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 anders.gustafsson@ftf.lth.se

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/***********************).

References

18

[1] Wernersson, L.-E.; Thelander, C.; Lind, E.; Samuelson, L. III-V nanowires - extending a narrowing road. Proceedings of the IEEE 2010,98, 2047-60.

[2] Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Gösele, U. et al. Nanowire-based one-dimensional electronics. Materials Today 2006,9, 28-35.

[3] Lu, W.; Xie, P.; Lieber, C. M. Nanowire transistor performance limits and applications. IEEE Transactions on Electron Devices 2008,55, 2859-76.

[4] Vandenbrouck, S.; Madjour, K.; Théron, D.; Dong, Y.; Li, Y.; Lieber, C. M.; Gaquiere, C. 12 GHz fmaxGaG/AlN/AlGaN nanowire MISFET. IEEE Electron Device Leters 2009,30, 322-4.

[5] Soci, C.; Zhang, A.; Bao, X. Y.; Kim, H.; Lo, Y.; Wang, D. Nanowire photodetectors. Journal of Nanoscience and Nanotechnology 2010,10, 1430-1449.

[6] Mi, Z.; Chang, Y. L. III-V compound semiconductor nanostructures on silicon: Epitaxial growth, properties, and applications in light emitting diodes and lasers. Journal of Nanophotonics 2009,3, 031602.

[7] Borgström, M. T.; Wallentin, J.; Heurlin, M.; Falt, S.; Wickert, P.; Leene, J.; Magnusson, M. H.; Deppert, K.; Samuelson, L. Nanowires with promise for photovoltaics. Selected Topics in Quantum Electronics, IEEE Journal of 2010,17, 1050-1061.

[8] Yan, R. X.; Gargas, D.; Yang, P. D. Nanowire photonics. Nat. Photonics 2009,3, 569-576.

[9] Tomioka, K.; Motohisa, J.; Hara, S.; Hiruma, K.; Fukui, T. GaAs/AlgaAs core multishell nanowire-based light-emitting diodes on Si. Nano Lett. 2010,10, 1639-1644.

[10] Yang, P. D.; Yan, R. X.; Fardy, M. Semiconductor nanowire: What's next? Nano Lett. 2010,10, 1529-1536.

[11] Tateno, K.; Gotoh, H.; Watanabe, Y. GaAs⁄AlGaAs nanowires capped with AlGaAs layers on GaAs(311)B substrates. Applied Physics Letters 2004,85, 1808-1810.

[12] Caroff, P.; Bolinsson, J.; Johansson, J. Crystal phases in III-V nanowires: From random toward engineered polytypism. IEEE Journal of Selected Topics in Quantum Electronics 2010,17, 829-846.

[13] Spirkoska, D.; Arbiol, J.; Gustafsson, A.; Conesa-Boj, S.; Glas, F.; Zardo, I.; Heigoldt, M.; Gass, M. H.; Bleloch, A. L.; Estrade, S. et al. Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures. Phys. Rev. B 2009,80, 245325.

[14] Dick, K. A.; Caroff, P.; Bolinsson, J.; Messing, M. E.; Johansson, J.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Control of III-V nanowire crystal structure by growth parameter tuning. Semicond. Sci. Technol. 2010,25, 024009.

[15] Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Fickenscher, M. A.; Perera, S.; Hoang, T. B.; Smith, L. M.; Jackson, H. E. et al. High purity gaas nanowires free of planar defects: Growth and characterization. Adv. Funct. Mater. 2008,18, 3794-3800.

[16] Bolinsson, J.; Ouattara, L.; Hofer, W. A.; Sk; amp; ld, N.; Lundgren, E.; Gustafsson, A.; Mikkelsen, A. Direct observation of atomic scale surface relaxation in ortho twin

structures inGaAs by XSTM. Journal of Physics: Condensed Matter 2009,21, 055404.

[17] Dheeraj, D. L.; Patriarche, G.; Zhou, H. L.; Hoang, T. B.; Moses, A. F.; Gronsberg, S.; van Helvoort, A. T. J.; Fimland, B. O.; Weman, H. Growth and characterization of wurtzite GaAs nanowires with defect-free zinc blende GaAsSb inserts. Nano Lett. 2008,8, 4459-4463.

[18] Persson, A. I.; Larsson, M. W.; Stenstrom, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Solid-phase diffusion mechanism for GaAsnanowire growth. Nat. Mater. 2004,3, 677-681.

[19] Plante, M. C.; LaPierre, R. R. Control of GaAs nanowire morphology and crystal structure. Nanotechnology 2008,19, 495603.

[20] Joyce, H. J.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C. Phase perfection in zinc blende and wurtzite III-V nanowires using basic growth parameters. Nano Lett. 2010,10, 908-915.

[21] Hiruma, K.; Yazawa, M.; Haraguchi, K.; Ogawa, K.; Katsuyama, T.; Koguchi, M.; Kakibayashi, H. GaAs freestanding quantum-size wires. J. Appl. Phys. 1993,74, 3162-3171.

[22] Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Zhang, X.; Guo, Y. N.; Zou, J. Twin-free uniform epitaxial GaAsnanowires grown by a two-temperature process. Nano Lett. 2007,7, 921-926.

[23] Tchernycheva, M.; Harmand, J. C.; Patriarche, G.; Travers, L.; Cirlin, G. E. Temperature conditions for GaAs nanowire formation by Au-assisted molecular beam epitaxy. Nanotechnology 2006,17, 4025-30.

[24] Dick, K. A.; Bolinsson, J.; Messing, M. E.; Lehmann, S.; Johansson, J.; Caroff, P.Parameter space mapping of InAs nanowire crystal structure.Journal of Vaccum Science and Technology B 2011,29, 04D103.

[25] Algra, R. E.; Verheijen, M. A.; Borgstrom, M. T.; Feiner, L. F.; Immink, G.; van Enckevort, W. J.; Vlieg, E.; Bakkers, E. P. Twinning superlattices in indium phosphide nanowires. Nature 2008,456, 369-72.

[26] Johansson, J.; Bolinsson, J.; Ek, M.; Caroff, P.; Dick, K. A. Combinatorial approaches to understanding polytypism in III-V nanowires. ACS Nano 2010,6, 6142-6149.

[27] Stringfellow, G. B. Organometallic vapor-phase epitaxy: Theory and practice. Academic Press: 1999.

[28] Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Fickenscher, M. A.; Perera, S.; Hoang, T. B.; Smith, L. M.; Jackson, H. E. et al. Unexpected benefits of rapid growth rate for III-V nanowires. Nano Lett. 2009,9, 695-701.

[29] Thelander, C.; Dick, K. A.; Borgstrom, M. T.; Froberg, L. E.; Caroff, P.; Nilsson, H. A.; Samuelson, L. The electrical and structural properties of n-type InAs nanowires grown from metal-organic precursors. Nanotechnology 2010,21, 205703.

[30] Chen; Shehata, S.; Fradin, C.; LaPierre, R.; Couteau, C.; Weihs, G. Self-directed growth of AlGaAs core−shell nanowires for visible light applications. Nano Lett. 2007,7, 2584-2589.

[31] Lim, S. K.; Tambe, M. J.; Brewster, M. M.; Gradecak, S. Controlled growth of ternary alloy nanowires using metalorganic chemical vapor deposition. Nano Lett. 2008,8,

19

1386-1392.

[32] Bolinsson, J.; Mergenthaler, K.; Samuelson, L.; Gustafsson, A. Diffusion length measurements in axial and radial heterostructured nanowires using cathodoluminescence. Journal of Crystal Growth 2011,315, 138-142.

[33] Magnusson, M. H.; Deppert, K.; Malm, J.-O.; Bovin, J.-O.; Samuelson, L. Gold nanoparticles: Production, reshaping, and thermal charging. Journal of Nanoparticle Research 1999,1, 243-251.

[34] Gustafsson, A.; Pistol, M.-E.; Montelius, L.; Samuelson, L. Local probe techniques for luminescence studies of low-dimensional semiconductor structures. J. Appl. Phys. 1998,84, 1715-1775.

[35] Todorovic, J.; Moses, A. F.; Karlberg, T.; Olk, P.; Dheeraj, D. L.; Fimland, B. O.; Weman, H.; van Helvoort, A. T. Correlated micro-photoluminescence and electron microscopy studies of the same individual heterostructured semiconductor nanowires. Nanotechnology 2011,22, 325707.

[36] Lehmann, S.; Jacobsson, D.; Deppert, K.; Dick, K. A. High crystal quality wurtzite-zinc blende heterostructures in metal-organic vapor phase epitaxy-grown gaas nanowires. Nano Research 2012,5, 470-476.

[37] Sköld, N.; Karlsson, L. S.; Larsson, M. W.; Pistol, M.-E.; Seifert, W.; Trägårdh, J.; Samuelson, L. Growth and optical properties of strained GaAs−GaxIn1-xP core−shell nanowires. Nano Lett. 2005,5, 1943-1947.

[38] Verheijen, M. A.; Immink, G.; Smet, T. d.; Borgström, M. T.; Bakkers, E. P. A. M. Growth kinetics of heterostructured GaP-GaAs nanowires. journal of American Chemica Society 2006,128, 1353-1359.

[39] Joyce, H. J.; Kim, Y.; Gao, Q.; Tan, H. H.; Jagadish, C. Growth, structural and optical properties of GaAs/AlGaAscore/shell nanowires with and without quantum well shells. In ICONN 2006, 2006.

[40] Gustafsson, A.; Sköld, N.; Seifert, W.; Samuelson, L. Cathodoluminescence studies of AlGaAs/GaAscore-shell nanowires. In Microscopy of Semiconducting Materials XIV, Oxford, UK, 2005, pp 463-466.

[41] Borgström, M. T.; Wallentin, J.; Trägårdh, J.; Ramvall, P.; Ek, M.; Wallenberg, L. R.; Samuelson, L.; Deppert, K. In situ etching for total control over axial and radial nanowire growth. Nano Research 2010,3, 264-270.

[42] Swaminathan, V.; Haren, D. L. V.; Zilko, J. L.; Lu, P. Y.; Schumaker, N. E. Characterization of GaAs films grown by metalorganic chemical vapor deposition. J. Appl. Phys. 1985,57, 5349-5353.

[43] Zou, J.; Paladugu, M.; Wang, H.; Auchterlonie, G. J.; Guo, Y. N.; Kim, Y.; Gao, Q.; Joyce, H. J.; Tan, H. H.; Jagadish, C. Growth mechanism of truncated triangular III-V nanowires. Small 2007,3, 389-93.

[44] Johansson, J.; Karlsson, L. S.; Svensson, C. P.; Martensson, T.; Wacaser, B. A.; Deppert, K.; Samuelson, L.; Seifert, W. Structural properties of <111>B -oriented III-V nanowires. Nat Mater 2006,5, 574-80.

[45] Ouattara, L.; Mikkelsen, A.; Sköld, N.; Eriksson, J.; Knaapen, T.; Cavar, E.; Seifert, W.; Samuelson, L.; Lundgren, E. GaAs/AlGaAs nanowire heterostructures studied by scanning tunneling microscopy. Nano Lett.

2007,7, 2859-2864.

[46] Biasiol, G.; Gustafsson, A.; Leifer, K.; Kapon, E. Mechanisms of self-ordering in nonplanar epitaxy of semiconductor nanostructures. Phys. Rev. B 2002,65,205306.

[47] Biasiol, G.; Reinhardt, F.; Gustafsson, A.; Martinet, E.; Kapon, E. Structure and formation mechanisms of algaas V-groove vertical quantum wells grown by low pressure organometallic chemical vapor deposition. Applied Physics Letters 1996,69, 2710-2712.

[48] Gustafsson, A.; Sköld, N.; Bolinsson, J.; Trägårdh, J.; Samuelson, L. Low-temperature cathodoluminescence studies of GaAsnanowires in the sem. Journal of Physics: Conference Series 2010,241, 012085.

[49] Gustafsson, A.; Bolinsson, J.; Sko ld, N.; Samuelson, L. Determination of diffusion lengths in nanowires using cathodoluminescence. Applied Physics Letters 2010,97, 072114.

[50] Kuech, T. F.; Veuhoff, E. Mechanism of carbon incorporation in MOCVDGaAs. Journal of Crystal Growth 1984,68, 148-156.

[51] Zhang, G.; Tateno, K.; Gotoh, H.; Sogawa, T.; Nakano, H. Structural, compositional, and optical characterizations of vertically aligned AlAs/GaAs/GaP heterostructure nanowires epitaxially grown on Si substrate. Japanese Journal of Applied Physics 2010,49, 015001.

[52] Pusep, Y. A.; Arakaki, H.; de Souza, C. A.; Rodrigues, A. D.; Haapamaki, C. M.; LaPierre, R. R. Crystal structure and optical characterization of heterostructured GaAs/AlGaAs/GaAs nanowires. J. Appl. Phys. 2013,113, 164311.

[53] Hoang, T. B.; Moses, A. F.; Zhou, H. L.; Dheeraj, D. L.; Fimland, B. O.; Weman, H. Observation of free exciton photoluminescence emission from single wurtzite GaAsnanowires. Applied Physics Letters 2009,94, 133105.

[54] Heiß, M.; Conesa-Boj, S.; Ren, J.; Tseng, H.-H.; Gali, A.; Rudolph, A.; Uccelli, E.; Peir, F.; Morante, J. R.; Schuh, D. et al. Direct correlation of crystal structure and optical properties in wurtzite/zinc-blende GaAsnanowire heterostructures. Phys. Rev. B 2011,83, 045303.

[55] Novikov, B. V.; Serov, S. Y.; Filosofov, N. G.; Shtrom, I. V.; Talalaev, V. G.; Vyvenko, O. F.; Ubyivovk, E. V.; Samsonenko, Y. B.; Bouravleuv, A. D.; Soshnikov, I. P. et al. Photoluminescence properties of GaAsnanowire ensembles with zincblende and wurtzite crystal structure. physica status solidi (RRL) - Rapid Research Letters 2010,4, 175-177.

[56] De, A.; Pryor, C. E. Predicted band structures of III-V semiconductors in the wurtzite phase. Phys. Rev. B 2010,81,155210.

[57] Akopian, N.; Patriarche, G.; Liu, L.; Harmand, J. C.; Zwiller, V. Crystal phase quantum dots. Nano Lett. 2010,10, 1198-1201.

[58] Zhang, L.; Luo, J.-W.; Zunger, A.; Akopian, N.; Zwiller, V.; Harmand, J.-C. Wide InP nanowires with wurtzite/zincblende superlattice segments are type-II whereas narrower nanowires become type-I: An atomistic pseudopotential calculation. Nano Lett. 2010,10, 4055-4060.

20

[59] Jancu, J. M.; Gauthron, K.; Largeau, L.; Patriarche, G.; Harmand, J. C.; Voisin, P. Type II heterostructures formed by zinc-blende inclusions in InP and GaAswurtzite nanowires. Applied Physics Letters 2010,97, 041910-3.

[60] Bao, J.; Bell, D. C.; Cap, F.; Wagner, J. B.; Martensson, T.; Tragardh, J.; Samuelson, L. Optical properties of rotationally twinned inp nanowire heterostructures. Nano Lett. 2008,8, 836-841.

[61] Ihn, S.-G.; Song, J.-I.; Kim, Y.-H.; Lee, J. Y.; Ahn, I.-H. Growth of GaAs nanowires on Si substrates using a molecular beam epitaxy. IEEE Transaction on Nanotechnology 2007,6, 384-389.

[62] Jahn, U.; Lähnemann, J.; Pfüller, C.; Brandt, O.; Breuer, S.; Jenichen, B.; Ramsteiner, M.; Geelhaar, L.; Riechert, H. Luminescence of GaAsnanowires consisting of wurtzite and zinc-blende segments. Phys. Rev. B 2012,85,045323.

[63] Plochocka, P.; Mitioglu, A. A.; Maude, D. K.; Rikken, G. L.; del Aguila, A. G.; Christianen, P. C.; Kacman, P.; Shtrikman, H. High magnetic field reveals the nature of

excitons in a single GaAs/AlAs core/shell nanowire. Nano Lett 2013,13, 2442-7.

[64] Ahtapodov, L.; Todorovic, J.; Olk, P.; Mjaland, T.; Slattnes, P.; Dheeraj, D. L.; van Helvoort, A. T.; Fimland, B. O.; Weman, H. A story told by a single nanowire: Optical properties of wurtzite GaAs. Nano Lett 2012,12, 6090-5.

[65] Fischbach, J. U.; Bimberg, D.; Bauser, E. Experimental determination of the anisotropy of the exciton wave function of GaAs in a magnetic field. Solid State Communications 1976, 18, 1255-1258.

[66] Salviati, G.; Albrecht, M.; Zanotti-Fregonara, C.; Armani, N.; Mayer, M.; Shreter, Y.; Guzzi, M.; Melnik,; Yu.V.; Vassilevski, K.; Dmitriev, V. A.; Strunk, H.P. Cathodoluminescence and transmission electron microscopy study of the influence of crystal defects on optical transitions in GaN. phys. stat. sol. (a) 1999,171, 325-9.