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Exploring Au droplet motion in nanowire growth: a simple route towards asymmetric GaP morphologies Bruno César Da Silva, Douglas Soares Oliveira, Fernando Iikawa, Odilon D. D. Couto Jr., Jefferson Bettini, Luiz Fernando Zagonel, and Monica Alonso Cotta Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02770 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017
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Exploring Au droplet motion in nanowire growth: a
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simple route towards asymmetric GaP morphologies
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Bruno C. da Silva1, Douglas S. Oliveira1, Fernando Iikawa1, Odilon D. D. Couto Jr1,
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Jefferson Bettini2, Luiz F. Zagonel1 and Mônica A. Cotta1.
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Campinas, São Paulo, Brazil
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Institute of Physics“Gleb Wataghin”, State University of Campinas, 13083-859
Brazilian Nanotechnology National Laboratory, National Center for Research in
Energy and Materials, C P 6192, 13083-970 Campinas, São Paulo, Brazil
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ABSTRACT:
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Here we show a new nanowire growth procedure, exploring the thermally-activated
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motion of Au droplets on III-V surfaces. We show that, by setting a single growth
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parameter, we can activate the crawling motion of Au droplets in vacuum and locally
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modify surface composition in order to enhance vapor-solid (VS) growth along oxide-
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free areas on the trail of the metal particle. Asymmetric VS growth rates are comparable
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in magnitude to the vapor-liquid-solid growth, producing unconventional wurtzite GaP
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morphologies, which shows negligible defect density as well as optical signal in the
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green spectral region. Finally, we demonstrate that this effect can also be explored in
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different substrate compositions and orientations, with the final shape finely tuned by
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group III flow and nanoparticle size. This distinct morphology for wurtzite GaP
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nanomaterials can be interesting for the design of nanophotonics devices.
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Gallium
Phosphide,
Asymmetrically-shaped
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KEYWORDS:
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Nanoparticle Crawling, Self-assembled Nanotrail
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Nanowire,
Gold
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Manuscript text:
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Nanowire morphologies provide an opportunity to access novel electronic and
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optical properties of the extensively studied III-V semiconductor materials due to
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nanoscale effects.1-4 In particular, direct band gap Gallium Phosphide (GaP) nanowires
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in the wurtzite (WZ) crystal structure have been demonstrated as a promising material
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for different purposes as hydrogen production by water splitting5, efficient solid-state
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green emitters6,7, light-harvesting applications8 and as compatible lattice substrate to
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epitaxial growth of hexagonal Si.9
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Actually,
non-conical
or
cylindrical
geometries
in
metal-catalyzed
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semiconductor nanostructures have emerged as a new trend to improve device
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performances, as demonstrated for high electron mobility InSb nanosails (or
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nanosheets),10,11 high efficiency light polarized sources in InP nanoflags12 and enhanced
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light scattering from V-shaped III-V nanomembranes.13 Nevertheless, one of the most
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promising benefits of alternative nanostructure geometries is in the improvement of
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optical absorption for photovoltaic devices, as extensively explored in Si
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nanomaterials.14-21 Despite those results, some phenomena are better suited to the
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traditional, high aspect ratio nanowire morphology, such as enhanced light absorption
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through light trapping, due to resonance effects when the structure diameter is
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comparable to the wavelength scale.22,23 On the other hand, different shapes, such as
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conical forms, can reduce reflection losses due to the gradual change of the refractive
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index.18-20 Thus, distinct geometries can lead to light-harvesting devices with different
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functionalities, and in order to achieve specific morphology control, new growth
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procedures or routes have to be created.24,25
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Recently, the spontaneous motion of Au droplets on III-V surfaces has been
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studied due to the role of this metal particle in Vapor-Liquid-Solid mechanism, usually
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employed in III-V nanowire growth.26-28 In particular, the ability to control droplet
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crawling opens up the possibility to create novel structures and to facilitate nanowire
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integration in device fabrication, through a process totally determined by an in situ
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mechanism, which can reach similar size limits as electron beam lithography. The
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fabrication of oxide free surfaces by Au droplet crawling has already been observed28;
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however, the direct impact of Au droplet motion in nanowire growth to create novel
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morphologies has not been determined yet. Here, we report the proof of concept for this
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idea, showing the growth of unconventional morphologies in defect-free WZ GaP
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nanomaterials by exploring the thermally-activated motion of small Au droplets on III-
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V surfaces.
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The annealing procedure which induces droplet motion used in this work was
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carried out with colloidal Au nanoparticles of 20 and 50 nm nominal diameters (in order
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to render visualization easier) deposited on (100) GaAs for 5 min either in vacuum (~10-
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450, 480, 510 and 540°C. Our GaP nanowires were grown on epiready (100) and
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(111)B GaAs, as well as (100) InP substrates by Chemical Beam Epitaxy (CBE) for 60
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minutes, without any previous surface treatment, and using colloidal Au nanoparticles
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with 5, 20 and 50 nm diameters in the same temperature range used for the annealing
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treatment. The substrate native oxide was not desorbed prior to GaP growth. Triethyl-
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gallium (TEG) with hydrogen (H2) as carrier gas and thermally decomposed phosphine
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(PH3) were used as group III and V precursor sources, respectively. Most samples were
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grown using TEG + H2 at 8 sccm and PH3 at 20 sccm; the sample was cooled down
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under the same PH3 flow used during growth. Different growth times have been studied
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for growth performed at 510°C using 20nm Au NP’s on (100) GaAs and same flow
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conditions described above. Group III and V precursor flows were varied in the ranges
torr) or under As2 overpressure (with thermally decomposed AsH3 flow of 13 sccm) at
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from 2 to 20 sccm and 5 to 50 sccm, respectively, at 510°C using 20 nm and 5 nm Au
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NP’s on (100) GaAs substrates.
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The nanowires were characterized by scanning electron microscopy (Inspect F50
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and Philips XL-30) and transmission electron microscopy (JEOL 2100 LaB6 and JEOL
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2100F both operated at 200kV). TEM samples were prepared by touching lacey carbon
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copper grids on the substrate, thus breaking off some nanowires. Samples for cross-
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sectional and side-view imaging were prepared after transferring the nanowires to a Si
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substrate and directly on cleaved GaAs as-grown substrates, respectively, using a dual
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beam Helios Nanolab 660 microscope. Macro-photoluminescence (PL) measurements
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were performed at 10K, respectively, using a 442 nm line of a He-Cd laser, with power
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of 500 µW and 1mm spot diameter on the as-grown sample. Micro (µ)-PL
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measurements on single nanowires were performed at 20 K using 405 nm solid-state
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laser source with 1mW power and ~ 1µm laser spot size. Atomic Force Microscopy
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(AFM) measurements were acquired in air with a Keysight model 5500 equipment
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operating in non-contact mode with conical Si tips.
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We first studied the effect of the gas phase on the annealed 20nm Au
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nanoparticles at 510°C. The AFM image in Fig 1A shows that no surface modification
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is observed when AsH3 is present; however, the annealing made in vacuum impacts the
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stability of the Au droplet, Fig. 1B. In this case, the majority (~70%) of the droplets
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move, creating trails on the surface. The depth profile across the trail shows a V-shape,
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with lateral planes forming ~ 45° with the substrate surface (Fig. 1C). The motion of
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the droplets occurs along the two equivalent directions ±[110]; droplets move within
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distances in the range of 0.1 – 1.3 µm, as indicated by white arrows in Fig.1D. It is well
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known that liquid Au pieces or droplets on III-V surfaces strongly interact with the
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substrate when heated, dissolving material beneath the particles and forming a faceted 4 ACS Paragon Plus Environment
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pitch below the droplet.28-30 Indeed, the volume of the particle observed after annealing
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is at least 30% greater than the nominal value for the colloidal particles used, indicating
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substrate material consumption. Fig. 1C shows that the trails are 2 nm deep; (100) GaAs
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native oxide layer is approximately 0.8 nm.31 This result also suggests that the oxide
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layer was totally removed inside the trail due to the droplet crawling.
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The most likely scenario thus is that liquid Au droplets form V-shaped grooves
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composed by {111} planes on (100) GaAs surfaces, similarly to the behavior observed
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for interfacial reactions using Au thin films.30 These faceted grooves present two sets of
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Ga and As-terminated {111} planes, with projection on the (100) GaAs surface along
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the directions ±[110] and ±[1-10], respectively, as schematically represented in Figs. 1E
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and F. These two different set of facets present distinct chemical etching rates,
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depending on the etchant used.32,33 The reaction of Au and Ga-terminated {111} facets
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is much faster than for As-terminated planes.31,34 This inhomogeneous dissolution can
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explain the crawling along ± [110] directions, as illustrated in Figs. 1G and H.
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In order to investigate the effect of temperature on the droplet stability, samples
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were annealed in vacuum conditions at different temperatures in the range 450-540°C,
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for 5 min. We observe that the crawling occurs only at or above 510°C, as schematically
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shown in Fig. 1I. The motion is preserved along the [110] direction in this temperature
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range (Support Information Fig. S1). This effect is also dependent on the nanoparticle
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size; at 510°C, for 50nm Au nanoparticles just a few noticeably smaller droplets move
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across the substrate surface, also along a [110] direction (Support Information Fig. S2).
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Additionally, symmetrical trails form on both sides of the droplet, what suggests an
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oscillatory motion around the center of the trail. In other words, the observed behavior
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seems to be characterized by a stick-slip motion, similarly to previous observations for
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Ga droplets.35 5 ACS Paragon Plus Environment
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Figure 1 – Atomic Force Microscopy (AFM) of annealed samples. Topography images
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of 20 nm Au nanoparticles on (100) GaAs substrates annealed at 510°C for 5 min (A)
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under As2 overpressure and (B) in vacuum. (C) Height profile of the image in (B),
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showing that the trail is approximately 2nm deep. (D) Sample annealed in vacuum;
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white arrows mark trails with different lengths caused by the crawling of the Au
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droplets. (E and F) Top-view and side-view illustration of the groove formed beneath
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the Au nanoparticle with four {111} facets, which present different atomic terminations.
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(G and H) Illustration of the crawling occurring in the specific ±[110] directions, due to
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the faster dissolution rate of Ga-terminated {111} facets by Au nanoparticles. (I)
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Representation of the effect of annealing temperature on the 20 nm Au nanoparticle
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stability for the annealing in vacuum; droplet crawling occurs at or above a critical
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temperature (510°C).
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Spontaneous temperature-activated motion of liquid metal droplets on III-V
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semiconductor surfaces has already been reported, especially for Ga droplets.35-38 In this
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case, droplet dynamics have been shown to be intrinsically associated with differences
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in surface roughness on solid surfaces, such as the local step density37,38; in general, the
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motion can be interpreted as a result of the surface free energy gradient under
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nonequilibrium conditions, caused by changes in surface roughness38. Recently, the
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crawling of liquid droplets has been observed and studied in more detail for Au
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nanoparticles, where vacuum conditions27 and the introduction of water vapor28 during
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the heating step have been demonstrated to affect droplet stability. Despite chemical and
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surface energy differences for Ga and Au droplets on III-V surfaces, we can expect
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some common features.
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Heating the substrate in vacuum can generate pits in the oxide layer of (100)
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GaAs surfaces at the temperature range used here.39-41 In fact, we observe an increase in
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surface roughness (~ 40%) when the annealing was performed in vacuum in comparison
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to As2 overpressure. Therefore, the driving force for droplet motion, in our case, can be
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partially explained by the larger surface roughness, caused by our heating procedure.
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Thus, crawling of liquid Au droplets could minimize the total surface energy,
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eliminating steps and smoothening the surface, while consuming part of the underlying
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material.29 However, we should point out that surface morphology has profound
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implications on the As and Ga evaporation rates, which may affect the droplet motion in
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a more complicated scenario42, where the gradient of chemical composition around the
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droplet play a significant role.35
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Next, we have carried out GaP growth in the same temperature range used to
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study droplet dynamics, in order to investigate the overall effect of nanoparticle
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crawling on nanowire growth. In this case, we have used 5, 20 and 50 nm Au
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nanoparticles deposited on (100) GaAs substrates. Figures 2A-C show the resulting
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morphologies obtained with 5nm Au droplets. For the lower temperatures (450 and
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480°C), symmetric nanowire shapes, with large tapering due to the reasonably low axial
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growth rate, are obtained. For temperatures from 510 up to 540°C, an asymmetric
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morphology appears, characterized by a quasi-two-fold symmetry (Support Information
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Fig. S3), tilted by 36° relative to the substrate surface, Fig. 2D. The base shows a
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parallelogram shape, with a conical top. Different structure populations appear on the
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sample; however, about 80% of them are asymmetric (statistical analysis is provided in
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Support Information Fig. S4), and present very similar shapes.
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Figure 2 – SEM images of GaP nanowires grown on different growth temperatures. (A-
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C) growth carried out with 5 nm Au nanoparticles on (100) GaAs substrates at 450, 480
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and 510°C, respectively. The transition in the morphology can be clearly observed at
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510°C. (D) Asymmetric nanowires with 36° tilt relative to the substrate surface. (E) 8 ACS Paragon Plus Environment
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Main growth parameters for the formation of the asymmetric morphology, which occurs
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for small droplets and appears at a critical temperature (510°C).
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The asymmetric morphology is also observed as the main population (~60%)
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when we use 20nm Au nanoparticles under the same GaP growth conditions (Support
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Information Fig. S5A). For larger (50nm) Au catalysts, droplet crawling also takes place
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on the substrate surface. However, during growth the metal catalyst keeps contact with
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the substrate, leading to the formation of irregularly-shaped planar nanowires.43
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Nevertheless, smaller Au dots, found along the planar structure, give rise to the growth
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of asymmetric structures as well (Support Information Fig. S5B).
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The general asymmetry characteristics of the grown structures are preserved
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under different V/III ratios, as shown in Support Information Fig. S6A and B.
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Specifically, our results show that the nanostructure aspect ratio can be altered. The
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characteristic dimensions are not very sensitive to the group V precursor (Support
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Information Fig. S6C) but the general nanostructure shape can be modified by TEG
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flow. Low values of group III precursor result in morphologies that resemble a
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nanosheet with high aspect ratio D/L (Support Information Fig. S6D), due to the
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enhanced diffusion along the surface. Furthermore, the conical segment is enhanced
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under these conditions as well. On the other hand, high TEG flows provide more
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asymmetric structures, with lower aspect ratios (Support Information Fig. S6D), most
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likely due to mass transport by diffusion preferentially localized on the nanostructure
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surface. Thus by controlling V/III ratio and TEG flow in particular, we can achieve a
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finer morphology control within the temperature and nanoparticle sizes windows
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studied here. Fig. 2E summarizes our results, showing that, for a critical temperature
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(510°C) and small Au droplets (d ≤ 20nm), asymmetric nanowires can be reliably
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obtained. 9 ACS Paragon Plus Environment
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Crystalline structure and quality of our asymmetric morphologies were further
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evaluated by TEM (Fig. 3A) and SAED (Fig. 3A, inset). The nanostructures grow in the
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wurtzite phase, along the usual direction observed for hexagonal III-V
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nanowires44. The Au droplet on the top of the structure (Fig. 3B) indicates the presence
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of axial VLS growth.45 However, we do not observe the formation of commonly
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reported defects or distinct crystal structure necks, usually associated with the change in
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supersaturation during sample cool down.46,47 Despite the unusual morphology, detailed
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HRTEM observations (Support Information Fig. S7 A and B) show that the asymmetric
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GaP nanowires have good crystal quality, with approximately 1-2 stacking faults per
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micrometer. Symmetric nanowires grown in the wurtzite phase, however, present a
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larger density of stacking faults (inset in Support Information Fig. S8A), which can also
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be observed from the redshift in the Raman spectra (Support Information Fig. S8A) due
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to the strain created by the zinc blende insertions.
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We also performed photoluminescence (PL) measurements at low temperature
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for samples grown at 510°C using 5nm Au nanoparticles. Fig. 3C shows macro-PL
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measurements which reveal the optical signature of the pseudo-direct wurtzite GaP
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nanowires in the green spectral range; the sharp optical emission at 2.09 and 2.04 eV are
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attributed to donor-acceptor pair (DAP) recombination and its phonon replica.7 The use
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of organometallic precursors in CBE technique leads to incorporation of carbon
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impurities48 and since the number of stacking faults seems to be very low, the PL band
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below 2 eV is possibly related to impurity incorporation. Low temperature µ-PL
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measurements carried out at single symmetric and asymmetric nanostructures, in
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general, show mainly two emission bands, centered at 2.4 eV and 1.9 eV (Support
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Information Fig. S8B). Nevertheless, in the asymmetric case, the low energy band is
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dominant, while the high energy band is more intense for the symmetric nanostructures. 10 ACS Paragon Plus Environment
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The different PL spectra for the two types of morphologies are most likely associated
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with the dominant growth mechanism leading to their formation, as we discuss further
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below. Moreover, the difference between the macro- and the general µ-PL spectra
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shown here could be attributed to statistical variations associated to the small number of
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nanowires probed in µ-PL measurements. While the larger laser spot size in macro-PL
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allows us to probe a considerable number of nanostructures, the mechanical removal of
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nanowires from GaAs substrate reduces our ensemble to nanowires grown on specific
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areas of the substrate which can suffer from local variations in impurities incorporation.
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Another possibility could be related to the high excitation used in our µ-PL
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measurements, which can quench the 2.09 eV emission.49 The weak signal emitted by
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our single nanowires is expected, since the asymmetric morphology was not passivated.
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The overall morphology of metal-catalyzed nanowires is determined by the
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balance of axial VLS and the VS growth on the exposed sidewalls. When nanowires
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expose facets with different polarities and hence distinct VS growth rates, the overall
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morphology can be quite distinct from the metal-catalyzed nanowire core.50-53 Wurtzite
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III-V nanowires grown in the direction usually exhibit low surface energy {10-
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10} or {11-20} planes as sidewalls, which results in a regular hexagonal cross-
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section.54,55 TEM images (Fig. 3D) of our asymmetric GaP nanowires present an
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irregularly-shaped cross section, which can be interpreted as two juxtaposed hexagons
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with intermediary facets between them. The main facets correspond to {11-20} family
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planes, according to the SAED pattern indexed in the [0001] zone axis (inset, Fig. 3D);
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however, the overall morphology shows a quasi two-fold symmetry. Thus, since these
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non-polar planes should exhibit similar VS growth rates, they cannot account for the
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formation of this singular morphology.
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Figure 3 – Structural properties of asymmetric GaP nanowires. (A) Low magnification
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TEM images and SAED pattern (inset) indexed as WZ crystal structure in the [10-10]
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zone axis, showing that the growth direction is . (B) HRTEM image showing the
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Au 5 nm droplet at the top of the structure and the absence of a neck region with
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different crystal structure. (C) 10 K photoluminescence, showing the WZ GaP emission
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in the green spectral range. (D) Cross-sectional low magnification TEM image and
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SAED pattern (inset) indexed in the [0001] zone axis; the sidewalls are formed by
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apolar {11-20} planes.
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The lack of a high density of crystallographic defects and polar facets for the
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asymmetric nanowires suggests their association with the droplet crawling process,
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particularly because their formation occurs in the same temperature range. In fact,
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enhanced VS growth can be expected in the oxide-free region associated with the
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droplet trail, as similar temperature values are usually employed for thin film growth.
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The axial VLS growth rate decreases with temperature (Support Information Fig. S9A),
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most likely due the increase in adatom incorporation directly on substrate surface;
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therefore the long axis of the grown structures seems smaller at larger temperatures
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(inset, Support information Fig. S3A-D). We also notice that the GaP axial growth rate
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is much smaller than for In-V nanowires grown in similar conditions, due to the low Ga
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solubility in the Au droplet.56 We can thus expect similar magnitudes to both VS and
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VLS growth processes. Furthermore, the larger growth temperatures should result in
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more pronounced VS growth also on the nanowire sidewalls, which present areas much
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larger than the growth front of the catalyst droplet. Indeed, approximately 98% of the
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nanowire volume obtained can be attributed to VS growth (schematics shown in Fig.
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S9B, inset). Our micro-PL results (Support Information Fig. S8B) can thus be
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interpreted as originating from different dopant incorporation mechanisms for VS and
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VLS growth and their corresponding volume ratios in each type of morphology.
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In order to better understand the effect of droplet crawling on nanowire growth
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and the driving mechanisms for the asymmetric morphologies observed, we analyzed
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the initial stages of GaP growth at 510°C by SEM, TEM and AFM using 20 nm Au
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nanoparticles in order to achieve a better image contrast. Fig. 4A shows the AFM
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topographic image for the sample with growth carried out during 30s on (100) GaAs 13 ACS Paragon Plus Environment
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substrates; in this case, the droplet not only remains in contact with the substrate, but is
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connected to a short trail on the surface, along the same ±[110] directions observed for
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the crawling in the annealed sample. AFM height profiles show that more material is
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deposited along the trail as compared to surrounding areas (~3 nm in height differences,
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Fig. 4B). Statistical analysis of the lengths of the grown trails show that most of them (~
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70%) lie in the range 100 - 400nm. These values agree well with the majority of the
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distances covered by the droplets during the annealing procedure (Support Information
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Fig. S10).
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Figure 4 – Asymmetric GaP nanowires grown at 510°C using 20 nm Au nanoparticles
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deposited on (100) GaAs substrates in the initial growth stages. (A) AFM topography
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image showing the growth performed for 30s. (B) Height profiles showing that more
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material is deposited in the droplet trail, and closer to the nanoparticle. (C-F) low
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magnification TEM and HRTEM images of the nanowire grown for 300s; the contrast 14 ACS Paragon Plus Environment
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variation at the nanowire pedestal is due to thickness differences during lamella
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preparation, which were confirmed analyzing the sample in different crystal
3
orientations. The asymmetry is already present, along with the formation of a
4
characteristic pedestal, with ZB crystal structure. (G-J) Illustration of the growth
5
scenario which leads to the asymmetric morphology.
6
For longer growth times (300s), the characteristic asymmetric shapes are already
7
formed, Fig. 4C, exhibiting a pedestal at their base. Side-view HRTEM images of the
8
structure directly on the substrate are shown in Fig. 4D-F; the GaP pedestal grows in the
9
zinc blend (ZB) phase and keeps an epitaxial relationship with the GaAs substrate, with
10
no crystallographic defects at this point. The formation of a pedestal at the nanowire
11
base is commonly attributed to the accumulation of material due to surface diffusion;
12
once a pedestal is formed below the droplet, the crystallographic phase may change
13
from ZB to WZ.57-59 The formation of the cubic phase in the initial stages is generally
14
associated to the low supersaturation condition of the droplet in the beginning of the
15
growth.46 Fig. 4D shows a sharp and defect-free interface between the two phases at the
16
beginning of the growth, and the wurtzite phase of the asymmetric nanowires appears
17
after the droplet loses contact with the substrate, Fig. 4E. Furthermore, stacking faults
18
and diameter variations appear in the top region of the structure; however, they are not
19
necessarily related, as we can see in Fig. 4F. These observations suggest that the
20
pedestal could act as a buffer layer, also releasing the elastic energy accumulated due to
21
the lattice mismatch between GaP and the GaAs substrate.60
22
Therefore, the droplet behavior observed in the initial growth stages may have
23
important implications to the entire nanowire growth process, as already demonstrated
24
for InSb nanosails.10 Based on our data, we interpret the growth scenario that leads to
25
the asymmetric morphology as follows. First, during the heating of the substrate in 15 ACS Paragon Plus Environment
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1
vacuum the droplet moves due to temperature-activated surface processes, Fig. 4(G and
2
H). However, in the temperature range used, the oxide layer on the remaining substrate
3
surface is not entirely removed.39 The droplet crawling alters the roughness of the
4
original surface, consuming the oxide layer and part of the underlying substrate surface.
5
This crawling process thus generates areas where the bare semiconductor surface is
6
totally exposed (Fig. 4H), hence effectively creating a selective area VS growth.61,62 Due
7
to the asymmetry created by droplet along the trail, preferential deposition occurs at the
8
side of the droplet-trail interface, with large VS growth rates due the high temperature
9
employed, Fig. 4I. After the final supersaturation condition is achieved and the
10
formation of the first VLS layers occurs, the droplet loses contact with the substrate,
11
originating the wurtzite phase. From this point on, the nanowire is supported by a
12
thicker ZB GaP originated from the enhanced deposition on the trail of the droplet. It is
13
interesting to note that the sidewalls opposite to the droplet trail are not supported by a
14
ZB base, and instead present a clear interface with the GaAs surface. In this scenario we
15
may assume that a stress field arises due to the lattice mismatch, creating a gradient in
16
the chemical potential which modifies adatom kinetics,63,64 favoring incorporation on
17
the thicker GaP regions. As both VLS and VS growth processes take place (Fig. 4J), the
18
asymmetric morphology appears. Diffusive processes create mass transport along the
19
different sidewalls of the growing structure, eventually leading to the formation of
20
intermediary facets with lower surface energy, as can be seen in Fig. 3D. The presence
21
of new facets other than {11-20} could also influence the VS growth rate for the non-
22
polar planes. However, if a nanowire shadows the stub originated by VS growth, the
23
structure does not form (Support Information Fig. S11). Therefore, in our interpretation
24
the crawling and the subsequent pedestal formation alter the adatom kinetics creating an
25
unbalanced VS growth on the apolar sidewalls. This process also generates enhanced
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VS growth originated from the trail, eventually leading to the asymmetric morphology.
2
A similar process has been reported for V-shaped InAs nanomembranes,13 originated
3
from growth at the opposite facets of a rectangular pyramidal island nucleus, obtained
4
via selective area epitaxy using nanoscale apertures on a dielectric mask. In our case,
5
the preferential Au dissolution on Ga-terminated {111} facets, and the resulting droplet
6
trail, determine the asymmetry of our nanostructures. Furthermore, the procedure
7
demonstrated here does not require lithographic processes before growth, while
8
providing a high yield of the asymmetric nanostructures, by exploring a much simpler
9
process.
10
Despite the important role of the droplet trail for the asymmetric growth
11
scenario, its length does not directly determine the final nanowire width (Support
12
Information Figure S10). As shown in Fig. 4B, the VS growth on the trail is not
13
uniform, and more material is deposited close to the droplet. The most likely reason is
14
that VLS growth continues during the deposition on oxide-free areas, and both the
15
nanowire pedestal and sidewalls can act as nucleation regions for VS growth. Thus,
16
substrate annealing conditions and the resulting broad range of droplet trail lengths
17
(Support Information Figure S10) are not necessarily the best parameters for growth
18
control. The final nanostructure morphology can be more effectively altered via V/III
19
flux ratios, and TEG flow in particular.
20
Device applications usually require growth on different substrates, which could
21
alter the asymmetric morphology due to the different interaction between surface and
22
nanoparticle. Fig. 5A-D shows GaP nanowires grown on (100) InP and (111)B GaAs
23
substrates, respectively. We observe that the asymmetric morphology is preserved in
24
both cases, despite different geometrical characteristics. The axial growth rate is
25
significantly altered when growth is carried out on (100) InP substrates, Fig. 5A, B. 17 ACS Paragon Plus Environment
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1
Consequently, the aspect ratio of the asymmetric structures is also modified, due to the
2
different diffusion lengths on the substrate surface and material availability for the
3
growth. Furthermore, vertical asymmetric nanowires with almost 100% yield are
4
possible when the growth is carried out using (111)B GaAs substrates and 5nm Au
5
nanoparticles, Fig. 5C, D. These observed changes are related to the interplay between
6
nanoparticle crawling and surface chemistry, as well as group III solubility in the
7
catalyst metal. In addition, the crawling process could be explored for creating in situ
8
preferential deposition areas for subsequent growth at higher temperatures than those
9
employed here, in order to minimize nanowire growth and provide morphologies such
10
as rectangular nanosheets, for example.
11 12
Figure 5 – SEM images of asymmetric GaP nanowires grown at 510°C using 5 nm Au
13
nanoparticles deposited on different substrates. (A and B) grown on (100) InP (C and D)
14
grown on (111)B GaAs.
15 16
In summary, we have shown a new morphology for wurtzite GaP nanomaterials,
17
which consist of asymmetric structures, with luminescence in the green spectral range.
18
We attribute the formation of this morphology due the thermally activated crawling of
19
the Au catalyzed droplet in the initial stages of the growth, which produces an oxide18 ACS Paragon Plus Environment
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1
free area, where the VS growth rate is enhanced compared to the surrounding substrate
2
and its native oxide. The shape asymmetry is related to the formation of a ZB pedestal
3
along the trail of the nanoparticle, which acts as a preferential nucleation for GaP
4
deposition thus creating enhanced VS growth rate of selected apolar facets. The unique
5
asymmetry demonstrated here could open up the possibility to use these nanowires in
6
photonics applications65-67 combined with the direct band gap nature of the material.
7 8
Associated Content
9
Support Information – A supplemental file is associated with this work. This
10
includes additional SEM, TEM and AFM images of the samples, statistical analysis
11
about the axial growth rate and the populations present in each sample and optical
12
measurements.
13 14
Corresponding Authors
15
[email protected],
[email protected] 16 17
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
18 19 20
Acknowledgements
21
We acknowledge the Brazilian Nanotechnology Laboratory (LNNano/CNPEM)
22
and Laboratory of Structural Characterization (LCS/UFSCAR) for granting access to
23
their electron microscopy facilities. B. C. da Silva acknowledges FAEPEX-UNICAMP
24
and CAPES for scholarships. We also acknowledge D. M. Ugarte for discussions on
25
transmission electron microscopy, H. T. Obata for technical assistance with CBE
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1
system and S. R. Araújo for lamella preparation. This work was financially supported
2
by the Brazilian agencies CAPES, CNPq (grants 479486/2012-3, 305769/2015-4 and
3
2012/10127-5); and FAPESP (grants 2012/11382-9, 2013/02300-1, 2014/2339-9,
4
2015/16611-4, and 2016/16365-6).
5 6
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[62] Heinecke, H. and Watcher, M., Mechanisms and Applications of Selective Area Growth by Metalorganic Molecular Beam Epitaxy (CBE), Applied Surface Science, 1997, 113, 1-8. [63] Barabási, A. L., Self-assembled Island Formation in Heteroepitaxial Growth, Applied Physics Letters, 1992, 70, 2565-2567. [64] Gutiérrez, H. R.; Cotta, M. A.; Bortoleto, R. R.; Carvalho, M. M. G. Role of Group V Exchange on the Shape and Size of InAs/InP Self-assembled Nanostructures, Journal of Applied Physics, 2002, 92, 7523-7526.
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[65] Khorasaninejad, M. and Saini, S. S., Silicon Nanowires Optical Waveguide, Optics Express, 2010, 18, 23442-23457.
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[67] Wang, B.; Stevens, E.; Leu, P. W. Strong Broadband Absorption GaAs Nanocone and Nanowire Arrays for Solar Cells, Optics Express, 2014, 22, A386-A395.
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Figure 1 – Atomic Force Microscopy (AFM) of annealed samples. Topography images of 20 nm Au nanoparticles on (100) GaAs substrates annealed at 510°C for 5 min (A) under As2 overpressure and (B) in vacuum. (C) Height profile of the image in (B), showing that the trail is approximately 2nm deep. (D) Sample annealed in vacuum; white arrows mark trails with different lengths caused by the crawling of the Au droplets. (E and F) Top-view and side-view illustration of the groove formed beneath the Au nanoparticle with four {111} facets, which present different atomic terminations. (G and H) Illustration of the crawling occurring in the specific ±[110] directions, due to the faster dissolution rate of Ga-terminated {111} facets by Au nanoparticles. (I) Representation of the effect of annealing temperature on the 20 nm Au nanoparticle stability for the annealing in vacuum; droplet crawling occurs at or above a critical temperature (510°C). 360x305mm (72 x 72 DPI)
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Figure 2 – SEM images of GaP nanowires grown on different growth temperatures. (A-C) growth carried out with 5 nm Au nanoparticles on (100) GaAs substrates at 450, 480 and 510°C, respectively. The transition in the morphology can be clearly observed at 510°C. (D) Asymmetric nanowires with 36° tilt relative to the substrate surface. (E) Main growth parameters for the formation of the asymmetric morphology, which occurs for small droplets and appears at a critical temperature (510°C). 541x294mm (72 x 72 DPI)
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Figure 3 – Structural properties of asymmetric GaP nanowires. (A) Low magnification TEM images and SAED pattern (inset) indexed as WZ crystal structure in the [10-10] zone axis, showing that the growth direction is . (B) HRTEM image showing the Au 5 nm droplet at the top of the structure and the absence of a neck region with different crystal structure. (C) 10 K photoluminescence, showing the WZ GaP emission in the green spectral range. (D) Cross-sectional low magnification TEM image and SAED pattern (inset) indexed in the [0001] zone axis; the sidewalls are formed by apolar {11-20} planes. 222x357mm (72 x 72 DPI)
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Figure 4 - Asymmetric GaP nanowires grown at 510°C using 20 nm Au nanoparticles deposited on (100) GaAs substrates in the initial growth stages. (A) AFM topography image showing the growth performed for 30s. (B) Height profiles showing that more material is deposited in the droplet trail, and closer to the nanoparticle. (C-F) low magnification TEM and HRTEM images of the growth performed for 300s; the contrast difference at the nanowire pedestal is due to thickness differences during lamella preparation by FIB, which were confirmed analyzing the sample in different crystal orientations. The asymmetry is already present, along with the formation of a characteristic pedestal, with ZB crystal structure. (G-J) Illustration of the growth scenario which leads to the asymmetric morphology. 797x577mm (72 x 72 DPI)
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Figure 5 – SEM images of asymmetric GaP nanowires grown at 510°C using 5 nm Au nanoparticles deposited on different substrates. (A and B) grown on (100) InP (C and D) grown on (111)B GaAs. 242x104mm (72 x 72 DPI)
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