Crystal Phases in Hybrid Metal–Semiconductor Nanowire Devices

Feb 23, 2017 - We investigate the metallic phases observed in hybrid metal-GaAs nanowire devices obtained by controlled thermal annealing of Ni/Au ele...
4 downloads 17 Views 3MB Size
Download PDF
Letter pubs.acs.org/NanoLett

Crystal Phases in Hybrid Metal−Semiconductor Nanowire Devices J. David,† F. Rossella,*,‡ M. Rocci,‡ D. Ercolani,‡ L. Sorba,‡ F. Beltram,‡ M. Gemmi,*,† and S. Roddaro‡,§ †

Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy NEST, Scuola Normale Superiore and Istituto Nanoscienze-CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy

‡

S Supporting Information *

ABSTRACT: We investigate the metallic phases observed in hybrid metal-GaAs nanowire devices obtained by controlled thermal annealing of Ni/Au electrodes. Devices are fabricated onto a SiN membrane compatible with transmission electron microscopy studies. Energy dispersive X-ray spectroscopy allows us to show that the nanowire body includes two Ni-rich phases that thanks to an innovative use of electron diffraction tomography can be unambiguously identified as Ni3GaAs and Ni5As2 crystals. The mechanisms of Ni incorporation leading to the observed phenomenology are discussed. KEYWORDS: Electron diffraction tomography, transmission electron microscopy, hybrid metal−semiconductor nanowires, thermal annealing, GaAs nanowires

T

he semiconductor nanowire (NW) technology1 represents a unique platform to engineer novel nanostructures yielding to advanced device functionalities ranging from nanoelectronic applications2,3 to optoelectronics,4−9 nanophotonics,10−12 nanoplasmonics,13,14 sensing,15,16 and energy harvesting17 applications. Recently, nanometer-scale axial metal−semiconductor device fabrication was demonstrated by exploiting ex situ thermal annealing. Metallic contacts were evaporated on individual NWs and then annealed at few 100 °C above room temperature for several minutes. This drives the migration of metal atoms from the contacts into the NW body, generating a metallic alloy in different segments of the NW. This method, first demonstrated in the SiNi/Si system,18 allowed to broaden the range of materials that can be axially combined in a single NW and opened the way to innovative hybrid metal−semiconductor nanoscale systems. Ex situ thermal annealing was in fact applied to a variety of nanosystems, not only Si-based NWs18−21 but also Ge NWs,22 Ge−Si core−shell NWs,23 InAs NWs,24 and GaAs NWs.25−27 Despite the manifest potential impact of hybrid metal− semiconductor NW devices, few studies investigated the exact crystal phases present in these nanojunctions.19,22,24,28 The main reason was likely the technical challenge of combining © XXXX American Chemical Society

thermally annealed devices with transmission electron microscopy (TEM). In particular, for the recently reported Ni-GaAs metallic alloy,27 the structural analysis of the newly formed metal phases is missing. In this work, we investigate the chemical composition and the crystalline nature of the Ni-rich phases occurring in hybrid metal−GaAs nanowire devices fabricated via the controlled thermal annealing of Ni/Au electrodes. GaAs NW-based devices were fabricated onto a 50 nm thick SiN membrane that is compatible with TEM investigations and they were thermally annealed in nitrogenrich atmosphere in order to promote Ni-diffusion into the desired portions of the NW. Using energy dispersive X-ray spectroscopy (EDX), we identify the pristine GaAs region and two Ni-rich phases characterized by different stoichiometry. Exploiting electron diffraction tomography (EDT),29 we reconstruct the reciprocal lattice and deduce the crystal parameters of each phase. In order to achieve this goal, in this Letter we present a novel protocol for the investigation of nanoscale crystal domains. Besides the expected GaAs phase, we identify two different structures that are compatible with Received: December 16, 2016 Revised: February 17, 2017 Published: February 23, 2017 A

DOI: 10.1021/acs.nanolett.6b05223 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters Ni3GaAs and Ni5As2. Quite strikingly, we find that for the Ni3GaAs phase an epitaxial relation is maintained between the pristine semiconductor and the metal alloy, despite the significant modification of the local crystal structure brought by Ni diffusion. We propose and discuss the mechanism of Ni incorporation underlying the observed phenomenology. A prototypical example of the NW-based devices used in our TEM investigations is shown in Figure 1. GaAs NWs with

crystalline phases occurring in the NW after thermal annealing, we exploited EDT. This technique consists of acquiring a set of diffraction patterns while tilting the sample around the goniometer axis of the microscope (x-axis in Figure 1c) within a certain angular range, in small angular steps, typically of 1° or less. Usually EDT measurements are performed while precessing the beam in order to integrate all reflections thus making the signal intensity suitable for structure-resolution analysis using methods developed for kinematical scattering (e.g., direct methods or charge flipping31). Here we present and apply an innovative approach for the identification of the crystal structure of domains with nanometric dimensions32 based on EDT analysis. For this purpose, we need both to precisely control the location of the electron beam (to avoid signal coming from nearby domains) and to derive exact unit-cell parameters from the selected data. Because of the small size of the investigated domains (usually extending for ∼150 nm in the axial direction), experimental patterns were too faint for data collection with a standard charge-coupled device. We therefore exploited last generation Timepix single-electron detection camera, which has zero background noise and is able to detect the signal from an individual electron.33 Dedicated software34 was used in order to merge the patterns measured at different angles (see Figure 1g) that contribute to the reconstruction of the reciprocal space in three dimensions. The latter in turn allows one to extract cell parameters and detect the relative crystallographic orientation of the phases.29 In our case, the sample was tilted around the x-axis (indicated in Figure 1c,g and lying in the SiN membrane plane) by a step interval of 1° in the range from −20° to +40°. Figure 1d−f reports, as an example, three diffraction patterns measured in a pristine GaAs NW at angles α = −18°, +3°, and +39°, respectively. It is worth mentioning that the smallest parallel beam available in our microscope was ∼150 nm, and this represents our spatial resolution limit. In principle, this limit could be pushed down to 10−20 nm, for example, using a field-emission electron gun (FEG) machine. As the dimensions of the investigated nanostructure shrink, achieving a good diffraction signal becomes more and more challenging and the characteristics of the single electron detector play a key role. In recent works,35,36 the crystal structure of grains having size of ∼100 nm was resolved using EDT data set. However, in these works data set were collected on a dispersion of nanocrystals, while in our case we performed EDT experiments in single nanowire devices. We note that metal−semiconductor junctions in thermally annealed NW devices were previously investigated using high-resolution (HR)-TEM.19,22,24,28 This technique, useful to gather local information on the junction, is however poorly suited for the crystallographic recognition of the phases of nanodomains, which is instead the purpose of our work. In fact, tomography provides 3D diffraction data and allows one to address the mutual orientation of neighboring crystal domains, avoiding the challenging search for projections along very specific directions, as it would be required in the case of HRTEM. As recently reported by some of us,27 rapid thermal annealing of the NiAu bilayer electrodes in nitrogen atmosphere (see Methods) leads to the formation of Ni-rich metallic phases inside the NW body; the interface between the metallized and pristine segments of the NW is clearly visible as a contrast step in the scanning electron micrograph. This scenario is illustrated in Figure 2, where the GaAs and Ni-rich NW sections are labeled as region I and II, respectively. Preliminarily, we used

Figure 1. Electron diffraction tomography in a single nanowire-based device. (a) Scanning electron micrograph of one of the studied devices and (b) schematic device cross-section. N-doped GaAs NWs were deposited onto a SiN membrane and contacted by two Ni/Au (10/ 100 nm) ∼300 nm wide electrodes. (c) Simplified sketch of the device and of the experimental setup. The SiN membrane, transparent to electrons, is supported by a thicker silica membrane with a central hole allowing the electrons to pass through the sample, which can be tilted by an angle α around the x-axis. The TEM (Timepix) and EDX detectors are indicated. (d−f) Diffraction patterns measured at different angles. (g) The merger of different diffraction patterns via dedicated software.

typical length of 1.5 μm and diameter of 65 nm were grown by metal-seeded chemical beam epitaxy30 using a Se-based precursor to introduce n-type doping (see Methods). The NWs were transferred onto a SiN membrane (∼50 nm thick) and lithographically defined Ni/Au (10/100 nm) electrodes were evaporated at the two ends of the nanostructure. Figure 1a shows a scanning electron microscopy (SEM) image of one of the investigated devices, while a schematic cross section of the electrode is shown in Figure 1b. Figure 1c schematically illustrates a cross section of the device architecture and depicts the experimental setup for the TEM investigation of single NWs. In order to determine the cell parameters of the different B

DOI: 10.1021/acs.nanolett.6b05223 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters Table 1. Lattice Parameters of the GaAs and Ni-Rich Phasesa phase GaAs a [Å] b [Å] c [Å] a/c α [deg] β [deg] γ [deg]

Ni3GaAs

Ni5As2

NW (meas.)

bulk [35]

NW (meas.)

bulk [36]

NW (meas.)

bulk [37]

3.95 3.95 6.27 0.63 88 91 120

3.989 3.989 6.564 0.61 90 90 120

3.92 3.97 5.07 0.78 90 88 123

3.87 3.87 5.03 0.77 90 90 120

6.75 6.73 12.31 0.55 90 90 120

6.815 6.815 12.506 0.54 90 90 120

a

The lattice parameters extracted from EDT analysis in different portions of hybrid metal−GaAs nanowires are reported, together with the corresponding parameters for the bulk counterpart from refs 37−39.

and this group of elements was used to generate atomic percent profiles (Supporting Information). The Ni atomic concentration profile indicates a strong diffusion of this element from the electrodes inside the NW body with the formation of a Nirich alloy and a corresponding depletion of Ni in the contact region near the NW (Supporting Information). Whereas the region I (far from the electrode) displays a composition compatible with the pristine semiconductor (50% Ga and 50% As) in the Ni-rich region II (close to the electrode), the chemical analysis indicates a Ni/Ga/As ratio of ∼3:1:1. EDT was then exploited in order to investigate the crystal structure in these two regions of the NW. In particular, because the transformed section (region II) of the NW usually displays an axial size of ∼200−400 nm, a condenser aperture of 5 μm was used in order to reduce the probed area down to a diameter of ∼150 nm. This in turn leads to the occurrence of weak and sparse diffraction spots that can be detected thanks to the high sensitivity of the Timepix camera and can successively be used to achieve a clear 3D reconstruction of the reciprocal space. More details about the experimental conditions are reported in the Supporting Information. The 3D reconstruction of the reciprocal space of region I (Figure 2a) is reported along the a* and c* axis projections in Figure 2b,c, respectively. The cell parameters deduced from this reconstruction are consistent with the GaAs phase (mainly wurtzite, see Table 1) with the occurrence of some streakings in the a* axis projection, indicating a slightly disordered stacking sequence along c, as usually observed in NWs.30 The reciprocal space 3D reconstruction of the phase containing nickel (region II in Figure 2a) is reported along the a* and c* axis projections in Figure 2d,e, respectively. In the region II, the calculated cell parameters are consistent with a NixGaAs structure with x ≈ 3 (see Table 11). It is worth noting that the data sets used for the reciprocalspace reconstruction of the two phases were gathered during the same data collection according to the following sequence: (i) measure a diffraction pattern in region I, (ii) measure a diffraction pattern in region II, (iii) tilt the sample of 1°, (iv) measure another pattern in region I, and (v) measure another pattern in region II. We iterated this sequence until the range from −20° to +40° was fully mapped. This makes it possible to compare directly the relative crystalline orientations of the two different phases. The orientation matrix of the two phases indicates an epitaxial relation with the two hexagonal cells equally oriented, having both c* axes lying parallel to the NW axis and both a* directions normal to the SiN plane.

Figure 2. Nickelization of a GaAs nanowire: the Ni3GaAs phase. (a) Scanning transmission electron micrograph of a NW device after thermal annealing. The pristine nanowire region (I) and the transformed nanowire region (II) are identified thanks to their different contrast. EDX spectroscopy identifies the GaAs composition in the region I, while it suggests the composition NixGaAs in region II with x ≈ 3 (Supporting Information). EDT allows reconstructing the crystalline unit cell of the two regions and confirms the structure of GaAs and of Ni3GaAs. (b−d) Reciprocal space 3D reconstruction collected on the region I projected along a* (panel b) and c* (panel d) directions of the GaAs reciprocal cell, which are normal and parallel to the NW axis, respectively. (c−e) Reciprocal space 3D reconstruction of the region II projected along a* (panel c) and c* (panel e) directions of the Ni3GaAs reciprocal cell, which are parallel to the projection displayed in (b,d), respectively.

EDX spectroscopy in order to identify the chemical composition of the different NW regions. The averaged EDX spectra obtained along the axial cross-section indicate, as expected, a sizable presence of Ga, As, Ni, and Au elements, C

DOI: 10.1021/acs.nanolett.6b05223 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters

reconstruction of the reciprocal space shows intense spots that can be indexed with a Ni3GaAs unit cell (red dashed hexagons in Figure 3d). Additional weaker spots are present in the (001) projection that are consistent with a Ni5As2 unit cell (blue dashed hexagon in Figure 3d; see also Table 1). In region IV, EDT data collection is cleaner and allows the identification of Ni5As2 structure only (Figure 3e). We link the simultaneous occurrence of both phases (Ni3GaAs and Ni5As2) in the region III to the electron beam probing the interface between regions III and IV that is indeed less abrupt with respect to the case of shorter annealing times. We did not observe any epitaxial relation between the Ni3GaAs and the Ni5As2 segments; in all the investigated devices, they display a random mutual orientation. Overall, we can describe our experimental findings invoking a mechanism of Ni diffusion inside the GaAs occurring in two steps: a first step in which a NixGaAs structure is formed with x fluctuating around 3, followed by a second step in which Ga desorbes from the NW and a Ni5As2 phase is formed. This NixGaAs phase was previously detected by TEM in the bulk as well as in thin films40−44 (obtaining x ≈ 241,42 to 3.5540,43,44) and correlated to annealing at temperatures exceeding 200 °C. On the contrary, the observation of the Ni5As2 phase is completely new in this system. The epitaxial relation between the pristine GaAs and the NixGaAs phase suggests that the diffusion of Ni into GaAs is a smooth process. The two structures are formed by stacking of similar layers (see Figure 4a,b), and the nickelization process can be understood as a

When the thermal annealing is carried out for longer times, typically 15−20 min (see also Supporting Information), the solid state reaction leads to a different scenario, as reported in Figure 3. The two regions of the NW labeled as III and IV in

Figure 3. Nickelization of a GaAs nanowire: the Ni5As2 phase. (a) Scanning transmission electron micrograph of a NW device after longer thermal annealing time. EDX spectroscopy carried out in the regions III and IV identifies the compositions (NixGaAs + NiyAsz) and NiyAsz with x ≈ 3, y ≈ 5 and z ≈ 2, respectively (Supporting Information). Crystalline unit cell reconstruction by means of electron diffraction tomography confirms the occurrence of the structures of Ni3GaAs and of Ni5As2. (b,d) Reciprocal space 3D reconstruction collected on the region III projected along a* (panel b) and c* (panel d) directions of the Ni3GaAs reciprocal cell. (c,e) Reciprocal space 3D reconstruction collected on the region IV projected along a* (panel c) and c* (panel e) directions of the Ni5As2 reciprocal cell.

Figure 4. Crystal structures of the observed phases. (a) GaAs. (Top) view of the structure along a*. (Bottom) view of one GaAs layer along c (NW axis). (b) NixGaAs with x ≈ 3. (Top) view of the structure along a*. (Bottom) view of the mixed Ni−Ga−As layer along c. (c) Ni5As2. (Top) view of the structure along a*. (Bottom) view of the structure along c.

Figure 3a display a similar contrast in the scanning transmission electron microscopy (STEM) micrograph but are characterized by a different chemical composition, as are the results from the EDX point analysis as well as in the averaged spectra collected along axial cross sections of the NW (Supporting Information). Ga, As, Ni, and Au elements are detected and the Ni atomic concentration profile suggests a more complex diffusion of this element inside the NW body with the formation of Ni-rich alloys with different Ni concentration in regions III and IV. Region III, which was not affected for shorter annealing times, displays after longer annealing a composition compatible with the occurrence of NixGaAs with x ≈ 3. Region IV, closer to the electrode, is also Ni-rich but displays an homogeneous composition NiyAsz with y ≈ 5 and z ≈ 2. EDT data sets were also collected in the regions III and IV, and the results are reported in panels (b−e) of Figure 3. In region III, the 3D

diffusion of Ni into the wurtzite GaAs structure. The NixGaAs is indeed easily formed by replacing one Ga/As site of the GaAs layers with a half occupied Ni site and by filling the gap in between with a Ni layer. The remaining semiconductor site in the mixed layer is statistically occupied either by Ga and As. In addition, differently from the case of the wurtzite structure, Ga and As lay exactly on the same plane. In the NixGaAs, the same mole of GaAs occupies a larger volume, which explains our frequent observation of slightly distorted NWs in the Ni3GaAs segment (see Figure 2a). On the contrary, the Ni5As2 phase cannot be arranged in a layered structure (see Figure 4c) and therefore its formation starting from NixGaAs requires a complete recrystallization and no epitaxial relation is retained. D

DOI: 10.1021/acs.nanolett.6b05223 Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters

■

Noticeably, the occurrence of an epitaxial relation between the pristine GaAs and the Ni3GaAs phase is reflected in the electrical transport properties of the investigated hybrid Ni3GaAs−GaAs NW devices. In fact, we performed transport experiments in a variety of devices similar to those investigated in the present work. This is widely described in the Supporting Information (Section 4 Electrical Transport Experiments). Although qualitatively similar transport features were observed in hybrid devices displaying the Ni3GaAs phase or the Ni5As2 phase, the best results in terms of stability and reproducibility of the electrical response were achieved for the case of the Ni3GaAs metal alloy. In conclusion, we investigated the chemical nature and the crystalline structure of the metallic phases occurring in hybrid metal−semiconductor NW-based devices, obtained thanks to the ex situ thermal annealing of Ni/Au contact electrodes evaporated on GaAs NWs. A solid-state reaction transforms portions of the NW from the pristine GaAs semiconductor into a Ni-rich metallic phase due to the migration of nickel atoms from the electrode into the NW body. The phase transformation evolves with time and two different stages were identified. In the early stage (short annealing time), an epitaxial and metallic Ni3GaAs phase is formed. In the later stage (longer annealing time), the Ni3GaAs phase is further transformed into Ni5As2. The present results provide better insight on the solidstate transformation yielding hybrid metal−GaAs NW-based devices that we believe is of much importance to optimize innovative device fabrication protocols and to open the way to the exploitation of the unique electronic properties of nanoscale metal−semiconductor junctions. Methods. GaAs NWs were grown by Au-assisted chemical beam epitaxy (CBE) in a Riber Compact 21 system. The system employs pressure control in the metalorganic (MO) lines to determine precursor fluxes during sample growth. A calibrated orifice at the injector insures the proportionality between line pressure and precursor flow through the injector. The precursors involved in the NW growth are triethylgallium (TEGa), tertiarybutylarsine (TBAs), and ditertiarybutyl selenide (DtBSe) as selenium source for n-type doping. A nominally 0.5 nm thick Au film was first deposited on (111)B GaAs wafers by thermal evaporation. Before the growth was initiated, the sample was annealed at 580 ± 10 °C under TBAs flow for 20 min in order to dewet the Au film into nanoparticles and to remove the surface oxide from the GaAs substrate. GaAs NWs were grown at a temperature of 650 ± 10 °C for 30 min and then the growth temperature was increased to 670 ± 10 °C and the GaAs NW growth was continued for another 90 min. The MO line pressures employed were 0.7, 1.0, and 1.0 Torr for TEGa, TBAs and DtBSe, respectively. Ex situ rapid thermal annealing (RTA) was performed in a controlled-atmosphere chamber using the following parameters: annealing temperature 280 °C, N2 atmosphere at pressure of 260 mbar. The TEM observations were performed on a Zeiss Libra 120 microscope operating at an accelerating voltage of 120 keV, equipped with an in-column omega filter for energy-filtered imaging. EDX spectroscopy analyses (point and line profiles) were carried out on the same microscope working in scanning mode (STEM) with a HAADF detector thanks to a Bruker XFlash 6T − 60 SDD detector. EDT data collection was carried out by recording the patterns with a Timepix single-electrondetection camera.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b05223. EDX spectra; TEM experimental conditions; thermal annealing (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

F. Rossella: 0000-0002-0601-4927 Present Address §

Dipartimento di Fisica “E. Fermi”, Università di Pisa, Largo Pontecorvo 3, I-56127 Pisa, Italy

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Scuola Normale Superiore, by CNR through the CNR-RFBR bilateral program 2015-2017, and by Regione Toscana through the project “Felix”. F.R. and M.R. acknowledge the support by the MIUR through the program “FIRB - Futuro in Ricerca 2013” - Project “UltraNano” (Grant RBFR13NEA4).

■

REFERENCES

(1) Zhang, A.; Zheng, G.; Lieber, C. M. Nanowires: Building blocks for nanoscience and nanotechnology; Springer, 2016. (2) Roddaro, S.; Pescaglini, A.; Ercolani, D.; Sorba, L.; Beltram, F. Manipulation of Electron Orbitals in Hard-Wall InAs/InP Nanowire Quantum Dots. Nano Lett. 2011, 11, 1695−1699. (3) Rossella, F.; Bertoni, A.; Ercolani, D.; Rontani, M.; Sorba, L.; Beltram, F.; Roddaro, S. Nanoscale spin rectifiers controlled by the Stark effect. Nat. Nanotechnol. 2014, 9, 997−1001. (4) Barrelet, C. J.; Greytak, A. B.; Lieber, C. M. Nanowire photonic circuit elements. Nano Lett. 2004, 4, 1981−1985. (5) Sirbuly, D. J.; Law, M.; Yan, H.; Yang, P. Semiconductor Nanowires for Subwavelength Photonics Integration. J. Phys. Chem. B 2005, 109, 15190−15213. (6) Agarwal, R.; Lieber, C. M. Semiconductor nanowires: optics and optoelectronics. Appl. Phys. A: Mater. Sci. Process. 2006, 85, 209−215. (7) Yan, R.; Gargas, D.; Yang, P. Nanowire photonics. Nat. Photonics 2009, 3, 569−576. (8) Guo, X.; Ying, Y.; Tong, L. Photonic Nanowires: From Subwavelength Waveguides to Optical Sensors. Acc. Chem. Res. 2014, 47, 656−666. (9) Brubaker, M. D.; Blanchard, P. T.; Schlager, J. B.; Sanders, A. W.; Roshko, A.; Duff, S. M.; Gray, J. M.; Bright, V. M.; Sanford, N. A.; Bertness, K. A. On-Chip Optical Interconnects Made with Gallium Nitride Nanowires. Nano Lett. 2013, 13, 374−377. (10) Qian, F.; Li, Y.; Gradeak, S.; Park, H. G.; Dong, Y.; Ding, Y.; Wang, Z. L.; Lieber, C. M. Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers. Nat. Mater. 2008, 7, 701− 706. (11) Heiss, M.; Fontana, Y.; Gustafsson, A.; Wust, G.; Magen, C.; O’Regan, D. D.; Luo, J.-W.; Ketterer, B.; Conesa-Boj, S.; Kuhlmann, A. V.; Houel, J.; Russo-Averchi, E.; Morante, J. R.; Cantoni, M.; Marzari, N.; Arbiol, J.; Zunger, A.; Warburton, R. J.; Fontcuberta i Morral, A. Self-assembled quantum dots in a nanowire system for quantum photonics. Nat. Mater. 2013, 12, 439−444.

E

DOI: 10.1021/acs.nanolett.6b05223 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters (12) Barrelet, C. J.; Bao, J.; Loncar, M.; Park, H.-G.; Capasso, F.; Lieber, C. M. Hybrid Single-Nanowire Photonic Crystal and Microresonator Structures. Nano Lett. 2006, 6, 11−15. (13) Arcangeli, A.; Rossella, F.; Tomadin, A.; Xu, J.; Ercolani, D.; Sorba, L.; Beltram; Tredicucci, A.; olini, M.; Roddaro, S. Gate-Tunable Spatial Modulation of Localized Plasmon Resonances. Nano Lett. 2016, 16, 5688−5693. (14) Stiegler, J. M.; Huber, A. J.; Diedenhofen, S. L.; Gomez Rivas, J.; Algra, R. E.; Bakkers, E. P. A. M.; Hillenbrand, R. Nanoscale free carrier profiling of individual semiconductor nanowires by infrared near-field nanoscopy. Nano Lett. 2010, 10, 1387−1392. (15) Hayden, O.; Agarwal, R.; Lieber, C. M. Nanoscale avalanche photodiodes for highly sensitive and spatially resolved photon detection. Nat. Mater. 2006, 5, 352−356. (16) 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. (17) Tang, J.; Huo, Z.; Brittman, S.; Gao, H.; Yang, P. Solutionprocessed core-shell nanowires for efficient photovoltaic cells. Nat. Nanotechnol. 2011, 6, 568−572. (18) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Single-crystal metallic nanowires and metal/semiconductor nanowire heterostructures. Nature 2004, 430, 61−65. (19) Lin, Y.-C.; Lu, K.-C.; Wu, W.-W.; Bai, J.; Chen, L. J.; Tu, K. N.; Huang, Y. Single Crystalline PtSi Nanowires, PtSi/Si/PtSi Nanowire Heterostructures, and Nanodevices. Nano Lett. 2008, 8, 913−918. (20) Mongillo, M.; Spathis, P.; Katsaros, G.; Gentile, P.; Sanquer, M.; de Franceschi, S. Joule-assisted silicidation for short-channel silicon nanowire devices. ACS Nano 2011, 5, 7117−7123. (21) Mongillo, M.; Spathis, P.; Katsaros, G.; Gentile, P.; de Franceschi, S. Multifunctional devices and logic gates with undoped silicon nanowires. Nano Lett. 2012, 12, 3074−3079. (22) Kral, S.; Zeiner, C.; Stoìger-Pollach, M.; Bertagnolli, E.; den Hertog, M. I.; Lopez-Haro, M.; Robin, E.; El Hajraoui, K-.; Lugstein, A. Abrupt Schottky Junctions in Al/Ge Nanowire Heterostructures. Nano Lett. 2015, 15, 4783−4787. (23) Hu, Y.; Xiang, J.; Liang, G.; Yan, H.; Lieber, C. M. Sub-100 Nanometer Channel Length Ge/Si Nanowire Transistors with Potential for 2 THz Switching Speed. Nano Lett. 2008, 8, 925−930. (24) Chueh, Y.-L.; Ford, A. C.; Ho, J. C.; Jacobson, Z. A.; Fan, Z.; Chen, C.-Y.; Chou, L.-Y.; Javey, A. Formation and Characterization of Ni xInAs/InAs Nanowire Heterostructures by Solid Source Reaction. Nano Lett. 2008, 8, 4528−4533. (25) Orru, M.; Rubini, S.; Roddaro, S. Formation of axial metalsemiconductor junctions in GaAs nanowires by thermal annealing. Semicond. Sci. Technol. 2014, 29, 054001. (26) Orru, M.; Piazza, V.; Rubini, S.; Roddaro, S. Rectification and Photoconduction Mapping of Axial Metal-Semiconductor Interfaces Embedded in GaAs Nanowires. Phys. Rev. Appl. 2015, 4, 044010. (27) Rossella, F.; Piazza, V.; Rocci, M.; Ercolani, D.; Sorba, L.; Beltram, F.; Roddaro, S. GHz Electroluminescence Modulation in Nanoscale Subwavelength Emitters. Nano Lett. 2016, 16, 5521−5527. (28) Fauske, V. T.; Huh, J.; Divitini, G.; Dheeraj, D. L.; Munshi, A. M.; Ducati, C.; Weman, H.; Fimland, B.-O.; van Helvoort, A. T. J. In Situ Heat-Induced Replacement of GaAs Nanowires by Au. Nano Lett. 2016, 16, 3051−3057. (29) Kolb, U.; Gorelik, T. E.; Otten, M. T. Towards automated diffraction tomography. Part II-Cell parameter determination. Ultramicroscopy 2008, 108, 763−772. (30) Zannier, V.; Ercolani, D.; Gomes, U. P.; David, J.; Gemmi, M.; Dubrovskii, V. G.; Sorba, L. Catalyst Composition Tuning: The Key for the Growth of Straight Axial Nanowire Heterostructures with Group III Interchange. Nano Lett. 2016, 16, 7183−7190. (31) Mugnaioli, E.; Gorelik, T. E.; Kolb, U. Ab initio structure solution from electron diffraction data obtained by a combination of automated diffraction tomography and precession technique. Ultramicroscopy 2009, 109, 758−765.

(32) Baraldi, A.; Buffagni, E.; Capelletti, R.; Mazzera, M.; Fasoli, M.; Lauria, A.; Moretti, F.; Vedda, A.; Gemmi, M. Eu incorporation into sol-gel silica for photonic applications: spectroscopic and TEM evidences of quartz and Eu pyrosilicate nanocrystal growth. J. Phys. Chem. C 2013, 117, 26831−26848. (33) van Genderen, E.; Clabbers, M. T. B.; Das, P. P.; Stewart, A.; Nederlof, I.; Barentsen, K. C.; Portillo, Q.; Pannu, N. S.; Nicolopoulos, S.; Gruene, T.; Abrahams, J. P. Ab initio structure determination of nanocrystals of organic pharmaceutical compounds by electron diffraction at room temperature using a Timepix quantum area direct electron detector. Acta Crystallogr., Sect. A: Found. Adv. 2016, 72, 236− 242. (34) Palatinus, L. PETS: a program for analysis of electron diffraction data; Institute of Physics: Prague, Czech Republic, 2011. (35) Birkel, C. S.; Mugnaioli, E.; Gorelik, T.; Kolb, U.; Panthöfer, M.; Tremel, W. Solution Synthesis of a New Thermoelectric Zn1+xSb Nanophase and Its Structure Determination Using Automated Electron Diffraction Tomography. J. Am. Chem. Soc. 2010, 132, 9881−9889. (36) Mugnaioli, E.; Andrusenko, I.; Schler, T.; Loges, N.; Dinnebier, R. E.; Panthöfer, M.; Tremel, W.; Kolb, U. Ab Initio Structure Determination of Vaterite by Automated Electron Diffraction. Angew. Chem., Int. Ed. 2012, 51, 7041−7045. (37) McMahon, M. I.; Nelmes, R. J. Observation of a Wurtzite Form of Gallium Arsenide. Phys. Rev. Lett. 2005, 95, 215505. (38) Zheng, X.-Y.; Lin, J.-C.; Swenson, D.; Hsieh, K.-C.; Chang, Y. A. Phase equilibria of Ga, Ni, As at 600°C and the structural relationships between Y-Ni3Ga2, Y′-Ni13Ga9 and T-Ni3GaAs. Mater. Sci. Eng., B 1989, 5, 63−72. (39) Kjekshus, A.; Skaug, K. E. On the crystal structure of Ni5As2. Acta Chem. Scand. 1973, 27, 582−588. (40) Sands, T.; Keramidas, V. G.; Washburn, J.; Gronsky, R. Structure and composition of NixGaAs. Appl. Phys. Lett. 1986, 48, 402−404. (41) Lahav, A.; Eizenberg, M.; Komem, Y. Interfacial reactions between Ni films and GaAs. J. Appl. Phys. 1986, 60, 991−1001. (42) Chen, S. H.; Carter, C. B.; Palmstrom, C. J.; Ohashi, T. Transmission electron microscopy studies on lateral reaction of GaAs with Ni. Appl. Phys. Lett. 1986, 48, 803−805. (43) Guarin, R.; Guivarch, A. Metallurgical study of Ni/GaAs contacts. I. Experimental determination of the solid portion of the NiGa-As ternary-phase diagram. J. Appl. Phys. 1989, 66, 2122−2128. (44) Guivarch, A.; Guerin, R.; Caulet, J.; Poudoulec, A.; Fontenille, J. Metallurgical study of Ni/GaAs contacts. II. Interfacial reactions of Ni thin films on (111) and (001) GaAs. J. Appl. Phys. 1989, 66, 2129− 2136.

F

DOI: 10.1021/acs.nanolett.6b05223 Nano Lett. XXXX, XXX, XXX−XXX