Nano Res

1

High resolution scanning gate microscopy

measurements on inas/gasb nanowire Esaki diode

devices

James L. Webb1 (), Olof Persson1, Kimberly A. Dick2, Claes Thelander3, Rainer Timm1 and Anders

Mikkelsen1

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0449-4

http://www.thenanoresearch.com on March 7, 2014

© Tsinghua University Press 2014

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer‐review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research DOI 10.1007/s12274‐014‐0449‐4

1

High Resolution Scanning Gate Microscopy

Measurements on InAs/GaSb Nanowire Esaki Diode

Devices

James L. Webba*, Olof Perssona, Kimberly A. Dickb, Claes Thelanderc, Rainer Timma and

Anders Mikkelsena

aDivision of Synchrotron Radiation Research, Lund University, Sweden,

bDivision of Solid State Physics, Lund University, Sweden,

cCenter for Analysis and Synthesis, Lund University, Sweden

e-mail: james.webb@sljus.lu.se

Keywords: Nanowire, Scanning gate microscopy, Esaki tunnel diode, InAs, GaSb, III-V,

heterostructure

Abstract

Gated transport measurements are the backbone of electrical characterization of nanoscale

electronic devices. Scanning gate microscopy (SGM) is one such gating technique that adds

crucial spatial information, accessing the localized properties of semiconductor devices.

Nanowires represent a central device concept due to the potential to combine very different

materials. However, SGM on semiconductor nanowires has been limited to a resolution in the

50-100nm range. Here, we present a study by SGM of newly developed III-V semiconductor

nanowire InAs/GaSb heterojunction Esaki tunnel diode devices under ultra-high vacuum. Sub

5nm resolution is demonstrated at room temperature via use of quartz resonator atomic force

microscopy sensors, with the capability to resolve InAs nanowire facets, the InAs/GaSb

*To whom correspondence should be addressed, e-mail: james.webb@sljus.lu.se

2

tunnel diode transition and nanoscale defects on the device. We demonstrate that such

measurements can rapidly give important insight into the device properties via use of a

simplified physical model, without the requirement for extensive calculation of the

electrostatics of the system. Interestingly, by precise spatial correlation of the device electrical

transport properties and surface structure we show the position and existence of a very abrupt

(<10nm) electrical transition across the InAs/GaSb junction despite the change in material

composition occurring only over 30-50nm. The direct and simultaneous link between

nanostructure composition and electrical properties helps set important limits for the precision

in structural control needed to achieve desired device performance.

Introduction

Semiconductor nanowires represent a highly active research field, for example with regard to

realizing advances in optoelectronic properties in applications such as solar cells[1][2] solid

state lighting[3] as well as electronic devices such as tunnel diodes[4] and nanowire field

effect transistors[5][6]. Nanowires are also used in research of more fundamental physics,

such as in recent studies of potential Majorana fermions[7][8] and exciton dynamics[9][10].

The antimonide compounds (InSb, GaSb) are of recent interest for a range of applications as

they form a broken bandstructure at the interface with InAs[11][12]. Such interfaces are of

relevance for a broad range of studies from high performance nanowire transistors[13][14] to

research on topological insulators[15].

Scanning probe microscopy (SPM) techniques offer an excellent means to characterize

semiconductor nanowires, with recent work on atomic force microscopy(AFM) and scanning

tunneling microscopy (STM) studies of electrically grounded individual nanowires deposited

on conducting surfaces[16-18]. However, semiconductor nanowires exhibit variability within

each growth set (of many thousands of wires). This makes it hard to identify an important

structural aspect specific to a single nanowire used for a device to the (general) properties of

3

other nanowires fabricated in the same growth set measured in parallel by complementary

techniques. These may include properties such as defects, doping and wire surface

composition alongside the effects on the wire of annealing, cleaning and processing

techniques and can have significant bearing on the behavior of the device. Imaging a single

device nanowire whilst measuring its electrical properties during device operation provides a

means to avoid this issue and directly link such specific nanowire properties to changes in the

device's electrical behavior and would be extremely useful in and identifying possible routes

for device improvement.

Scanning gate microscopy, by which the conductance of a semiconductor device can

be modified by the local gating effect produced by an STM or AFM tip close to the wire, is

one such method that nanowire devices may be characterized during operation. SGM is

especially useful due to the dependence of the gating on the (capacitive) coupling between

wire and tip, in turn dependent on the surface and bulk properties of a nanowire. This allows

both of these to be probed, providing information such as the local carrier type and a

qualitative indication of the carrier concentration in the nanowire under the tip. SGM is a

well-established technique, having been used previously to study III-V semiconductor

nanowire devices at low temperature[19], for InAs nanowires[20][21], for InN[22] and Si

nanowires[23],to study carbon nanotube devices[24] and work on low dimensional

systems[25][26]. Alternative complementary techniques have also been used to characterize

operational devices, including SPM methods such as Kelvin force microscopy[27][28] and by

other means such as focused laser excitation by scanning photocurrent microscopy[29].

Here, we focus on SGM measurements on the InAs/GaSb nanowire Esaki (tunnel)

diode, consisting of a single InAs-GaSb heterojunction (in a single nanowire) with excellent

tunnel diode properties[4]. The structure is also of relevance for complementary field effect

transistor structures[30][31]. Electrical and SPM measurements of such a device can be

performed in a lateral geometry, with the wire flat on an insulating (Si/SiO2) substrate surface

4

and contacted at 2 or more points by metallic electrodes patterned by conventional

lithographic techniques (see Figure 1). In this work we measure the gating effect on such

devices using a tip at positive or negative bias Vtip in either a fixed position over the nanowire,

or scanning in AFM mode across its width or along its length whilst measuring changes in

device conductance. By this method we can resolve nm-scale structural features of the wire

itself, particularly those difficult to observe by standard AFM such as localized defects, in

addition to investigating the physics of the device operation through reproducing trends in

device electrical transport behavior under gating using a simplified model of the device.

Results and discussion

A first verification of gating effect was performed by measuring Isd with the tip

stationary a) above (on top) and b) away from (off) the nanowire. Figure 2 shows the current

through the device Isd as a function of applied bias Vsd at different Vtip with the tip placed

directly above the InAs nanowire section or 5μm off the wire. In the latter case (and for

Vtip=0), we observed electrical behavior characteristic of Esaki diode operation with a

negative dI/dV region (with peak Isd in this region at around Vp=Vsd=0.62V) in forward bias

(Vsd>0). Off the wire, the conductance behavior of the device was unaltered by any bias

applied to the tip. With the tip placed on top of the InAs section (approximately 500nm from

the heterojunction) an increasingly negative Vtip was found to suppress overall device

conductance. This included the elimination of the negative dI/dV region, an effect we attribute

to electron carrier depletion in the InAs under the tip reducing those available for tunneling at

the junction, plus reduction of reverse bias current at negative Vsd .Over GaSb, no clear tip

gating effect was observed.

In the ungated case, we observe a peak-to-valley ratio for the negative dI/dV region of

1.3 with typical device resistance was in the 1-3MΩ range. Current densities of 0.98kA/cm2 at

reverse bias -0.3V were observed (assuming a 35nm InAs part diameter junction). These

5

figures are comparable to other axial nanowire tunnel junction devices[32] at room

temperature but less than the InAsSb/GaSb Esaki diodes reported by Ganjipour et al.[33]

specifically designed for high current operation. In this latter work, with a high-κ

lithographically patterned top gate over the entire nanowire, the same negative gate

suppression of conductance was observed, with the threshold for significant effect at V<-1.3V

and complete suppression of the negative dI/dV region at V<-2V. For the tip gate on InAs, we

require a higher tip gate bias to achieve the same effect, likely due to the more localized

nature of the gate and absence of the dielectric material.

Further measurements were performed on another device by setting a fixed Vtip and

measuring the change in Isd through the nanowire whilst scanning in AFM mode the tip across

the wire width (in x, Figure 3), simultaneously measuring the height profile beginning with an

overview scan encompassing a large section of the nanowire covering both InAs and GaSb

sections and part of the Ti/Au contacts. This can be seen in Figure 3. For Vsd>0 (forward bias)

and Vtip<0, conductance was suppressed over the n-type InAs wire section, a feature we again

attribute to the local depletion of carriers in the wire under the tip. The opposite enhancement

effect was observed for positive tip bias Vtip>0, behavior identical to that observed in the work

by Zhou et al. [34] for InAs-only nanowires. In contrast to their work, we observed strong,

homogeneous gating along the length of the InAs segment with minimal interference from the

metallic electrodes, which we attribute to low resistance nanowire-electrode contacts. Little

effect was observed on the GaSb, an aspect requiring further measurement detailed below.

The unexpected gating effect on the Au pad was an artifact produced by incomplete Ti/Au

coverage of the nanowire combined with the increasing unstable oscillation of the tip

scanning on the embedded wire causing it to move into closer proximity with the nanowire,

producing a strong gating effect.

A notable effect observed during measurement was the presence of a larger gating

effect with the tip at the edges of the nanowire rather than on the wire top at lower tip bias

6

(Figure 4,d). Based on Poisson equation simulations of the electrostatic potential, we attribute

this effect to the additional electric field and tip-wire coupling on the wire edge due to the

geometry of the tip and reduction in mean tip-wire distance when scanned across the

nanowire. At increasingly negative tip bias Vtip→-4V a strong gating effect could be observed

not only at the edges but on top of the wire itself.

Higher resolution scans were performed on the InAs part of the wire. At such high

resolutions, it became possible to observe the crystal facets of the InAs part of the nanowire

(Figure 4,c with structure shown schematically in Figure 4,a) due to the change in tip-wire

coupling and hence gating effect at the side facet edges. This demonstrated the extremely

good nm-scale resolution of the microscope and a direct link between surface topography of

the wire and the electrical transport properties of the device. We estimate the maximum

achievable resolution to be sub-5nm from the images and linescans over the wire step edge.

This high resolution represents a significant improvement over previous SGM studies of

nanowires devices, where only a broad effect at the approximate wire position was observed

with little detail on the wire itself, comparative examples of which include the work by Yang

et al. on Si nanowires[35], Liu et al. for InN nanowires[22] or Martin et al. for Si/NiSi2

nanowires[23].

We attribute the improved resolution as compared to earlier work to the different

design of our system, which utilizes high stiffness and high Q factor qPlus sensors based on a

resonant quartz tuning fork with an all-metallic tungsten tip, suitable for multi-mode

STM/AFM/SGM operation. Such AFM systems have previously been demonstrated to

achieve atomic scale resolution in standard AFM imaging mode[36][37]. The smaller

vibrational amplitudes associated with quartz sensors and scanning in a low-noise UHV

system designed for atomic resolution STM operation are extremely beneficial to high

resolution scanning. In addition all-metallic tungsten tips are more robust against degradation

than metal coated Si tips used in alternative cantilever systems[38]. We consider that this

7

setup offers considerable advantages over alternative laser-based AFM systems for

SGM[39][40].

In order to investigate gating in more detail across the heterojunction, we performed a

series of scans along the wire length with the tip directly above the center of the wire (x=0).

Despite the weaker observed gating effect, we considered this to be the best method to avoid

anomalous variation in gating strength due to identified enhanced gating effects arising from

scanning over the nanowire edge. We considered this to be due to the relatively flat nature

(<10nm roughness) on the top crystalline facet of the nanowire. This reduced the likelihood of

any anomalous change in gating strength during measurement arising from changes in

tip-nanowire distance through the reaction of the AFM feedback system, which could result

from a scan across the entire height of the nanowire. Such a method also ensured applying a

consistent shape of electric potential to the nanowire from the tip centered above it,

minimizing variation arising from inhomogeneity in tip or nanowire geometry that could

otherwise arise by scanning across the full nanowire width.

Figure 5 shows the change in Isd as a function of position y along the wire and as a

function of bias across the wire Vsd for Vtip=0 relative to -3V. A negligible drift was observed

during the 20-30min scan time, confirmed by simultaneous imaging, tip height monitoring

and by repeated scans to verify the data. We observe a transition at the interface from current

suppression in the InAs to weak current enhancement in the GaSb, the latter an aspect not

seen in detail for scans across the wire. The effect was localized in the Vsd region with

negative dI/dV, in the tunneling regime of operation. For this device, the bias at peak

tunneling current Vp was reduced slightly to 0.4V as compared to 0.6V for the device in

Figure 2, an effect we attribute to variability in series resistance to the nanowire produced

during the fabrication or measurement process.

To provide some explanation for these observations we consider a simplified

bandstructure model of the heterojunction, as shown schematically in Figure 6. In this form

8

we consider a simple broken bandstructure, neglecting bandbending at the interface, which we

consider to be abrupt at y=100nm. We model the tip gate to act as an effective potential

Veff ≤Vtip decaying exponentially in y along the wire and perturbing the energy of the

conductance and valence bands in each material - to higher energy for Vtip<0 and lower for

Vtip>0. For Vtip<0 and far from the junction, this effect can be considered as to introduce an

additional energy barrier acting as an additional resistance (with enhanced conductance for

Vtip>0) leading to the general trends in gating effect observed. At the junction, this effect can

reduce the potential required to align conduction states in InAs with empty valence states

available for tunneling on the GaSb side. In the tunneling regime, for Vtip<0 and tip on InAs,

this has the effect of shifting the position of peak tunnel current, with maximum alignment in

states, Vsd=Vp0 (without gating) to lower Vsd=Vp1 as detailed on the schematic in Figure 6.

Due to the size and geometry of the tip we consider the perturbation of the band

energy to act over a significant proportion of the wire, producing a weak effect at the junction

even when scanning several hundred nm away. This small effect at the junction has a

significant effect on Isd in the tunneling regime due to the low number of states available and

the minimal potential required to shift Vp, altering the position at which peak tunnel current

occurs, and reducing overlap between conduction and valence bands. For the tip over the

GaSb section, the same perturbation effect is produced but acting to increase Vp→Vp2.

We consider the idealized device current to take the form Isd=It + Id with tunnel current

given by the approximate form[41] pVVppt VVII 1e combined with the standard

expression for p-n diode current 1e0 thVVd II and where Isd is normalized to the zero

tip bias peak current Isd(Vsd=Vp0). Using these empirical formulas allows the qualitative

reproduction of tunnel diode current in the different regimes of operation without requiring

extensive calculation of the device electrostatics, permitting comparison to the experimental

data and giving some insight into the physics of the device.

9

By modeling device behavior using this model to derive Vp and evaluating the

empirical expressions for tunnel and conventional diode current in forward bias, the effect on

current can be qualitatively mapped as a function of tip position. Figure 7 shows the result for

Vtip=-1V close to the junction on InAs and GaSb as compared to Vtip=0, demonstrating the

resulting shift in device current ΔIsd in the tunneling regime. The changes in Vp act to reduce

Isd over InAs and increase it over GaSb, qualitatively replicating the effect measured by

experiment. At low bias, significant gate independent tunneling dominates and at high bias the

device operates as a conventional p-n diode, reproducing the absence of tip gate dependence

in these regimes that we also observed experimentally.

The dependence on tip position can be seen in Figure 8 showing the modeled shift in

Vp relative to the ungated state Vp0 as a function of position along the nanowire, as compared

to the experimentally derived position of Vp from the data shown in Figure 5 (dashed gray

line). For both modeled and experimental data, the size of the shift in Vp and hence strength of

gating effect increases as the tip moves towards the heterojunction, with a change in sign of

the effect moving from one material to the other. Although qualitatively matching the

observed behavior (with some quantitative agreement over InAs), the model overestimates the

strength of the effect in GaSb at the junction, predicting a maximum ΔVp=0.75. We attribute

this to the inadequacy of such a simplified model in calculating the complex effects produced

with the tip extremely close to or on the heterojunction combined with the fact that the

junction is not necessarily as abrupt as modeled, with a gradual shift in material composition

between each part, as shown in Figure 1. Considering a uniform strength of gating effect

between the two materials may also be invalid, due to this gradual change in composition and

due to the higher n-type doping concentration of the InAs section. Despite the gradual

material composition change of the wire, the junction remains abrupt with a shift in Vp

behavior over a range of 5-10nm, significantly less than the 30-50nm change from InAs to

GaSb as shown in Figure 1. Notably, this can be resolved using the SGM technique despite

10

the large scale effect of the tip gating. Using SGM, we can identify the precise position of the

tunnel junction within the wider material transition from GaSb to InAs. We find the junction

to be coincident with the initial increase in nanowire width, as can be seen by comparing to

the AFM height profile in Figure 8. This demonstrates the effectiveness of the technique in

correlating localized structural change to device function.

Identification of defects by scanning probe techniques in a device is an aspect of

considerable interest[27][42][43]. This is particularly the case for defects that cannot be easily

identified by ordinary AFM, such as those with little surface prominence. In order to test the

use of SGM in defect identification, artificial defects were induced in a InAs/GaSb device by

running the tip into the left hand side of the nanowire close to the heterojunction (on the InAs

side), removing a small section of the wire. Figure 9 shows the resulting SGM measurement

on that device for both positive and negative tip biases. The defect can be clearly observed by

the scanning gate technique, due to its effect on the local gating produced by the local

disruption of the surface, despite being near-invisible under AFM.

Conclusions

We have fabricated a series of InAs/GaSb Esaki diode devices consisting of a

semiconductor nanowire with lateral Ti/Au contacts, demonstrating good tunnel diode

behavior. Using scanning gate microscopy combined with non-contact AFM, we have

characterized the electrical transport properties of such devices whilst simultaneously

scanning and imaging using AFM. We observed a strong gating effect over the InAs wire part

with near complete suppression of Isd at a tip bias of Vtip→-4V and enhancement of

conductance at positive tip bias conforming to trends seen in previous studies[34]. As a

function of Vsd, this gating effect was greatest in the region with the device in the tunneling

regime of operation and thus most sensitive to changes produced by the local tip gate. A

simple model considering the effect of shifting the bias required to reach peak tunnel current

11

Vp and using standard empirical formulas for tunnel diode current was able to qualitatively

reproduce the trends in conductance behavior observed in the different regimes of device

operation, giving significant insight into the physics of the device operation that could not be

obtained from an electrically grounded single nanowire. A clear link between wire surface

structure and electrical transport was demonstrated by imaging the nanowire facets by SGM,

also highlighting the very high resolution possible using the technique. This allowed

observation of wire detail and gating measurements down to the sub-5nm scale, significantly

better than other SGM works performed at room temperature[22][34]. This also permitted

identification of defects introduced into the nanowire that could not be observed by AFM

alone. No aspect of the work was limited to III-V nanowire systems, with standard fabrication

and SPM techniques used throughout, permitting its potential application to the

characterization of a wide range of semiconductor devices.

Such measurements demonstrate the potential of our SGM setup for non-destructive

characterization, potentially during and after alteration of the device surface by deposition,

oxidation, passivation and other in situ processing under highly stable UHV conditions. This

can be performed whilst also working alongside complimentary methods such as TEM and

XPS to give high quality information about the behavior of a specific nanowire device. Such

data allows a simultaneous connection between device nanowire structural changes and

electrical properties, something which is not possible by separate SPM and transport studies

alone.

Methods

Nanowires were grown by low-pressure metal-organic vapor phase epitaxy

(LP-MOVPE, 10 kPa) in hydrogen carrier with a total flow of 6 L/min, using size-selected

gold aerosol particles with nominal diameter 30 nm, on InAs (111)B substrates. The InAs

nanowire section was grown at 450 C for 13 minutes using trimethylindium (molar flow

12

4.5x10-6) and arsine (1.9x10-4). Tetraethyltin (1.9x10-7) was added after 4 minutes of growth

to n-dope the InAs, and switched off 1 minute before the end of the InAs. The GaSb section

was grown at 500C for 20 minutes using trimethylgallium (3.4x10-5) and trimethylantimony

(3.5x10-5); diethylzinc with a molar flow of 1.9x10-5 was used to p-dope the GaSb. Further

details of the growth can be found in the work by Ek et al .[44]. Figure 1 shows a TEM image

through the nanowire heterojunction, showing the change in crystal growth and wire diameter

(50 nm for the GaSb, 35 nm for the InAs) and across the heterojunction. The composition of

the nanowire does not shift abruptly between InAs and GaSb - instead a gradual change over

50-100nm occurs, as can be seen from the energy-dispersive X-ray spectroscopy (XEDS)

measurements of the composition taken across the interface between the two materials.

Device fabrication was performed on n++ doped Si/SiO2 substrates with SiO2 oxide

thickness 200nm. Dry deposition of wires was performed onto pre-fabricated 80nm thickness

Ti/Au electrical contacts patterned by mask aligned UV photolithography, in addition to

markers for nanowire location patterned by electron beam lithography (EBL). Nanowire

position was identified by scanning electron microscopy (SEM) using a FEI Nova NanoLab

600 SEM to image a 100x100μm area containing the EBL pre-patterned alignment markers as

a position reference. Nanowire position was then identified from the marker grid to create a

contact electrode pattern to it, from 10μm width contacts to the initial photolithographic

electrodes down to two 250nm width contacts to opposite ends (one to GaSb, one to InAs) of

the nanowire. Deposition of these contacts was performed by vacuum thermal evaporation of

40nm of Ti and 80nm of Au at 5x10-7mbar base pressure. Prior to deposition, the wire was

exposed to oxygen plasma (at 5mTorr O2 pressure) for 45s and placed in 1:9 HCl:H2O

solution for 30s in order to remove any residual resist and to reduce native oxide thickness.

Measurements were performed at room temperature in an Omicron XT dual

AFM/STM at 10-11mbar ultra-high vacuum (UHV) using Omicron qPlus[45] sensors of a

damped quartz tuning fork resonator design, with an all-metallic tungsten combined

13

AFM/STM tip, capable of tip bias Vtip up to +/-10V relative to ground. All SGM

measurements were performed in non-contact AFM mode. 4 electrical contacts to a

custom-made sample plate were utilized, with the device chip adhered to the sample plate

using indium and Al wire bonded to the plate electrical connections via additional Au foil

pads. Due to the sensitivity of the devices to electrical shorting, external grounding was

provided by means of a break out box to each contact, permitting separate grounding of both

device and measurement equipment. Electrical measurement was performed using a Keithley

2401 sourcemeter to apply a bias Vsd across the nanowire device. A Stanford SR570 low noise

current preamplifier was used to measure current Isd through the device, such that Vsd>0

represented a positive bias applied to the p-type (GaSb) end of the nanowire (forward bias).

Acknowledgement

This work was performed within the Nanometer Structure Consortium at Lund

University (nmC@LU), and was supported by the Swedish Research Council (VR), the

Swedish Foundation for Strategic Research (SSF), the Crafoord Foundation, the Knut and

Alice Wallenberg Foundation, and the European Research Council under the European

Union's Seventh Framework Programme Grant Agreement 259141. One of the authors (R.T.)

acknowledges support from the European Commission under a Marie Curie Intra-European

Fellowship.

References

1. Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Åberg, I.; Magnusson, M. H.; Siefer, G.;

Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B. et al. InP Nanowire Array Solar Cells

Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit. Science 2013, 339,

1057-1060.

14

2. Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Indium phosphide nanowires as

building blocks for nanoscale electronic and optoelectronic devices. Nature 2011, 409,

66-69.

3. 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.

4. Borg, B. M.; Ek, M.; Ganjipour, B.; Dey, A. W.; Dick, K. A.; Wernersson, L.-E.;

Thelander, C. Influence of doping on the electronic transport in GaSb/InAs(Sb) nanowire

tunnel devices. Appl. Phys. Lett. 2012, 101, 043508.

5. Egard, M.; Johansson, S.; Johansson, A.-C.; Persson, K.-M.; Dey, A. W.; Borg, B. M.;

Thelander, C.; Wernersson, L.-E.; Lind, E. Vertical InAs Nanowire Wrap Gate Transistors

with ft > 7 GHz and fmax > 20 GHz. Appl. Phys. Lett. 2010, 10, 809-812.

6. Ford, A. C.; Ho, J. C.; Chueh, Y.-L.; Tseng, Y.-C.; Fan, Z.; Guo, J.; Bokor, J.; Javey, A.

Diameter-Dependent Electron Mobility of InAs Nanowires. Nano Lett. 2009, 9, 360-365.

7. Mourik, V.; Zuo, K.; Frolov, S. M.; Plissard, S. R.; Bakkers, E. P. A. M.; Kouwenhoven,

L. P. Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor

Nanowire Devices. Science 2012, 336, 1003-1007.

8. Deng, M. T.; Yu, C. L.; Huang, G. Y.; Larsson, M.; Caroff, P.; Xu, H. Q. Anomalous

Zero-Bias Conductance Peak in a Nb-InSb Nanowire-Nb Hybrid Device. Nano Lett.

2012, 12, 6414-6419.

9. Bulgarini, G.; Reimer, M. E.; Hocevar, M.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.;

Zwiller, V. Avalanche amplification of a single exciton in a semiconductor nanowire.

Nat. Photonics 2012, 6, 455-458.

10. 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.

15

11. Sakaki, H.; Chang, L. L.; Ludeke, R.; Chang, C.-A.; Sai-Halasz, G. A.; Esaki, L.

In1-xGaxAs-GaSb1-yAsy heterojunctions by molecular beam epitaxy. Appl. Phys. Lett. 1977, 31,

211.

12. Ek, M.; Borg, B. M.; Dey, A. W.; Ganjipour, B.; Thelander, C.; Wernersson, L.-E.;

Dick, K. A. Formation of the Axial Heterojunction in GaSb/InAs(Sb) Nanowires with

High Crystal Quality. Cryst. Growth Des. 2011, 11, 4588-4593.

13. Wernersson, L.-E.; Thelander, C.; Lind, E.; Samuelson, L. III-V Nanowires: Extending a

Narrowing Road. P. IEEE 2010, 98, 2047-2060.

14. Dey, A.; Borg, M.; Ganjipour, B.; Ek, M.; Thelander, K. D.; Lind, E.; Thelander, C.;

Wernersson, L.-E. High-Current GaSb/InAs(Sb) Nanowire Tunnel Field-Effect Transistors.

Electron Devic. Lett. 2013, 34, 211-213.

15. Knez, I.; Du, R.-R. Quantum spin Hall effect in inverted InAs/GaSb quantum wells.

Frontiers of Physics 2012, 7, 200-207.

16. Hjort, M.; Wallentin, J.; Timm, R.; Zakharov, A. A.; Håkanson, U.; Andersen, J. N.;

Lundgren, E.; Samuelson, L.; Borgström, M. T.; Mikkelsen, A. Surface Chemistry, Structure,

and Electronic Properties from Microns to the Atomic Scale of Axially Doped

Semiconductor Nanowires. ACS Nano 2012, 6, 9679-9689.

17. Hilner, E.; Håkanson, U.; Fröberg, L. E.; Karlsson, M.; Kratzer, P.; Lundgren, E.;

Samuelson, L.; Mikkelsen, A. Direct Atomic Scale Imaging of III-V Nanowire Surfaces.

Nano Lett. 2008, 8, 3978-3982.

18. Hjort, M.; Lehmann, S.; Knutsson, J.; Timm, R.; Jacobsson, D.; Lundgren, E.; Dick, K.;

Mikkelsen, A. Direct Imaging of Atomic Scale Structure and Electronic Properties of

GaAs Wurtzite and Zinc Blende Nanowire Surfaces. Nano Lett. 2013, 13, 4492-4498.

19. Boyd, E. E.; Storm, K.; Samuelson, L.; Westervelt, R. M. Scanning gate imaging of

quantum dots in 1D ultra-thin InAs/InP nanowires. Nanotechnology 2011, 22, 185201.

16

20. Zhukov, A. A.; Volk, C.; Winden, A.; Hardtdegen, H.; Schapers, T. Low-temperature

conductance of the weak junction in InAs nanowire in the field of AFM scanning gate.

JETP Lett. 2011, 93, 10-14.

21. Zhukov, A.; Volk, C.; Winden, A.; Hardtdegen, H.; Schapers, T. Distortions of the

Coulomb Blockade Conductance Line in Scanning Gate Measurements of InAs Nanowire

Based Quantum Dots. J. Exp. Theor. Phys. 2013, 116, 138-144.

22. Liu, J.; Cai, Z.; Koley, G. Charge transport and trapping in InN nanowires investigated

by scanning probe microscopy. J. Appl. Phys. 2009, 106, 124907.

23. Martin, D.; Heinzig, A.; Grube, M.; Geelhaar, L.; Mikolajick, T.; Riechert, H.; Weber,

W. M. Direct Probing of Schottky Barriers in Si Nanowire Schottky Barrier Field

Effect Transistors. Phys. Rev. Lett. 2011, 107, 216807.

24. Kim, Y.; Oh, Y. M.; Park, J.-Y.; Kahng, S.-J. Mapping potential landscapes of

semiconducting carbon nanotubes with scanning gate microscopy. Nanotechnology 2007, 18,

475712.

25. Aoki, N.; Burke, A.; da Cunha, C. R.; Akis, R.; Ferry, D. K.; Ochiai, Y. Study of

quantum point contact via low temperature scanning gate microscopy. J. Phys. Conf. Ser.

2006, 38, 79-82.

26. Aoki, N.; da Cunha, C. R.; Akis, R.; Ferry, D. K.; Ochiai, Y. Scanning gate microscopy

investigations on an InGaAs quantum point contact. Appl. Phys. Lett. 2005, 87,

223501.

27. Bae, S.-S.; Prokopuk, N.; Quitoriano, N. J.; Adams, S. M.; Ragan, R. Characterizing

defects and transport in Si nanowire devices using Kelvin probe force microscopy.

Nanotechnology 2012, 23, 5706.

28. Fuller, E. J.; Pan, D.; Corso, B. L.; Tolga Gul, O.; Gomez, J. R.; Collins, P. G.

Quantitative Kelvin probe force microscopy of current-carrying devices. Appl. Phys. Lett.

2013, 102, 083503.

17

29. Allen, J. E.; Hemesath, E. R.; Lauhon, L. J. Scanning Photocurrent Microscopy Analysis

of Si Nanowire Field-Effect Transistors Fabricated by Surface Etching of the Channel.

Nano Lett. 2009, 9, 1903-1908.

30. Dey, A. W.; Svensson, J.; Borg, B. M.; Ek, M.; Wernersson, L.-E. Single InAs/GaSb

Nanowire Low-Power CMOS Inverter. Nano Lett. 2012, 12, 5593-5597.

31. Borg, B. M.; Dick, K. A.; Ganjipour, B.; Pistol, M.-E.; Wernersson, L.-E.; Thelander, C.

InAs/GaSb Heterostructure Nanowires for Tunnel Field-Effect Transistors. Nano Lett.

2010, 10, 4080-4085.

32. Wallentin, J.; Persson, J. M.; Wagner, J. B.; Samuelson, L.; Deppert, K.;

Borgström, M. T. High-Performance Single Nanowire Tunnel Diodes. Nano Lett. 2010,

10, 974-979.

33. Ganjipour, B.; Dey, A. W.; Borg, B. M.; Ek, M.; Pistol, M.-E.; Dick, K. A.; Wernersson,

L.-E.; Thelander, C. High Current Density Esaki Tunnel Diodes Based on GaSb-

InAsSb Heterostructure Nanowires. Nano Lett. 2011, 11, 4222-4226.

34. Zhou, X.; Dayeh, S. A.; Wang, D.; Yu, E. T. Scanning gate microscopy of InAs

nanowires. Appl. Phys. Lett. 2007, 90, 233118.

35. Yang, C.; Barrelet, C. J.; Capasso, F.; Lieber, C. M. Single p-Type/Intrinsic/n-Type

Silicon Nanowires as Nanoscale Avalanche Photodetectors. Nano Lett. 2006, 6, 2929-2934.

36. Giessibl, F. J. Atomic resolution on Si(111)-7x7 by noncontact atomic force microscopy

with a force sensor based on a quartz tuning fork. Appl. Phys. Lett. 2000, 76, 1470.

37. Ternes, M.; Lutz, C. P.; Hirjibehedin, C. F.; Giessibl, F. J.; Heinrich, A. J. The Force

Needed to Move an Atom on a Surface. Science 2008, 319, 1066-1069.

38. Morita, S.; Giessibl, F. J.; Wiesendanger, R. Noncontact Atomic Force Microscopy ;

NanoScience and Technology; Springer Berlin Heidelberg, 2009; 121-142.

18

39. Pelliccione, M.; Sciambi, A.; Bartel, J.; Keller, A. J.; Goldhaber-Gordon, D. Design of

a scanning gate microscope for mesoscopic electron systems in a cryogen-free dilution

refrigerator. Rev. Sci. Instrum. 2013, 84, 033703.

40. Wilson, N. R.; Cobden, D. H. Tip-Modulation Scanned Gate Microscopy. Nano Lett. 2008,

8, 2161-2165.

41. Sze, S. M. Physics of Semiconductor devices, 2nd ed.; Wiley, 1981.

42. Schroer, M. D.; Petta, J. R. Correlating the Nanostructure and Electronic Properties of

InAs Nanowires. Nano Lett. 2010, 10, 1618-1622.

43. Howell, S. L.; Padalkar, S.; Yoon, K.; Li, Q.; Koleske, D. D.; Wierer, J. J.; Wang, G. T.;

Lauhon, L. J. Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells

Using Scanning Photocurrent Microscopy. Nano Lett. 2013, 13, 5123-5128.

44. Ek, M.; Borg, B. M.; Johansson, J.; Dick, K. A. Diameter Limitation in Growth of

III-Sb-Containing Nanowire Heterostructures. ACS Nano 2013, 7, 3668-3675.

45. Giessibl, F. Advances in atomic force microscopy. Rev. Mod. Phys. 2003, 75,

949-983.

19

Figures

Abstract Figure: Schematic of SGM on a InAS/GaSb device (SEM, below) with current

suppression (darker) and enhancement (lighter) shown alongside AFM profile.

20

Figure 1: (Left)SEM image example of InAs/GaSb nanowire lateral device on Si/SiO2,

contacted by Ti/Au electrodes. The wider section of the wire is the GaSb part, the narrower

the InAs part with the heterojunction (A) between materials indicated. Nanowire diameter is

50nm for the GaSb, 35nm for the InAs. (Center) TEM image across the heterojunction A

between the narrower InAs and wider GaSb parts of the wire (Right) Compositional analysis

of the nanowire heterojunction by XEDS showing the variation in nanowire composition

across the interface.

21

Figure 2: Isd through the nanowire device as a function of Vsd for Vtip=0→-4V both on and off

the InAs nanowire section (schematically indicated). The device showed Esaki diode

characteristics, with a negative dI/dV region, which was dependent on tip bias.

22

Figure 3: Overview SGM (top) and AFM (middle) scan of an entire nanowire device plus part

of the Ti/Au contacts, with electrical connections schematically shown (bottom) for low

forward bias Vsd=0.1V and Vtip=-2V. The conductance is shown in a normalized form in order

to highlight the nanowire position, with darker color reflecting a lower conductance.

Reduction in (normalized) conductance (Isd→0.5-0.6, darker) was observed for the tip over

the InAs wire part, with little effect observed on GaSb or off the wire (Isd→1).

23

Figure 4: a) Schematic (SC) diagram of hexagonal wire structure, b) AFM scan of the wire

(with overlaid line profile, black) and c) high resolution SGM on InAs part. The SGM

resolution is sufficient to image individual surface steps within the wire side facets due to the

change in gating effect as the tip crosses them, which cannot be observed by AFM alone. d)

Higher detail SGM on the wire across the InAs/GaSb junction, showing reduction (darker)

and enhancement (lighter) of conductance when scanning across the wire at negative and

positive tip bias respectively. The strongest effect was observed at the edges at low bias

(Vsd=0.1V), with additional strong gating with the tip on top of the wire at Vtip<-3V. For

Vtip=-4V, a tip change is observed as a shift in gating effect position, produced by the strong

electrostatic forces scanning at high tip bias close to the nanowire surface.

24

Figure 5: Difference in current through the wire between zero and positive tip bias

ΔI=Isd(Vtip=-3V) - Isd(Vtip=0V) as a function of bias across the wire Vsd and position y(in nm)

along the wire length, with the tip centered on top of the wire. The junction is located at

approximately y=450nm. Conductance is reduced on the InAs part (y<425nm) and weakly

enhanced on GaSb (y>475nm), with a transition region inbetween. This is also shown on the

local plots of current vs. Vsd (right) with their measurement position indicated by an arrow.

The peak absolute effect on ΔI averaged in 10nm intervals is indicated as guide by the grey

dashed line.

25

Figure 6: Model schematic of the InAs/GaSb heterojunction bandstructure. represents the

subtracted position dependent voltage drop across the junction and E energy, with E- in eV.

The perturbation in conduction band (CB) and valence band (VB) produced by a negative tip

(Veff=Vtip<0) on InAs (top) and GaSb (bottom) is shown. The (forward) bias needed to reach

peak tunnel current with maximum overlap of available states Vsd=Vp is indicated. The effect

of the tip bias is to decrease (increase) Vp on InAs (GaSb), producing a decrease (increase) in

device current Isd. Here, Vp1<Vp0<Vp2, where Vp0 is the peak tunnel current bias for Vtip =0, Vp1

the lower bias with the tip on InAs and Vp2 the higher bias on GaSb. Tip position is indicated

(blue arrow).

26

Figure 7: Plot of modeled device current Isd. The effect of the tip on InAs is to reduce

Vp0→Vp1, resulting in a reduction in current in the negative dI/dV tunneling regime as

observed for the device. The opposite effect is produced by the tip on GaSb where Vp0→Vp2

and a positive ΔI results. At high Vsd, It→0 and conventional diode behavior dominates.

27

Figure 8: (Left vertical axis) Change in peak tunnel current ΔVp =Vp-Vp0 as a function of

position y along the wire (at x=0) across the heterojunction (at y=100nm) for Veff=Vtip<0 and

uniform gating on both materials. A sign change in effect is produced between InAs and GaSb

- moving from current suppression to enhancement. Overlaid is the peak position data from

experiment, represented by the gray dashed line in Figure 5. (Right axis) AFM height profile,

showing the sharp structural change (increase in wire width) across the heterojunction.

28

Figure 9: Identification of defects. An artificial defect was produced in the device by a

standard AFM/STM tip, which can be clearly observed under SGM due to the enhancement of

the local gating effect around it due to changes in wire structure. In contrast, the defect cannot

be clearly seen under AFM.