for sample A, and 710 and 360 nm for sample B,respectively. These values are close to the measured ones.
The calculated dispersion relations are shown in Figs.
2共a兲 and 2共b兲. The region deeper in color represents larger
dissipated power. The modes for which the power dissipation

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101907-3

Appl. Phys. Lett. 89, 101907 共2006兲

Takeda et al.

is too small to show by color gradation are represented bysolid curves. In both samples, the strong dissipation shownby color gradients corresponds to the SPP modes propagating
at the interface between Au and active layer 共Au/ Sinc: SiO2 SPP兲, while weak dissipation shown by solid curves
to the SPP modes propagating at the interface between Au
and photoresist 共PR兲 layer 共Au/PR SPP兲. The calculated dispersion relations of the SPP modes agree well with those of
the experimentally obtained ones. We can thus assign theorigin of the peaks in Fig. 1. The assignments of the modes
are written in Figs. 1共b兲 and 1共c兲. The overall agreement
implies that the strong PL peaks in Figs. 1共b兲 and 1共c兲 arise
from the coupling of excitons with SPPs. The slight disagreement between experiments and calculation in Fig. 2 is probably due to the wavelength dispersion of the refractive index
of the photoresist layer, which is not taken into account in
the calculation.
In Figs. 2共a兲 and 2共b兲, the power dissipation of Au/ Sinc: SiO2 SPP is always much larger than that of Au/PR SPP.
This is consistent with Fig. 1共b兲 where PL intensity ofAu/ Si-nc: SiO2 SPP is always larger than that of Au/PR SPP.
However, in Fig. 1共c兲, at particular angles the PL intensity ofAu/PR SPP is larger than that of the Au/ Si-nc: SiO2 SPP.
This means that larger fraction of power coupled to SPP
modes does not always result in larger PL enhancement. Theintensity depends also on the penetration depth of evanescentwaves associated with SPP modes and the distance to corrugated surface. In sample A, the Au and photoresist layers arethin enough for the Au/ Si-nc: SiO2 SPP mode to efficiently
couple to the corrugated surface. On the other hand, insample B, despite strong coupling with excitons in Si-nc’s,
the Au/ Si-nc: SiO2 SPP mode cannot efficiently couple tothe corrugated surface, resulting in smaller intensity than thatof the Au/PR SPP mode.
As described above, the PL enhancement factor is different between samples A and B. This arises mainly from different PL backgrounds between them. The Au layer ofsample A is thinner than that of sample B, resulting in higher
transmission of the emission light. The transmitted light appears as a background signal, and the larger background results in seemingly smaller PL enhancement.
In addition to the SPP modes, waveguided modes exist
in the photoresist layer. The calculated dispersion of thewaveguided modes are shown by broken curves in Figs. 2共a兲and 2共b兲. The thickness of the photoresist layer for sample Aallows only the zeroth order transverse electric 共TE0兲 modeto exist. On the other hand, for sample B both the TE0 andthe zeroth order TM 共TM0兲 modes can exist in the photoresist layer. In Figs. 2共a兲 and 2共b兲, experimentally obtained
data agree well with the calculated dispersion except for theTE0 mode in Fig. 2共b兲 which is not observed experimentally.
This is simply due to the existence of the very strong PL
peak of the Au/ Si-nc: SiO2 SPP mode nearby.

In conclusion, we observed directional PL from Si-nc’s
on which a thin Au layer and an organic grating structure areformed. From the angular dependence of the PL spectra, thedispersion relations were obtained. The experimentally obtained dispersion relations agreed well with the calculated
ones. The overall agreement allowed us to unambiguously
conclude that the directional PL is mediated by SPPs excitedby the energy transfer from excitons confined in Si-nc’s. The
present results suggest that the coupling with SPPs is one ofthe possible approaches to enhance PL properties of Si-nc’s.
However, within the present work, i.e., coupling with a onedimensional grating, the enhancement was not very large.
This may be further improved by introducing a twodimensional grating24 or matching the effective refractive indices of the media at both sides of metal layer interfaces.9
This work is supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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