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SURFACE AND INTERFACE ANALYSIS
Surf. Interface Anal. 27, 491È494 (1999)
SNOM-induced Photoluminescence of Individual
InGaAs Quantum Dots Using Etched
Metal-coated Fibre Tips
I. Manke,* J. Lorbacher, J. L. Spithoven,¤ F. Heinrichsdor† and M. DaŽ hne-Prietsch
Institut fuŽr FestkoŽrperphysik, Technische UniversitaŽt Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany
Scanning near-Ðeld optical microscopy is used to investigate the photoluminescence of individual In Ga As
0.4 0.6
quantum dots. The high transmission efficiency of etched Ðbre tips allows investigations of quantum dots at room
temperature with sufficient intensity, even after coating the tip with metal for background elimination. The pure
background-free signal of single quantum dots results in Lorentzian photoluminescence spectra with Ðnite line
widths of 10–20 meV at room temperature to Æ1 meV at 4 K. This is in contrast to the expectation of extremely
narrow lines in the microelectron-volt region. Copyright ( 1999 John Wiley & Sons, Ltd.
KEYWORDS : quantum dots ; semiconductor nanostructures ; SNOM ; NSOM
INTRODUCTION
Scanning near-Ðeld optical microscopy (SNOM),1,2
with its outstanding spatial resolution, allows the
optical investigation of individual semiconductor
nanostructures that are very much less than 1 lm apart.
Quantum dots are of special interest in this Ðeld
because their discrete electronic energy levels are
expected to result in extremely narrow photoluminescence lines.3h5 Strained quantum dots can be prepared
in a self-organized way by StranskiÈKrastanov growth,
resulting in an inhomogeneous size distribution of the
dots on top of a wetting layer. Spatially averaging photoluminescence experiments of such a dot ensemble
regularly yield inhomogeneously broadened transition
lines,6,7 whereas SNOM allows the study of speciÐc
characteristics of individual dots.8
In this work, we present SNOM-induced photoluminescence results of In Ga As quantum dots
0.6etched Ðbre tips, the
embedded in a GaAs matrix.0.4Using
photoluminescence intensity in internal-reÑection
geometry is considerably higher than in the case of
pulled tips. This enables measurements of single
quantum dots with sufficient intensity, even after
coating the Ðbre tips with metal for background elimination. In this way, the pure signal of single quantum
dots could be observed. A strongly reduced spatial
resolution is observed in experiments using far-Ðeld
detection of the photoluminescence signal, which is
related to extensive di†usion of the excited carriers. The
photoluminescence spectra of individual quantum dots
are characterized by Lorentzian lines of Ðnite width
10È20 meV at room temperature, decreasing substan* Correspondence to : I. Manke, Institut fuŽr FestkoŽrperphysik,
Technische UniversitaŽt Berlin, Hardenbergstrasse 36, D-10623 Berlin,
Germany.
¤ On leave from : Faculteit Natuur- & Sterrenkunde, Universiteit
Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands.
CCC 0142È2421/99/050491È04 $17.50
Copyright ( 1999 John Wiley & Sons, Ltd.
tially with decreasing temperatures to \1 meV at 4 K.
This observation is in contrast both to the theoretical
prediction of extremely narrow lines3,4 and to experimental results on similar InAs quantum dot samples.5
The broadening can be related to a lifetime e†ect of
thermally activated electrons in the dots.
EXPERIMENTAL
For the experiments, two di†erent home-built SNOM
set-ups were used. One design, mounted in a glass
dewar, is used for measurements down to liquid nitrogen temperature.8 A second design is built in a
continuous-Ñow He cryostat for temperatures down to
4 K.9 Scanning of the sample is performed by a
quadrant-sectored tube piezo (EBL 2) with 25 mm
length and 3.2 mm diameter, yielding a scan range of
D100 lm ] 100 lm at 300 K and D20 lm ] 20 lm at
4 K. The tip height is controlled by non-optical shearforce detection using a tuning fork with the tip glued to
one side.8,10 A vertical resolution of D0.5 nm and a
lateral resolution of D15 nm are obtained. At constant
temperatures, the thermal drift is D0.1 lm h~1.
In contrast to our previous work performed with
uncoated pulled tips,8,11 the SNOM tips used here were
prepared by chemical etching and subsequent metal
coating. Multimode Ðbres were etched in 48% hydroÑuoric acid covered with a layer of baby oil12, resulting
in an opening angle of D20¡. Metal coating was performed in a high-vacuum chamber at 10~6 mbar by
evaporation of 100È200 nm thick Ag or Al Ðlms under
an angle of D75¡ from the Ðbre axis while rotating the
tip. We did not observe signiÐcant di†erences between
Al- and Ag-coated tips, except that the latter were
stable for a longer time. Because the apex of these tips
was often not transparent, an opening had to be prepared by gently polishing the tip apex at the sample
surface using the lateral vibration of the tuning fork.
Received 15 September 1998
Accepted 8 December 1998
492
I. MANKE ET AL .
The opening of the aperture was then detected simply
by the onset of the photoluminescence signal.
The sample was excited optically by a HeÈNe laser
with hl \ 1.96 eV, and the signal was detected using a
monochromator with a resolution of D1 meV and a
Peltier-cooled InGaAs photodiode with a noise level of
D8 ] 10~17 W JHz ~1. For spectrally integrated photoluminescence imaging, the monochromator was
replaced by a 1.12 eV low-pass Ðlter in order to
suppress the radiation from the laser and from recombination outside the dots. Measurements in internalreÑection geometry were performed using a 2 ] 1 Ðbre
coupler for feeding the laser radiation through the tip
onto the sample and collecting the photoluminescence
light by the tip for detection. For far-Ðeld excitation or
detection, a cleaved Ðbre was used in close position to
the tip apex.
The In Ga As quantum dot sample was prepared
0.4 0.6
using metal-organic
chemical vapour deposition by
growing 0.94 nm In Ga As on an undoped
0.6
GaAs(001) substrate and0.4
subsequent
capping with a 20
nm thick GaAs and AlGaAs layer.6,7 They were characterized by transmission electron microscopy (TEM),
yielding an area dot density of D10 lm~2, and by conventional spatially averaging photoluminescence.7 In
the latter experiment, transitions at hl \ 0.93, 1.02 and
1.10 eV were observed at room temperature, which were
inhomogeneously broadened by D40 meV. These three
lines were explained on the basis of a term scheme with
one electronic quantum state for the electrons and at
least three states for the holes,13 resulting in the socalled ground-state transition and two or more transitions involving excited hole states.
RESULTS AND DISCUSSION
Plate 1 shows a bird view of the spectrally integrated
photoluminescence intensity of the quantum dots taken
with an etched, metal-coated tip at room temperature in
internal-reÑection geometry. Most peaks represent the
emission from individual dots, demonstrating a lateral
resolution (FWHM) of D300 nm, whereas some
broader and brighter peaks can be assigned to groups
consisting of a few dots. The emission intensities of the
quantum dots are usually very similar. At tip positions
between the dots the signal reduces almost to zero, so
that there is only a negligible background signal from
neighbouring dots, which is in strong contrast to our
previous data obtained using uncoated pulled Ðbre
tips.8,11 In addition, the photoluminescence intensity is
now about two orders of magnitude higher. Thus it is
clearly demonstrated that etched coated Ðbre tips operated in internal-reÑection geometry are very useful for
resolving the pure signal from individual quantum dots
with sufficient intensity.
In Plate 2 we present alternative imaging results
using far-Ðeld excitation or detection. Far-Ðeld excitation results are shown in Plate 2(a), where the quantum
dots are observed with a lateral resolution similar to
that in internal-reÑection geometry. When reversing the
light direction, the quantum dots are excited in the near
Ðeld and the photoluminescence is collected in the far
Ðeld. In this case, di†usion e†ects of the carriers will
Surf. Interface Anal. 27, 491È494 (1999)
have a strong inÑuence on the lateral resolution. Indeed,
such a behaviour is observed in Plate 2(b), where the
quantum dots are no longer resolved. Only small variations in intensity are found when going from the lower
left of the image to the upper right, which is related to a
higher dot density in the lower left region. Plate 2(c)
shows contour lines through the two images. Because
the observed lateral variations in far-Ðeld detection
extend over [1 lm, a di†usion length of the excited
carriers far above 1 lm is derived.
With the exception of di†usion length studies, the
internal-reÑection geometry is much more useful for
studying quantum dots with high spatial resolution and
sufficient intensity, because of more easy handling and
the lack of tip-shadowing e†ects, yielding more reproducible results. Even in the case of small amounts of
light penetrating through the tip Ñanks, e.g. due to
incomplete coverage or holes in the metal coating, only
a small, laterally constant background was observed in
internal-reÑection geometry. Therefore, we preferred
this mode in all further experiments.
In Plate 3, spectrally resolved photoluminescence
images in internal reÑection mode are shown at three
di†erent detection energies. Some dots appear at only
one photon energy, e.g. the dot marked 1, but others
can be seen at two or even at all three di†erent energies,
marked 2 and 3, respectively. This observation indicates
a Ðnite spectral width of the dot emission in the
millielectron-volt range, in contrast to the expectation
of extremely narrow lines of the order of several
microelectron-volts.3h5
In order to analyse this behaviour in more detail,
photoluminescence spectra of single dots at di†erent
positions on the sample were recorded, as shown in Fig.
1. In Fig. 1(a) the room-temperature emission from a
single dot is displayed, including the transitions from
ground state and Ðrst excited state at 0.935 eV and 1.02
eV, respectively. The spectrum shown in Fig. 1(b) displays the ground-state transitions from two neighbouring dots. All spectra are characterized by relatively
broad symmetric lines that can be well described by a
Lorentzian lineshape, as revealed from a least-squares
Figure 1. (a,b) Photoluminescence spectra of individual
quantum dots, taken at 300 K in internal-reflection geometry (data
points), together with Lorentzian fit curves (solid lines). (c) Spectrum taken at 4 K. The different line widths (FWHM) are indicated,
varying between 10 and 20 meV from dot to dot at room temperature down to below 1 meV at 4 K.
Copyright ( 1999 John Wiley & Sons, Ltd.
Plate 1. Spectrally-integrated photoluminescence
image of In0.4Ga0.6As quantum dots taken at 300 K
in internal-reflection geometry.
Plate 3. Spectrally-resolved photoluminscence
image In0.4Ga0.6As quantum dots perfomed at
300 K in internal-reflection geometry at the same
sample with different detection energies. Three
positions are marked with numbers, indicating a
quantum dot in (1) only one image, (2) two
images and (3) all three images.
Plate 2. Images of the spectrally-integrated
photoluminescence intensity taken at 300 K, using
(a) farfield excitation and (b) farfield detection.
Both images were taken at the same sample area.
(c) Cross sections along the lines in (a) and in (b).
Copyright © 1999 John Wiley & Sons, Ltd.
Surf. Interface Anal. 27 (1999)
SNOM OF InGaAs QUANTUM DOTS
Ðt analysis (solid lines in Fig. 1). The analysis of the
spectra from several dots results in Lorentzian line
widths (FWHM) scattered between 10 and 20 meV at
room temperature. It is noted that the Ðrst excited-state
transition also shows comparable line widths.
In addition, a strong temperature dependence was
observed. Figure 1(c) shows the photoluminescence of a
few dots taken at 4 K. Around 1.0 eV, i.e. in the spectral
range of the ground-state transition at this temperature,
it is clearly observed that the line width is now \1
meV, mainly determined by the resolution of the monochromator.
In order to investigate this transition in more detail,
spectra of the same dots were taken at di†erent temperatures. It is difficult to track the photoluminescence
emission from selected quantum dots in the complete
temperature range 4È300 K because of the high thermal
drift, amounting to D100 lm in our experimental
set-up. Therefore, an experiment was performed where
an uncoated tip was Ðxed mechanically at a certain
position of the sample by gently touching the surface, in
this way compensating for drift e†ects and allowing
observation of the temperature dependence of the
spectra from a certain group of quantum dots.
Figure 2 shows the resulting photoluminescence
spectra at di†erent temperatures, originating mainly
from three dots. In the ground-state emission, the photoluminescence signal of the three dots cannot be
resolved very well because of the use of an uncoated
Ðbre tip, resulting in a higher background from neigh-
Figure 2. Temperature dependence of the photoluminescence of
a group of about three quantum dots, taken with an etched
uncoated tip positioned at a fixed location of the surface. The line
width (FWHM) decreases from Á20 meV at 300 K down to the
monochromator resolution of Á4 meV at 5 K.
Copyright ( 1999 John Wiley & Sons, Ltd.
493
Figure 3. Arrhenius plot of the line width as a function of temperature.
bouring dots.8,11 In the excited-state photoluminescence, in contrast, sufficient excitation intensity is
required to observe this transition, so the background is
more suppressed. Here, it is clearly observed that the
line width is decreasing with decreasing temperature
from D20 meV at 300 K to \4 meV at 5 K. It should
be noted that the spectral resolution in this experiment
was only D4 meV, so the actual line width at low temperatures will be much lower, as has been demonstrated
already in Fig. 1(c). Furthermore, the photoluminescence intensity decreases by less than one order of
magnitude when going to higher temperatures, and the
photoluminescence energies show the expected red shift
by D60 meV.
The Ðnite temperature-dependent linewidth and the
Lorentzian line shape can be explained by thermal excitation of carriers in the dot, reducing the lifetime particularly at higher temperatures. In this case, the thermal
excitation probability (and therewith the line width) is
given by ! \ ! exp([E /k T ), where E is the activa0 Arrhenius
A B plot of theA line width is
tion energy. An
shown in Fig. 3. Because of the spread in the observed
line widths of di†erent dots at a given temperature, the
activation energy can only be determined with a rather
large error bar to 3 ^ 2 meV.
Based on the model that the photoluminescence is
described by transitions between one electron state and
three hole states,7,13 and because the di†erent optical
transitions are a†ected by the thermal broadening in a
similar way, it can be speculated that the electron state
is only weakly bound by the activation energy as determined above. A model of the band structure is shown in
Fig. 4. The thermal activation of electrons in the dot,
either to excited states within the dot or to continuum
states in the two-dimensional wetting layer, can result in
the observed broadening. This model requires that there
is no signiÐcant optical-transition probability from the
excited electron states to the hole states, e.g. due to a
strongly reduced wavefunction overlap, otherwise
intense additional lines would be observed. An activation energy in this range also agrees with the intensity
variation observed in Fig. 2 : at thermal energies exceeding the activation energy, a signiÐcant fraction of the
Surf. Interface Anal. 27, 491È494 (1999)
494
I. MANKE ET AL .
Boltzmann distribution, i.e. strongly asymmetric, and
the line width would be comparable for all dots, this
model can be rejected.
It should be noted that calculations for InAs
quantum dots result in much higher electron binding
energies of [100 meV.13 The di†erent values derived
here for In Ga As dots can be assigned to di†erent
0.4 0.6 modifying the electronic structure
structural properties,
as compared with InAs dots. Because of this discrepancy and because of the lack of theoretical investigations on the electronic structure of InGaAs dots, the
presentation of a conclusive model of the electronic
structure for explaining the observed line width behaviour is presently not advisable. In order to investigate
the line shape variation in more detail, more experiments at low temperatures are currently under way.
CONCLUSIONS
Figure 4. Model of the electronic structure of the InGaAs
quantum dots. The electron is only weakly bound with a binding
energy of a few millielectron-volts, resulting in a thermal activation
at higher temperatures to excited electron states, e.g. in the
wetting layer. The line width is thus identical for all three optical
transitions.
electrons is excited and therefore does not contribute to
the photoluminescence signal.
It can also be assumed that there are no bound electron states at all in the dot. In this case, the dot hole
state would recombine with an electron from the twodimensional wetting layer, also leading to a
temperature-dependent line broadening of the emission
lines. Because the expected line shape would be the
result of the two-dimensional density of state and the
In this work we have demonstrated that SNOM experiments with etched metal-coated Ðbre tips provide sufficient spatial resolution and signal intensity to separate
photoluminescence features from individual quantum
dots. We clearly observe that the spectral line width of
the dots is much broader than expected and can be
described by a Lorentzian. It is assumed that the
broadening is due to a lifetime e†ect, possibly related to
a low binding energy of the electron in the dots.
Acknowledgements
W. Busse, C. K. Kim, T. Kalka and A. Bauer are acknowledged for
assistance during the experiment and M. Grundmann and D. Bimberg
for discussion of the results. Part of this work was supported by Sonderforschungsbereich 296 of the Deutsche Forschungsgemeinschaft.
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4. U. Woggon, Optical Properties of Semiconductor Quantum
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Surf. Interface Anal. 27, 491È494 (1999)
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Copyright ( 1999 John Wiley & Sons, Ltd.
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