Journal of Crystal Growth 245 (2002) 212–218

Self-organized InGaAs quantum dots grown on
GaAs (3 1 1)B substrate studied by conductive atomic
force microscope technique
Yoshitaka Okadaa,*, Masashi Miyagia, Kouichi Akahanea, Mitsuo Kawabea,
Hidemi ShigekawabbaInstitute of Applied Physics, University of Tsukuba, 1–1–1 Tennodai, Tsukuba 305-8573, Japan
CREST, Japan Science and Technology Corporation (JST), Institute of Applied Physics, University of Tsukuba,
1–1–1 Tennodai, Tsukuba 305-8573, Japan

Received 30 April 2002; accepted 26 July 2002
Communicated by H. Ohno

Abstract
We have used conductive atomic force microscope (AFM) in a high vacuum in order to investigate the electronic
properties of self-organized InxGa1–xAs quantum dots (QDs) on GaAs (3 1 1)B substrates. The QDs were fabricated byatomic H-assisted molecular beam epitaxy, and Si AFM tips coated with Au, which warrants electrical conductivity
were used to measure both the topographic and current images of QDs surface simultaneously. The conductive AFM
measurements were performed in vacuum at room temperature and at lowered temperatures. With this technique,the current–voltage (I–V ) characteristics of QDs of varying sizes, and of any other arbitrary positions on the QDs
surface can also be studied by using the same conductive AFM tip. It was found that the center of a QD is
more conductive than its periphery, and the surface in between the QDs is highly resistive. The differences in the
conductance are thought to be due to the local modiﬁcation of surface bending associated with the surface states.
Further, we have shown that the conductance becomes spatially uniform at all points over the packed and ordered QDs
at low temperatures, which could be explained by lateral coupling of these strained QDs.
r 2002 Elsevier Science B.V. All rights reserved.
PACS: 73.21.La; 73.23.b; 73.63.Kv
Keywords: A1. Low dimensional structures; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials

1. Introduction*Corresponding author. Tel.: +81-298-53-5293; fax: +81298-53-5205.
E-mail address: okada@ims.tsukuba.ac.jp (Y. Okada).

Recently, the studies of the electronic andoptical properties of self-assembled semiconductor
quantum dots (QDs) have gained increasing
attention [1–5]. While the common characterization techniques such as photoluminescence (PL)0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 7 0 2 - 5

Y. Okada et al. / Journal of Crystal Growth 245 (2002) 212–218

and capacitance spectroscopy are particularly
useful in probing the local properties containing
an ensemble of QDs, the scanning probe microscope (SPM) technique becomes more advantageous, if the properties of individual QDs are to be
exploited with nm-scale size resolution [6–8].
Meanwhile, we have previously reported that
InxGa1–xAs QDs self-organized on high-index
GaAs (3 1 1)B substrates exhibit a unique structural and optical characteristics compared to themore commonly studied InAs QDs grown on(0 0 1) substrates [9,10]. The QDs growth mode on
GaAs (3 1 1)B surface has been shown to befundamentally different from the well-established
Stranski–Krastanov (S–K) growth mode, and infact, a complex phase separation and strain-relief
mechanism are responsible for the formation ofsuch high-density and well-ordered QDs arrays on(3 1 1)B surface. However, the electronic properties
of individual QDs as well as of QDs surface itself
have not been fully understood at present. Our aim
is to investigate the surface properties of such aunique ordered InxGa1–xAs QDs array fabricatedon GaAs (3 1 1)B substrate by using conductive
atomic force microscope (AFM) technique.
A conductive AFM tip touches the substratesurface with an nm-scale contact area, typically
5–10 nm in diameter. Though the tip/surface
contact area has not been determined exactly, it
can be estimated with a reasonable accuracy byassuming a deformable metallic sphere representing the tip in contact with a deformable substratesurface [11]. The use of conductive AFM tip allows
us to modify the local band proﬁle of a given QD
structure with external applied bias, whose size
varies typically in the range of 20–50 nm indiameter. Consequently, the current–voltage (I–V ) characteristics of QDs of varying sizes, and of
any other arbitrary positions on the surface can bestudied with conductive AFM tips. Furthermore,
the electronic properties such as artiﬁcial atom-likeenergy states and shell ﬁlling of QDs with single
electrons can be investigated with this type oftechnique [5,12]. Our previous study reported inRef. [13] was conducted in an uncontrolled
ambient in air. Thus, the obtained results weresensitive to the effect of surface oxidation due tothe presence of an adsorbed thin layer of water213

and hence lacked in reproducibility. As a continuing effort, we have employed an UHV–AFM andall the samples were pre-heated in vacuum toremove the water layers prior to the measurement.

2. Experiments
Self-organized QDs were fabricated by depositing B12.7 monolayers of undoped In0.4Ga0.6As onn-type GaAs (3 1 1)B substrate (doped to1  1018 cm–3) by atomic H-assisted molecularbeam epitaxy (H-MBE), as reported in detailelsewhere [9,10]. To be particularly noted isthe unique self-organization characteristics of
In0.4Ga0.6As QDs grown on GaAs (3 1 1)B substrate, and that the QDs size and density can be
modulated simply by controlling the deposition
temperature, while retaining the structural ordering of QDs array. Self-organized QDs studied inthis work were fabricated at 5201C, and the QDs
density and average size were B3.3  1010 cm–2
and 50 nm, respectively. The growth temperature
was higher than that used in Ref. [13], for which
the QDs were fabricated at 5001C resulting in ashorter inter-dot spacing. The hydrogen backpressure was kept constant at B6  10–6 Torrduring substrate cleaning and H-MBE growth.
Si tips coated with Au, which warrants electricalconductivity of the tip, were used to measure boththe topographic and current images of QDs
surface. The I–V characteristics of QDs of varying
sizes and of any other arbitrary positions on the
QDs surface were also studied by using the same
conductive tip. Though the tip radius of an Aucoated tip was larger than that of an uncoated tip,
this did not affect 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. During the SPM
image and I–V curve acquisition, a constantcontact force was applied via an electronics
feedback control, and the measurements wereconducted in contact mode in vacuum either atroom temperature, or cooled temperature at
173 K. The samples were transferred from theMBE chamber to our SPM system in dry nitrogen214

Y. Okada et al. / Journal of Crystal Growth 245 (2002) 212–218

ambient, which was then pumped down with aturbo-molecular pump to a base pressure ofoB5  10–7 Torr.
In this work, the following fundamental issues
were investigated; (1) effect of thermal treatmentof the sample on the subsequent SPM measurements, and (2) effect of structural ordering of
InxGa1–xAs QDs on GaAs (3 1 1)B surface on local
conductance measurements. These studies arethought to be essential, if the electronic properties
such as artiﬁcial atom-like energy states and shellﬁlling of QDs with single electrons are to bereliably investigated.

3. Results and discussion
First, it is known that applying a positive bias
above threshold to a semiconductor surface withrespect to SPM tip would lead to oxidation of thesurface in the vicinity of tip apex due to thepresence of a thin surface water layer. In fact, SPM
tip-induced oxidation has been studied in detail inboth Si [14,15], and GaAs [16–19]. If the conductance measurements are to be performed in air,
one therefore needs to take into consideration of
the upper limit of bias permissible for reliablemeasurements, of around o4 V in our case [13],thereby avoiding undesirable oxidation of the QDs
surface. Furthermore, the surface water ﬁlm isknown to result in a strong adhesive force anddefocusing of applied electric ﬁeld between the tip
and sample [15]. For this reason, each QDs sample
was thermally treated in vacuum at 701C for 1 h
before proceeding to SPM characterization in
vacuum. In order to prevent the tip from crashinginto the sample during heating due to thermal
expansion, the tip was placed close to, but not in a
direct contact to the heated sample. Fig. 1 shows
the traces of force curves measured in a highvacuum at room temperature (a) before, and (b)after the thermal treatment. The horizontal axis drepresents the distance between the sample and tip.
A minus d means that the tip is pushed against the
sample. The vertical axis represents the atomic
force generated between the sample and tip. Hencea positive or negative force means a repulsive oradhesive force. It was observed that before thermal
treatment, the maximum adhesive force between
the tip and sample under our scanning condition
was B16 nN, which was then reduced by a factorof 5 after thermal treatment at 701C for 1 h. Thus,
this moderate thermal treatment is useful inremoving the surface water layer efﬁciently fromthe QDs surface in vacuum and this condition wasused throughout this work. A more rigorous heat
treatment of 2001C for 1 h was recently reported
by Ono et al. [20].
Next, the topographic and current images, andthe respective line scan images of In0.4Ga0.6As
QDs on GaAs (3 1 1)B substrate measured at room
temperature are shown in Fig. 2(a) and (b). BothFig. 1. Traces of force curves measured in high vacuum at room temperature (a) before, and (b) after the thermal treatment at 701C for
1 h. The horizontal axis d represents the distance between the sample and tip. A minus d means that the tip is pushed against the
sample. The vertical axis represents the atomic force generated between the sample and tip. Hence a positive or negative force means arepulsive or adhesive force. The maximum adhesive force before the thermal treatment between Au-coated Si AFM tip and QD was
reduced by a factor of 5 after the treatment.

Y. Okada et al. / Journal of Crystal Growth 245 (2002) 212–218

215

Fig. 2. (a) Topographic, and (b) current image of In0.4Ga0.6As QDs grown on GaAs (3 1 1)B substrate measured in vacuum at room
temperature after thermal treatment. Respective cross-sectional images along solid lines are also shown. Both images were obtained
simultaneously at a sample bias of V ¼ 1 V and the scan size was 1.0 mm  1.0 mm.

images were obtained simultaneously at a sample
bias of V ¼ 1 V and the scan size was 1.0 mm 
1.0 mm. It can be observed that the topographic
QDs size and height are relatively uniform andaverage size is B50 nm. The QDs size was alsomeasured in a separate experiment by operating
the AFM in tapping mode which would provide an
improved lateral resolution, and we have conﬁrmed the observed QDs sizes in contact modeand tapping mode were identical. A partial
structural ordering is also evident from Fig. 2(a).
Also shown in Fig. 2(b), the local surface potential
is dependent on the position along the top of QDs
as well as on the position on the surface. Near thecenter of a QD was more conductive than its
periphery, and if the tip was placed in between the
QDs, the surface was found to be highly resistive.
These results are much similar to, and in fact
have reproduced the observations reported for
self-assembled InAs QDs grown on GaAs (0 0 1)
by Tanaka et al. [7], in which the differences in the

conductance were attributed to the local modiﬁcation of surface bending associated with the surface
states in InAs QDs and wetting layer. Though thegrowth mechanism of InAs QD on GaAs (0 0 1) by
S–K mode differs from that of InxGa1–xAs QD on
GaAs (3 1 1)B as discussed in the previous section,
the fundamental features that are common to andresponsible for the obtained results in both cases
could be summarized as follows; (1) The area with
no QDs is expected to be similar to that of bulk
GaAs surface. On a bare GaAs surface, Fermi
level would be pinned strongly at the mid bandgap
energy by the negatively charged surface states,
resulting in the formation of a surface depletion
layer and hence suppression of current ﬂow or
conductance. (2) An electron accumulation occurs
on the surface of InAs due to the positively
charged surface states [21], which consequently
leads to the lowering of Schottky barrier height.
(3) The effect of surface potential lowering would
be larger at the center of QD than its periphery.

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Y. Okada et al. / Journal of Crystal Growth 245 (2002) 212–218

This would in turn suggest that the effect of
surface potential lowering would be larger for alarge QD than a small QD [7]. Our present results
as shown in Fig. 2 should be carefully comparedwith those previously reported in Ref. [13]. The
QDs sample studied in Ref. [13] was fabricated at alower growth temperature than that in Fig. 2, forwhich the QDs were smaller in average size ofB40 nm with a small size ﬂuctuation, and almost
twice in density B6  1010 cm–2. In such a closely
packed QDs array surface on GaAs (3 1 1)B, theSchottky barriers were small and the local surface
potential was thus practically uniform at allmeasured points over the whole (3 1 1)B surface
at room temperature as of Fig. 5 in Ref. [13].
Next, the topographic and current images, andthe respective line scan images of In0.4Ga0.6As
QDs on GaAs (3 1 1)B substrate measured at alower temperature of 173 K are shown in Fig. 3(a)and (b). Here the QDs substrate was the same asthat of Fig. 2, but an area with closely spaced QDs

was deliberately chosen. By lowering the temperature to 173 K, the thermal current or Schottky
conduction would be reduced at least by a factorof B2 compared to room temperature. Bothimages were again obtained simultaneously at asample bias of V ¼ 1 V as before, but the scan size
was reduced from Fig. 2 and it was 0.5 mm 
0.5 mm in this case. It can be observed from theﬁgure that, (1) the general characteristics observedat room temperature and at 173 K are essentially
identical in the case of isolated QDs and at
positions in between neighboring QDs as seen in
Fig. 2. (2) However, for packed and ordered QDs,
which are, for example, indicated by a solid line inthe ﬁgure, the conductance and hence local surface
potential were observed to be spatially uniform at
all measured points over these QDs. Thus, forclosely packed and ordered QDs surface structure,neighboring QDs are thought to be laterally
coupled at low temperatures, which would thenlead to a higher electrical conductance than that ofFig. 3. (a) Topographic, and (b) current image of closely spaced In0.4Ga0.6As QDs grown on GaAs (3 1 1)B substrate measured in
vacuum at 173 K. Respective cross-sectional images along solid lines are also shown. Both were obtained simultaneously at a sample
bias of V ¼ 1 V and scan size was 0.5 mm  0.5 mm.

Y. Okada et al. / Journal of Crystal Growth 245 (2002) 212–218

isolated QDs [22]. This is further evidenced by the
local I–V measurements conducted at the respective points on QDs surface. The results are plottedin Fig. 4: on top of isolated In0.4Ga0.6As QD(marked 1), on top of packed QD (marked 2), andin between packed QDs (marked 3), respectively. It
can be seen that a closely packed pair of QDs has ahigher conductance and also a non-linear step-likecharacteristics than an isolated QD. A physicalmodeling of the observed non-linear I–V characteristics in view of coupling of QDs requires athorough analysis as reported previously [22], andthe modeling together with clariﬁcation of interdot coupling strength on dot separation iscurrently under study.

217

The coupled QDs are expected to become theimportant elements in future quantum computing
and QD memory applications [23–26]. In order fora strong coupling of neighboring QDs to occur,
clean and identical electronic barriers must exist in
between each QDs. Not only the size uniformity,
but also the structural ordering of QDs
becomes an important factor and to this regard,
we have convincingly shown that self-organized
In0.4Ga0.6As QDs on GaAs (3 1 1)B substrates
are promising candidate.

4. Summary
We have used conductive AFM technique toinvestigate the electronic state of InxGa1–xAs QDs
surface grown on GaAs (3 1 1)B substrate. In order
to ideally minimize the effect of adsorbed waterlayer on the surface, we ﬁrst demonstrated, in
terms of adhesive force acting between the tip andsample surface, that a moderate thermal treatment
of QDs sample at 701C for 1 h is very useful in
suppressing the effect of surface water layer. The
maximum adhesive force before thermal treatment
between the tip and sample under our scanning
condition was reduced by a factor of 5 after the
given treatment. Second, the center of QD wasfound to be more conductive than its periphery,
and the surface in between the QDs was measured
to be highly resistive. The differences in the
conductance were attributed to the local modiﬁcation of surface bending associated with the surface
states. Third, we showed that closely spaced andordered QDs, which are characteristic of thosefabricated on GaAs (3 1 1)B could be electronicallycoupled at low temperatures. A physical modelingof such a non-linear I–V characteristics in view ofcoupling state of QDs is currently under study,
however, we have shown that the properties of
QDs surface can be studied by this local probe
technique.

Fig. 4. I–V characteristics measured at respective points on
QDs surface: on top of isolated In0.4Ga0.6As QD (marked 1), on
top of packed QD (marked 2), and in between packed QDs(marked 3).

Acknowledgements
The authors would like to gratefully acknowledge
Profs. K. Yamabe, K. Akimoto, K. Murakami,

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Y. Okada et al. / Journal of Crystal Growth 245 (2002) 212–218

and T. Makimura of the University of Tsukuba
for their support. One of the authors (YO)was supported in part by TEPCO Research
Foundation.

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