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Appl. Phys. Lett. 117, 163101 (2020); https://doi.org/10.1063/5.0021520 117, 163101
© 2020 Author(s).
Low temperature scanning tunneling
microscopy and spectroscopy on laterally
grown InxGa1−xAs nanowire devices
Cite as: Appl. Phys. Lett. 117, 163101 (2020); https://doi.org/10.1063/5.0021520
Submitted: 13 July 2020 . Accepted: 02 October 2020 . Published Online: 19 October 2020
Yen-Po Liu , Lasse Södergren , S. Fatemeh Mousavi , Yi Liu , Fredrik Lindelöw, Erik Lind , Rainer Timm ,
and Anders Mikkelsen
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Low temperature scanning tunneling microscopy
and spectroscopy on laterally grown In
x
Ga
1x
As
nanowire devices
Cite as: Appl. Phys. Lett. 117, 163101 (2020); doi: 10.1063/5.0021520
Submitted: 13 July 2020 .Accepted: 2 October 2020 .
Published Online: 19 October 2020
Yen-Po Liu,
1,2
Lasse S€
odergren,
2,3
S. Fatemeh Mousavi,
1,2
Yi Liu,
1,2
Fredrik Lindel€
ow,
2,3
Erik Lind,
2,3
Rainer Timm,
1,2
and Anders Mikkelsen
1,2,a)
AFFILIATIONS
1
Department of Physics, Lund University, 22100 Lund, Sweden
2
NanoLund, Lund University, 22100 Lund, Sweden
3
Department of Electrical and Information Technology, Lund University, 22100 Lund, Sweden
a)
Author to whom correspondence should be addressed: anders.mikkelsen@sljus.lu.se
ABSTRACT
Laterally grown In
x
Ga
1x
As nanowires (NWs) are promising candidates for radio frequency and quantum computing applications, which,
however, can require atomic scale surface and interface control. This is challenging to obtain, not least due to ambient air exposure between
fabrication steps, which induces surface oxidation. The geometric and electronic surface structures of In
x
Ga
1x
As NWs and contacts, which
were grown directly in a planar configuration, exposed to air, and then subsequently cleaned using atomic hydrogen, are studied using low-
temperature scanning tunneling microscopy and spectroscopy (STM/S). Atomically flat facets with a root mean square roughness of 0.12 nm
and the InGaAs (001) 4 2 surface reconstruction are observed on the top facet of the NWs and the contacts. STS shows a surface bandgap
variation of 30 meV from the middle to the end of the NWs, which is attributed to a compositional variation of the In/Ga element concentra-
tion. The well-defined facets and small bandgap variations found after area selective growth and atomic hydrogen cleaning are a good starting
point for achieving high-quality interfaces during further processing.
V
C2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://
creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/5.0021520
III–V semiconductor nanowires (NWs) have demonstrated sig-
nificant promise for applications in (opto)electronics and now quan-
tum computing.
1–6
In particular, the surfaces of the NWs play a
significant role in defining their function due to the large surface-to-
bulk ratio. This can result in Fermi level pinning, recombination,
changes in conductivity, as well as carrier scattering at the surfaces.
Recently, selectively grown lateral NWs have gained significant inter-
est. This so-called template-assisted selective epitaxy (TASE)
7,8
or
selective area growth (SAG)
9–11
is especially promising for applications
such as metal–oxide–semiconductor field-effect transistors (MOSFET)
for radio frequency applications
9,12,13
and for quantum transport in
lateral NWs for Majorana-based semiconductor/superconductor
qubits.
1–5
In this work, we focus on metalorganic vapor phase epitaxy
(MOVPE) laterally grown In
x
Ga
1x
As NWs, relevant for both radio
frequency and quantum computing applications.
2,9
MOVPE is the
preferred technique for commercial synthesis of III–Vs. However, after
the MOVPE growth of the NWs, the sample will be exposed to air
as it is moved to other deposition systems, for example, to apply super-
conducting contacts and perform lithography involved in device fabri-
cation. Therefore, the surface will oxidize, potentially introducing
defects in the heterojunction contacts in the system, which likely
reduces the performance in particular, important for quantum com-
puting devices. For this reason, Molecular Beam Epitaxy (MBE) has
until now been the dominant method for growing semiconductor/
superconductor quantum devices, where contact formation by, e.g., Al
deposition, can be integrated in the same vacuum chamber.
8,14
For
MOVPE-grown devices, one would need to remove the surface oxide
while still maintaining highly perfected surfaces and not ruining the
morphology. Atomic hydrogen cleaning has achieved this goal for sev-
eral binary compound NW surfaces.
15–18
However, it has not been
explored if this strategy works for ternary NWs and wires grown using
TASE/SAG. It is a non-trivial question as the surface structure and
morphology have been found to vary depending on III-V material and
Appl. Phys. Lett. 117, 163101 (2020); doi: 10.1063/5.0021520 117, 163101-1
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Applied Physics Letters ARTICLE scitation.org/journal/apl
nanostructuring,
19
and in some cases, it has been difficult to obtain
crystalline surfaces.
20
Another open question for laterally grown ter-
nary NWs is the material composition along and across the wire,
which can potentially vary, inducing gradients in the surface bandgap.
Detecting such local surface changes in lateral NWs is a challenge
compared to free-standing NWs, which can more easily be peeled off
and investigated.
Scanning tunneling microscopy (STM) and spectroscopy (STS)
are unique techniques for mapping both precise topography and elec-
tronic properties down to the atom scale on surfaces. STM/S studies of
NWs have become more common.
15–18,21–23
While a number of STM
studies exist on nanowire {110} and {11-20} side facets,
15,17,18,24–28
{001} facets of laterally grown III–V semiconductor NWs have not
been studied by STM yet. In particular, there are only a few studies
found for NW devices.
21–23
The In
x
Ga
1x
As NW devices were selectively, laterally grown on
a semi-insulating InP:Fe (001) substrate by MOVPE as described pre-
viously
9
and in the supplementary material. Schematic illustrations of
the device are shown in Fig. 1. Scanning electron microscopy (SEM)
was performed using a FEI Nova NanoLab
TM
600 SEM. In-air atomic
force microscopy (AFM) was performed using a JPK AFM in tapping
mode. STM/S was performed in ultra-high vacuum using a low tem-
perature closed-cycle SIGMA Surface Science Infinity STM running at
9 K. Measurement details are found in the supplementary material.A
commercial hydrogen cracker (MBE Komponenten) was operated at
1700 C with a hydrogen pressure of 5 10
6
mbar in the UHV
chamber.
In order to confirm the in-air overall surface topography of the
device, SEM and AFM imaging was performed [see the supplementary
material, Figs. S1(a) and S1(b)]. This shows that the structures even
after air exposure and, thus, formation of a 1–2 nm thick native oxide
film
29
are regular and flat. After AFM measurements, the sample was
inserted into the STM UHV and hydrogen cleaning was used to
remove the surface oxides. The sample was held at 420Cduring
hydrogen cleaning for 50 min; afterward, t he hydrogen cracker was
turned off, and subsequently, the sample temperature was reduced to
room temperature. The complete cleaning process consisted of 2
cleaning cycles of 50 min each. This has previously been sufficient to
clean InAs and GaAs NWs.
15–17,21–23
After the cleaning, the sample
was inserted into the LT-STM where a temperature of 10 K was
reached in about 15 min.
Initially, STM imaging was performed on the contacts to verify
that the surface of the sample was clean. Then, the tip was navigated to
the NW device region by a procedure described in the supplementary
material. In the device region, the area with a regular pattern of parallel
NWs separated by trenches could be recognized as seen in Fig. 2(a).
The image compares well with the AFM images [Fig. S1(b)], and thus,
the structure appears intact after the cleaning procedure and all the
NWs appear similar in morphology. To study the NWs in more detail,
we zoom in as shown in Fig. 2(b).Theheightprofiletakenatthesame
area [Fig. 2(c)] indicates that the NWs have a flat top facet and two
side facets at an angle of 45. The measured height of the NW in the
image is 10 nm. This is a little less than the expected height of 13 nm,
which can be attributed to the width of the tip, which prevents it from
scanning the bottom of the trenches between the wires. Still, we can
get clear images of the top and side facets of the NWs. The flatness of
the NW top facet is highlighted in Fig. 2(d), with additional height
profiles given in Fig. S2 of the supplementary material.Thecentral
20 nm wide area of the facet has islands (5nm wide and 0.5 nm
high). The height of the islands is comparable to the lattice constant of
InGaAs. The surface roughness of this central part of the NW top
facet, as indicated by the dashed purple rectangle in Fig. 2(d),hasa
root mean square (RMS) value of 0.12 nm. These values confirm a flat
surface with few major defects and a height variation corresponding to
one to two atomic layers. This is consistent with previous studies of
NWs after hydrogen cleaning.
19,30
This low level of roughness is a
good starting point for developing good semiconductor/superconduc-
tor interfaces and can be further carefully improved by optimization of
the cleaning procedure.
17
FIG. 1. Lateral NW device schematics: (a) Top view illustration of the NW sample
design and material. x ¼0.63 for the composition of the contact material, while x is
higher in the NW material. (b) Cross-sectional schematic of the NWs and contacts.
The dashed lines show where the NWs end.
FIG. 2. STM topography images of the lateral NWs: (a) A 700 700 nm
2
STM
image shows the geometry of contacts and NWs (aligned vertically in this image).
The 200 200 nm
2
large area marked by the blue square is shown by a zoom-in
image in (b). (c) A height profile taken across a NW from image (b) indicates a flat
top facet and steep side facets of the NW. Green dots mark the positions where
line spectroscopy data were obtained. (d) The 30 30 nm
2
STM image, acquired
on a single NW as indicated by the red arrow in (c), shows the flatness of the NW
top facet.
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We can further zoom in to the top facet to investigate the
atomic-scale surface structure of the In
x
Ga
1x
As NW. Figure 3(a)
shows periodic stripes and disordered areas, the latter likely due to
random variations in the ternary alloying
31
or remaining Ga-rich
oxide clusters (most stable types of oxides). The periodic stripes indi-
cate atomically ordered regions on top of the NW. The height profile
shown in Fig. 3(b) reveals a distance between neighboring stripes of
1.7 nm and a height of 0.15nm. A similar surface structure has
been observed on the clean contact a few micrometers away from
the NWs [indicated by the light blue dot in Fig. S1(a)], as is shown in
Figs. 3(c) and 3(d), with a distance between t he peaks of about 1.5 nm
and a height of 0.13–0.17 nm. The observed surface pattern can be
interpreted as a (4 2) surface reconstruction, which is common for
GaAs or InAs (001) surfaces.
32–36
This surface reconstruction is
another indication that it is possible to obtain well-ordered NW surfa-
ces after atomic hydrogen cleaning. The (001) surface direction of the
NW top facet is unique for laterally grown NWs, while vertically
grown III–V Zincblende NWs typically have {110} or {111}A/B twin
plane facets.
15–17,19
The angle between the NW (001) top facet and its
side facets amounts to 45, indicating that the side facets consist of
{011} surface planes.
In order to investigate the possible surface band structure varia-
tion along the NWs, STS point spectra were acquired at three positions
on the NW. These positions are separated by about 75 nm, reaching
from the middle of the NW toward its end, as shown in the inset of
Fig. 4. Reference spectra were recorded on the contact, which is
denoted position 4. From these four different places, we show spectra
of the normalized differential conductance (dI/dV)/(I=V), which is
proportional to the local density of states around the Fermi level.
30
Each of the spectra shown is an average over 15 individual measure-
ments. More measurement details are given in the supplementary
material.
In all spectra, the central area with none or very few states signi-
fies the bandgap at the surface, while the steep increase in states for
both negative and positive sample bias can be interpreted as the sur-
face valence band (VB) and conduction band (CB) onsets, respectively.
The slopes of these band onsets are fitted linearly, as shown in Fig. 4,
and from the intersection of these lines with y¼0, the band edges can
be determined, as described in Refs. 30 and 37. The absolute values of
the obtained band onsets might be influenced by electronic effects
such as tip-induced band-bending and by the presence of surface
states, but the precision of the band edge determination from the
experimental data and, thus, the relative error between band
onsets measured at different positions is 3 meV as discussed in the
supplementary material. From Fig. 4,anincreaseintheapparent
bandgap from the NW center to the contact can be seen. The blue
spectra recorded at position 1 (middle of the NW) show the VB edge
at 210 meV and the CB edge at 200 meV, resulting in a surface
bandgap of 410meV. The VB edge remains constant for all three posi-
tions on the NW, while bandgap changes are due to the shifting of the
CB edge. The fitted CB edges of positions 2 and 3 are at 210meV and
230 meV. This results in a bandgap increase from the middle to the
end of the wire with values of 410, 420, and 440meV (position 1,2,3).
On the contact (position 4), both the VB edge at 230 meV and the
CB edge at 260 meV are shifted to larger values, resulting in a bandgap
of 490 meV. In order to rule out the fact that the bandgap change is
caused by tip changes, the first and the last dI/dV-V spectra of the
entire STS series are recorded at the same area and found to be similar
[Fig. S3(c) of the supplementary material].
As the variation in the bandgap is robust on top of the wire and
no significant change in surface morphology is observed along the
wire, the most likely explanation is a variation of the Ga/In concentra-
tion at least in the surface region. Along the NWs, the In to Ga compo-
sition ratio especially at the surface can vary, as diffusion along the
wire might occur during both growth and annealing steps. Optical
characterization of similarly grown lateral devices showed a higher In
content in the NWs as compared to the contacts.
9
Accordingly, the In
concentration in the NWs might be expected to increase from the end
to the middle, leading to a narrowing of the bandgap in the middle
FIG. 3. Surface reconstruction: (a) 15 15 nm
2
STM image zoomed in from
Fig. 2(b) at the location of the blue square shown in the inset. (b) Height profile
along the blue arrow shown in (a). (c) 25 25 nm
2
STM image taken on the con-
tact. (d) Height profile along the blue arrow shown in (c). STM images were taken
with sample biases of 3.0 V (a) and 5.5 V (c) and tunneling currents of 80 pA
(a) and 50 pA (c).
FIG. 4. Bandgap variations along the NW: Averaged (dI/dV)/(I=V) spectra at posi-
tions 1–4, as indicated by blue squares in the inset. Dashed black lines show a lin-
ear fit of the VB (left) and CB (right) onset.
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and a significantly larger bandgap at the contacts. This is in qualitative
agreement with the STS observations.
As a baseline for discussing the surface bandgap change in more
detail, we calculated “bulk” bandgap values depending on the In/Ga
concentrationinaNWasshowninthesupplementary material,
Fig. S4. The calculation is based on 8-band k ptheoryassuminga
50 nm wide, 14nm high, and infinitely long fully strained NW
placed along the [100] direction on InP (001), as shown in the inset of
supplementary material Fig. S4. The absolute values of the apparent
bandgap found by STS along the NW and at the contact would indi-
cate nearly pure InAs material at the middle of the NW and a Ga con-
tent of only about 12% at the contact material, which is far below the
expected values. Tip-induced band bending (TIBB) can affect the mea-
sured bandgap, but this would result in larger experimental values, in
contrast to what is observed (see the supplementary material for more
discussions on this). Instead, the too low apparent bandgap can be
related to the presence of surface states, giving rise to additional states
at the band edges and then shifting the apparent bandgap to smaller
values. A similar effect of the bandgap reduction has been observed in
a previous STS study on an In
0.53
Ga
0.47
As (001) surface
38
and found
to be caused by the (4 2) surface reconstruction. Even in that study,
a pure InAs (001)–(4 2) surface was measured, and the change in
the surface bandgap between the InAs (001) and the In
0.53
Ga
0.47
As
(001) samples, as observed in the STS spectra, was of approximately
the same size as the change in the bulk bandgap between both materi-
als.
38
Additional studies of defects on the In
x
Ga
1x
As (001) surface
39,40
indicated a local perturbation of the electronic surface structure by
defects, but a consistent average behavior was observed. Thus, there
are several indications that a gradient in the bulk bandgap will be
reflected in the surface bandgap for the present surface. In addition,
defects that would give rise to mid-gap states can be ruled out.
38,39
The
kp bulk bandgap calculations show that the increase in the Ga con-
tent mainly affects the CB edge, where the energy shift is about three
times larger than that at the VB edge. This is consistent with the exper-
imental observation that the surface VB onset is almost constant along
the wire, while the surface CB onset changes. By quantitatively com-
paring the experimentally observed increase in the bandgap by
30 meV along the NW with the k p calculation, we find that this
corresponds to an increase in the Ga content by about 5% from the
center to the ends of the wire. Compared to the values observed at the
contact and assuming that this follows the nominal values of 63% In,
the wires have an In content of 73%–78%. However, it must be
stressed that STS measurements probe the surface region, and thus, it
could be that the compositional variation is only present in the near
surface region. Still, the present study shows that variations in the
bandgap are small in the important surface region, and previous STS
work on the surface
38,39
indicates that a significant change in the bulk
bandgap would be reflected in the STS as well, unless some complex
In interdiffusion occurs during H cleaning.
In addition to the observed change in the bandgap along the NW
long axis, we also evaluate changes in the surface electronic structure
across the NW. The surface band edge values measured across the
NW top facet are very stable, varying less than 50 meV between indi-
vidual (non-averaged) spectra. However, we find an increase in the
bandgap from the top facet to the side facet of the NW by nearly
100 meV, which we attribute to the absence of reconstruction-induced
surface states on the {011} facets and to the possible influence of the
tip shape. More details are found in the supplementary material and
Fig. S5, including a calculation of the tip-induced band bending as a
function of tip shape and surface state density.
In summary, STM and STS measurements have been performed
on SAG lateral NW devices, including atomically resolved characteri-
zation of the surface structure and an analysis of the local variation of
the surface bandgap. After native oxide removal by atomic hydrogen
cleaning, the surface is found to be flat with an RMS roughness of only
0.12 nm. A (4 2) surface reconstruction is observed both on the con-
tacts and on the (001) top facet of the In
x
Ga
1x
As NW. A gradual
increase in the surface bandgap by 30 63 meV is found from the mid-
dle of the In
x
Ga
1x
As NW toward its ends, which is consistent with a
decrease in the In content by 5%, at least in the near surface region.
This study demonstrates that a high surface quality and moderate ele-
mental variations can be obtained after SAG and hydrogen cleaning,
which opens a path toward further improved surface quality with
atomic scale perfection.
See the supplementary material for details on sample preparation,
AFM/STM imaging, navigation of the STM to the devices, and further
details on STS measurements and their interpretation.
This work was supported by the Swedish Research Council
(VR) and the Swedish Foundation for Strategic Research (SSF).
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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Applied Physics Letters ARTICLE scitation.org/journal/apl
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