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Nano Res
1
GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence
Jessica Bolinsson1,†, Martin Ek2, Johanna Trägårdh1,‡, Kilian Mergenthaler1, Daniel Jacobsson1,
Mats-Erik Pistol1, Lars Samuelson1, and Anders Gustafsson1 () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0414-2
http://www.thenanoresearch.com on January 13 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0414-2
1
GaAs/AlGaAs heterostructure nanowires studied by
cathodoluminescence
Jessica Bolinsson, Martin Ek, Johanna Trägårdh,
Kilian Mergenthaler, Daniel Jacobsson, Mats-Erik
Pistol, Lars Samuelson, and Anders Gustafsson*
Lund University, Sweden
Page Numbers. The font is
ArialMT 16 (automatically
inserted by the publisher)
Cathodoluminescence investigations show that the optical
quality of GaAs-based nanowires increases with growth
temperature. There is a link between emission at 1.48 eV and
twin planes, preventing near-bandgap emission from the
nanowires.
2
GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence
Jessica Bolinsson1,*, Martin Ek2, Johanna Trägårdh1,**, Kilian Mergenthaler1, Daniel Jacobsson1, Mats-Erik Pistol1, Lars Samuelson1, and Anders Gustafsson1 (). 1Solid State Physics and the Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden. 2Polymer and Materials Chemistry, Lund University, Box 124, SE-221 00 Lund, Sweden. *Present address:Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark. **Present address: Strathclyde Institute Of Pharmacy And Biomedical Sciences, University of Strathclyde,161 Cathedral Street
Glasgow G4 0RE, UK.
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT In this report we explore the structural and optical properties of GaAs/AlGaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodoluminescence (CL) in a scanning electron microscope (SEM).We perform a systematic investigation of how thenanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires.We investigated the total photon emission, as well as variations in the emission energy and intensity along the length of GaAs/AlGaAs nanowires grown at different temperatures. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy (TEM). In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in-situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.
KEYWORDS GaAs/AlGaAs core shell nanowires, Metalorganic vapour phase epitaxy(MOVPE), Cathodoluminescence, Twin
defects, Transmission electron microscopy
1. Introduction
III-V semiconductor nanowires have great promise
to become important building blocks in future
electronic [1-4] and optical devices [5-10]. In
particular, heterostructure nanowires are interesting
Nano Res DOI (automatically inserted by the publisher) Research Article
———————————— Address correspondence to [email protected]
3
for many optoelectronic applications such as light
emitting diodes [6, 9], photodetectors [5] and solar
cells [7]. In this work we focus on investigating the
optical properties of GaAs/AlGaAs heterostructure
nanowires. This is one of the most important III-V
semiconductor material combinations for
nanowire-based optoelectronic devices and band
gap engineering. The small lattice mismatch
between GaAs and AlAs (~0.12%) means that
GaAs/AlxGa1-xAs heterostructures have very little
strain, even when x is close to 1, while the
difference in band gaps between the two binary
materials is around 0.7 eV. The AlGaAs shell
increases the emission efficiency of GaAs nanowires
by reducing the non-radiative recombination
associated with surface states [11]. A serious
concern often mentioned in connection with III-V
nanowires is the quality of their crystal structure
with respect to the majoreffectimpactthat crystal
defects and irregularities can have on the their
optical, electrical and transport properties [12, 13].
For device applications it is of paramount
importance to understand how local variations in
the nanowire crystal structure, such as twin planes
and crystal phase mixing, influence their
functionality. The crystal structure of bulk GaAs is
zinc blende (ZB), while GaAs nanowires, like the
majority of III-V nanowires, are generally not
composed of a single defect-free crystal phase for
most growth conditions. In GaAs nanowires there
are often insertions of rotationally twinned
segments along the <111>B growth direction [14-16]
and the wurtzite (WZ) phase is sometimes found in
this type of nanowires [14, 17-20]. Recently it has
been shown that it is possible to tune the crystal
phase of GaAs nanowires as well as other III-V
nanowires by carefully choosing the right
parameters for the growth [14, 20]. For example, by
increasing the core growth temperature it is
possible to favour the formation of the WZ phase
for GaAs nanowires, and by decreasing the
temperature to favour the ZB phase [14, 21-23].
Other means of influencing the crystal structure of
nanowires during growth is by changing the molar
flows and V to III ratio of the group V and group III
source gases [15, 20, 24] as well as by doping [25].
Despite recent progress in controlling the crystal
structure of III-V nanowires, it is still by no means
straightforward to design the nanowire fabrication
process to obtain a 100% yield of GaAs nanowires
with a single crystal phase throughout their entire
length [26]. As mentioned above, growth
parameters such as temperature, molar flows of the
sources and the V/III ratio tend to influence the
crystal structure of the nanowires. However, these
parameters canalso have a significant effect on the
axial and radial growth rates and theincorporation
of impurities, for example carbon, during epitaxial
growth of GaAs using metalorganic vapour phase
epitaxy (MOVPE) [27]. In some cases, the growth
parameters that favour a particular crystal structure
can also result in significant radial overgrowth on
the side facets of the nanowires, leading to tapered
nanowires. Whether a materialislayer grows axially
or radiallymay have a significant influence on
thenanowireproperties in terms of impurityand
defect concentrations [28, 29] and, for ternaries or
higher order alloys, its composition [30-32].For
particle-seedednanowires in particular, where the
axial growthproceeds from the interaction with the
seed particle, it cannot be expected that the material
properties of theaxial and radial grown layers are
identical. As we show in this report, the optical
quality and the emission intensity of the nanowires
can differ significantly between the growth
conditions for radial and axial growth. This means
that it can be difficult to find parameters that fulfill
the requirements of excellent crystal quality, as well
as high material quality in terms of homogenous
composition, low density of impurities, and optical
quality.
In this work, we focus on GaAs/AlGaAs
heterostructure nanowires grown by metalorganic
vapour phase epitaxy (MOVPE), seeded by gold
aerosol-particles deposited onto GaAs substrates.
We explore how the optical properties of the
nanowires are affected by the growth temperature
of the GaAs nanowire core, radial growth, and
pre-growth annealing by carrying out low
temperature cathodoluminescence (CL)
measurements in a scanning electron microscope
(SEM). Using this type of measurement we study
the spatial variations in the photon emission energy
and intensity along the length of the nanowires.
4
Transmission electron microscopy (TEM) is used to
characterize the crystal structure of the nanowires.
We also use a combination of TEM and CL
measurements on the same individual nanowires to
directly correlate specific photon emission
characteristics with local variations in the crystal
structure within single nanowires.
2. Experimental details
2.1 Nanowire fabrication
The nanowires were seeded by gold
aerosol-particles and the growth was carried out by
low-pressure (100 mbar) MOVPE using hydrogen
as the carrier gas. Size-selected gold aerosol
nanoparticles with a diameter of 50 nm were
deposited onto epi-ready Si-doped (111)B-oriented
GaAs substrates [33]. The nanowire growth
procedure can be divided into four parts, which are
described in detail below: 1) in-situ pre-growth
annealing; 2) GaAs nanowire core growth; 3)
optional GaAs shell growth; and finally 4) capping
by growing an AlGaAs shell. Typical structures
used in this study are illustrated in Fig 1.
Figure 1 Schematic illustration of the different nanowire
structures that were studied in this work: (a) Uncapped GaAs
nanowire cores. (b) AlGaAs capped GaAs nanowires
(GaAs(c)/AlGaAs(s) nanowires). (c) GaAs nanowires capped
with a high temperature grown GaAs shell followed by an
AlGaAs capping (GaAs(c)/GaAs(s)/AlGaAs(s) nanowires).
Note that (b) and (c) illustrates that material also grows
(unintentionally) underneath the seed-particle while the
nanowire shells are being grown.
The in-situ pre-growth annealing step was typically
performed at 650°C for 10 min. in a background of
arsine (AsH3). In order to investigate the effect of
the temperature used during the annealing step,
samples were also prepared using annealing
temperatures of 580°C and 550°C.In this case the
core growth temperature was 480°C. After the
annealing step, the temperature was ramped down
to the nanowire growth temperature and the
particle-seeded nanowire growth was initiated by
turning on the group III source (i.e.
trimethylgallium).
A series of samples of GaAs nanowire cores were
grown at temperatures between 380°C and 500°C in
steps of 20°C. As the nanowire growth rate depends
strongly on the growth temperature, the nanowire
core growth time was different for the different
temperatures (ranging from 90 min. for 380°C down
to 12 min. for 500°C, giving approximate growth
rates from 30 to 250 nm / min.) to give nanowire
lengths in the order of a few(typically 4) μm. One
nanowire sample was also prepared by using a
two-temperature growth sequence for the core,
similar to that used in ref. [22]: First the nanowires
were nucleated and grown for 6 minutes at 480°C.
The growth was interrupted by turning off the
group III supply while ramping down the
temperature to 380°C. The nanowire growth was
re-initiated by turning on the group III source again
and was continued for 45 minutes.
In order to investigate the effect of side-facet
roughness of the GaAs core, wealso prepared GaAs
nanowires with smooth side-facets by growing a
thin GaAs “shell” on the GaAs nanowire cores
grown at 440°C (see schematics in Fig. 1(c)). Two
different temperatures for the GaAs nanowire shells
were used: 630°C and 690°C. All GaAs nanowires,
unless specifically noted, were capped with an
AlGaAs shell in order to form a GaAs/AlGaAs
core-shell structure (see Fig. 1(b)). The AlGaAs
shells were grown for 3 min. at a temperature of
630°C, resulting in an AlGaAs shell thickness of
about 20-30 nm. From monochromatic CL imaging
of thicker shell growth, we find that the shell has an
Al content of about 15-20%.
The four types of structures that we have studied
5
will be referred to as GaAs(c) for uncapped GaAs
nanowire cores; GaAs(c)/GaAs(s) for GaAs
nanowire cores capped with a GaAs shell;
GaAs(c)/AlGaAs(s) for GaAs nanowire cores
capped with an AlGaAs shell and finally
GaAs(c)/GaAs(s)/AlGaAs(s) for a GaAs nanowire
core capped with a GaAs shell followed by an
AlGaAs shell. Trimethylgallium (TMG),
trimethylaluminium (TMA) and AsH3 were used as
sources for gallium, aluminium and arsenic. Molar
fractions of 9 × 10-6, 2 × 10-6 and 9 × 10-4 for TMG,
TMA and AsH3 were used respectively, giving an
input V/III ratio of ~100 for the GaAs and ~80 for
the AlGaAs.
The nanowires that were studied in this work are
shown schematically in Fig. 1. In this figure we also
illustrate how growth occurs axially underneath the
seed-particle when forming the nanowire shell
during the high temperature growth steps. The top
part of the core-shell nanowires is grown axially
under the conditions employed for the nanowire
shell growth. Although not shown explicitly in this
figure, it should be noted that for particle-seeded
GaAs nanowires grown by MOVPE, radial
overgrowth takes place on the nanowire side facets
while growing the GaAs nanowire cores at the
lower temperatures (i.e. between 380 - 500°C). This
unintentional radial overgrowth increases with
nanowire growth temperature and leads to a
tapering of the nanowires.
2.2 Characterization of the nanowire crystal
structure and morphology
The morphology and crystal structure of the
nanowires were analysed by SEM and TEM. An FEI
Nova Nanolab 600 SEM was used for the SEM
imaging and the TEM imaging was performed
using a 300 kV JEOL 3000F. The TEM was used for
high resolution (HR), conventional dark field (DF)
and high-angle annular dark field (HAADF)
imaging. Samples for TEM and for CL/TEM
characterization were prepared by direct transfer of
the nanowires to a lacey-carbon Cu grid by gently
pressing the grid onto the substrate. The
high-resolution images were acquired in a <110>
projection.The stacking sequence visible in high
resolution images can be used to assign the crystal
structure of the segments to either WZ or one of the
two ZB twins. In overview images the segments are
visible as bands of different grey levels across the
nanowires.
2.3 Optical characterization
The optical properties were assessed by CL in a
dedicated set-up using an SEM (Cambridge
Instruments S250) equipped with a liquid-He
coldstage. Single and multiple nanowires were
studied at 6-8 K and the emission was detected
using a GaAs photomultiplier coupled to a
monochromator. Monochromatic CL and SEM
images were recorded simultaneously. The
nanowires were mainly studied as-grown in
side-view by cleaving the substrates and mounting
the fresh cleave facing the electron beam and the
detection mirror. This configuration gives
information of tens or hundreds of individual
nanowires in a single image. Typical CL images in
this mode are presented in Fig. 8. The nanowires
were also broken off and transferred to
non-emitting substrates, as in Fig. 6.As it is difficult
to control where the nanowires break, only a part of
the nanowire will be broken off. The broken off
nanowires will often appear shorter. It is therefore
preferable to study the nanowires as grown on the
substrate. Studying individual nanowires as grown
on the substrate ismade possible by the high spatial
resolution of the CL technique. More details of the
experimental set-up and the CL measurements can
be found in ref. [34].
Another importance of the substrate is that it can be
used as a reference for the intensity when
comparing the average emission from nanowires
from different growth runs. We typically scan an
area with 20-30 nanowires in side view and scan the
same size area on the side of the substrate, typically
100-200 μm away from the surface. This means that
the intensities from different growth runs can be
compared via the intensity relative to the substrate
emission without any ambiguity due to slight
differences in alignment of the detection system.The
nanowire and the reference spectra areboth
recorded under identical alignment conditions. We
have also compared many different substrates both
before and after growth, with no noticeable
6
difference in intensity and peak position.It is
reasonable that we observe no difference before or
after growth, since we detect emission from far
below the surface where the growth takes place,
and the emission is therefore unaffected by the
growth on the surface. The use of the substrate as a
reference for the intensity of the nanowire emission
is described in more detail in the supplementary
material.
2.4 Combined optical and crystal structure
characterization of single nanowires
In order to correlate features observed in CL
imaging with structural properties, we have
developed a scheme to perform CL and TEM
studies on the same nanowire. The nanowires are
transferred to a lacey-carbon TEM grid and then
analysed by CL. At the same time as the CL
measurements are carried out, a number of SEM
maps are recorded at various magnifications. These
SEM maps are used to locate the same nanowire in
the TEM, enabling us to correlate the CL
measurements and the crystal structure or other
features of an individual nanowire. The order of
these studies is essential, as the electron beam of the
TEM tends to damage the emission from the
nanowires [35].
3. Results
3.1 Influence of growth temperature on the crystal
structure of GaAs/AlGAs heterostructure
nanowires
The growth temperature for GaAs nanowire cores is
known to influence the crystal structure, as well as
the axial and radial growth rates [14, 15, 22, 36]. Fig.
2 (a) and (b) show TEM images of
GaAs(c)/AlGaAs(s) nanowires for which the GaAs
nanowire cores where grown at 380°C and 500°C
respectively. At 380°C the crystal structure is purely
ZB with some twin planes (around 10 per μm) and
as the temperature is increased the density of twin
planes increases (around 400 per μm at 500°C).
When a nanowire coregrowth temperature of 480°C
or higher was used, a few short WZ segments were
occasionally observed (less than 1 per μm and 5
bilayers or shorter), but not for all nanowires
investigated by TEM and not for the one shown in
Fig. 2(b). In Fig. 2(c) we show TEM images of a
GaAs(c)/AlGaAs(s) nanowire grown using the
two-temperature growth sequence for the core, as
described in section 2.1. It is evident from these
images that the first part of the nanowire, grown at
480°C, has a crystal structure with a high density of
twin planes, similar to the nanowire seen in Fig.
2(b). The second part of the nanowire, grown at
380°C, is ZB with the inclusion of a few short
twinned segments, similar to the nanowire seen in
Fig. 2(a). These results are in agreement with what
has been reported earlier for particle-seeded GaAs
nanowires in this temperature range [14, 15, 22, 36].
As mentioned above, the nanowire growth
temperature also influences the radial growth on
the nanowire side facets [37-39]. In the temperature
range we have used to grow the GaAs nanowire
cores, both the radial and the axial growth rates
increase with temperature, leading to more radial
growth of GaAs on the nanowire side facets with
increasing temperature. This can be seen in Fig. 2,
where the nanowire in (b) (grown at 500°C) clearly
has a much more tapered shape compared with the
nanowire seen in (a) (grown at 380°C).In the range
380 to 440°C, there is only slight tapering, whereas
the tapering increases significantly from 460 to
500°C. At 500°C, the diameter increases about twice
from tip to base. See supplementary material for an
analysis and discussion of the degree of tapering.
7
Figure 2 TEM images of GaAs(c)/AlGaAs(s) nanowires for which different temperatures were used while growing the GaAs
nanowire core.The right part of each image is a higher magnification image of the nanowires on the left. (a) GaAs nanowire cores
grown at 380°C and (b) at 500°C. Note the difference in crystal structure. At the lower temperature the crystal structure is ZB with
occasional twins and at the higher temperature the twin density is increased by a factor of around 40. (c) Nanowire grown using a
two-temperature growth sequence. The first part of the GaAs nanowire core was grown at 480°C and the second part was grown at
380°C. There is a clear change in the crystal structure along the length of the nanowire. The lower part in the TEM images show a
high density of twins and the nanowire crystal structure is comparable to the structure seen in the nanowire shown in (b). The upper
part shows an abrupt reduction in the density of twins and the crystal structure is similar to what is seen in the nanowire in
(a).Different twin orientations show up with different grey levels. Arrows indicate twin planes in the high magnification image of (a).
For the GaAs(c)/AlGaAs(s) and
GaAs(c)/GaAs(s)/AlGaAs(s) nanowires we find that
as the shells are formed, there is simultaneously
significant axial growth underneath the gold
particle [32, 40, 41]. From HRTEM
investigations(not shown here), we observe that the
axial segments that grow at high temperature
during the shell growth of the core-shell nanowires
usually have a crystal structure that is highly mixed
between ZB and WZ, and it includes many twin
planes – as expected from the high growth
temperature. In addition, smaller segments of
higher polytypes (4H and 6H) are observed in these
parts of the nanowires [26]. This is the case for both
GaAs and AlGaAs segments grown at high
temperature. We will refer to the intentional axial
growth as the core (core growth) and the intentional
radial growth as the shell (shell growth). The
unintentional growth will be referred to as high
temperature axial segments and radial overgrowth,
respectively.
3.2 Effect of GaAs nanowire growth temperature
on the photon emission
The emission of the GaAs nanowire cores was
studied as a function of nanowire core growth
temperature, with respect to intensity and peak
position.This part of the study was carried out on
nanowires with a single AlGaAs shell. Spectra of
the average emission from 20-25 nanowireson the
substrate in side view for each sample are shown in
Fig. 3(a). The main peak emission energy from the
nanowires is centered at about 1.48 eV for all
nanowires that were studied in this work. The peak
is rather broad, with a full width at half maximum
of ≈ 30 meV, when using a 20 meV spectral
resolution for the measurements.There are a
number of factors contributing to this line width.
There is a variation from nanowire to nanowire, as
well as along a single nanowire. These variations
8
are about 5-10 meV in both cases. Another
contributing factor is the high excitation density
inherent to the CL technique.The emission energy
expected for the free exciton of ZB GaAs (bulk) is
higher, at about 1.51-52 eV [28, 42]. We will address
the possible origins of the 1.48 eV emission in
section 3.4.
For the 380°C and 440°C nanowires, there is an
additional emission peak at1.53-1.55 eV. The
emission pattern is spotty in CL images at this
energy (not shown here).We assign these peaks to
areas in the AlGaAs shell with a locally higher Ga
content. The variations arecaused by Ga segregation
during the growth ofthe shell on the uneven side
facets. A careful CL/TEM investigation of the
emission shows a correlation between the intensity
and the local twin density, where we can expect
Ga-rich pocketscaused by the uneven side facets
due to the presence of the twins. The peak is
present in all samples grown at 380°C and 440°C.
However, it is absent from the samples with the
additional GaAs shell. With the additional shell, the
uneven side facets are smoothed, removing the
drive for the Ga segregation during the AlGaAs
growth.
Figure 3 (a) series of CL spectra measured at 8K with
increasing growth temperature for the GaAs core of
GaAs(c)/AlGaAs(s) nanowire structures. The spectra are
normalized and vertical offsets are used for clarity. For all
nanowires, the emission is centred at around 1.48eV and is
quite broad. For the 380°C and the 440°C cores, there is an
additional peak at around 1.55eV, unrelated to the core. (b) the
peak intensity as a function of growth temperature for the GaAs
nanowire core emission The intensity exhibits an exponential
increase with growth temperature. The intensity increases from
1/10 of the corresponding substrate intensity at 380°C to 3
times the substrate intensity at 500°C.
In order to compare the intensity of the emission
from the nanowires between different samples we
used the intensity of the emission from the cleaved
side of the substrate as a reference for each sample,
as described in section 2.3 and in the supplementary
material.Fig. 3(b) shows the intensity of the main
emission peak as a function of nanowire core
growth temperature.The samples have the same
density of nanowires.We have also chosen not to
correct fordifferences in the volume due to
tapering.As seen from this figure, there is a strong
correlation between the GaAs nanowire core
growth temperature and the emission intensity
from the nanowires. The emission intensity is
observed to increase exponentially by a factor of 30
as a function of the nanowire core growth
temperature.As discussed above, the tapering is
almost constant in the range 380 to 440°C and at the
highest temperature, the volume difference is less
than a factor of two.The observed increase in
intensity cannot be explained by the tapering alone.
To further investigate why the emission intensity is
so strongly influenced by the nanowire core growth
9
temperature, CL measurements were carried out on
nanowires fabricated using a two-temperature
growth sequence for the GaAs nanowire core (see
section 2.1 for the growth conditions and section 3.1
for the crystal structure). The first part of the GaAs
nanowire core was grown at 480°C (the lower part
of the nanowire) and the second part was grown at
380°C (the upper part of the nanowire), see Fig. 2(c).
From the measurements on the single-temperature
nanowire cores (see Fig. 3), we would expect to
observe a bright emission from the lower part of the
nanowires and a much weaker emission from their
upper part. However, these nanowires show very
weak emission intensity from both parts of the
GaAs nanowire core and no significant variation of
the emission intensity or energy could be detected
along the length of the nanowires. The whole GaAs
nanowire core is observed to have similar emission
intensity as when the GaAs nanowire core is grown
using the single growth temperature of 380°C, see
Fig. 4.
Figure 4 (a) Normalized CL spectra of three sets of nanowires
grown at different nanowire core temperatures: 480°C (top),
380°C (middle), and at 480°C and 380°C (bottom) using a
two-temperature growth sequence. Vertical offsets are used for
clarity. In (b) the same three spectra are plotted using the same
absolute intensity scale.
3.3 Influence of side facet morphology and radial
growth on the photon emission
When the crystal structure varies along the length
of the nanowires, the shape and orientation of the
side facets of the nanowires also change. In fact,
there is a strong correlation between the nanowire
crystal structure and its side facets [14]. As
discussed in the literature [43, 44], a highly twinned
ZB crystal structure results in side facets composed
of (111) micro-facets. A ZB GaAs nanowire with
twin planes tends to have a zigzag outer shape of
the nanowire core. This in turn leads to rough
interfaces between the GaAs and the AlGaAs
material. This can cause variations in the Al
concentration and the thickness of the AlGaAs shell
[45-47].
To investigate if, and how the presence of
micro-facets of the GaAs nanowires influences their
optical properties, we prepared GaAs nanowires
containing a high density of twin planes but with
flat side facets at the interface to the AlGaAs shell.
This was done by including a high temperature
growth step of GaAs after growing the GaAs
nanowire cores(grown at 440°C) to create a thin
GaAs shell.Two different temperaturesfor the GaAs
shell growth were investigated, 630°C and 690°C.
The GaAs shell fills the “side pockets” formed due
tothe presence of the micro-facets [45]. In Fig. 5(a)
and (b) two SEM images show the difference
between the side facet morphology of GaAs
nanowires prepared with (GaAs(c)/GaAs(s)), and
without(GaAs(c))the high temperature GaAs shell.
Fig. 5(c) and (d) illustrate the difference between the
GaAs-AlGaAs interface of these two types of
nanowires with an additional AlGaAs shell.
10
Figure 5 (a) SEM images of uncapped GaAs nanowires and (b) of GaAs capped GaAs nanowires. The insets show single nanowires
at higher magnification. Note the difference in side-facet morphology of uncapped and capped nanowire cores when the overall
nanowire crystal structure is ZB with a high density of twins. (c) and (d) illustrates how this difference give rise to significant
differences inaffectsthe appearance of the interface between the GaAs and the AlGaAs material when adding an AlGaAs shell onto
nanowire structures as those shown in (a) and (b) respectively.The SEM images are tilted 20° off the nanowire axis.
CL measurements show that adding the GaAs shell
to make the side facets smooth before adding the
AlGaAs shell does not improve the optical
properties of the GaAs nanowire cores.In fact, the
emission intensity from most parts of the nanowires
is slightly reduced compared with when growing
the AlGaAs shell directly on a micro-facetted GaAs
nanowire core.This is somewhat surprising
considering the increased volume of GaAs material
when including a GaAs shell in the nanowire
structures. However, in this case, a
high-temperature GaAs shell is grown on the
low-temperature GaAs radial overgrowth. This
low-quality layer with its non-radiative
recombination has a negative effect on the emission,
even after the capping.The intensity is reduced,
despite the larger volume of the GaAs resulting
from the additional GaAs shell.Two different
temperatures for the GaAs shell growth were
investigated, 630°C and 690°C.
Our CL measurements indicate that there are no
noticeable differences in the optical properties with
respect to the temperature used for the GaAs
nanowire shell growth. It is difficult to decouple
radial and axial growth for particle-seeded
nanowire growth. This means that as the GaAs shell
is grown on the GaAs nanowire core, growth also
takes place axially under the seed-particle.
Therefore, nanowires that have a high temperature
grown GaAs shell will also have a high-temperature
axial GaAs segment formed during the shell growth.
Fig. 6 shows an SEM image (a) and the
corresponding CL image (b) of a
GaAs(c)/GaAs(s)/AlGaAs(s) nanowire transferred
to a Si substrate. As seen from the CL image and the
line scan of the intensity shown in Fig. 6(c), the top
part of the nanowire has much stronger emission
intensity than the rest of the nanowire. This is the
segment of the GaAs nanowire core that was grown
at high temperature underneath the seed-particle
while the GaAs shell was formed. The strong
emission intensity agrees with the trend of
increasing emission intensity with nanowire core
growth temperature showed in the graph in Fig.
3(b).
As mentioned above, the nanowires with the extra
GaAs shells show no 1.55 eV emission. We associate
this emission with local variations in the Ga content
of the AlGaAs shell due to growth on the uneven
side facets. This indicates that smooth side facets
result in more homogeneous shell compositions.
This has a limited impact for a lattice matched
AlGaAs shell on GaAs, but becomes important for
lattice mismatched shells, where local variations in
the composition can lead to local variations in the
strain.
11
Figure 6 (a) SEM image of GaAs(c)/GaAs(s)/AlGaAs(s)
nanowires that have been transferred to a Si substrates. (b) CL
image of the same area as in (a). (c) Line scan of the CL
intensity along the length of one of the nanowires seen in the
SEM image in (a) as indicated by the dashed line. The emission
originates mainly from a segment at the top of the nanowire
structures. This upper part of the nanowires corresponds to
GaAs material being grown underneath the seed-particle during
the GaAs shell growth at 630°C.Growth direction left to right.
3.4 Influence of crystal structure on the photon
emission
As mentioned earlier, the nanowires that were
studied in this work have their main emission peak
at 1.48 eV. However, we find that one important
difference between the GaAs(c)/GaAs(s)/AlGaAs(s)
and the GaAs(c)/AlGaAs(s) nanowires is that the
former actually show weak emission at 1.52 eV in
the average spectrum of many nanowires. In order
to investigate the origin of the 1.48 eV emission and
why theweak emission at 1.52 eV isonly found in a
few nanowires in one particular type of samples, CL
measurements were carried out on single nanowires
that were subsequently characterized by TEM (see
section 2.4). Fig 7. shows two different nanowires
from the same sample. Fig. 7(a) shows an SEM
image of the upper part of a nanowire on a TEM
grid and (b) and (c) show two CL images of the
same nanowire, where (b) is recorded at 1.48 eV
and (c) at 1.52 eV. (d) shows the corresponding
DF-TEM image from the same part of the nanowire
using one of the ZB specific (002) reflections. In this
image one of the two twin orientations shows up
bright while the other one is dark. (e) shows line
traces of the intensities of (b - red) and (c - blue).
The nanowire shown in Fig. 7(a) to (d) has an
almost 100 nm-long twin-free ZB segment, formed
during the growth of the GaAs shell at 630°C (see
Fig. 1(c)). This twin free ZB segment clearly
overlaps with the region where the emission at 1.52
eV is the strongest. The 1.48 eV emission is
observed from the whole part of the
high-temperature axial GaAs segment. Fig. 7(f)
shows a TEM image of another nanowire from the
same sample without any extended ZB segment in
the high-temperature segment of the core. Fig. 7(g)
shows the spectra from the two nanowires. Both
nanowires show the 1.48 eV emission, but only the
one with the extended ZB segment (I) shows
significant 1.52 eV emission. Not all of the
nanowires from this sample showthe 1.52 eV
emission. For those with this emission, we observe a
correlation between the 1.52 eV intensity and the
presence and extension of twin-free ZB segments.
Segments up to 25 nm do not result in any 1.52 eV
emission, whereas segments longer than 35 nm do.
This indicates that the apparent minimum length of
twin-free ZB segments which gives noticeable 1.52
eV emission is somewhere between 25 and 35 nm.
Interestingly, this length scale is comparable with
the excition diameter in GaAs.All the nanowires
investigated by this combined method show a
varying degree of 1.52 eV emission and we observe
a correlation between the 1.52 eV intensity and the
presence and extension of twin-free ZB segments.
12
Figure 7 A combined CL and TEM study of two differentsingle
nanowires.(a-e) is from nanowire I and (f) is from nanowire II.
(a) shows an SEM image, and (b) and (c) monochromatic CL
images. (b) was recorded at 1.48 and (c) at 1.52 eV. Both
emissions originate in an unintentionally grown segment near
the top of the nanowire. (d) The corresponding (002) DF TEM
image of the same nanowire. The contrast reveals that there is
an about 100 nm long GaAs segment of pure ZB near the top.
(e) Intensities of the emission in (b red) and (c blue). (f)
different nanowire with no extended ZB segments. (g) Spectra
from the two nanowires above, I is from the nanowire in (d)
and II in (f).Comparison between the emission from the
nanowire in (d) (I) and in (f) (II).Growth direction left to right.
The spatialextensions of the 1.52 eV and the 1.48 eV
emissions observed in the CL images of Fig. 7 need
some explanation.It is important to recall that in CL
images, the photon emission is recorded as a
function of where the charge carriers were
generated (the position of the electron beam). Two
emission peaks originating in different spatial
features normally show complementary behaviour
in CL images, as long as the features are larger than
the spatial resolution. The spatial resolution can be
influenced by diffusion of carriers. In addition, the
diffusion can be directional, usually from areas of
high to low energy. In the present case that means
from the 1.52 eV to the 1.48 eV feature, but not in
the opposite direction. From our previous
measurements of diffusion of carriers in similar
nanowires [32], we would expect a diffusion length
of around 1 μm, which is longer than the entire
high-temperature segment. This should lead to 1.48
eV emission appearing to come from the entire
segment, and the 1.52 eV emission localized to the
extended twin-free ZB section. This is consistent
with what we observe in the images in Fig. 7.
3.5 Effect of pre-growth annealing temperature on
the photon emission
The effect of pre-growth annealing on the optical
properties of GaAs nanowires was investigated by
performing CL measurements on a set of nanowire
samples that involved three different in-situ
pre-growth annealing temperatures (550°C, 580°C
and 650°C) before initiating nanowire growth.The
core growth was carried out at 480°C. In Fig. 8 we
show CL images and intensity profiles along the
length of the nanowires recorded for the GaAs
nanowire core emission. The lower, middle and
upper line scans represent the intensity profiles for
nanowires annealed at 550°C, 580°C and 650°C,
respectively. The upper parts of the nanowires show
similar emission intensities for all three annealing
13
conditions. In contrast, the 1-2 μm closest to the
substrate is significantly brighter than the upper
parts for the highest annealing temperature and
much weaker than the upper parts for the lowest
annealing temperature. It is important to point out
that the nanowires appear to be identical in the
SEM images for all three annealing temperatures.
Similar observations of weak emission from the
lower part of the nanowire cores when using a
low-temperature annealing (580°C) has been
reported earlier from our laboratory [48]. As the
annealing temperature is increased, the optical
quality of the lower parts of the nanowires is
improved. At the highest annealing temperature
(650°C), they exhibit even stronger emission
intensity than the upper parts of the nanowires (see
Fig. 8(b)). This gradient reflects the larger diameter
of the lower part due to the tapering of the
nanowires. In addition, these nanowires also show
a slight decrease in the emission intensity at about
2μm away from the interface between the
nanowires and the substrate surface, as seen in the
upper line scan in Fig. 8(a) and the CL image in Fig.
8(b). The origin of this dip is currently unknown.
Though we show data for nanowires attached to
their substrates in Fig. 8, the same emission patterns
are recorded for nanowires transferred to Si
substrates. This rules out the effects of transfer of
excitation to/from the substrate.
Figure 8 (a) Line scans of the emission from
GaAs(c)/AlGaAs(s) nanowires when grown after applying
different pre-growth annealing temperatures: 550 °C, 580 °C
and 650°C (see section 1.1). Vertical offsets are used for clarity.
(b) and (c) are CL images of nanowire samples annealed at,
respectively, 650°C and 550°C. For an annealing temperature of
550 °C we observe a significantly weaker emission from the
first section of the nanowire structures.Growth direction left to
right.
4. Discussion
The most striking feature of this study is the
exponential increase in the emission intensity with
the nanowire core growth temperature. From 380°C
to 500°C there is 30-fold increase in the intensity of
the emission from the nanowires.There are several
potential sources for this effect. The 500°C
nanowires are more tapered than the ones at 380°C,
but the volume increase alone cannot explain the
30-fold increase in intensity.Especially since there is
a five times increase in the intensity in the range 380
to 440°C, where there is almost no increase in the
tapering. When making a line trace of the intensity
along the nanowires grown at 500°C (our most
14
tapered nanowires), the intensity increases by about
a factor of two from tip to base in this sample, so the
overall increase in intensity from 380 to 500°C due
to the increased volume at the base should be less
than a factor of two. A similar factor can be arrived
at when considering the overall volume increase
due to the tapering of the nanowire core grown at
500°C. The deterioration of the emission with
reduced nanowire core growth temperature can
therefore either be related to the core itself or to the
radial overgrowth. A key sample in determining the
cause of this deterioration is the nanowires grown
with the two-temperature nanowire core growth
sequence. The nanowires in this sample show an
even and low intensity, similar to the intensity from
the cores grown at 380°C. If the deterioration were
related to the nanowire core, we would expect to
see two sections with a ten-fold difference in
intensity between them. The fact that the entire
two-temperature nanowire core shows an even and
low intensity leads us to conclude that it is the
radial overgrowth rather than the quality of the
core that causes the reduced intensity. The growth
of the upper segmentat 380°C has even reduced the
intensity from the 480°C segment. Even at a
nanowire core growth temperature of 380°C, there
is some growth of material on the side facets of the
nanowires and this radial growth is likely to be
responsible for the reduction in the intensity of the
segment grown at 480°C.Radial growth on the
nanowire side facets can, to some extent, be
compared with conventional layer-by-layer growth
of GaAs by MOVPE. In general, GaAs shows the
highest material quality at significantly higher
growth temperatures (around 600 - 650°C), as
compared to particle-seeded GaAs nanowire
growth by MOVPE [27]. In the temperature regime
used for the nanowire core growth in this work, it is
therefore expected that the radial overgrowth
occurs at conditions that are far from optimum. The
GaAs that grows on the side facets at low
temperature is probably of low quality and might
contain significant amounts of impurities resulting
from incomplete decomposition of the metalorganic
precursor (i.e. TMG)[27], as well as containing other
types of defects, like vacancies and antisites. This is
likely to introduce non-radiative recombination
centres. Increasing the nanowire core growth
temperature is likely to improve the quality of this
radial layer, reducing the density of non-radiative
recombination centres. This could explain why the
emission intensity is improved with nanowire core
growth temperature, as the properties of the
radially overgrown GaAs material improves with
increasing growth temperature.
The optical and electrical properties of GaAs is very
sensitive to surface states and it is known that a
high-bandgapmaterial capping like AlGaAs of
GaAs nanowires leads to a significant enhancement
in the emission intensity [37]. In recent reports we
have shown that an AlGaAs capping not only leads
to a significantly increased brightness of the photon
emission, but also to an increased diffusion length
of the charge carriers within GaAs nanowire
structures [32, 49]. The low emission intensity and
the short diffusion length of uncapped GaAs
nanowires were attributed to the presence of
non-radiative surface states on the sidefacets of the
nanowires. In the present study, non-radiative
recombination (and thereby low emission intensity)
is most probably related to the poor material
quality of the radial overgrowth, as mentioned in
the previous paragraph.This would be the case for
both the low photon emission intensity from the
480°C segment of the GaAs nanowires grown using
a two-temperature core growth sequence and the
weak photon emission observed for the major part
of the GaAs(c)/GaAs(s)/AlGaAs(s) nanowires.In
contrast, the growth of a high temperature core on a
low temperature core shows a much higher
intensity from the high temperature part. This
isillustrated, for instance,in fig 6, though in this case
the temperatures were 440/630°C.
The next issue to address is the origin of the 1.48 eV
emission energy. As presented in section 3.2 and 3.4,
the main peak emission energy from the nanowires
is centred at about 1.48 eV for all nanowires that
were studied in this work, and not at the energy
related to the bandgap emission of GaAs (i.e.
1.51-1.52 eV [28]).The origin of the emission from
the nanowires could be related to impurities. For
MOVPE using metalorganics it is necessary to
consider whether unintentional carbon is
responsible. Carbon incorporation may occur from
the gallium source (TMG) due to incomplete
15
decomposition of the precursor molecules at the
relatively low temperature used for the nanowire
growth here [50].Although it is believed thatcarbon
incorporation in general is small for particle-seeded
nanowire growth, carbon impurities may
incorporate in the radially formed layers [28].
However, the carbon related emission is normally
reported to occur at a higher energy than what we
observe – at 1.494 eV [28, 50]. The emission below
the bandgap of ZB GaAs was identified by Zhang et
al. as the effect of carbon introduced into the GaAs
from the AlGaAs shell, as uncapped nanowires
show the expected bandgap emission [51]. Similar
observations were interpreted by Pusep et al. as
band bending from the core to the surface and
spatially indirect emission from the edges to the
centre of the GaAs core [52]. In addition to
impurities, another possible explanation for the 1.48
eV emission is that it is related to the crystal
structure of the nanowires. From the literature it is
known that the crystal structure of nanowires can
affect their optical properties (see for example[12, 13,
53-55] and references therein). There are reports of
both theoretical studies and experimental
measurements, which strongly suggest that when
III-V materials adopt different crystal structures this
give rises to detectable differences in their
bandgaps [12, 56]. In some cases like GaAs, a type II
band alignment between the ZB and the WZ crystal
phases forms [13, 57-60] and spatially indirect
emission can be observed at an energy below the
smaller of the two bandgaps. Spirkoska et al.
performed photoluminescence spectroscopy on
GaAs nanowires with crystal structures ranging
from pure ZB to what they refer to as “wurtzite-rich
zinc-blende/wurtzite heterostructures” and found
that the emission energy shifts from 1.515 eV down
to 1.43 eV as the percentage of the WZ phase and
the lengths of the segments increase [13]. Heiss et al
reports on measurements of confocal
micro-photoluminescence combined with TEM
measurements on individual GaAs nanowires [54].
When investigating polytypic GaAs nanowires with
a mixture of ZB and WZ segments they found an
emission down to 1.455 eV. For ZB nanowires with
varying densities of twins they observed emission
in the range 1.48 to 1.51 eV, where the emission
energy decreased with increasing twin density.
Novikov et al. report on PL measurements of GaAs
nanowires with emission peaks at 1.52 eV and 1.48
eV, where the later emission peak was found in
nanowires characterized as having predominantly
the WZ crystal phase [55]. Ihn et al. also report on
photon emission in GaAs nanowires at 1.48 eV [61]
and the nanowires are reported to include both WZ
and ZB crystal phases as well as twins. In mixed
WZ/ZB GaAs nanowires, Jahn et al. observed both
spatially indirect emission and carbon related
emission [62]. The conclusion from this literature
study is that it is therefore not straightforward to
identify the cause of the emission below the ZB
GaAs bandgap.
There are some important indications in the
literature regarding the conditions needed to
observe bandgap emission. Joyce et al. have carried
out studies on ZB GaAs nanowires where they
reduced the density of twins by using a
two-temperature nanowire growth process [22].
They have also explored the effect on the twin
density when changing the V to III ratio [15]. In
both these reports and other reports by the same
group, their PL data is mostly dominated by a peak
at around 1.515 eV. Plochocka et al. studied
individual ZB nanowires with a low density of
twins [63]. Using magnetic fields, they were able to
identify the near-bandgap emission as different
closely spaced excitonic emission lines (free and
bound). In a recent study, Ahtapov et al. have
studied pure WZ GaAs with a low density of
stacking faults (less than 10 per μm) [64]. The
emission is dominated by one peak that the authors
identify as excitonic emission from the WZ GaAs.
These publications show the importance of a clean
crystal structure with a low density of structural
defects to be able to observe excitonic emission
from GaAs cores.
Our measurements on the high-temperature axial
GaAs segments of the GaAs(c)/GaAs(s)/AlGaAs(s)
in the present study clearly indicate that when the
density of rotational twins is in the range of less
than 40 twins per μm in combination with a high
growth temperature it is possible to detect an
emission associated with the free exciton of ZB
GaAs. Nevertheless, it is often weak in comparison
to the 1.48 eV emission. Unlike most publications
16
showing an emission below the bandgap of ZB
GaAs, we observed only very few and short WZ
segments in some of the nanowires. We can
therefore conclude that the 1.48 eV emission is not
related to spatially indirect emission due to the type
II band alignment caused by ZB/WZ crystal phase
mixing. We do see a correlation between the 1.52 eV
emission and the presence of longer twin-free ZB
segments in the high-temperature axial segment of
the GaAs(c)/GaAs(s)/AlGaAs(s) nanowires. We also
observe a direct correlation between the length of
the twin-free ZB segments and the ratio between
the 1.52 eV and 1.48 eV peaks. The shortest segment
we observe the 1.52 eV emission from is about 35
nm (though very weak), and the longest segment
that does not result in any detectable 1.52 eV
emission is about 25nm. The length scale could be
of importance as the diameter of an exciton in bulk
GaAs is expected to be about 27 nm [65]. With a
shorter distance between the twins than this, we
would not expect to see any excitonic emission as
the exciton would be trapped by the twin and
recombine with the lower energy.
If the 1.48 eV emission were related to individual
twins, the increase in this twin-related emission
with core growth temperature could be related the
corresponding increase in twin density. We should
then also observe a corresponding reduction in a
competing emission. As we do not observe any
other emission, the competing recombination path
is most likely non-radiative. We do observe two
emisson paths from the samplewith the double
GaAs shell and then only from the
high-temperature axial GaAs segment. The length
of the pure ZB segment that we observe 1.52 eV
emission from in Fig. 7(d) is about 100 nm. This is
similar to the twin density we observe in the
samples grown at 380°C. Despite segments of
similar lengths, we do not observe any emission at
1.52 eV from these nanowires under normal
excitation conditions. However, there is one
significant difference in the emission from the
nanowires of the two growth runs. The emission
intensity is much higher (several orders of
magnitude) from the high-temperature axial
segment than from the low-temperature core. The
much higher intensity is an indication that there is
less competing non-radiative recombination. When
we increase the probe current to above 1 nA, we
saturate the non-radiative recombination enough to
observe the free exciton emission from the
nanowires with a core grown at 380°C. The
observation of free exciton emission from
nanowires with a significantly lower twin density
by Joyce et al. [15] supports our conclusion that the
main emission from our nanowires at 1.48 eV is
primarily related to the fact that the nanowires we
studied here have too high a twin density, where
the twins act as emission centres at 1.48 eV. This
emission can be related to an electronic level
introduced by the twin plane, or related to
impurities that getter at the twin planes. We are
currently unable to determine which.
We note that in an oversimplified picture, the twin
with its ...ABCABCBACBA... stacking could be
regarded as a very thin WZ quantum well, creating
a potential well for the holes, effectively acting as an
isoelectronic trap or an acceptor[60]. This model is
used in the literature to explain the emission
associated with stacking faults in WZ GaN with
reasonable agreement with the data [66]. A more
rigorous model however,would be required to find
the actual energies. A calculation using this model
can be found in the supplementary material.
For particle-seeded growth of nanowires, the
common practice is to anneal the particle coated
substrate in-situ before growth. This has several
benefits as it alloys the particle with the substrate,
ensures perfect contact between the substrate and
the nanowire and it also removes the native oxide
and remnants from the particle deposition and any
other processing. It is important to get this step
right in order to maximize the optical and electrical
properties of the nanowires. In our study of
nanowires grown with different annealing
temperatures, we conclude that a temperature of
650°C improves the optical properties of the lower
part of the nanowires, as compared to annealing at
lower temperatures (550°C and 580°C). We believe
that the lower temperatures lead to incomplete
removal of the native oxide on the substrate and/or
remnants from the particle deposition. These
residues can be dislodged from the surface and
incorporated into the nanowires during growth.
17
When these residues are incorporated into the
nanowires, they are likely to act as non-radiative
recombination centres, leading to a reduced
intensity in the first few micrometres of the
nanowires. The improved intensity along the length
is either due to a limited diffusion length of these
residues, preventing them from reaching further up,
or simply that the residues have been cleaned off
the surface. The reduced intensity from the lower
parts of the nanowires agrees well with our
conclusion that it is poor quality of the radial
overgrowth that reduces the emission intensity
from the nanowire cores that were grown at lower
temperature.
The last issue that we discuss relates to observations
made when studying the
GaAs(c)/GaAs(s)/AlGaAs(s) nanowires. Although
the GaAs shell resulted in a weak photon emission
from most parts of the nanowires, the top part of
these structures showed very strong emission. Even
though we focus on CL measurements in this study
we also carried out μPL measurements on several of
the different types of nanowires in this study. A
comparison of μPL spectra from single
GaAs(c)/AlGaAs(s) and GaAs(c)/GaAs(s)/AlGaAs(s)
nanowires shows an increase of the emission
intensity for the latter type of nanowires. However,
from the CL data we know that this apparent
improvement is only related to the top part of the
nanowire. This highlights the importance of
including a characterization method where it is
possible to determine not only the absolute
intensity but also the spatial origin and the spatial
variations of the emission. This is essential for
investigating complex heterostructure nanowires.
5. Conclusions
We have investigated the optical and structural
properties of Au-particle seeded GaAs/AlGaAs
heterostructure nanowires using CL, SEM and TEM
measurements. We found that the crystal structure
as well as the optical properties varies with GaAs
nanowire core growth temperature. The density of
rotational twins in the predominantly ZB nanowires
increases exponentially with GaAs nanowire core
growth temperature, from around 10 per μm for a
nanowire core growth temperature of 380°C to
around 400 per μm for a temperature of 500°C. In
the same temperature range the photon emission
intensity was found to increase by a factor of 30.
The main emission peak was centred at around 1.48
eV for all investigated nanowires. We only observe
emission related to the free exciton of GaAs from
high temperature grown axial GaAs segments in a
few of the nanowires which included an additional
GaAs shell (i.e. GaAs(c)/GaAs(s)/AlGaAs(s)
nanowires). From combined TEM and CL
measurements we observe a correlation between the
free exciton emission and the presence of longer
twin free ZB segments grown at high temperature.
Our results strongly suggest that when the GaAs
nanowires have a crystal structure with a density of
rotational twins of less than40 per μm (segments
longer than 25 nm) or higher, the photon emission
will be dominated by a twin related emission peak
at around 1.48 eV. This study also shows that radial
overgrowth on the nanowire side facets can have a
deteriorating effect on the optical quality of the
nanowires, in particular radial overgrowth at lower
growth temperatures. Finally, an important
conclusion from our investigation is that using a
high enough temperature during the pre-growth
annealing step before initiating nanowire growth is
necessary to ensure a high optical quality of the
nanowires, especially for their lower parts.
Acknowledgements
This work was supported by the Nanometer
Consortium at Lund University (nm@LU), the
Swedish Research Council (VR), the Swedish
Foundation for Strategic Research (SFF) and the
Knut and Alice Wallenberg foundation (KAW). The
growth was done using a MOVPE of Lund
Nanofabrication Lab. The authors would like to
thank Niklas Sköld for valuable inputs to the
discussions.
Electronic Supplementary Material:
Supplementary material (On tapering as a function
of growth temperature, WZ quantum wells in ZB
and how the CL measurements were performed) is
available in the online version of this article at
http://dx.doi.org/10.1007/***********************).
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18
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