Robust Epitaxial Al Coating of Reclined InAs Nanowires - Nano

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Robust Epitaxial Al Coating of Reclined InAs Nanowires Jung-Hyun Kang, Anna Grivin, Ella Bor, Jonathan Reiner, Nurit Avraham, Yuval Ronen, Yonatan Cohen, Perla Kacman, Hadas Shtrikman, and Haim Beidenkopf Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03444 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Robust Epitaxial Al Coating of Reclined InAs Nanowires Jung-Hyun Kang†*, Anna Grivin†, Ella Bor†, Jonathan Reiner†, Nurit Avraham†, Yuval Ronen†, Yonatan Cohen†, Perla Kacman‡, Hadas Shtrikman†, and Haim Beidenkopf† † Dept. of Condensed Matter Physics, Braun Center for Submicron Research, Weizmann Institute of Science, Rehovot 76100, Israel ‡ Institute of Physics Polish Academy of Science, Al. Lotnikow 32/46, 02-668 Warsaw, Poland KEYWORDS: MBE, (001) substrate, reclining InAs nanowires, in-situ Al side-coating, nanowire intersections ABSTRACT: It was recently shown that in-situ epitaxial aluminum coating of indiumarsenide nanowires is possible and yields superior properties relative to ex-situ evaporation of aluminum (Nature Matt. 2015, 14, 400-406). We demonstrate a robust and adaptive epitaxial growth protocol satisfying the need for producing an intimate contact between the aluminum superconductor and the indium-arsenide nanowire. We show that the (001) indium-arsenide substrate allows successful aluminum side-coating of reclined indium-arsenide nanowires that emerge from (111)B micro-facets. The existence of a robust, induced hard superconducting gap in the obtained indium-arsenide/aluminum core/partial-shell nanowires is clearly demonstrated. We compare epitaxial side-coating of round and hexagonal cross-section nanowires, and find the surface roughness of the round nanowires to induce a more uniform 1 ACS Paragon Plus Environment

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aluminum profile. Consequently, the extended aluminum grains result in increased strain at the interface with the indium-arsenide nanowire which is found to induces dislocations penetrating into round nanowires only. An unique feature of proposed growth protocol is that it supports in-situ epitaxial deposition of aluminum on all three arms of indium-arsenide nanowire intersections in a single growth step. Such aluminum coated intersections play a key role in engineering topologically superconducting networks required for Majorana based quantum computation schemes. INTRODUCTION: Majorana quasiparticles were suggested1 to appear at the edges of a semiconductor nanowire (NW) coupled to a superconductor.2, 3 Subsequently, a number of experimental evidences for Majorana modes in structures based on NWs have been reported. In particular, indium-arsenide (InAs) and indium-antimonite (InSb) NWs have become key ingredients of semiconductor/superconductor hybrid devices used in the quest for Majorana fermions, thanks to their strong spin−orbit coupling and Zeeman splitting.4-8 In these realizations, however, low-energy states were observed below the proximity-induced superconducting gap. Such soft-gap states are a source of detrimental decoherence of Majorana modes. As these states were ascribed to disorder at the interface,8, 9 for observing Majorana particles it is essential to utilize devices with an improved intimate contact between the superconducting metal and the semiconductor. It was also suggested that such a metallic contact enhances the control over the chemical potential inside the NW. The growth technology of NW heterostructures is very well developed and various semiconductor NW structures with perfect interfaces, like e.g., silicon/germanium (Si-Ge) core-shell and i-Si/SiOx/p-Si core-shell-shell NWs,10 InAs NWs with embedded vertical indium-phosphide (InP) segments,11 InAs/InSb hetero-NWs,12 InAs/InP core-shell NWs,13 indium-arsenide/gallium-arsenide (InAs/GaAs) core-shell NWs,14 and InAs/InP core-shell NWs with Al in the shell,15 have been reported. However, combining the epitaxy of a 2 ACS Paragon Plus Environment

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superconductor metal with the epitaxial growth of semiconductor NWs is still a challenging task. The first in-situ deposition of an aluminum (Al) shell on the sidewalls of InAs NWs by molecular beam epitaxy (MBE) has been reported by Krogstrup et al.16 The process detailed in Ref. [16] is, however, rather demanding due to the particular geometry which is not easy to adapt to any MBE system alongside the low temperature required to avoid migration of the Al. As the (111) oriented NWs grow perpendicular to the (111)B substrate the impinging angle of the Al flux on the side facets can be small depending on the growth chamber geometry and substrate configuration. In Ref. [16] the authors deal with the orientation of the Al flux with respect to the NWs side facet by rotating the sample and are subsequently able to form a full shell where all side facets are Al coated. Our approach is different in the sense that we initially grow reclined NWs on a (001) substrate, which allows us to obtain both round and hexagonal in-situ Al-coated NWs and provides a unique way of achieving Al covered InAs NW intersections where all three arms are in-situ coated in a single growth step. The Al evaporation presented in this work is a more robust process, in cases where the angle at which the Al atoms impinge on the NWs surface is too small. Our approach satisfies the basic need for producing an intimate contact between the Al superconductor and the InAs NW. Although our method does not allow a full Al shell coverage, for chemical potential tuning purposes half-shell coverage is anyhow beneficial. Inclined NWs provide the possibility of growing Al coated III-V NWs in most common MBE systems where the angle of the incoming beam is not large enough to form a uniform layer on the NW side facets. Furthermore, the high impinge angle of the Al flux moderates the requisite to cool the sample down during Al evaporation – our approach does not require such low temperatures. Most importantly, this approach provides the means for in-situ Al evaporation of branched NWs such as Y-shape intersections, i.e., growing hybrid branched NW structures in a single growth step, without using pre-processed substrates. The growth of all structures mentioned above 3 ACS Paragon Plus Environment

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has become a very important subject and several solutions are currently proposed, like growth of different NW networks on pre-processed substrates;17 and NW networks with a predefined number of superconducting islands.18 Finally, in parallel to our work the Julich group investigated in-situ growth of the superconductor Al on vapor-solid (VS) grown InAs NWs using MBE.19 While their attempt allows to extend the findings from metal deposition on phase pure wires16 to NWs with alternating zinc-blende (ZB) and wurtzite (WZ) phases, our results describe the dependence on the shape of the NW, i.e., Al side coated NWs with round as well as hexagonal cross-section have been obtained. In Ref. [19] the conclusion of Ref. [16] that the temperature has a major influence on the morphology of the NW shell is confirmed. Studying NWs with different shape, and thus different strain at the superconductor/semiconductor interface, allowed us to show that strain plays a crucial role in the Al shell morphology and the presence of dislocations thus formed. EXPERIMENT: The tilted InAs NWs were grown by the gold-assisted, vapor-liquid-solid (VLS) process in a high purity MBE (RIBER-32) system. The epi-ready (001) InAs substrate was initially heated in the introduction chamber to ~180 ºC for water desorption, and then degassed at ~350 ºC. This was followed by an oxide blow-off with no intentional arsenic overpressure in a dedicated treatment chamber, where a gold layer of less than 1 nm thickness was subsequently evaporated. After the transfer to the growth chamber, the substrate temperature was ramped to 550 ºC (ΔT: 20 ºC/min.) for ripening of the gold into droplets with the desired size and density. The InAs NWs were grown at 400 ºC with group V/III ratio of ~100 for ~2 hours, which provides NWs nearly free of stacking faults.17 The NW crosssection is either round or hexagonal. The latter is formed by reducing the substrate temperature by 100 °C after growth of the round NWs. During 30 minutes of additional side growth at lower temperature the NW thickness increases and six {110} facets are formed. After the growth of NWs the sample is kept at 300 °C until arsenic overpressure reduces to 4 ACS Paragon Plus Environment

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1e-10 Torr in order to avoid arsenic condensation on the NWs surface, which in turn induces the formation of a resistive AlAs layer at the interface once the Al shutter is opened. The sample is then cooled down to -30 to -40 °C which is necessary to avoid migration of the Al (we found that cooling down to 0 was sufficient in order to suppress Al migration on the reclined NWs but since the growth is so complex we prefer cool down to the lowest reachable temperature to assure a maximum yield). Al is evaporated from a Knudsen cell at an angle of ~20 ° from the substrate normal (~70 ° from substrate surface) and at a temperature of ~1000 °C for ~100 sec (giving approximately 10 sec/nm), while watching the reflection high energy electron diffraction (RHEED) smear as a result of the bending of the NWs. When the coating of NWs with Al is completed the sample is rapidly pulled out of the growth chamber and oxidized immediately to keep Al from migrating. The Al coated InAs NWs were characterized by field emission scanning electron microscopy (FE-SEM, Zeiss Supra-55, 3 kV, working-distance ~4 mm), transmission electron microscopy (TEM), high resolution (HR-) TEM (JEOL JEM-2100 TEM, 200 kV, Gatan Ultrascan XP 2k × 2k CCD camera) and ZEISS Supra 55 scanning tunneling microscopy (STM). The structural properties, including lattice constant and atomic configuration, were analyzed using CrystalMaker® for Windows (Ver. 9.2.7, CrystalMaker Software Ltd.). For conductance measurements, normal (titanium/gold; Ti/Au) and superconducting (Ti/Al) metallic contacts were evaporated on the in-situ Al coated InAs NW with an electron beam metal evaporation and standard lift-off techniques. The pattern was formed by an electron beam lithography system (JEOL 9100). For the normal contacts, prior to metal evaporation, the aluminum shell was removed using type-D etchant. Then, the samples were treated by an ammonium polysulfide ((NH4)2Sx) solution to remove the oxide layer and passivate the NW surface with sulfur atoms. For the superconducting contact argon ion milling was performed inside the evaporation chamber prior to evaporation, in order to

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remove the native aluminum oxide. A thin metallic gate (Ti/Au) between the normal contact and the Al covered NW was evaporated to form an electrostatic barrier in the superconductor – normal (S-N) junction. Measurements were carried out in a He3 cryostat at a temperature of 300 mK and in a He4 dilution fridge at ~25 mK.

Figure 1. (a) An SEM image showing different Al deposition for inclined and vertical NWs despite the very same growth conditions on same growth surface. Enlarged SEM images showing morphology of deposited Al and schematic of Al deposition (inset) for the (b) vertical and (c) inclined InAs NWs grown on (001) substrate.


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The main result of the process presented here, which is based on growth of InAs NWs on a (001) rather than the conventional (111)B substrate, is captured by the SEM image presented in Figure 1a. It shows the ensuing Al coverage of two NW orientations under the very same growth conditions, both emerging from a (001) surface. The striking differences in the resulting Al coverage, in spite of the identical local growth conditions, are highlighted by the respective Figures 1b and 1c. Occasionally, we find NWs that grow normal to the substrate surface, along the direction. At this geometry, illustrated in Figure 1b, only a little amount of Al vapor condenses on the cold side facets and nano-size Al droplets form on the NW side walls. To improve the Al coverage of vertical NWs a substantial tilt of the substrate/MBE manipulator, to increase the Al impinging angle is necessary. This, however, would typically shift the substrate with the NWs far out of the Al beam. The large reclining angle of 55 ° of the NWs grown on the (001) provides a large impinging angle without any manipulation, between NW growth and Al coating stage. The InAs NWs emerge from two opposite {111} micro-facets (as depicted by Figure S1 of the Supplementary Information); at an angle of 35° to the (001) surface. Accordingly the projection of the impinging Al flux on the NW side facet grows from 34 % on the NWs grown on (111) substrate to an average of 82 % on the reclining NWs grown on (001) substrates. The Al flux impinging on the side facets of the reclined NWs (illustrated schematically in Figure 1c) forms a continuous Al half-shell. As a matter of fact, even though reproducibility and uniformity are not as reassured we managed to obtain a uniform Al side coating. Thanks to the high impinging angle of the Al flux the uniform side coating was possible even when the substrate was cooled only to 0°C, without having to cool it down to subzero temperatures which comes at the price of more impurities adhering to the NWs surface. As was demonstrated before in Ref. [16], a fundamental parameter governing the Al growth is the diffusion length of the impinging Al atoms on the InAs side facets. Increasing the growth temperature increases the surface


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diffusion length which suppresses Al adatoms incorporation. This can be balanced off with increased impinging flux of Al which decreases the diffusion length due to adatom collisions. Consequently, the higher Al flux impinging on the reclining NW side walls is what enables us to increase the growth temperature.

Figure 2. In-situ Al coated ‘Y’-shape InAs NW intersection grown on the (001) InAs substrate: (a) SEM image showing the intersection morphology and (b) TEM image showing its crystal structures along with the Al side coating of all three arms. 8 ACS Paragon Plus Environment

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An important additional advantage of the growth of reclined NWs on a (001) surface over the commonly used (111)B surface is the spontaneous formatting of InAs NW intersections.20, 21

NW intersections having at least three arms are a crucial resource as they are likely to

facilitate braiding of Majorana end states, which underlies any topological protected quantum computation scheme.5 NW growth on (001) presented here is, therefore, unique also in the sense that it allows, on demand, in-situ Al side coating of InAs intersections. Such Al coated ‘Y’-shape intersection can be seen in the SEM image in Figure 2a. The NWs have pure WZ structure while a ZB structure is seen in the core of the intersection.16, 20 A TEM image clearly showing the crystal structure of the NW intersection, as well as the Al side coating of the surface of all three NW arms is given in Figure 2b. The in-situ Al side-coated three way NW intersection are planned to be utilized in a future mesoscopic study, nevertheless such bare intersections whose conductance has been shown in Ref. [20] and a recent experiment showing coherent splitting of Cooper pairs.22 As already mentioned, the facets can be healed to form a hexagonal cross-section by adding a growth stage at a somewhat reduced temperature. Faceting of the NWs is related to the surface energies, which are determined by the dangling bonds of the {1100}WZ planes.23 The core NWs’ morphologies are visualized in Figure S2 by STM. Also as seen at the base of thick 80-100 nm NWs in Figure S3, the surface of the round NWs is constructed of a series of rather rough step edges with randomly oriented dangling bonds. The surfaces of the faceted NWs are atomically flat, apart for a few ad-atoms, and the dangling bonds are accordingly colinear. The profiles, round or faceted, of reclined NWs have an effect on the properties of the Al coating as demonstrated in SEM images and schematics (insets) of Figures 3a and 3b. TEM images in Figure S4 in the Supplementary Information show a comparison of the overall


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morphology of Al shells deposited on the round and faceted InAs NWs. In contrast to a round shape NW which generally shows smooth Al surface (Figures 3a and S4a), very rough Al surface is observed in a hexagonal faceted NW (Figures 3b and S4b). In the intentionally faceted NWs grown on the (001) substrate the three facets having a line of sight with the Al source are coated as shown in Figure 3b. This is in contrast to the case of perpendicular growth on a (111)B substrate, where coating of two facets is feasible as demonstrated in Ref [16]. The Al half-shell initially grows in grains having a different crystal orientation. At a later stage of growth the crystallites merge to form a continuous film structure, in the so called Volmer-Weber growth mode.16,

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The Al half-shell deposited on the round shaped NWs

shows quite long and continuous crystalline segments (Figure 3c). However, in the faceted NWs the crystalline segments are shorter and modulate in thickness; thus the resulting profile is more rough (Figure 3d). With respect to the morphology difference, comparison of different Al shell thicknesses is presented in Figures S5 and S6 showing that the Al shell deposited on the faceted NWs has a significantly rougher surface independent of the mean thickness of the Al shell. Remarkably, the comparison between round and faceted NW profiles suggests that rough InAs side surfaces result in a more uniform Al coating with extended grains. We attribute this to the effect surface roughness has on the diffusion length of the impinging Al adatoms. Naturally, the rougher surfaces of the round shaped NWs suppress the Al surface diffusion. Accordingly surface roughness has an effect similar to that of lowering the temperature, which was indeed shown in Ref. [16] to result in larger crystallographically oriented Al grains. The roughness of the round surface similarly results in the initial nucleation of a dense distribution of small Al grains. As the growth continues they quickly merge into a thin film. It is thin enough for the growth dynamics to be dominated by the surface energy, which suppresses polycrystallinity and orients the grains. In contrast, the Al growth on faceted


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surfaces resembles the high temperature dynamics, initially nucleating well separated large Al grains. These merge at a later stage of the growth into a thicker film. At such thickness surface energy is second to strain (captured by the NW bending seen in Figures 3e and 3f). Therefore, the grains do not re-orient and a grainier Al layer forms. The grainy structure of the Al layer on the faceted NWs leads to a large modulation (7-15 nm) of its overall thickness and in thinner Al coating results in discontinuous coverage (demonstrated in Figure S7). While rough NW surfaces are considered undesirable because they promote surface scattering in conductance experiments, when it comes to Al coating surface roughness seems to promote Al coating uniformity and crystallinity. Thus we so far chose to carry our transport experiments on the round rather than the faceted NWs. Our previous reports of detection of zero bias conductance peaks, possibly signifying Majorana fermions5, 19 were measured only on round shape NWs which repeatedly showed low noise performance.

An uniform and thin layer is essential for having a high critical parallel magnetic field, which is, in turn, necessary for the research of both trivial and topological superconductivity. We obtain additional information on the crystallites and the nature of their interface with InAs by HR-TEM imaging. The WZ structure of the round shape InAs NWs is shown in Figure 4a (darker on the left). It interfaces with two types of face-centered cubic (FCC) crystal structures of the Al layer, covered by a 3-4 nm thick amorphous oxide layer. The atomic structure model of the plane view on {2110}WZ planes of InAs NW and {110}FCC of Al shell (Figure 4b) depicts the atomic arrangement in both adjacent grains and the relative atomic order, in particular at the interface. The grainy crystalline structure of the Al shell deposited on the InAs NWs can be categorized into two types: type-α, with parallel FCC orientation to the NW (↑WZ

InAs

║ ↑FCC Al) and type-β, with a perpendicular

FCC orientation to the NW (↑WZ InAs ┴ ↑FCC Al). As shown in Figure S8,


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the Al shell deposited on the faceted NWs has the same crystal structures, thus the same analysis can be applied.

Figure 3. SEM images showing surface morphology and schematic cross section (inset) of (a) a single round and (b) hexagonal faceted InAs NWs. HR-TEM images which show crystal structures of (c) a single round and (d) hexagonal faceted InAs NW in-situ coated with Al. The SEM images showing overall morphology bending of the (e) round and (f) faceted InAs NWs. The notch at the end of the faceted wires forms during the faceting process. The red arrows point at a NW intersection.


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Figure 4. (a) HR-TEM image and (b) atomic structure model of polycrystalline Al shells deposited on a round shape InAs NW: {2110}WZ (InAs) and {110}FCC (Al) plan views. (c) An atomic structure model and (d) a HR-TEM image of a segment of Al shell having parallel orientation to the InAs NW (Type-α); (e) an enlarged HR-TEM image showing good matching of 2 InAs atomic layers with 3 Al atomic layers. (f) Atomic structure model and (g) 13 ACS Paragon Plus Environment

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HR-TEM image of polycrystalline Al shell with perpendicular orientation to the InAs NW (type-β), showing high mismatch between InAs NW and Al shell and, in consequence, semiperiodic dislocations on the InAs side of the interface (inset). The red arrows point at dislocations.

The relationship between the atomic layers of the InAs NW grown in the WZ direction and the Al half-shell having FCC orientation parallel to the NW axis (type-α) that was already demonstrated in Ref. [16] is depicted in Figures 4c-e. Although the materials seemingly have 33 % lattice mismatch (d: 4.05 Å (Al) / 6.06 Å (InAs)), interfacing 2 InAs layers (the so called WZ/2H structure stacking order)25 with 3 Al monolayers yields a periodic interface with residual lattice mismatch of 0.36 %. Accordingly, the resulting interface is free from any structural defects for a small enough layer thickness. However, this is no longer true for the type-β interface, when the interface between the two materials is formed by InAs {1100}WZ and Al {111}FCC planes. In the direction of the FCC structure the Al atoms do not form a super-structure (see Figure 4f). As illustrated in Figures 4f and S9, the best fit occurs for ‘4 InAs MLs vs. 6 Al atoms’, but even in this case the mismatch is high, reaching 5.7 %. This large mismatch is accommodated mostly in the more elastic Al layer. Nevertheless, a sequence of semi-periodic dislocations a few dozens of nm apart (pointed by red arrows) are induced within the InAs NW, as shown in Figure 4g. An edge dislocation penetrating about 5 lattice sites into the NW is shown in the inset. Interestingly, these dislocations are observed only in round shaped InAs NWs deposited with type-β Al shell regardless of the thickness of the NWs or their Al shell as shown in Figure S10. We have not detected them in the faceted NWs. We attribute this to the large Al grain sizes formed on the round-shaped NWs relative to those found on the hexagonal NWs. The strain due to the interfacial lattice mismatch is relaxed at grain boundaries. Therefore, larger 14 ACS Paragon Plus Environment

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grains result in greater total strain buildup, which will induce dislocations beyond a certain threshold. We cannot rule out their presence in the Al crystal because its small lattice constant is comparable to the TEM resolution limit. Still, their penetration into the InAs NW is somewhat surprising considering it is more than twice stiffer than bulk Al (shear moduli of 58 GPa relative to 26 GPa, respectively). However, as the TEM images show, the Al layer is imaged after oxidation during which a 3-4 nm thick alumina layer forms. Alumina is one of the stiffest materials (bulk shear moduli of 124 GPa) and thus forms a stiff backbone to the Al layer, pushing the crystallographic deformation into the InAs core. Edge dislocations that penetrate selectively into the core were reported in the study of the InAs/GaAs core-shell NWs.26 TEM images have shown there semi-periodic edge dislocations penetrating few monolayers into the InAs core with an extra nucleated plane. Apparently, both in InAs/GaAs and in InAs/Al the interfacial bonds are strong enough to avoid formation of interfacial dislocations. It could very well be, that the surface roughness of the round shaped NWs plays a role here too either by limiting the propagation of surface dislocation or by modifying the wetting properties of the InAs surface and improving the adhesion at the interface. Finally, we note that the main purpose of growing InAs NWs with in-situ epitaxial Al halfshell is to optimize the proximity induced superconductivity in the semiconductor wire. We show that this is indeed achieved with the reclined NW growth protocol by demonstrating both an induced hard superconducting gap and multiple Andreev reflections in a half-shell Josephson junction device.25, 27 Schematic and SEM images of a device fabricated from a round-profile Al-coated NW are shown in Figures 5a and 5b, respectively. The measured differential conductance (G) as a function of source-drain bias (VSD) of an in-situ Al coated InAs NW is shown in Figure 5c. For lower top gate bias, the tunneling barrier is decreased and Andreev reflection probability is enhanced resulting in sub-gap conductance (red line).28


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In this particular configuration an Andreev state was observed at ~100 µV within an induced Al gap of ~210 µV. In the high barrier configuration (blue line) Andreev reflection is suppressed. In its absence a hard induced gap is observed. The existence of a hard gap demonstrates the intimate and homogeneous contact formed between the wire and the epitaxial Al layer.9 In a subsequently fabricated, partial-shell, Josephson junction device we are able to demonstrate also multiple Andreev reflections (see Figure S11).27 It yields a lower bound for the inelastic scattering length of 1.8 mm. The results of further detailed studies of transport properties of these Al-coated Y-shape InAs NWs will be published elsewhere.


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Figure 5. (a) Schematic illustration and (b) SEM image of the S-N junction device used for transport measurements. (c) Differential conductance as function of SD voltage with two topgate biases: low barrier (VTG = -0.2 V, red line) shows an Anreev state at ~100 μV and an induced gap of ~210 μV; high barrier (VTG = -0.3 V, blue line) shows a hard superconducting gap in the absence of Andreev reflection.

In summary, we have demonstrated an efficient and universal in-situ deposition method of Al half-shell on reclined InAs NWs grown in MBE on (001) InAs substrates. The shape of the


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NWs (round or faceted), the thickness of the Al shell as well as their polycrystalline structure can be well controlled. Depending on the relative crystal orientations of the Al shell and the NW, either a very smooth interface between the InAs NW and the Al shell can be obtained or dislocations on the surface of the InAs NW are semi-periodically formed. From our HR-TEM we conclude that the formation of an AlAs resistive layer at the interface between the Al layer and the InAs NW is avoided. We further show that NWs growth on (001) uniquely allows to form InAs intersections instantaneously in-situ side-coated by Al. The hard superconducting gap conductance as well as the sub-gap tunneling conductance (the so-called soft gap) in S-N junction device and the conductance of the multiple Andreev reflection in the S-S partial-shell Josephson junction device, fabricated using these Al/InAs NWs, were demonstrated. The results of SEM, STM and HR-TEM studies as well as the results of conductance measurements prove formation of an intimate contact between the NW and the epitaxial Al. The quality of this contact is essential for many mesoscopic physics experiments and in particular for observation of the Majorana particles.

Supporting Information Figure S1 is a SEM image showing the {111} faceted surface where Au droplets will sit and an illustration of the WZ InAs NW grown on the {111}ZB surface. Figure S2 shows STM images of a round shape and faceted shape InAs NW. Figure S3 explains surface planes and calculated free energy of a round shape and hexagonal faceted WZ InAs NW. Figure S4 shows overall morphology of Al shells deposited on round shape and hexagonal shape (faceted) InAs NWs, respectively. Figure S5 shows different thickness of Al shells deposited on round shape InAs NWs. Figure S6 demonstrates different thickness of Al shells deposited on hexagonal shape (faceted) InAs NWs. Figure S7 is HR-TEM image showing disconnection of thin Al shell deposited on faceted InAs NWs. Figure S8 shows 18 ACS Paragon Plus Environment

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polycrystalline Al shells deposited on a faceted InAs NW which are consisted with type-α and -β. Figure S9 is atomic structure models show how they are overlapped at the interface plane (InAs {1100}WZ and Al {111}FCC). Figure S10 shows FE-SEM, TEM, and HR-TEM images of single round shape InAs NW having small diameter with thick Al shell and large diameter with thin Al shell, respectively. Figure S11 demonstrates the S-S partial-shell Josephson junction device: a schematic illustration and a SEM image and multiple Andreev reflection peaks. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author Dr. Jung-Hyun Kang, Department of Condensed Matter Physics, Braun Center for Submicron Research, Weizmann Institute of Science, Rehovot 76100, Israel. E-mail: [email protected], Tel: +972-8-934-2519, Fax: +972-8-934-4128

ACKNOWLEDGMENT The authors are grateful to Moty Heiblum for making this research possible. We deeply thank Michael Fourmansky for his unstinting technical assistance. All authors acknowledge partial financial supports of the Israeli Science Foundation (Grant No. 532/12 and Grant No. 36799), BSF grant No. 2014098, and the Polish National Science Center (grant No. 2013/11/B/ST3/03934).

Dr. Hadas Shtrikman, incumbent of the Henry and Gertrude F.

Rothschild Research Fellow Chair.

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References

[1] Wilczek, F. Nat. Phys. 2009, 5, 614-618 [2] Lutchyn, R. M.; Sau, J. D. and Das Sarma, S. Phys. Rev. Lett. 2010, 105, 077001. [3] Oreg, Y.; Refael, G. and von Oppen, F. Phys. Rev. Lett. 2010, 105, 177002. [4] Mourik, V.; Zuo, K.; Frolov, S. M.; Plissard, S. R.; Bakkers, E. P. M. A. and Kouwenhoven, L. P. Science 2012, 336, 1003-1007. [5] Das, A.; Ronen, Y.; Most, Y.; Oreg, Y.; Heiblum, M. Nat. Phys. 2012, 8, 887. [6] Rokhinson, L. P.; Liu, X. and Furdyna, J. K. Nat. Phys. 2012, 8, 795. [7] Churchill, H. O. H.; Fatemi, V.; Grove-Rasmussen, K.; Deng, M. T.; Caroff, P.; Xu, H. Q. and Marcus, C. M. Phys. Rev. B 2013, 87, 241401. [8] Chang, W.; Albercht, S. M.; Jespersen, T. S.; Kuemmeth, F.; Krogstrup, P.; Nygard, J. and Marcus, C. M. Nat. Nanotechnology 2015, 10, 232-236.


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[9] Takei, S.; Fregoso, B. M.; Hui, H.-Y.; Lobos, A. M. and Das Sarma, S. Phys. Rev. Lett. 2013, 110, 186803. [10] Lauhon, L. J.; Gudiksen, M. S.;Wang, D. and Lieber, C. M., Nature 2002, 420, 57-61. [11] Björk, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R. and Samuelson, L. Nano Lett. 2002, 2, 87-89. [12] Ercolami, D.; Rossi, F.; Li, A.; Roddaro, S.; Grillo, V.; Salviati, G.; Beltram, F. and Sorba, L. Nanotechnology 2009, 20, 505605. [13] Ghalamestani, S. G.; Heurlin, M.; Wernersson, L.-E.; Lehmann, S. and Dick, K. A. Nanotechnology 2012, 23, 285601. [14] Popovitz-Biro, R.; Kretinin, A.; Huth, P. V. and Shtrikman, H. Cryst. Growth Des. 2011, 11, 3858-3865. [15] Haapamaki, C. N.; Baugh, J. and LaPierre, R. R. J. Cryst. Growth 2012, 345, 11-15. [16] Krogstrup, P.; Ziino, N. L. B.; Chang, W.; Albrecht, S. M.; Madsen, M. H.; Johnson, E.; Nygard, J.; Marcus, C. M. and Jespersen, T. S. Nature Matt. 2015, 14, 400-406. [17] Krizek, F.; Kanne, T.; Razmadze, D.; Johnson, E.; Nygård, J.; Marcus, C. M. and Krogstrup, P. Nano Lett. 2017, 10.1021/acs.nanolett.7b02604. [18] Gazibegovic, A.; Car, D.; Zhang, H.; Balk, S. C.; Logan, J. A.; de Moor, M. W. A.; Cassidy, M. C.; Schmits, R.; Xu, D.; Wang, G.; Krogstrup, P.; Op het Veld, R. L. M.; Zuo, K.; Vos, Y.; Shen, J.; Bouman, D.; Shojaei, B.; Pennachio, D.; Lee, J. S.; van Veldhoven, P. J.; Koelling, S.; Verheijen, M. A.; Kouwenhoven, L. P.; Palmstrøm, C. J. and Bakkers, E. P. A. M. Nature.2017, 548, 434-438.


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[19] Güsken, N. A.; Rieger, T.; Zellekens, P.; Bennemann, B.; Neumann, E.; Lepsa, M.; Schäpers, T. and Gruetzmacher, D. ArXiv 2017, arXiv:1707.02629. [20] Kang, J. H.; Cohen, Y.; Ronen, Y.; Heiblum, M.; Buczko, R.; Kacman, P.; PopovitzBiro, R. and Shtrikman, H. Nano Lett. 2013, 13, 5190−5196. [21] Kang, J. H.; Galicka M.; Kacman, P. and Shtrikman, H., Nano Lett. 2017, 17, 531−537. [22] Cohen, Y.; Ronen, Y.; Kang, J. H.; Heiblum, M.; Feinberg, D.; Mélin, R. and Shtrikman, H. ArXiv 2017, arXiv:1606.08436. [23] Luqani, L.; Ercolani, D.; Sorba, L.; Sibirev, N. V.; Timofeeva, M. A. and Dubrovskii, V. G., Nanotechnology 2012, 9, 095602. [24] Venables, J. A.; Spiller G. D. T. and Hanbücken, M., Rep. Prog. Phys. 1984, 47, 399459. [25] Irmer, B.; Blick, R. H.; Simmel, F.; Gödel, W.; Lorenz, H. and Kotthaus, J. P. Appl. Phys. Lett. 1998, 73, 2051. [26] Popovitz-Biro, R.; Kretinin, A.; Huth, P. V. and Shtrikman, H. Cryst. Growth Des. 2011, 11, 3858-3865. [27] Kleinsasser, A. W.; Miller, R. E.; Mallison, W. H. and Arnold, G. B. Phys. Rev. Lett. 1994, 72, 1738–1741. [28] Kjaergaard, M.; Nichele, F.; Suominen, H. J.; Nowak, M. P.; Wimmer, M.; Akhmerov, A. R.; Folk, J. A.; Flensberg, K.; Shabani, J.; Palmstrøm, w. C. J. and Marcus, C. M. Nat. Commun. 2016, 7, 12841.


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Figure 1. (a) An SEM image showing different Al deposition for inclined and vertical NWs despite the very same growth conditions on same growth surface. Enlarged SEM images showing morphology of deposited Al and schematic of Al deposition (inset) for the (b) vertical and (c) inclined InAs NWs grown on (001) substrate. 148x172mm (150 x 150 DPI)

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Figure 2. In-situ Al coated ‘Y’-shape InAs NW intersection grown on the (001) InAs substrate: (a) SEM image showing the intersection morphology and (b) TEM image showing its crystal structures along with the Al side coating of all three arms. 76x153mm (150 x 150 DPI)


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Figure 3. SEM images showing surface morphology and schematic cross section (inset) of (a) a single round and (b) hexagonal faceted InAs NWs. HR-TEM images which show crystal structures of (c) a single round and (d) hexagonal faceted InAs NW in-situ coated with Al. The SEM images showing overall morphology bending of the (e) round and (f) faceted InAs NWs. The notch at the end of the faceted wires forms during the faceting process. The red arrows point at a NW intersection. 126x153mm (150 x 150 DPI)


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Figure 4. (a) HR-TEM image and (b) atomic structure model of polycrystalline Al shells deposited on a round shape InAs NW: {2110}WZ (InAs) and {110}FCC (Al) plan views. (c) An atomic structure model and (d) a HR-TEM image of a segment of Al shell having parallel orientation to the InAs NW (Type-α); (e) an enlarged HR-TEM image showing good matching of 2 InAs atomic layers with 3 Al atomic layers. (f) Atomic structure model and (g) HR-TEM image of polycrystalline Al shell with perpendicular orientation to the InAs NW (typeβ), showing high mismatch between InAs NW and Al shell and, in consequence, semi-periodic dislocations on the InAs side of the interface (inset). The red arrows point at dislocations. 230x281mm (150 x 150 DPI)


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Figure 5. (a) Schematic illustration and (b) SEM image of the S-N junction device used for transport measurements. (c) Differential conductance as function of SD voltage with two top-gate biases: low barrier (VTG = -0.2 V, red line) shows an Anreev state at ~100 µV and an induced gap of ~210 µV; high barrier (VTG = -0.3 V, blue line) shows a hard superconducting gap in the absence of Andreev reflection. 127x179mm (150 x 150 DPI)


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Robust Epitaxial Al Coating of Reclined InAs Nanowires - Nano

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