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Light intensity imaging of single InAs quantum dots using scanning
tunneling microscope
T. Tsuruoka,
a)
Y. Ohizumi, and S. Ushioda
b)
Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
共Received 3 February 2003; accepted 19 March 2003兲
Light intensity images of self-assembled p-type InAs quantum dots 共QDs兲 embedded in
Al
0.6
Ga
0.4
As were measured by injecting electrons from the tip of a scanning tunneling microscope
at room temperature. Bright round features appeared in the images for different photon energies. The
light emission spectrum measured over each bright feature showed a single emission peak with
different peak energy. By comparing the emission peak energies with the transition energies
calculated for pyramidal shaped QD structures, we found that the observed bright features
correspond to individual InAs QDs. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1576509兴
Semiconductor quantum dot 共QD兲 structures with sizes
on the order of several tens of nanometers can be easily
fabricated by means of self-organized growth in lattice-
mismatched heteroepitaxy.
1
These self-assembled QDs are
well known to show broad emission peaks in the conven-
tional photoluminescence spectra; the broadening is under-
stood to mainly result from size fluctuations of QDs.
2
Thus,
it is desirable to be able to measure the emission from indi-
vidual QDs. To evaluate the optical properties of individual
QDs, one needs another technique having a high spatial reso-
lution with the length scale of the QD structure. Many at-
tempts have been made to develop means to achieve the
desired spatial resolution.
Scanning-tunneling-microscope light emission 共STM–
LE兲 spectroscopy is one of the most powerful techniques for
investigating semiconductor quantum structures. This
method has been applied to evaluate the optical properties of
single quantum wells 共QWs兲
3–5
and wires.
6
The transport
properties of the carriers injected from the STM tip were also
studied by using the light emission from QWs.
7,8
For QD
structures, there are only few reports on self-assembled InP
9
and InAs QDs.
10
In these studies high bias voltages of 4–10
V and high tunneling currents of 1–10
A were used for
measuring the light emission spectra. Under these conduc-
tions, both the sample surface and the STM tip are probably
damaged so that it is difficult to obtain light emission from
QDs under stable conditions. Recently Håkanson et al. mea-
sured the light emission spectra of InP QDs covered with 5
nm of GaInP, using tunneling currents of 10–20 nA.
11
They
observed two emission peaks from single QDs and assigned
them to the transitions involving different electronic states of
the dots.
In this letter we report on light emission measurements
of self-assembled InAs QDs using low tunneling current and
low bias voltage levels of STM. The local injection of low-
energy carriers from the STM tip enables one to obtain light
intensity images from single QDs and to investigate the op-
tical properties of individual dots. We also evaluate the spa-
tial resolution of the light intensity image.
The samples were grown on p-type GaAs共100兲 sub-
strates by molecular-beam epitaxy. First a GaAs buffer layer
and an Al
0.6
Ga
0.4
As layer 共50 nm thick兲 were grown at a
substrate temperature of 600 °C. After growing the AlGaAs
layer the substrate temperature was lowered to 530 °C, and 2
monolayers 共ML兲 of InAs was deposited at a growth rate of
0.15 ML/s. For deposition above ⬃1.6 ML, the transition
from two-dimensional to three-dimensional growth was ob-
served in the reflection high-energy electron diffraction
pattern.
1
Then a 20-nm-thick Al
0.6
Ga
0.4
As layer and a 5-nm-
thick GaAs layer were grown as capping layers, keeping the
substrate temperature at 530°C. All the layers were Be
doped to have a hole concentration of 2⫻ 10
18
cm
⫺ 3
for Al-
GaAs. Finally, the sample surface was passivated by expos-
ing it to a molecular beam of As
4
below room temperature,
in order to avoid surface contamination during sample trans-
portation through atmosphere.
The STM–LE measurements were performed in an
UHV chamber with a base pressure below 5⫻ 10
⫺ 9
Pa at
room temperature. After introducing the sample into the
STM chamber, the As overlayer was removed by annealing
the sample at 330 °C for 30 min in UHV. The STM was
operated in a constant-tunneling-current mode, using me-
chanically polished PtIr tips. The light emitted from the
sample was collected by an optical fiber with a core diameter
of 600
m along the direction 70° from the surface normal.
The collected light was then analyzed with a grating spec-
trograph equipped with a liquid-nitrogen-cooled charge-
coupled device camera. Our detection system operates effi-
ciently in the spectral range between 1.3 and 2.3 eV.
Prior to the STM–LE measurements, we observed the
surface morphology of 2-ML-thick InAs grown on
Al
0.6
Ga
0.4
As using atomic force microscopy 共AFM兲. Figure 1
shows an AFM image (200⫻100 nm
2
) measured in air for
the sample without the capping layers. We see clearly that
self-assembled QDs are formed. From the measured AFM
images, the base length l and height h of the dots were
estimated to be in the range of 30–50 and 1.5–5.5 nm, re-
a兲
Also at: RIKEN Photodynamics Research Center, Sendai 980-0845, Japan;
electronic mail: tsuruoka@ushioda.riec.tohoku.ac.jp
b兲
Also at: RIKEN Photodynamics Research Center, Sendai 980-0845, Japan.
APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 19 12 MAY 2003
32570003-6951/2003/82(19)/3257/3/$20.00 © 2003 American Institute of Physics
Downloaded 11 May 2003 to 130.34.208.217. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
spectively. The size parameters l and h were found to be
related by the following equation:
l⫽4.14h⫹26. 共1兲
The surface density of the observed QDs was about 4
⫻ 10
10
cm
⫺ 2
.
Figure 2共a兲 shows a STM image (200⫻ 100 nm
2
) of the
sample surface after removing the As overlayer. The image
was obtained with a sample bias voltage of 2 V and a tun-
neling current of 0.15 nA. Several terraces are observed,
where each step corresponds to one atomic layer step
(⬃0.3 nm). The position of underlying InAs QDs could not
be identified in the STM image.
To obtain the light intensity image of the InAs QDs, we
divided the area of Fig. 2共a兲 into 30⫻ 15 pixels and measured
the light emission spectra by injecting electrons from the
STM tip at the center point of each pixel. Each pixel had an
area of 6.7⫻ 6.7 nm
2
. The sample bias voltage and the tun-
neling current were set at 3 V and 1 nA, respectively. The
integration time was 10 s for each spectrum. All the spectra
were corrected for the energy-dependent sensitivity of our
detection system. By plotting the emission intensity at fixed
photon energies of interest on the respective pixels, the light
intensity image can be obtained simultaneously for different
photon energies.
Figures 2共b兲–2共d兲 show the light intensity images at the
photon energies of 1.40, 1.53, and 1.63 eV, respectively, with
a bandwidth of 60 meV for the same surface area as in Fig.
2共a兲. The emission intensities are normalized to the maxi-
mum emission intensity of each image. Bright round features
are observed to appear on various sites in the images for the
three photon energies. The total number of bright features in
the image is estimated to be 10–12, in good agreement with
the number of QDs of the uncapped sample as shown in Fig.
1.
Figure 3 shows the light emission spectra measured at
three points on the bright features labeled A, B, and C in
Figs. 2共b兲–2共d兲. We see that the emission peaks at 1.40, 1.54,
and 1.62 eV correspond to the bright features A, B, and C,
respectively. It was found that all the observed bright fea-
tures are associated with single emission peaks having dif-
ferent peak energies. The spectra of Fig. 3 also contain emis-
sion peaks arising from neighboring bright features 共for
example, the peaks at 1.54 and 1.62 eV appear in both of the
spectra measured at points B and C.兲.
To identify the observed bright features, we have inves-
tigated the correlation between the emission peaks and the
size of QDs by performing simulations for the energy levels
of a pyramidal shaped QD structure using a finite element
method.
12
The transition energies between the electron and
heavy-hole ground states were calculated using the size pa-
rameters evaluated from the AFM images 关see Eq. 共1兲兴.A
detailed description of the simulation will be given in a sepa-
FIG. 1. AFM image of 2-ML-thick InAs quantum dots grown on
Al
0.6
Ga
0.4
As.
FIG. 2. STM image 共a兲 of the 共100兲 surface of InAs quantum dots capped
with 20 nm of Al
0.6
Ga
0.4
As and 5 nm of GaAs layers and spectrally resolved
light intensity images taken at the photon energies of 1.4共b兲, 1.53 共c兲, and
1.63 eV 共d兲 with a bandwidth of 60 meV.
FIG. 3. Light emission spectra measured at three points on the bright fea-
tures labeled A, B, and C in Figs. 2共b兲–2共d兲.
3258 Appl. Phys. Lett., Vol. 82, No. 19, 12 May 2003 Tsuruoka, Ohizumi, and Ushioda
Downloaded 11 May 2003 to 130.34.208.217. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
rate article.
13
From this simulation we found that the QDs
with l⫽ 40 and 34 nm (h⫽ 3.4 and 2.2 nm兲 have transition
energies at 1.40 and 1.62 eV, respectively. This theoretical
prediction agrees well with the experimental observation.
Thus, we conclude that the bright features observed in the
light intensity images correspond to individual InAs QDs.
The intrinsic linewidth of the emission peaks in Fig. 3 is
estimated to be 30–45 meV by taking account of the energy
resolution of the spectrum 共34 meV at 1.5 eV兲. This line-
width significantly broader than that of the InGaAs QDs em-
bedded in GaAs barriers 共10–20 meV兲
14
may result from the
effects of high doping concentration (⬃1⫻ 10
19
cm
⫺ 3
)in
the dots
15
and interface roughness due to high Al mole frac-
tion of the AlGaAs barriers.
16
Finally, we wish to discuss the spatial resolution of the
light intensity image. Figure 4 plots a cross-sectional line
profile across the dot labeled D in Fig. 2共d兲, as indicated by
the dashed line in the figure. The full width at half maximum
of the emission intensity profile is ⬃35 nm for this dot. From
the emission peak energy of the dot D 共1.59 eV兲, the base
length is expected to be 36 nm according to the transition
energy calculation mentioned earlier. By fitting a pyramidal
curve having this base length convoluted with the Gaussian
function to the intensity profile of Fig. 4, the spatial resolu-
tion was estimated to be about 30 nm. In our experiments,
photon emission occurs by radiative recombination between
the electrons injected from the STM tip and the holes in the
QDs. Hence, the spatial resolution is determined by the spa-
tial spread of the electrons from the injection point before
being captured in the QDs. The spatial spreading of the in-
jected electrons can be characterized by two transport param-
eters in real space, the thermalization and diffusion
lengths.
7,8
The thermalization length increases from a few
nanometers to ⬃30 nm with the increase of the initial energy
of electron injection, while the diffusion length is several
hundred nm independent of the injection energy.
7,17
The
depth of the QDs from the surface 共25 nm兲 is in the same
range as the thermalization length in our sample. Thus, we
believe that the light emission occurs mainly from the elec-
trons that have just thermalized.
In summary, we have measured the light emission spec-
tra and the light intensity images of self-assembled InAs
QDs by injecting electrons from the STM tip. Bright round
features were observed on various sites of the spectrally re-
solved light intensity images. They were associated with
single emission peaks having different peak energies. From
the calculation of the transition energies between electron
and heavy-hole ground states of pyramidal shaped QD struc-
tures, we have identified the observed bright features with
individual InAs QDs. The spatial resolution of the image is
estimated to be about 30 nm. This resolution appears to be
determined by the thermalization length of the injected elec-
trons.
The authors gratefully acknowledge valuable advice
from Professor J. Nishizawa. They would also like to thank
Professor M. Ogawa for giving them useful advice on the
transition energy calculation of pyramidal shaped QD struc-
tures. This work was supported in part by a Grant-in Aid for
Scientific Research A 共No. 13304022兲 from the Japan Soci-
ety for the Promotion of Science.
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FIG. 4. Cross-sectional emission intensity profile across the bright feature
labeled D in Fig. 2共d兲, indicated by the dashed line. The emission intensity
is integrated in the energy range between 1.58 and 1.60 eV.
3259Appl. Phys. Lett., Vol. 82, No. 19, 12 May 2003 Tsuruoka, Ohizumi, and Ushioda
Downloaded 11 May 2003 to 130.34.208.217. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
