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This is the published version of a paper presented at Conference on Nanotechnology VI, APR 24-25,
2013, Grenoble, France.
Citation for the original published paper:
Sangghaleh, F., Bruhn, B., Sychugov, I., Linnros, J. (2013)
Optical absorption cross section and quantum efficiency of a single silicon quantum dot.
In: Nanotechnology VI (pp. 876607-). SPIE - International Society for Optical Engineering
Proceedings of SPIE
http://dx.doi.org/10.1117/12.2017483
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
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Optical absorption cross section and quantum efficiency of a single
silicon quantum dot
F. Sangghaleh*a, B. Bruhna, I. Sychugova, J. Linnrosa
Materials and Nanophysics, School of ICT, KTH Royal Institute of Technology, SE-164 40 Kista,
Sweden
a
ABSTRACT
Direct measurements of the optical absorption cross section (σ) and exciton lifetime are performed on a single silicon
quantum dot fabricated by electron beam lithography (EBL), reactive ion etching (RIE) and oxidation. For this aim,
single photon counting using, an avalanche photodiode detector (APD) is applied to record photoluminescence (PL)
intensity traces under pulsed excitation. The PL decay is found to be of a mono-exponential character with a lifetime of
6.5 µs. By recording the photoluminescence rise time at different photon fluxes the absorption cross could be extracted
yielding a value of 1.46×10-14cm2 under 405 nm excitation wavelength. The PL quantum efficiency is found to be about
9% for the specified single silicon quantum dot.
Keywords: Single silicon quantum dot, absorption cross section, quantum efficiency (QE), photoluminescence (PL)
decay, luminescence rise time, silicon nanocrystals
1. INTRODUCTION
Quantum efficiency estimation of individual silicon quantum dots is of considerable importance, since it is one of the
essential parameters influencing their applicability in optoelectronic and photonic devices. In order to determine the
efficiency of an optical process in a nanocrystal, the physics of the photon absorption process as well as the emission
mechanism must be well understood.
For years bulk silicon was considered as an inefficient light emitter due to its indirect band gap nature. After the
discovery of strong photoluminescence (PL) emission of porous silicon at room temperature1,2, silicon nanocrystals
became the center of interest for many researchers3,4. The quantum confinement model is believed to be relevant for
silicon nanocrystals5,6, coupling the observed increase of the band gap energy with the decrease of nanocrystal size. The
development of the single-dot spectroscopy technique7 led to a deeper insight of the photophysics of nanocrystals,
however, many aspects of the light emission mechanisms are still not well understood. In particular, studies of
photoluminescence absorption and decay have been mainly focusing on ensembles of silicon nanocrystals so far8,9,10, in
which the effects of size and Si-SiO2 interface nature of individual dots are averaged.
In the current work the results of PL lifetime and absorption cross section measurements of a single silicon quantum dot
are presented. The samples were fabricated by electron beam lithography (EBL) and reactive ion etching (RIE), followed
by self-limiting oxidation11. The well-defined positions of individual silicon quantum dots formed by this method enable
repeatable single-dot spectroscopy. The measurements were carried out under different excitation laser power densities,
using an avalanche photodiode detector (APD) for recording luminescence traces. Blinking and spectroscopy
experiments were also performed on the same dot as complementary measurements. An estimate of the quantum
efficiency of this nanocrystal, taking into account the detectivity of the PL setup, is presented.
2. EXPERIMENTAL
The experimental PL setup consists of a conventional wide field optical microscope system connected to a spectrograph
(Andor, Shamrock 500i) and an EMCCD camera (Andor, iXon3) at one port and to an avalanche photodiode (Becker &
Hickl, DPC-230) at another port. A flipping mirror inside the microscope enables switching between these two ports.
*[email protected]; phone 46 8 790 4159; fax 46 8 79043000;
Nanotechnology VI, edited by Rainer Adelung, Proc. of SPIE Vol. 8766, 876607
© 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2017483
Proc. of SPIE Vol. 8766 876607-1
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Figure 1 show
ws a simple scchematic of th
he PL setup. T
The sample is excited using
g a UV laser (4405 nm). Dep
pending on thee
type of the P
PL measuremeents, the laserr is operated iin continuouss or pulsed mode. For PL ddecay and absorption crosss
section measuurements, pullsed excitation
n with a repettition rate of 20
2 kHz and on
n-time duratioon of 10 µs iss used. This iss
done at differrent excitationn laser powerr densities (8- 80 Wcm-2). On
O the other hand, for blinnking measureements the PL
L
intensity traces are recordeed by the APD
D under contiinuous excitattion (8 Wcm-22). The laser bbeam has an angle
a
of ~ 30º
t emission iis collected th
hrough a high numerical apperture lens (N
NA= 0.7) withh
with respect tto the sample surface and the
100x magniffication. Appropriate opticcal componennts such as high quality filters and m
mirrors are used
u
to blockk
background luuminescence.
ilicroscope
Flippinç
J
APD
-pass filters
E;Kcitation Las(
Spe( Itrcr,ete!-
CCD
405 nm
Figure 1. A Schematic off the experimen
ntal PL setup. T
The excitation laaser beam (405 nm) impinges oon the sample surface
s
at
mitted light is guided
g
through aan optical micro
oscope and colllected by an APPD or spectrom
meter
an angle oof ~ 30º. The em
connectedd to an EMCCD
D camera for ph
hoton counting oor spectral meaasurements, resp
pectively.
The samples are fabricatedd in three steps. Electron beeam lithograph
hy is used to define
d
a patterrn of undulatiing nano-wallss
on an n-type silicon waferr. Walls with three differennt thicknessess (40, 65 and 90 nm) are thhen formed by
b reactive ionn
etching (RIE)). A simple schematic
s
of one
o undulatinng nano-wall is
i shown in figure
fi
2a. Finaally, self-limiting oxidationn
inhomogeneoously reduces the silicon strructures, yieldding individuaal silicon nano
ocrystals withiin the oxidized walls11. Duee
to large pree-selected separation betw
ween the doots (~ 2 µm
m), it is po
ossible to peerform severral repeatablee
photoluminesscence measurrements on ex
xactly the sam
me dot. Apply
ying short or long oxidationn duration, sin
ngle or doublee
quantum dotss can be form
med in the thin
nnest and meddium parts of the
t walls resp
pectively11. Fig
igure 2c show
ws a typical PL
L
image of thee sample afterr short oxidattion and undeer cw UV ex
xcitation. Figu
ure 2b is an eenlarged scan
nning electronn
microscope (SEM) image of the area sp
pecified by thee gray rectang
gle in figure 2c.
2 All the meeasurements are
a focused onn
m dot indicated
d by the red arrrow in figure 2c.
one single silicon quantum
3.. RESULT
TS
Figure 2d shows the room
m temperaturee PL spectrum
m of the singlle silicon quantum dot, speecified by thee red arrow inn
c excitation power densitty. The emissiion energy peaks at 1.695 eeV with a linee width of 1000
figure 2c, undder 8 Wcm-2 cw
meV. This is typical for siingle silicon nanocrystals
n
aat room tempeerature6. The low-energy
l
saatellite peak at a distance of
i ascribed to TO phonon iinteractions. Previously
P
it was
w shown thaat at lower tem
mperatures thee
60 meV to thhe main peak is
line-width off the main peak is narroweer (~ 0.4 meV
V at 10K) and
d the TO-pho
onon replica ccan be better resolved 12,133.
Blinking studdies are also conducted
c
on the same quanntum dot baseed on single photon
p
countinng. For this purpose
p
the PL
L
intensity tracces are recordded using an APD detectoor under contiinuous excitattion of 8 Wccm-2. For the particular doot
studied, the eemission intennsity is relatively constant aand blinking is
i only observ
ved once at thee end of the recorded
r
tracee,
see Figure 3aa.
Proc. of SPIE Vol. 8766 876607-2
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(a)
1.695eV
(
E2= 1.63E )Ë,
M n
M I
me
1
I
-441400geAr
II
1.5
1,6
1,7
1
18
Eneirgy (eV)
Figure 2. (a) Schematic of
o an undulating
g silicon nano-w
wall with three different thickn
nesses. (b) SEM
M image of such
h nanohe thinnest
walls afterr RIE and beforre oxidation. (c) Photoluminesscence image off single silicon quantum dots fformed inside th
part of thee walls after shoort oxidation11. The SEM imagge in (b) shows the details of th
he similar area determined by the grey
rectangle. The single quaantum dot under investigation is specified by a red arrow. (d) PL emission sspectrum at roo
om
temperatuure and Gaussiaan fits to main (rred) and satellitte (blue) peak. Emission peakss at 1.695 eV w
with full width half
h
maximum
m (FWHM) of 100 meV at an excitation
e
poweer density of 8 Wcm
W -2, the TO-phonon sidebannd is shifted 60
0 meV to
lower eneergy with respecct to the main peak.
(a)
CT,
PL intensity (arb. units)
NJ
n w
n an n COn v
n
CO
n
From this, tw
wo distinct inteensity levels, denoted as O
ON and OFF, can
c clearly bee defined from
m its histogram
m (Figure 3b)).
The dashed liine in betweeen ON and OF
FF indicates thhe threshold that
t
is used fo
or the distinctition between the
t two statess.
Blinking stattistics of simiilar single siliicon quantum
m dots have been
b
studied in detail in eaarlier work14. Blinking andd
spectral meassurements connfirm that the luminescent
l
oobject under in
nvestigation iss indeed a “sinngle” silicon quantum
q
dot.
I'l1 Ill 'i
Ill
1111111i
' °l
ON
n
Thresholc
n
91-F
mo
Th n e
¡sl
Counts
Figure 3. (a) PL Intensityy traces of the studied
s
single siilicon quantum dot recorded att room temperaature. (b) Corressponding
histogram
m of the blinkingg traces, demon
nstrating the twoo distinct statess (ON and OFF)). A threshold liline is considereed in
between th
the two levels.
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i-
-2
.
10
J.
20
Figure 4. Photoluminescence decay and
d rise of the singgle silicon dot specified
s
by thee red arrow in fiigure 2c, under two
W -2. The soliid lines are mon
no-exponential fits. (a) The phhotoluminescencce decay
excitationn power densitiees of 8 and 64 Wcm
exhibits a lifetime of 6.5 µs for both laser power densitties. whereas th
he rise time stro
ongly depends oon excitation.
The photolum
minescence traansient decay and rise is sttudied under pulsed
p
laser ex
xcitation at a frequency off 20 kHz usingg
an on-time duuration of 10 µs.
µ The emitteed photons aree collected by the APD deteector based onn the single ph
hoton countingg
technique. Thhe results are shown
s
in figure 4 for two ex
excitation pow
wer densities (8
8 and 64 Wcm
m-2).
A clear monno-exponentiall character with
w a lifetimee of ~ 6.5 µss can be obseerved for thee PL decay under
u
differennt
excitation lasser power dennsities. This is
i consistent with the resu
ults obtained from an earliier study on several singlee
quantum dotss performing time-resolved
t
measurementts15.
The kinetics of the lumineescence rise, as
a the laser is switched on, follows a mo
ono-exponentiial function, as
a well. In thiss
me (τrise) valuees under 8 annd 64 Wcm-2 excitation po
ower densitiess are 3.76 µss and 0.49 µss,
case, the obttained rise tim
respectively.
ws the dependdence of the luminescencee rise rate on the excitation
n power densitity. A linear increase of thee
Figure 5 show
luminescencee rise rate witth excitation power
p
can cleearly be seen
n. This is expeected from thhe equation off the temporaal
shape of the lluminescence rise time16:
⎧
⎡ ⎛
1 ⎞ ⎤⎫
I (t ) = I 0 ⎨1 − exp ⎢ − ⎜ σφ + ⎟ t ⎥ ⎬
τ ⎠ ⎦⎭
⎣ ⎝
⎩
(1)
fl and τ is th
he photoluminnescence decay
y time.
Where ϕ is thhe excitation flux
Thus, the sloppe of the lineaar function in figure 5 yieldds the absorptiion cross sectiion to a value of σ=1.46×10
0-14cm2 for thee
specified singgle silicon quaantum dot und
der 405 nm exxcitation. This is within an order
o
of magnnitude in agreeement with thee
values reporteed by other grroups9,10 from measurementts on ensemble of silicon naanocrystals.
The luminesccence quantum
m efficiency (Q
QE) is calculaated using the formula for th
he excitation rregime below saturation:
I = P × σ × D × QE
E
(2)
17
Where P is thhe excitation flux,
f
σ the abssorption crosss section and D the system detectivity
d
.
The system ddetectivity is measured
m
for different optiical componen
nts in the PL setup as the ppercentage off efficient lighht
transmission//reflection throough each off them. Approoximately 10 % of the ligh
ht is considereed to be emittted efficientlyy
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into the upperr hemisphere of the single quantum
q
dot ffrom the surfaace18. Finally the
t obtained qquantum efficiency is ~ 9%
%,
which is closee to a previouusly reported value
v
(~ 15%) 19.
4
G= 1.46x10 -141 - m
.I
X
A
o
4
-
ï
Excit ration
1
flux (c m's
1 í;
)x10
Figure 5. Reciprocal of thhe luminescencce rise time verssus photon flux
x under the saturation level. Thhe luminescencee rise rate
x.
is a linearr function of thee excitation flux
4. C
CONCLUS
SION
Measurementts of luminescence decay and rise timee under differrent laser exciitation powerr densities on an individuaal
silicon quantuum dot, usingg the photon counting techhnique, were performed. Two
T
distinct eemission inten
nsity states, ass
well as specttral measurem
ments, confirm
m the observat
ation of a sing
gle silicon quantum dot. Th
The decay tim
me is of monoexponential ccharacter withh 6.5 µs decay
y time, which is in agreemeent with resultts obtained froom time-resollved PL decayy
measurementts on similar single silicon quantum dotts15. The slope of the lineaar increase off the luminesccence rise ratee
yields an absoorption cross section value of 1.46×10-144cm2. The meaasured value of
o the absorptiion cross-sectiion allowed uss
to estimate thhe quantum effficiency of thiis particular ddot to be ~ 9%
%.
It could be sshown that luuminescence transients
t
cann be successffully measured for a single
le silicon nan
nocrystal. Thiss
enables studiees on many inndividual objeects and subseequent comparrison of their properties
p
am
mongst each other, as well ass
to ensembless. Individual variations
v
betw
ween single eemitters can be
b resolved an
nd possibly ccorrelated to their
t
structuraal
t density off states of a naanocrystal can
n possibly be pprobed using this techniquee
differences inn future work.. In addition, the
by applying ddifferent excitation wavelen
ngths.
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