SUPPLEMENTAL INFORMATION
for
Three-dimensional nanoscale study of Al segregation
and quantum dot formation in GaAs/AlGaAs core-shell
nanowires
L. Mancini1, Y. Fontana2, S. Conesa-Boj2, I. Blum1, F. Vurpillot1, L. Francaviglia2, E. RussoAverchi2, M. Heiss2, J. Arbiol3,4, A. Fontcuberta i Morral2, L. Rigutti1
1
Groupe de Physique des Matériaux, UMR CNRS 6634, University and INSA of Rouen, Normandie University,
76800 St. Etienne du Rouvray, France
2 Laboratoire des Matériaux Semiconducteurs, École Polytechnique Fédérale de Lausanne, 1015 Lausanne,
Switzerland
3
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, 08193 Bellaterra, CAT,
Spain
4
Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, CAT, Spain
1. Nanowire growth details
The nanowire core-shell structures were grown by the Ga-assisted method using a DCA P600
Molecular beam epitaxy machine. The GaAs nanowire core structures were obtained at a
temperature of 640 °C under a flux of Ga equivalent to a planar growth rate of 0.03 nm s −1 and a V/III
flux ratio of 60. After one hour of growth, the conditions were switched to radial growth by
increasing the As pressure to 2x10-5 mbar and reducing the substrate temperature down to 460C.
Multi shells with a thickness of 50 nm were grown by alternating AlxGa1−xAs (3) and GaAs (2). The Al
composition (correspondent to the AlAs alloy fraction in the main text) was x = 0.33. The last
AlxGa1−xAs layer was capped with 5 nm GaAs to prevent oxidation.
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3D atom probe analysis of Al segregation and quantum dot formation in GaAs/AlGaAs core-multishell nanowires
2. Sample preparation protocol
The core-shell nanowires were thus prepared for the experiment in the following way. (i) As a first
step, sharp tungsten (W) tips were prepared by electrochemical polishing in NaOH; subsequently, a
400 nm wide and 4-5 µm long cut was milled by focused ion beam (FIB) along their axis. (ii) Single
NWs were collected with the prepared W tips by micromanipulation under an optical microscope. As
shown in fig. 1-(d) of the main text, the cut in the W tip allowed for a good alignment of the NW
along the tip axis. This minimizes the stress related to the application of an intense electric field in
the atom probe, which can easily lead to sample failure at the first stages of analysis. (iii) The NWs
were glued to the W tip either by FIB-assisted Pt-C soldering or by conductive epoxy and (iv) the NW
tip was eventually sharpened by FIB annular milling at 30 keV followed by a cleaning step at 2kV. This
procedure yields a tip shape as shown in fig. 1-(d) of the main text and with a minimal amount of
implanted Ga limited to the first few nanometers from the tip surface.
3. Considerations on tip reconstruction in atom probe tomography
The analyzed sample NW1 tip is shown in the scanning electron microscopy (SEM) images of fig. [S1] before and after the APT analysis. The comparison of the tip shape before and after the analysis
allows for the best possible 3D reconstruction. This data acquisition took around 5h, with around 5
millions of atoms collected.
The tip was reconstructed with the so-called standard E-ϐ algorithm [1], with the following
parameters: curvature factor 1.00, projection point m+1=1.6, E-ϐ factor E-ϐ=21 V/Å and detector
efficiency =0.30. The best reconstruction was obtained by comparing the tip shapes before and
after APT analysis. It is worth noticing that a correct reconstruction depth was only retrieved by
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3D atom probe analysis of Al segregation and quantum dot formation in GaAs/AlGaAs core-multishell nanowires
assuming a detector efficiency equal to about one half of the theoretical efficiency, meaning that one
over two atoms from the analyzed volume are lost either as uncorrelated evaporated ions or as
neutrally evaporated/desorbed species. This mechanism has already been found in InGaN/GaN multiQW specimens, but should still be investigated in non-nitride III-V materials [2].
Notice that due to the fragility of the tips, often the APT analysis was performed until the tip flashed.
Thus, for several analyzed tips only the SEM image of the tip before APT is available. However, as all
tips were analyzed within a limited range of experimental parameters (detection rate, temperature
and laser power), the reconstruction parameters have been assumed as stable – with the exception
of the E-ϐ factor, which is function of the known tip radius before the analysis .
Figure [S-1]. SEM images of the field-emission tip NW1 (a) before and (b) after APT analysis. (c) shows the
superposition of a) and b), evidencing the evaporated part of the tip.
4. Analyzed tips, experimental conditions and main observations in Atom Probe
The detailed information about the analyzed tips in terms of APT experimental parameters is
reported in table S-I; It is worth noticing that only tips NW1 and NW3 could be analyzed without
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3D atom probe analysis of Al segregation and quantum dot formation in GaAs/AlGaAs core-multishell nanowires
being flashed. Tip NW3 was analyzed twice (run NW3-I and NW3-II) and eventually flashed during the
second run.
The tips were analyzed under green (λ=515 nm) fs laser pulses, keeping the laser energy per pulse in
the range 15-80 pJ (1 pJ corresponds to a peak intensity of around 0.22 mW/µm 2 during the 140 fs
pulse).
A typical mass histogram is reported in fig. [S-2]. The species occurring with highest frequency are
Al1+, Ga1+, and different atomic and molecular cluster As-containing species. Ga2+ is present in
concentrations under 1%, while events corresponding to unidentified ions and background noise
constitute about 1.8% of the counts.
Ga1+
Al1+
105
Counts
4
10
Al
2+
As
As1+
2+
As32+
As21+
As31+
As52+
3
10
102
101
0
20
40
60
80 100 150 200 250
Mass/Charge ratio (amu)
Figure [S-2].A typical mass spectrum issued from the atom probe analysis of a nanowire sample.
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3D atom probe analysis of Al segregation and quantum dot formation in GaAs/AlGaAs core-multishell nanowires
Table S-I: Details of analyzed samples and experimental conditions. All runs have been performed at
al laser wavelength λ=515 nm and at a constant detection rate =2 x 10-3 Ions/pulse.
Sample
Tip
NW1
Millions of
atoms
collected and
(State of the
tip after
analysis)
Depth (nm)
x Width
(nm) of
analysis
T (K)
Laser
pulse
energy
5.7
200x40
60
30
1 Al segregation area at the hexagon edge,
no evidence of QDs.
270x64
60
72
2 Al segregation areas at the hexagon edges
well defined. Fluctuation of Al concentration
near the base of the analyzed area.
145x40
50
30
2 Al segregation areas visible but rougher
than in sample A. No alloy fluctuations can
be related to the presence of QD.
342x47
40
30
Better defined Al seg. areas than in the
volume analyzed at 50K (C-I) but still rougher
than in sample A. One of the two
segregation areas is considerably rougher,
with strong AlAs alloy fluctuations, than the
other. Presence of a QD close ot the Al-rich
region on one of the hexagon corners.
188x52
40
15
2 Al segregation areas at the hexagon edges
are visible. Segregation outside the two
main areas may be due to artefacts. No QD
could be assessed.
Main observations
(pJ)
(Preserved)
NW2
16.7
(Flashed)
NW3-I
3.3
(Preserved)
NW3-II
8.5
(Flashed)
NW4
13.0
(Flashed)
5. Atom Probe analysis of sample NW1
The 3D reconstructed volume issued by the second APT analysis of sample NW1 is reported in fig.[S3]. The analyzed region was centered at the interface between the innermost AlGaAs shell and one
intermediate GaAs shell. The interface between the GaAs shell and an outer AlGaAs shell is also
visible, though very close to the limit of the field of view. As in sample NW3 shown in the main text,
Al segregation occurs on the planes crossing the hexagon vertices along the (101) planes.
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Figure [S-3].(a) 3D reconstruction of APT data issued from sample NW1 and (b) 2D AlAs alloy fraction maps
defined for the three 2 nm thick slices displayed in (a). An animated version of the 3D reconstructed volume
is also available as a supporting information video.
For sample NW1, the Al segregation along the planes crossing the hexagon vertices can be assessed
through the proximity histogram reported in fig. [S-4], which was calculated as follows: first, an
isoconcentration surface is defined by a threshold Al elemental concentration of 30%; then, the
elemental concentration is calculated as a function of the distance from this surface. The proxigram
shows that the Al concentration can become as high as 80% in the segregation volume, with an
interface defined over around 2 nm. These data are consistent with those relative to sample NW3
reported in the main text.
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140
Element fraction (at %)
60
x=0 in correspondence of the
isoconcentration surface with
30% Al threshold concentration
120
100
40
80
60
20
40
AlAs, GaAs fraction (%)
Al %
Ga %
As %
20
0
0
-10
-8
-6
-4
-2
0
2
Distance (nm)
Figure [S-4]. Proximity histogram showing the concentration profile across the iso-concentration surface defined
by the threshold elemental Al concentration of 30% (60% AlAs fraction) in sample NW1 : the region at x>0
corresponds to the Al segregation region on the plane connecting the vertices of the hexagons.
6. Analysis of alloy fluctuations and interface definition in the Al-rich segregation region
In the following, we report the results of the characterization of the Al-rich segregation region
defined along the planes connecting the internal and external vertices of the hexagon in an AlGaAs
shell. This analysis was motivated by previous scanning transmission electron microscopy (STEM)
analyses indicating that Ga-rich regions (i.e. quantum dots) could form at the external edge of the Alsegregation line, at the interface with the GaAs shell [3]. We could not perform an analogous
observation by APT, possibly because of the relatively small total volume analyzed. However, we
could ascertain that the interface of the Al segregation planes present a certain roughness and alloy
fluctuations, which could be related to the formation of the QDs observed in ref. [3].
Sample NW2
The reconstructed volume of sample NW2 allows for the identification of two hexagon edges, for
both of which the Al segregation area can be easily identified. In fig. [S-5], the 3D distribution of Al
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3D atom probe analysis of Al segregation and quantum dot formation in GaAs/AlGaAs core-multishell nanowires
atoms is presented. The isoconcentration surface, defined for a threshold Al elemental concentration
of 22%, envelops the high Al concentration region along the hexagon edge.
Figure [S-5]. APT analysis of sample NW2: (a) 3D reconstruction of the Al distribution (blue dots) in a side view
without and (b) with a superposed isoconcentration surface defined by the threshold of Al elemental
concentration equal to 22% (~44% AlAs alloy fraction)); (c,d) Same as (a,b) but top view.
In figure [S-6], we report a series of isosurface with progressively increasing Al threshold
concentration. From these isosurfaces, some points can be identified (e.g. the one marked by the red
arrow) where the spatial fluctuations of the Al concentration are more pronounced. For these
regions, we applied then a 2D concentration analysis on slices parallel and perpendicular to the NW
axis. The details of the distribution are reported in figs. [S-7] and [S-8]. Within this wire, we found no
evidence of QD formation. However, the spatial fluctuations we assessed could be related to QD
formation: it is possible that QDs only form where these fluctuations become more pronounced. This
could happen quite rarely along a single nanowire, as previous cathodoluminescence (CL) analysis
showed [3].
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Figure [S-6] Isoconcentration surfaces with progressively higher Al elemental concentration (i.e. half of the AlAs
alloy fraction) threshold indicating fluctuations in the spatial definition of the Al-rich region at the hexagon edge
in sample NW2. The red arrow indicates a feature potentially related with the mechanism of formation of QDs
at the corner of the shell hexagon.
Figure [S-7] Sample NW2: 2D distribution of Al within a slice parallel to the high concentration region along the
hexagon edge (a) with and (b) without a 21%-Al isosurface superposed. The red arrow indicates the same
location as in the previous figure. The color scale indicates the Al elemental concentration (approximately 1/2 of
the AlAs alloy fraction).
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Figure [S-8] Sample NW2: 2D distribution of the Al atoms in slices approximately perpendicular to the NW axis,
around the location indicated by the red arrow in this and in the previous images. No evidence of QD can be
assessed. The color scale indicated the Al elemental concentration ( ½ of the AlAs alloy fraction).
Note on the protrusion of the Al-rich region at the corner of the hexagon. From the 2D concentration
maps shown in fig. [S-7], it seems that the Al-rich region tends to protrude towards the surrounding
GaAs shell (but also, more moderately, towards the inner GaAs shell). This is a residual
reconstruction artifact known as “local magnification effect” and is due to the different evaporation
fields of the GaAs and of the AlGaAs phase [1] [4]. The Al richer regions may be slightly distorted and
look larger than the al-poorer regions. At present, there are no ways to completely eliminate this sort
of artifacts in complex 3D structures with abrupt phase interfaces; in our case, the effect was limited
by a careful choice of the experimental parameters and does not play any role in the assessment of
QD formation and interface roughness.
Sample NW3
The 3D analysis of the Al segregation regions imaged by APT in sample NW3-II is reported in fig. [S-9].
This sample exhibits similar structural features as the previously shown sample NW2. In the figure,
we show the position of the two boxes (5 nm thick) over which the Al elemental concentration was
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3D atom probe analysis of Al segregation and quantum dot formation in GaAs/AlGaAs core-multishell nanowires
mapped in the 2D color plots. Box 1 shows a more regular distribution than box2. In box 2, in
particular, it is possible to identify strong alloy fluctuations which correspond to Al-poorer regions
with a size of several nanometers. However, the AlAs fraction in these regions was still quite high,
around 15-20%, and therefore hardly compatible with a properly defined QD.
Figure [S-9] (a) 3D distribution of Al atoms in sample NW3, with the position of the two 5 nm thick boxes used
for the concentration mapping along the Al-rich region of the AlGaAs shell; (b) same as (a) but with 22% Al
isosurfaces superposed; (c) 2D Al elemental concentration extracted from box 1 and (d) extracted from box 2;
(e) zoom of (d) in a region exhibiting strong concentration fluctuations at the interface between the Al-rich
region of the AlGaAs shell and the GaAs surrounding shell.
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References
[1] F. Vurpillot, B. Gault, B. Geiser et D. J. Larson, «Reconstructing atom probe data: A review,»
Ultramicroscopy, vol. 132, pp. 19-30, 2013.
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Wide Bandgap Semiconductor Materials and Nanostructures Measured by Atom Probe
Tomography and Its Dependence on the Surface Electric Field,» Journal of Physical Chemistry C,
vol. 118, n° %141, pp. 24136-24151, 2014.
[3] M. Heiss, Y. Fontana, A. Gustafsson, ... et A. Fontcuberta i Morral, «Self-assembled quantum dots
in a nanowire system for photonics,» Nature Materials, 2012.
[4] F. Vurpillot, M. Gruber, G. Da Costa, I. Martin, L. Renaud et A. Bostel, «Pragmatic reconstruction
methods in atom probe tomography,» Ultramicroscopy, vol. 111, p. 1286, 2011.
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