Ge(Si)/Si(001) quantum dots produced by gas-source molecular beam epitaxy at 575 degreesC were investigated using energy-filtering transmission electron microscopy and x-ray energy dispersive spectrometry. Results show a nonuniform composition distribution in the quantum dots with the highest Ge content at the dot center. The average Ge content in the quantum dots is much higher than in the wetting layer. The quantum dot/substrate interface has been moved to the substrate side. A growth mechanism of the quantum dots is discussed based on the composition distribution and interfacial structures

Cockayne, D. J. H.

Jiang, Z. M.

Jin, G.

Liao, X. Z.

Wan, J.

Wang, K. L.

Zou, J.

Physical Review B

University of Queensland eSpace

PHYSICAL REVIEW B, VOLUME 65, 153306Alloying, elemental enrichment, and interdiffusion during the growth
of GeSiÕSi001 quantum dots
X. Z. Liao,1,2,* J. Zou,1 D. J. H. Cockayne,3 J. Wan,4 Z. M. Jiang,4,5 G. Jin,4 and Kang L. Wang4
1Australian Key Center for Microscopy & Microanalysis, The University of Sydney, Sydney NSW 2006, Australia
2Division of Materials Science and Technology, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
3Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, England
4Device Research Laboratory, Electrical Engineering Department, University of California at Los Angeles,
Los Angeles, California 90095-1594
5Surface Physics Laboratory, Fudan University, Shanghai 200433, China
~Received 2 October 2001; published 27 March 2002!
Ge~Si!/Si~001! quantum dots produced by gas-source molecular beam epitaxy at 575 °C were investigated
using energy-filtering transmission electron microscopy and x-ray energy dispersive spectrometry. Results
show a nonuniform composition distribution in the quantum dots with the highest Ge content at the dot center.
The average Ge content in the quantum dots is much higher than in the wetting layer. The quantum dot/
substrate interface has been moved to the substrate side. A growth mechanism of the quantum dots is discussed
based on the composition distribution and interfacial structures.
DOI: 10.1103/PhysRevB.65.153306 PACS number~s!: 68.65.Hb, 68.35.Dv, 68.37.LpSince the discovery that coherent ~dislocation-free! semi-
conductor quantum dot ~QD! islands can be formed through
Stranski-Krastanow ~SK! growth, in which a layer-by-layer
grown flat wetting layer is followed by island formation in
lattice-mismatched heteroepitaxial systems,1 the SK growth
QD islands have attracted considerable attention because of
their potential electronic and optoelectronic applications.2
The composition of QD’s has been the subject of intense
investigation3–6 because of its importance in understanding
the structure-property relationship of QD’s.7 However, there
has been relatively little investigation of the relationship be-
tween the nature of QD growth and the composition
distribution.8,9
Many transmission electron microscopy ~TEM! tech-
niques, including spectrum techniques6,10 and imaging
techniques,3,9,11 have been used for QD local composition
investigations. Electron-energy-filtering imaging ~EFI! in the
analytical TEM is of particular importance for the investiga-
tion of heteroepitaxial structures,9 because it can provide in-
formation not only about the elemental distribution at na-
nometer resolution,12 but also about the interfacial
morphology. In this paper, we report an EFI investigation of
the microstructure and chemistry of Ge~Si!/Si~001! QD’s and
discuss a possible growth mechanism that leads to the ob-
served results.
A Ge/Si~001! sample consisting of ten layers of Ge QD’s
separated by about 40 nm of Si spacer layers was grown
using gas-source molecular beam epitaxy ~MBE! with a
Si2H6 gas source and a Ge effusion cell at a temperature of
575 °C. The Ge deposition thickness at each layer was 1.6
nm with a growth rate of 0.4 nm/min. In this paper, we
concentrate only on the top unburied QD islands. Cross-
section TEM specimens were prepared using mechanical
thinning followed by Ar1-ion-beam thinning in a Gatan pre-
cision ion polishing system ~PIPS! with an accelerating en-
ergy of 3 keV. A cross-section TEM investigation was carried
out using a Philips CM120 operated at 120 kV equipped with
a Gatan imaging filter ~GIF! system and also using a VG0163-1829/2002/65~15!/153306~4!/$20.00 65 1533601B scanning transmission electron microscope operated at
100 kV equipped with an Oxford x-ray energy dispersive
spectroscopy ~EDX! system. Elemental mapping was per-
formed with the GIF using the three-window technique.13
The Si L2,3 edge at 99.2 eV in the electron energy loss spec-
trum was used for Si mapping, and the centers of two pre-
edge windows were set at 60 and 80 eV with a slit width of
20 eV. Ge maps were obtained using the Ge L2,3 edge at
1217 eV with the centers of two pre-edge windows set at
1117 and 1177 eV and a larger slit width of 60 eV to increase
the image intensity at higher energy loss so that focusing of
the images is possible.
Because the background removal procedure in the three-
window technique is unable to totally eliminate the intensity
changes caused by diffraction contrast variations that occur
between images acquired at different energy losses, leaving
artifacts in the final EFI,14,15 strong diffraction conditions
were avoided by orienting the specimen away from any main
zone axis, but keeping the QD/substrate interface aligned
with the electron beam.
To carry out quantitative elemental distribution investiga-
tion using EFI, it is important that the TEM specimen be
sufficiently thin for the electrons detected to be dominated by
single scattering.12 For elemental analysis using edges above
1000 eV, the specimen can be relatively thick because mul-
tiple scattering can be ignored if t/l,2,16 where t is the
specimen thickness and l is the mean free path for a plasmon
excitation. However, for edges below 1000 eV, t/l needs to
be smaller than 0.5.17 The specimen thickness can be mea-
sured from the electron energy loss spectrum using the rela-
tionship t/l5ln(Itotal /I0),18 where I total is the total spectrum
intensity and I0 is the integrated intensity of the zero-loss
peak.
Figure 1~a! shows a typical unfiltered TEM image of an
island, i.e. I total . Figure 1~b! shows the same island imaged
using zero-loss electrons, i.e. I0 . Figure 1~c! shows I total /I0 ,
and Fig. 1~d! shows the values of I total /I0 along the white©2002 The American Physical Society06-1
BRIEF REPORTS PHYSICAL REVIEW B 65 153306line in Fig. 1~c!. From Fig. 1~d! it is clear that (I total /I0) in
the Si substrate @the island/substrate interface is marked with
a white arrow and a dark arrow in Figs. 1~c! and 1~d!, re-
spectively# is below 1.40 and increases to about 1.46 above
the interface because of the change of chemical composition.
The value of 1.40 at the substrate gives t/l,0.34 in the
substrate. Using l5115 nm for Si at 100 keV,19 the substrate
thickness t539 nm. In contrast to the relatively smooth in-
tensity curve in the substrate area, (I total /I0) in the QD island
fluctuates, with the highest value of 1.46 near the QD/
substrate interface and lowest value of 1.30 at the edge of the
QD island, giving 0.26,t/l,0.38. Two possible reasons
may be responsible for the relatively large fluctuation of t/l:
~1! local thickness ~t! variations and/or ~2! local composition
variations that result in the change of l. Because we are not
sure if the local specimen thickness is a constant, a direct
comparison of local composition within the island using the
intensity of EFI images will be less reliable. To cancel the
intensity change in EFI images possibly induced by thick-
ness variations, the atomic ratio map technique12 is used, in
which the atomic ratio of two elements is related to the ratio
of their elemental map intensities by a k factor ~ratio of par-
tial ionization cross sections of the two elements!. However,
because the parameters ~recording time, beam convergence,
slit width, etc.! for acquiring the Ge and Si maps are differ-
ent, determining a k factor is very difficult. As a result, em-
FIG. 1. ~a! Unfiltered TEM image of a Ge~Si!/Si~001! QD island
giving a total electron intensity I total , ~b! filtered TEM image of the
same area obtained from zero-loss electron beam providing I0 , ~c!
result of (I total /I0) which can be related to specimen thickness at
local area, and ~d! value of (I total /I0) along the white line in ~c!. A
white arrow in ~c! and a dark arrow in ~d! mark the position of the
island/substrate interface in the sample.15330ploying this technique in this investigation provides only a
relative composition distribution.
Figure 2~a! is a typical EFI of the island showing a Ge
map where the brightness corresponds to the Ge content pro-
jected normal to the image. It is clear that the image intensity
~brightness! of the island is much higher than that of the
wetting layer, which appears as a fuzzy white line, implying
a much higher Ge concentration in the island than in the
wetting layer. Note that because of the relatively thick wet-
ting layer @see Fig. 2~b!#, the image intensity will not be
smeared out by a loss in the image resolution. Figure 2~b! is
a typical Si map of the island. A wetting layer of about 3 nm
thickness, as marked between a dark and a white line in Fig.
2~b!, with semitransparent contrast above the Si substrate is
seen in the Si map. The thickness and contrast seen in the Si
map all imply that the wetting layer is a GeSi alloy. Figure
2~c! shows the result of the Ge map in Fig. 2~a! divided by
the Si map in Fig. 2~b!. The intensity in Fig. 2~c! can be
directly related to the local atomic ratio of Ge and Si by a k
factor. The intensity profile along the dark line ~marked with
‘‘1’’! passing through the island in Fig. 2~c! is plotted in Fig.
3 ~also marked with ‘‘1’’!, showing that the highest intensity
is located at the middle of the island. The intensity profile
FIG. 2. ~Color! ~a! Ge elemental map of the island shown in Fig.
1 and ~b! Si map of the same island. A dark line and a white line are
drawn along the lower and upper boundaries of the semitransparent
wetting layer, respectively, to show the layer thickness. ~c! Result of
the Ge map divided by the Si map. ~d! Pseudocolor ~spectrum!
image showing the intensity ~brightness! distribution of ~c!. A dark
line is drawn passing through the wetting layer/substrate interface.6-2
BRIEF REPORTS PHYSICAL REVIEW B 65 153306along the white rectangle ~marked with ‘‘2’’! in a wetting
layer area in Fig. 2~c! is also plotted in Fig. 3 ~marked with
‘‘2’’ also! and the full width at half maximum of the wetting
layer peak conforms to a wetting layer thickness of about
3 nm.
To further confirm the results obtained from EFI, an EDX
measurement of Ge and Si was carried out along the dark
line in Fig. 2~c! and the result of the intensity ratio of the
Ge-K peak and Si-K peak is also presented in Fig. 3, dem-
onstrating a very similar relative composition concentration
distribution profile to the results obtained from EFI.
To see more clearly the intensity distribution and the in-
terfacial structure in Fig. 2~c!, a pseudocolor ~spectrum! im-
age of Fig. 2~c! is shown in Fig. 2~d! where the highest
intensity is presented in red color and the lowest intensity in
purple. A horizontal dark line is drawn in Fig. 2~d! along the
wetting layer/substrate interface. It is seen that Ge has dif-
fused down below the dark line and the area with the highest
intensity is above the dark line.
To explain the above experimental phenomena, we note
that Tersoff8 has suggested that when islands nucleate on a
strained alloy, segregation of the larger-mismatch component
to the islands occurs to reduce the nucleation barrier and,
because the optimum composition is the same at any island
size, the enrichment of the larger-misfit element to the is-
lands will continue during the island growth. This will result
in a compositionally depleted wetting layer if the island
growth continues after the incident flux is turned off and will
result in progressively reducing the larger-misfit component
in the outmost layers of the islands. Tersoff also commented8
FIG. 3. Intensity profile along the dark line drawn in Fig. 2~c!
(IGe /ISi) and EDX values along the same line. The left coordinate
is the EFI intensity ratio, and the right coordinate is the EDX in-
tensity ratio of Ge-K/Si-K . The intensity profile along the white
rectangle area in Fig. 2~c! is also presented, and the wetting layer
thickness is obtained from the full width at half maximum of the
wetting layer peak.15330that if the growth is limited by surface diffusion, then since
Ge diffuses much more quickly than Si, even greater Ge
enrichment will occur. Although in this work nominally pure
Ge is deposited on Si~001!, it is believed that, from the above
experimental evidence and previous reports,20,21 the forming
wetting layer is in fact a GeSi alloy due to the intermixing
with the substrate. Ge buildup by segregation on the surface
of this initial flat layer is considered the driving force for
islanding. However, while the mechanism suggested by
Tersoff8 can explain the observed results, the extent of seg-
regation reported here appears to exceed the values predicted
by his work.
The enrichment of Ge in the islands reduces the energetic
barrier of islanding. On the other hand, it increases the strain
energy between the island and substrate. This strain energy is
then reduced by further alloying of the island material with
the substrate, and this is evidenced by the interdiffusion of
Ge and Si at the island/substrate interface. The interdiffusion
results in the island/substrate interface moving down to the
substrate side, as seen in Fig. 2~d!, and the highest Ge con-
tent area in the islands, which is originally located at the
initial island/substrate interface, as predicted by Tersoff,8
moving up to the middle of the islands.
It is not surprising that this composition distribution is
different from our previous results on Ge~Si!/Si~001! grown
at 700 °C in which ~i! the island top has the highest Ge
content and the island bottom has the lowest Ge content10
and ~ii! a trench was observed around each island,22 but is
not seen in our current investigation. Chaparro et al.23 also
reported similar trenches and similar composition distribu-
tions to our previous reports. The explanation for these ob-
servations is that high temperatures result in increased el-
emental interdiffusion at the island/substrate interface as has
been evidenced in our previous report,22 and this results in
the high Ge content moving further up the island. The larger
elemental activation energies at high temperatures allows an
elemental redistribution so that the system can release strain
energy as much as possible.
In conclusion, the epitaxial growth of Ge~Si!/Si~001! QD
islands involves a complex series of processes including al-
loying of the deposited material with the substrate material,
enrichment of the larger-mismatch element ~Ge! into the is-
lands, elemental interdiffusion between the islands and sub-
strate, and elemental redistribution within the islands. Differ-
ent growth kinetics may result in totally different
composition distributions within the islands.
The authors thank the Australian Research Council for
financial support. This project is also partially supported by
ARO and the Semiconductor Research Corporation.*Electronic address: xzliao@lanl.gov
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Alloying, elemental enrichment, and interdiffusion during the growth of Ge(Si)/Si(001) quantum dots

Alloying, elemental enrichment, and interdiffusion during the growth of Ge(Si)/Si(001) quantum dots

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