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Physica E 21 (2004) 414 – 418
www.elsevier.com/locate/physe

Electronic states of self-organized InGaAs quantum dots onGaAs (3 1 1)B studied by conductive scanningprobe microscope
R. Oshima∗ , N. Kurihara, H. Shigekawa, Y. Okada
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan

Abstract
We have used conductive scanning probe microscope (SPM) in high vacuum and operated at 173 K in order to investigate
the electronic properties of self-organized InGaAs quantum dots (QDs) grown on GaAs (3 1 1)B and (0 0 1) substrates.
Ordered InGaAs quantum dot arrays on GaAs (3 1 1)B surface were fabricated by atomic-H assisted molecular beam epitaxy(H-MBE), and Si SPM tips coated with Au which warrants electrical conductivity were used to measure simultaneously boththe topographic and current images of QDs surface. From the current–voltage (I –V ) curves, unique and di:erent plateau
features were observed for QDs formed on GaAs (3 1 1)B and (0 0 1) substrates. The results suggested that a high degreeof symmetry of InGaAs QDs on (3 1 1)B was responsible for the observed degeneracy of electronic states and arti<cialatom-like states. We demonstrate that this conductive SPM technique becomes a powerful tool in studies of single electron
charging of individual dots.
? 2003 Elsevier B.V. All rights reserved.
PACS: 81.05.Ea; 73.61.−r; 85.30.Vw
Keywords: Quantum dots; Self-organized growth; MBE; Conductive SPM

1. Introduction
Recently, the studies on low-dimensional quantumdot (QD) structures have attracted intense research
for applications to QD devices such as single-electrontransistors [1], far-infrared detectors [2], and QD
lasers [3]. Further, resonant tunneling QD diodes
are regarded as excellent candidate for digital circuit applications because of their superior switching
speed and low-power consumption. On the other,∗ Corresponding author. Tel.: +81-29-853-6902; fax: +81-29853-6902.
E-mail address: bk981495@s.bk.tsukuba.ac.jp (R. Oshima).

unique electronic states of QDs analogous to the real
atoms have been investigated by Tarucha et al. [4]who studied resonant tunneling characteristics of a
GaAs-based double barrier QD diode fabricated bylithography and etching technique. And for this reason, QDs are referred to as arti<cial atoms. However,
the electronic properties of self-organized QDs fabricated by strain-induced heteroepitaxy have not been
fully understood and thus have been the target of research recently at various institutions.
While the common characterization techniques suchas photoluminescence (PL) and capacitance spectroscopy are particularly useful in probing the local
properties containing an ensemble of dots, scanning1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physe.2003.11.063

R. Oshima et al. / Physica E 21 (2004) 414 – 418

probe microscope (SPM) techniques would be advantageous, if the properties of individual dots are to be
exploited with nm-scale size resolution. The use ofconductive SPM tip allows us to modify the local band
pro<le of a given QD structure with external appliedbias. Furthermore, the electronic properties such asarti<cial atom-like energy states and shell <lling ofdots with single electron can be investigated using thistechnique [5]. We have previously reported that theelectronic properties of QDs surface can be studied
by conductive SPM with metal-coated Si tips [6]. In
this work, we focused our attention on the transport
properties such as single electron charging and related
Coulomb e:ects in self-organized InGaAs QDs grown
on GaAs substrates, which are potential candidates forthe future components in optoelectronics and quantum
information processing technology.
2. Experiments
Three stacked layers of self-organized In0:4 Ga0:6 As
quantum dots used in this study were grown on n-type
GaAs (3 1 1)B and (0 0 1) substrates (doped to 1 ×1018 cm−3 ) by atomic-H-assisted molecular beam epitaxy (H-MBE). After standard degreasing and etchingsteps, each GaAs substrate was subjected to cleaning
in UHV by using atomic H irradiation at 580◦ C for
10 min in order to obtain an atomically Lat surface.
Then, a 300-nm-thick GaAs bu:er layer (Si-doped,2:5 × 1018 cm−3 ) was grown at 580◦ C followed by a50-nm-thick undoped GaAs spacer layer. After lowering the substrate temperature to 500◦ C, 8:8 ML ofIn0:4 Ga0:6 As QDs layer was grown at a growth rateof ∼0:1 ML=s. The hydrogen backpressure was kept
constant at ∼6 × 10−6 Torr during substrate cleaningand H-MBE growth.
To be particularly noted in the In0:4 Ga0:6 As QDsgrown on GaAs (3 1 1)B substrates that di:ers remarkably from those assembled on (0 0 1) surface is theirunique self-organization mechanism [7]. The quantum
dot growth on GaAs (3 1 1)B is known to be fundamentally di:erent from the well-established Stranski–
Krastanov (S–K) growth mode, and in fact, a complex
phase separation and strain-relief mechanism are responsible for the formation of such high-density andwell-ordered dot arrays on (3 1 1)B surface. Further,
we can control the dot size and density by simply

415

controlling the deposition temperature while maintaining the structural ordering of QDs arrays [7].
After growth, samples were transferred from MBE
chamber to our SPM system in dry nitrogen ambientin order to minimize natural oxidation, and pumped
down by using a turbo-molecular pump to a base pressure of ¡5 × 10−7 Torr [6]. Si tips coated with Au,
which warrants electrical conductivity were used tomeasure both the topographic and current images of
QDs surface. The current–voltage (I –V ) characteristics of dots of varying size and of any other arbitrary
positions on the dots surface were also studied by using the same conductive tip. Though the tip radius ofan Au-coated tip was certainly larger than that of an
uncoated tip, this did not a:ect the lateral resolution
required for the topographic and current image measurements done in this work. Further, removal of Au
from the tip during scanning was not observed under
our operating conditions.
Each QDs sample was thermally treated in SPM
chamber at 70◦ C for 1 h before proceeding to SPM
characterization in order to remove residual surface
water <lm [6]. The surface water <lm is known toresult in a strong adhesive force as well as defocusing of applied electric <eld between the tip and sample [8]. In order to prevent the tip from crashing into
the sample during heating due to thermal expansion,
the tip was placed close to, but not in a direct contact
to the heated sample. During the SPM image and I –V
curve acquisition, a constant contact force was applied
via an electronics feedback control, and the measurements were conducted in contact mode in vacuum andusing a liquid nitrogen cooled stage at 173 K.
In this work, we focused our attention to clarifying
the transport properties such as single electron charging and related Coulomb e:ects in self-organized InGaAs QDs grown on GaAs substrates by using simpleand reliable method of conductive SPM technique.
3. Results and discussion
Fig. 1 shows (a) schematic sample structure, and(b) calculated ideal conduction band diagrams underno bias and a <nite forward bias V , respectively. The
samples were con<gured to form a double tunnel barrier junction structure. The bottom-most dot layer wasintended to improve the quality of the top two QDs

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R. Oshima et al. / Physica E 21 (2004) 414 – 418

Fig. 2. AFM images of topmost In0:4 Ga0:6 As quantum dots surfaceof 3-stack sample on (a) GaAs (3 1 1)B and (b) (0 0 1) substrate,respectively. Scan size is 1 m × 1 m.

(a)
0.06

(a) r = 22.44nm, h = 9.9nm
on GaAs(311)B

p - shell

Current (nA)(b) r = 14.05nm, h = 9.7nm
on GaAs(001)Fig. 1. (a) Schematic sample structure, and (b) calculated ideal
conduction band diagrams under no bias (V = 0) and a <niteforward bias (V ¿ 0), respectively.

(c) Wetting layer

0.05(b)0.04(c)2.0layers, and to align them vertically during stackedgrowth. No dislocations were observed in transmission
electron microscope (TEM) measurements. As seenin Fig. 1(b), the ground state of electrons in the middle QD would lie above the Fermi level of tip underno bias condition, which is one of the necessary conditions in order to observe Coulomb related e:ects intransport studies. Moreover, it is known that an electron accumulation occurs on the surface of InAs due tothe positively charged surface states in InAs, and thise:ect consequently leads to a lowering of Schottky
barrier height between InAs and tip [5]. Therefore, thetopmost dots were used in e:ect as zero-dimensional
nano-emitters to inject electrons to single QDs thatwere buried directly below the topmost dots [9].
Fig. 2 show the atomic force microscope (AFM)
images of topmost In0:4 Ga0:6 As QDs surface of 3-stackQDs sample on (a) GaAs (3 1 1)B and (b) (0 0 1) substrate, respectively. The average QD diameter, height,
and arial density were (a) 49:45 nm, 8:82 nm and1:3×1010 cm−2 on (3 1 1)B and (b) 54:82 nm, 9:23 nms - shell

2.5

3.0
Bias (V)

3.5Fig. 3. I –V curves measured at 173 K on a single In0:4 Ga0:6 As
QD on (a) GaAs (3 1 1)B, (b) (0 0 1) and (c) wetting layer,
respectively.

and 1:69×1010 cm−2 on (0 0 1), respectively, andthus were nearly identical. However, it can be seenthat an ordered QDs array structure was maintainedon (3 1 1)B substrate after strain accumulation during
stacked growth. Further, each QD was circular-disk
shaped with a higher degree of structural in-planesymmetry. On the other hand, wire-like islands elongated along [1–10] were observed for QDs grown on
GaAs (0 0 1) substrate after stacking of 3 dot layers.
Next, I –V curves were taken by placing the tip on
top of various isolated QDs. Fig. 3(a) shows typical I –V curve measured on a single QD formed on
GaAs (3 1 1)B. The QD size was 22:44 nm in radius and 9:9 nm in height, respectively. Similarly,Fig. 3(b) shows a curve obtained for a single QD
formed on (0 0 1) with 14:05 nm in radius and 9:7 nm

R. Oshima et al. / Physica E 21 (2004) 414 – 418(a)p - shell

0.06
Current (nA)in height, respectively. As the QDs on (0 0 1) wereellipse-shaped as shown in Fig. 2(b), we approximatedthem to circular disks in discussing the QD size in thelater section. Staircase-like characteristics were clearlyobserved in Figs. 3(a) and (b), while the I –V curve (c)that was measured on a wetting layer region showed
no apparent feature.
Staircase characteristics generally correspond toCoulomb blockade and related tunneling phenomena,i.e. charging of single electrons into a QD. As shownin Fig. 3(a), two closely spaced plateaus were <rstobserved immediately after onset of current, followed
by a larger gap and four closely spaced and equidistant plateaus. This characteristic feature was in good
agreement with the recent reports of single electron
charging of a QD as an arti<cial atom [4,10]. Thus,
we attribute the <rst doublet of plateaus to tunnelingthrough the ground state (s-shell), and the quartet totunneling through the <rst excited state (p-shell) ofQD. The gap between the two should then correspondto energy spacing between the ground and <rst excitedstate in a dot due to quantum con<nement. The mean
width of plateau NV was estimated to be 138:6 mVfrom Fig. 3(a). Thus, single electron charging energy NEex can be estimated as 8:64 meV by taking
the bias-to-energy conversion factor of , which wasgiven as fractional amount of bias that changes theenergy of QD to be ∼6:23 meV=V into account.
Meanwhile, calculated charging energy NEcal was
7:34 meV by approximating the QD as a circular
metal disk of capacitance given by C = 8 R, where
R is the QD radius. Therefore the approximation
showed a good agreement with our measured result.
On the other hand, I –V curve shown in Fig. 3(b)exhibited only three equidistant plateaus and wasremarkably di:erent from Fig. 3(a). We think thatthese plateaus originated from single electron charging of QD, however, degeneracy of energy states orshell structure of arti<cial atom was not formed inthis sample. Following a similar approximation asabove, the mean width of plateau NV was estimatedto be 164:51 mV from Fig. 3(b). This would giveNEex ∼9:81 meV, which was again in fair agreementwith calculation of NEcal ∼11:72 meV, taking thebias-to-energy conversion factor of ∼4:50 meV=V
into account in this case. We attribute the di:erence
between Fig. 3(a) and (b) to clear di:erence of dot
shape. It is known that a high degree of symmetry417

p - shell(b)0.05

0.04(a) r = 22.44nm, h = 9.9nm(b) r = 13.96nm, h = 9.43nm

2.02.53.0

3.5

Bias (V)Fig. 4. I –V curves measured at 173 K with the tip placed on single
QDs self-organized on GaAs (3 1 1)B substrate for two di:erent
QD sizes (a) 22:44 nm in radius and 9:9 nm in height and (b)13:96 nm in radius and 9:43 nm in height, respectively.

of dot is in fact responsible for the degeneracy ofquantized energy level. This is because the symmetry of two-dimensional harmonic potential requires acylindrical symmetry with a soft boundary pro<le [4].
We have then investigated the dependence ofI –V characteristics on dot size. Fig. 4 represent theI –V curves measured on QDs self-organized on
GaAs (3 1 1)B substrate for two di:erent dot sizes(a) 22:44 nm in radius and 9:9 nm in height and(b) 13:96 nm in radius and 9:43 nm in height, respectively. First, it can be observed that a larger dot ismore conductive than a smaller dot. This is due to thedi:erence in local surface potentials, which would
be larger for small dots [5,11]. Second, four plateaus(p-shell) were clearly seen in both I –V curves but theseparation was larger for a smaller dot than a larger
dot. This is due to the size e:ect since smaller dot
would have a larger charging energy.
Finally, Fig. 5 plots the measured single electron
charging energies for several In0:4 Ga0:6 As QDs formed
on GaAs (3 1 1)B and (0 0 1). The solid line represents the calculated charging energies by approximating QDs as circular metal disks of capacitance given
by C = 8 R. It can be seen that the experimental results obtained in this work were in fair agreement withthis simple calculation. Major origins for deviation included: (1) approximating the shape of more of anelliptical dot formed on (0 0 1) to a circular disk, and(2) approximating the size of middle QD which was

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R. Oshima et al. / Physica E 21 (2004) 414 – 418

Charging Energy (meV)40
Calculation
QD on GaAs(311)B
QD on GaAs(001)2000102030

From the I –V curves, unique plateau features wereobserved for QDs formed on GaAs (3 1 1)B and (0 0 1)
substrates. The results suggested that a high degree ofsymmetry of InGaAs QDs formed on GaAs (3 1 1)B
is responsible for the observed degeneracy of electronic states and arti<cial atom-like shell structure.
The QDs formed on (0 0 1) did not show such degeneracy. These considerations were in good agreement
between measured and simple calculation model ofcharging energies of single electrons in to a quantum
dot.

Radius (nm)Fig. 5. Measured single electron charging energies for several
In0:4 Ga0:6 As QDs formed on GaAs (3 1 1)B and (0 0 1). The solidline shows the calculated charging energies by approximating QDs
as circular metal disks.

buried below a topmost QD the same as the topmost
QD size. From TEM studies, upper QDs were larger
in radius than dots in lower layers due to strain accumulation with stacking of dot layers. Nonetheless, wehave successfully demonstrated that conductive SPM
technique becomes a powerful tool in studies of single
electron charging of individual quantum dots.
4. Summary
We have used conductive SPM in high vacuum andoperated at 173 K in order to investigate the electronic
properties of self-organized In0:4 Ga0:6 As QDs grown
on GaAs (3 1 1)B and (0 0 1) substrates. The 3-stacked
layers of self-organized InGaAs QDs were grown by
H-MBE. The bottom-most QDs layer was intended toimprove the quality of the top two dot layers, and toalign them vertically during stacked growth. On theother, the topmost QD was used as a zero-dimensional
nano-emitter to inject electrons to one QD that wasburied directly below this topmost dot. This con<guration is advantageous in studies of single electron
charging of individual QDs.

Acknowledgements
The authors would like to gratefully acknowledge
Profs. K. Yamabe, K. Akimoto, K. Murakami, and T.
Makimura of University of Tsukuba for their support.
One of the authors (YO) was supported in part by
TEPCO Research Foundation.
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