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Polarity-driven Non-uniform Composition in InGaAs Nanowires Yanan Guo, Tim Burgess, Qiang Gao, Hark Hoe Tan, Chennupati Jagadish, and Jin Zou Nano Lett., Just Accepted Manuscript • Publication Date (Web): 17 Oct 2013 Downloaded from http://pubs.acs.org on October 18, 2013
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Polarity-driven Non-uniform Composition in InGaAs Nanowires Ya-Nan Guo,† Timothy Burgess,‡ Qiang Gao,‡ H. Hoe Tan,‡ Chennupati Jagadish,‡ and Jin Zou*,†,§ †
‡
Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia
§
Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia
* Corresponding author. E-mail: [email protected].
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Manipulating the composition and morphology of semiconductor nanowires in a precisely controlled fashion is critical in developing nanowire devices. This is particularly true for ternary III-V nanowires. Many studies have shown the complexities within those nanowires. Here we report our findings of compositional irregularity in the shells of core-shell InGaAs nanowires with zinc-blende structure. Such an effect is caused by the crystal polarity within III-V zincblende lattice and the one-dimensional nature of nanowires that allows the formation of opposite polar surfaces simultaneously on the nanowire sidewalls. This polarity-driven effect in III-V nanowires may be utilized in manipulating the composition and morphology of III-V nanowires for device applications.
Key words: InGaAs nanowires, zinc blende, polarity, misfit strain, non-uniform composition, core-shell
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Semiconductor nanowires are attracting research interests due to their distinct physical and chemical properties for many potential applications.1-4 Among the various nanowire systems, ternary III-V nanowires have the potential to expand the applications of semiconductor nanowires into new dimensions since their band gaps may be continuously tuned by varying the composition of the nanowires.5-7 As one of the most important ternary semiconductor materials, InxGa1-xAs is an ideal candidate for long wavelength optoelectronic applications due to its direct bandgap covering a broad spectral range.8, investigated intensively in recent years.5,
9
10-13
Therefore, InxGa1-xAs nanowires have been Currently, the most widely used approach to
synthesize nanowires is through the vapor-liquid-solid (VLS) mechanism.14,
15
where metallic
nanoparticles catalyze the anisotropic crystal growth. Au nanoparticles have been the most widely used catalyst, which remain at the growth front of the nanowires and confines the diameter of the grown nanowires. Simultaneously, unintentional epitaxial growth on the nanowire sidewalls via vapor-solid (VS) growth can occur,16 leading to the nanowires growing laterally and subsequently a tapered morphology. By employing VLS growth, InxGa1-xAs nanowires can be grown by metal-organic chemical vapor deposition (MOCVD) and compositionally homogeneous InxGa1-xAs nanowires should be achieved. The values of x should be defined by the given flow rates of different precursors in the vapor phase. However, compositional inhomogeneity has been a long-standing issue in the ternary III-V nanowire systems.5, 17-19 Recent study revealed that the observed inhomogeneity is caused by a simultaneous formation of core-shell nanowires, which is due to the different natures of VLS growth of cores and VS growth of shells, as well as an effect of catalysts selectively hindering the precipitation of certain group-III element at the growth interface.20 These studies
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suggest that the lateral growth need to be eliminated in order to produce compositionally homogeneous ternary III-V nanowires.18, 20 Here, we reveal that there can be another dimension of inhomogeneity within the core-shell InxGa1-xAs ternary nanowires, where the composition of the shells varies along different crystallographic directions. Transmission electron microscopy (TEM) investigations on individual nanowire cross-sections revealed that the zinc-blende phase InGaAs nanowires have six 112 sidewall facets, where three Ga-rich zones are formed along 112A sidewalls in the shell, while three In-enriched zones along 112B sidewalls. This compositional segregation in nanowire shells is determined to be driven by the polarity of the zinc-blende structure. This polarity-driven effect in nanowire growth may be utilized as new approach in creating multiwavelength emission or detection, which in turn provides new insight into understanding and utilizing III-V ternary nanowires for device applications. Epitaxial InxGa1-xAs (with the nominal vapor molar fraction of In, xv= 0.1) nanowires were grown on GaAs (111)B substrates pre-treated with poly-L-lysine (PLL), using a horizontal flow MOCVD reactor at a constant pressure of 100 mbar. The growth temperature was 450 °C, and the flow rate of hydrogen carrier gas was 15 slm. Au nanoparticles with a diameter of 60 nm (from commercially available colloidal Au solution) were used as catalysts for inducing the nanowire growth. Trimethylindium (TMI), trimethylgallium (TMG) and AsH3 were used as the precursors for In, Ga and As, respectively. The growth time was 30 min, and the V/III ratio was set to 46 for the growth. The morphological, structural and chemical characteristics of the nanowires were investigated by field-emission scanning electron microscopy (SEM, JEOL-7001) and TEM (Philips Tecnai-F20) equipped with X-ray energy-dispersive spectroscopy (EDS). Individual nanowires for TEM observations were prepared by mechanically dispersing the
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nanowires onto holey carbon films supported by Cu grids. The preparation of cross-sections of individual nanowires for TEM investigations involves first embedding the nanowires into resin (EPON), and then sectioning them using an ultra-microtome (Leica EM UC6). The orientation of nanowires with respect to the original substrates was well preserved during the sectioning process. Additionally, the sectioning was initiated from the bottom of the nanowires, and the number of the sections being cut as well as the thicknesses of the sections were carefully controlled, so that the region where those cross-sections were obtained from the nanowires can be estimated. The thin sections were then transferred onto the carbon-coated formvar films, supported by Cu grids. SEM studies suggest that most nanowires are vertically freestanding on the GaAs 111B substrate with tapered morphology. To understand their structural characteristics, TEM investigation is employed.
Our extensive transmission electron microscopy (TEM)
characterization indicates that the nanowires have zinc-blende crystal structure. Figure 1a is a bright-field TEM image of a typical nanowire with insets being enlarged TEM images. Figure 1b is the EDS profiles taken from different sections along the nanowire and shows that the In concentration increases from the top to the bottom of the nanowire. From Figure 1a, the nanowire is tapered and Moiré fringes can be clearly observed towards the bottom of the nanowire, which suggests the formation of a nanowire core and a shell within the nanowire with the core and shell having different lattice parameters (hence different compositions). Moreover, the diameter of the core measured from the Moiré fringes was found to be uniform throughout the nanowire, which is about the size of the catalyst (~ 70 nm as shown in the insets of Figure 1a); while the shell was found to be tapered, as evidenced by Figure 1a. Therefore, the as-grown In0.1Ga0.9As nanowires should have core-shell structure with uniform cores being Ga-rich and
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tapered shells being In-enriched (containing relatively more In than cores). Additionally, the growth of cores is induced directly by catalysts via VLS growth, while the growth of shells is resulted from lateral growth on nanowire sidewalls via VS growth.20 To further understand the compositional characteristics of the cores and shells, TEM investigations on the cross-sections of the nanowires were carried out. Figure 2a is a bright-field TEM image of a nanowire cross-section taken from the bottom region of a nanowire, showing a hexagonal core in the middle, surrounded by a truncated-triangular shell. Figure 2b is a corresponding selective area electron diffraction (SAED) pattern; from which the six sidewalls of the nanowire can be determined to be 112. Our previous study21 suggested that the lateral growth on the nanowire 112 sidewalls tends to transform the cross-section of GaAs nanowire from a hexagon to a truncated-triangle, which is in an excellent agreement with the observed lateral growth of nanowire shell forming a truncated-triangular profile. It is of interest to note that there are some clear contrasts dividing the shell into six zones. To clarify the origin of those zones, EDS analysis was preformed. Figure 2c shows the typical EDS spectra taken from different regions of the nanowire cross-section shown in Figure 2a, in which the three zones with longer 112 sidewalls are found to contain more In (named In-enriched zones) than the other three zones with shorter 112 sidewalls (named Ga-rich zones). The comparison of the EDS spectra shown in Figure 2c indicates that the In concentration in three Ga-rich zones is close to that in the Ga-rich core, but drastically different from the adjacent In-enriched zones. Such a phenomenon was further confirmed by scanning transmission electron microscopy (STEM) equipped with high angle annular dark field (HAADF) detector, which produces STEM images with the atomic-number contrast (Z contrast). Figures 2d-2f are respectively bright-field STEM image, HAADF image and EDS In map taken from a typical nanowire cross-section. The Ga-
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rich core, as well as six compositional zones in the shell, can be clearly identified from the HAADF image (Figure 2e) and EDS In map (Figure 2f). Our TEM investigations on over a dozen of nanowire cross-sections taken from different regions along their axial directions showed the similar compositional profile. Therefore, we concluded that (1) the ternary nanowires are indeed core-shell nanowires with cores containing small amount of In and shells containing more In; and (2) the nanowire shells have a 3-fold symmetry with three Ga-rich and three Inenriched zones. To understand the formation of the non-uniform nanowire shells, we note that the six 112 sidewalls of a nanowire are polar surfaces, suggesting that the polarity may play an important role in the observed phenomenon. Therefore, determining the polarities of nanowire sidewalls is necessary. Figure 3a is a bright-field TEM image taken along a zone-axis from side view. Since nanowires were epitaxially grown on GaAs 111B substrate and the crystalline polarity of nanowires does not change across rotational twins,22 the nanowire growth direction can be unambiguously determined to be [111]. Based on the observations that (1) the nanowire sidewall with thicker shell when viewing along zone axis is 112A surface (deduced from Figures 3a,b), (2) the thickness of truncated-triangular shell is thicker towards the smaller sidewall when viewing along [111] zone axis (deduced from Figure 2a), and (3) six nanowire sidewalls are of 112 (Figure 2b); the polarity of the SAED pattern in Figure 2b can then be determined, and from which the polarity of nanowire sidewalls can be determined with the three longer facets being 112B and the three shorter ones being 112A. Based on the above investigations, a schematic illustration (Figure 4a) can be drawn to outline the observed compositional variations in nanowire cross-section showing Ga atoms being preferentially incorporated into the lattice on 112A surfaces, while In atoms being
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preferentially incorporated into the lattice on 112B surfaces during VS growth of shells. It is of interest to note that some compositional segregation in VS-grown nanowire shells has been reported in core-shell nanowires.23-26 However, the explanations from these studies are based on curvature-induced difference in the diffusion of different group-III adatoms,26-28 and cannot explain our observed phenomenon, because the compositional zones we observed within nanowire shells are not along corrugated apexes, but along the entire 112 surfaces. This strongly suggests that the polarity of zinc-blende lattice is the determining factor in the observed phenomenon. To understand how crystal polarity plays a role in such preferential incorporation of group-III elements, the atomic arrangement of the sidewalls of a nanowire core was considered. Figure 4b is an atomic model of a zinc-blende GaAs lattice viewing along the [011] zone axis, corresponding to Figure 3. To simplify the argument, we consider the lateral growth of InxGa1xAs
shell around the GaAs core on the 112A and 112B sidewall surfaces. It is important to
note that 112 surfaces are not atomically flat, but can be dissociated into 111 and 002 surfaces, as illustrated in Figure 4b. For type-1 sites, i.e. the atomic sites for group-III atoms to be incorporated on the 002 surfaces (marked by green circles in Figure 4b), two new bonds are formed for every incoming group-III atom for both 112A and 112B sidewalls. However, for type-2 sites, i.e. the atomic sites for group-III atom to be incorporated on the 111 surfaces (marked by yellow circles in Figure 4b), three new bonds are formed for every incoming groupIII atom for the 112A surface, while only one new bond is formed for the 112B sidewalls. Therefore, during the epitaxial growth of ternary shells, with In and Ga atoms competitively incorporating onto GaAs 112 surfaces (surfaces of existing GaAs cores), the extra strain energy caused by the incorporation of each In atom on the GaAs surface is expected to be higher
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on 112A surface than that on 112B surface, because the strain energy created by incorporation of In atoms with GaAs lattice increases with increasing number of newly formed strained bonds on the surface. Based on this consideration, during the growth of InxGa1-xAs shells, In atoms are energetically more favorable to be incorporated on the 111B surface dissociated from 112B sidewall, while Ga atoms are energetically favorable to be incorporated on 111A surface dissociated from 112A sidewall, in order to minimize the system energy. As a consequence, during the growth of our In0.1Ga0.9As nanowires, Ga-rich regions prefer to grow on the 112A sidewalls, and In-enriched regions prefer to grow on the 112B sidewalls, resulting in the compositional zones along the nanowire cross-section. It should be noted that the above consideration is only valid if such preferential deposition is the only effective mechanism for minimizing the system strain energy caused by the lattice mismatch between the core and shell. In this study, no misfit dislocations were experimentally found at the core-shell interface along the nanowire, as evidenced by the side-view TEM results (TEM image shown in Figure 3a and high-resolution TEM image shown in Figure 3c). Additionally, geometric phase analyses (GPA) derived from high-resolution TEM images on nanowire cross-sections were carried out (refer to the supporting information) that confirm the strain level in 112A facets is similar to that in the core, while the strain level in 112B facets is slightly higher (refer to the strain map shown in Figure S3). These findings strongly support the proposed polarity-driven effect. It is of interest to note other polarity-driven effects have been observed in III-V lateral nanowire heterostructures.29-31 For example, Zheng et al.31 observed six phase-segregated Al-rich bands (with identical composition) along directions alternating in thickness in the AlGaAs shell, and significant morphological difference in 112A and 112B facets, which alters the hexagonal core-multishell cross-section to a 3-fold symmetrical shape.
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However, such finding is distinct from our observation, where different compositional zones are formed along the entire prominent facets of nanowire shells in different polar directions. We believe that the polarity-driven compositional variations we observed could be applicable to core-shell nanowire systems with zinc-blende structure that have small lattice mismatch between core and shell, e.g. A/A1-xBx binary/ternary core-shell nanowires with low value of x, where the lattice parameters of core and shell are slightly different and core-shell interfaces remain coherent (no misfit dislocations at the interface for strain relief). The consideration of lattice polarity may hold the key in understanding the selective sites of quantum dots formed on the nanowire sidewalls29,30 or at core-shell interface.23 Additionally, the discovery of this effect demonstrates the uniqueness of the nanowire geometry for studying the fundamental physics of epitaxy as opposite polar surfaces may be available as vicinal surfaces, which is impossible in the case of standard 2D epitaxy. As for understanding the growth of ternary III-V nanowires, our study shows that, apart from the commonly observed formation of core-shell structure, VLS-grown ternary nanowires might develop complex shells that are chemically non-uniform along the radial direction. Understanding of such phenomenon may provide new opportunities in utilizing crystal polarity for fabricating nanowire devices. With appropriate surface passivation, these complex nanowire structures may be used for more controlled fabrication of hierarchical nanowire heterostructures23, 32, 33 and nanowire networks34, 35 for complex nanowire devices.
Acknowledgements The authors acknowledge the financial support from the Australian Research Council. Facilities used in this work are supported by the Australian Microscopy & Microanalysis Research Facility
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at the University of Queensland and Australian National University, and the Australian National Fabrication Facility, ACT Node. Dr Jennifer Wong-Leung from Australian National University is thanked for helping with the GPA included in the supporting information.
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Figure 1. (a) TEM bright-field (BF) image of a typical InGaAs nanowire. The insets show magnified TEM phase contrast images of the three areas marked by colored rectangles in (a). ] zone axis, showing Moiré fringes along the The phase-contrast images were taken along [ nanowire. (b) The corresponding XEDS spectra from the top (point 1), middle (point 2) and bottom (point 3) areas of the nanowire as indicated by the circles and numbers in (a).
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Figure 2. (a) TEM BF image of a typical cross-section of an InGaAs nanowire obtained from the bottom region of the nanowire, taken along [111] zone axis, showing some contrasts of the coreshell structure, where the shell is divided into 6 zones by straight lines. (b) The corresponding SAED pattern taken along [111] zone axis on the nanowire cross-section shown in (a). (c) Typical EDS spectra taken on the areas of the core (point 1) and two different areas in the shell (points 2 and 3 in (a)). (d) and (e) are the STEM BF and HAADF Z-contrast images,
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respectively, taken on another cross-section of an InGaAs nanowire obtained from the middle region of the nanowire. (f) The corresponding cross-sectional In map of the nanowire.
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1] zone axis at the bottom region of a Figure 3. (a) Side-view TEM BF image taken along [ nanowire, showing the clear boundaries that demarcate the core and shell of the nanowire. The nanowire growth direction [111]B and the boundaries between core and shell are indicated in the 1] on the area shown in (a) and figure. (b) SAED pattern taken with the beam direction of [ indexed based on the growth direction being [111]. All directions can be assigned with polarities as indicated in the figure. The polarities of the two 211 sidewall surfaces shown in (a) can be determined, with the left one being (211)B and the right one being (211)A. (c) High-resolution 1] zone axis showing TEM image of the highlighted rectangular area in (a) taken along the [ the dislocation-free core-shell interface.
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Figure 4. (a) Cross-sectional illustration of the core-shell nanowire, with the polarities of all six sidewall surfaces being indicated. Three In-enriched and three Ga-rich zones within the shell surround the Ga-rich hexagonal core in the middle of the nanowire. (b) A 3D crystal model of ] direction (side view of nanowire lattice), zinc-blende GaAs lattice projected along the [ ] zone axis indicated in both (a) and (b). Type 1 (green with the viewing direction of [ circles) and type 2 (yellow circles) atomic positions are highlighted as possible sites for group-III atoms to be incorporated on A and B surfaces.
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TABLE OF CONTENTS GRAPHIC TEM image of a typical nanowire cross-section and the illustration of its compositional profile
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Figure S2. (a) High-resolution TEM image taken along [111] zone axis, focusing on one of the {112}A corners of the nanowire cross-section, corresponding to the square-marked area in Figure S1. (b) Diffractogram generated by the GPA software package through FFT used for phase calculation. Two {220} spots (marked with circles) were used for phase calculation. (c) Magnified high-resolution TEM image of the square-marked area in (a) highlighting the {220} lattice fringes. 434x654mm (72 x 72 DPI)
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Communication
Polarity-driven Non-uniform Composition in InGaAs Nanowires Yanan Guo, Tim Burgess, Qiang Gao, Hark Hoe Tan, Chennupati Jagadish, and Jin Zou Nano Lett., Just Accepted Manuscript • Publication Date (Web): 17 Oct 2013 Downloaded from http://pubs.acs.org on October 18, 2013
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Polarity-driven Non-uniform Composition in InGaAs Nanowires Ya-Nan Guo,† Timothy Burgess,‡ Qiang Gao,‡ H. Hoe Tan,‡ Chennupati Jagadish,‡ and Jin Zou*,†,§ †
‡
Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia
§
Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia
* Corresponding author. E-mail: [email protected].
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Manipulating the composition and morphology of semiconductor nanowires in a precisely controlled fashion is critical in developing nanowire devices. This is particularly true for ternary III-V nanowires. Many studies have shown the complexities within those nanowires. Here we report our findings of compositional irregularity in the shells of core-shell InGaAs nanowires with zinc-blende structure. Such an effect is caused by the crystal polarity within III-V zincblende lattice and the one-dimensional nature of nanowires that allows the formation of opposite polar surfaces simultaneously on the nanowire sidewalls. This polarity-driven effect in III-V nanowires may be utilized in manipulating the composition and morphology of III-V nanowires for device applications.
Key words: InGaAs nanowires, zinc blende, polarity, misfit strain, non-uniform composition, core-shell
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Semiconductor nanowires are attracting research interests due to their distinct physical and chemical properties for many potential applications.1-4 Among the various nanowire systems, ternary III-V nanowires have the potential to expand the applications of semiconductor nanowires into new dimensions since their band gaps may be continuously tuned by varying the composition of the nanowires.5-7 As one of the most important ternary semiconductor materials, InxGa1-xAs is an ideal candidate for long wavelength optoelectronic applications due to its direct bandgap covering a broad spectral range.8, investigated intensively in recent years.5,
9
10-13
Therefore, InxGa1-xAs nanowires have been Currently, the most widely used approach to
synthesize nanowires is through the vapor-liquid-solid (VLS) mechanism.14,
15
where metallic
nanoparticles catalyze the anisotropic crystal growth. Au nanoparticles have been the most widely used catalyst, which remain at the growth front of the nanowires and confines the diameter of the grown nanowires. Simultaneously, unintentional epitaxial growth on the nanowire sidewalls via vapor-solid (VS) growth can occur,16 leading to the nanowires growing laterally and subsequently a tapered morphology. By employing VLS growth, InxGa1-xAs nanowires can be grown by metal-organic chemical vapor deposition (MOCVD) and compositionally homogeneous InxGa1-xAs nanowires should be achieved. The values of x should be defined by the given flow rates of different precursors in the vapor phase. However, compositional inhomogeneity has been a long-standing issue in the ternary III-V nanowire systems.5, 17-19 Recent study revealed that the observed inhomogeneity is caused by a simultaneous formation of core-shell nanowires, which is due to the different natures of VLS growth of cores and VS growth of shells, as well as an effect of catalysts selectively hindering the precipitation of certain group-III element at the growth interface.20 These studies
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suggest that the lateral growth need to be eliminated in order to produce compositionally homogeneous ternary III-V nanowires.18, 20 Here, we reveal that there can be another dimension of inhomogeneity within the core-shell InxGa1-xAs ternary nanowires, where the composition of the shells varies along different crystallographic directions. Transmission electron microscopy (TEM) investigations on individual nanowire cross-sections revealed that the zinc-blende phase InGaAs nanowires have six 112 sidewall facets, where three Ga-rich zones are formed along 112A sidewalls in the shell, while three In-enriched zones along 112B sidewalls. This compositional segregation in nanowire shells is determined to be driven by the polarity of the zinc-blende structure. This polarity-driven effect in nanowire growth may be utilized as new approach in creating multiwavelength emission or detection, which in turn provides new insight into understanding and utilizing III-V ternary nanowires for device applications. Epitaxial InxGa1-xAs (with the nominal vapor molar fraction of In, xv= 0.1) nanowires were grown on GaAs (111)B substrates pre-treated with poly-L-lysine (PLL), using a horizontal flow MOCVD reactor at a constant pressure of 100 mbar. The growth temperature was 450 °C, and the flow rate of hydrogen carrier gas was 15 slm. Au nanoparticles with a diameter of 60 nm (from commercially available colloidal Au solution) were used as catalysts for inducing the nanowire growth. Trimethylindium (TMI), trimethylgallium (TMG) and AsH3 were used as the precursors for In, Ga and As, respectively. The growth time was 30 min, and the V/III ratio was set to 46 for the growth. The morphological, structural and chemical characteristics of the nanowires were investigated by field-emission scanning electron microscopy (SEM, JEOL-7001) and TEM (Philips Tecnai-F20) equipped with X-ray energy-dispersive spectroscopy (EDS). Individual nanowires for TEM observations were prepared by mechanically dispersing the
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nanowires onto holey carbon films supported by Cu grids. The preparation of cross-sections of individual nanowires for TEM investigations involves first embedding the nanowires into resin (EPON), and then sectioning them using an ultra-microtome (Leica EM UC6). The orientation of nanowires with respect to the original substrates was well preserved during the sectioning process. Additionally, the sectioning was initiated from the bottom of the nanowires, and the number of the sections being cut as well as the thicknesses of the sections were carefully controlled, so that the region where those cross-sections were obtained from the nanowires can be estimated. The thin sections were then transferred onto the carbon-coated formvar films, supported by Cu grids. SEM studies suggest that most nanowires are vertically freestanding on the GaAs 111B substrate with tapered morphology. To understand their structural characteristics, TEM investigation is employed.
Our extensive transmission electron microscopy (TEM)
characterization indicates that the nanowires have zinc-blende crystal structure. Figure 1a is a bright-field TEM image of a typical nanowire with insets being enlarged TEM images. Figure 1b is the EDS profiles taken from different sections along the nanowire and shows that the In concentration increases from the top to the bottom of the nanowire. From Figure 1a, the nanowire is tapered and Moiré fringes can be clearly observed towards the bottom of the nanowire, which suggests the formation of a nanowire core and a shell within the nanowire with the core and shell having different lattice parameters (hence different compositions). Moreover, the diameter of the core measured from the Moiré fringes was found to be uniform throughout the nanowire, which is about the size of the catalyst (~ 70 nm as shown in the insets of Figure 1a); while the shell was found to be tapered, as evidenced by Figure 1a. Therefore, the as-grown In0.1Ga0.9As nanowires should have core-shell structure with uniform cores being Ga-rich and
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tapered shells being In-enriched (containing relatively more In than cores). Additionally, the growth of cores is induced directly by catalysts via VLS growth, while the growth of shells is resulted from lateral growth on nanowire sidewalls via VS growth.20 To further understand the compositional characteristics of the cores and shells, TEM investigations on the cross-sections of the nanowires were carried out. Figure 2a is a bright-field TEM image of a nanowire cross-section taken from the bottom region of a nanowire, showing a hexagonal core in the middle, surrounded by a truncated-triangular shell. Figure 2b is a corresponding selective area electron diffraction (SAED) pattern; from which the six sidewalls of the nanowire can be determined to be 112. Our previous study21 suggested that the lateral growth on the nanowire 112 sidewalls tends to transform the cross-section of GaAs nanowire from a hexagon to a truncated-triangle, which is in an excellent agreement with the observed lateral growth of nanowire shell forming a truncated-triangular profile. It is of interest to note that there are some clear contrasts dividing the shell into six zones. To clarify the origin of those zones, EDS analysis was preformed. Figure 2c shows the typical EDS spectra taken from different regions of the nanowire cross-section shown in Figure 2a, in which the three zones with longer 112 sidewalls are found to contain more In (named In-enriched zones) than the other three zones with shorter 112 sidewalls (named Ga-rich zones). The comparison of the EDS spectra shown in Figure 2c indicates that the In concentration in three Ga-rich zones is close to that in the Ga-rich core, but drastically different from the adjacent In-enriched zones. Such a phenomenon was further confirmed by scanning transmission electron microscopy (STEM) equipped with high angle annular dark field (HAADF) detector, which produces STEM images with the atomic-number contrast (Z contrast). Figures 2d-2f are respectively bright-field STEM image, HAADF image and EDS In map taken from a typical nanowire cross-section. The Ga-
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rich core, as well as six compositional zones in the shell, can be clearly identified from the HAADF image (Figure 2e) and EDS In map (Figure 2f). Our TEM investigations on over a dozen of nanowire cross-sections taken from different regions along their axial directions showed the similar compositional profile. Therefore, we concluded that (1) the ternary nanowires are indeed core-shell nanowires with cores containing small amount of In and shells containing more In; and (2) the nanowire shells have a 3-fold symmetry with three Ga-rich and three Inenriched zones. To understand the formation of the non-uniform nanowire shells, we note that the six 112 sidewalls of a nanowire are polar surfaces, suggesting that the polarity may play an important role in the observed phenomenon. Therefore, determining the polarities of nanowire sidewalls is necessary. Figure 3a is a bright-field TEM image taken along a zone-axis from side view. Since nanowires were epitaxially grown on GaAs 111B substrate and the crystalline polarity of nanowires does not change across rotational twins,22 the nanowire growth direction can be unambiguously determined to be [111]. Based on the observations that (1) the nanowire sidewall with thicker shell when viewing along zone axis is 112A surface (deduced from Figures 3a,b), (2) the thickness of truncated-triangular shell is thicker towards the smaller sidewall when viewing along [111] zone axis (deduced from Figure 2a), and (3) six nanowire sidewalls are of 112 (Figure 2b); the polarity of the SAED pattern in Figure 2b can then be determined, and from which the polarity of nanowire sidewalls can be determined with the three longer facets being 112B and the three shorter ones being 112A. Based on the above investigations, a schematic illustration (Figure 4a) can be drawn to outline the observed compositional variations in nanowire cross-section showing Ga atoms being preferentially incorporated into the lattice on 112A surfaces, while In atoms being
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preferentially incorporated into the lattice on 112B surfaces during VS growth of shells. It is of interest to note that some compositional segregation in VS-grown nanowire shells has been reported in core-shell nanowires.23-26 However, the explanations from these studies are based on curvature-induced difference in the diffusion of different group-III adatoms,26-28 and cannot explain our observed phenomenon, because the compositional zones we observed within nanowire shells are not along corrugated apexes, but along the entire 112 surfaces. This strongly suggests that the polarity of zinc-blende lattice is the determining factor in the observed phenomenon. To understand how crystal polarity plays a role in such preferential incorporation of group-III elements, the atomic arrangement of the sidewalls of a nanowire core was considered. Figure 4b is an atomic model of a zinc-blende GaAs lattice viewing along the [011] zone axis, corresponding to Figure 3. To simplify the argument, we consider the lateral growth of InxGa1xAs
shell around the GaAs core on the 112A and 112B sidewall surfaces. It is important to
note that 112 surfaces are not atomically flat, but can be dissociated into 111 and 002 surfaces, as illustrated in Figure 4b. For type-1 sites, i.e. the atomic sites for group-III atoms to be incorporated on the 002 surfaces (marked by green circles in Figure 4b), two new bonds are formed for every incoming group-III atom for both 112A and 112B sidewalls. However, for type-2 sites, i.e. the atomic sites for group-III atom to be incorporated on the 111 surfaces (marked by yellow circles in Figure 4b), three new bonds are formed for every incoming groupIII atom for the 112A surface, while only one new bond is formed for the 112B sidewalls. Therefore, during the epitaxial growth of ternary shells, with In and Ga atoms competitively incorporating onto GaAs 112 surfaces (surfaces of existing GaAs cores), the extra strain energy caused by the incorporation of each In atom on the GaAs surface is expected to be higher
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on 112A surface than that on 112B surface, because the strain energy created by incorporation of In atoms with GaAs lattice increases with increasing number of newly formed strained bonds on the surface. Based on this consideration, during the growth of InxGa1-xAs shells, In atoms are energetically more favorable to be incorporated on the 111B surface dissociated from 112B sidewall, while Ga atoms are energetically favorable to be incorporated on 111A surface dissociated from 112A sidewall, in order to minimize the system energy. As a consequence, during the growth of our In0.1Ga0.9As nanowires, Ga-rich regions prefer to grow on the 112A sidewalls, and In-enriched regions prefer to grow on the 112B sidewalls, resulting in the compositional zones along the nanowire cross-section. It should be noted that the above consideration is only valid if such preferential deposition is the only effective mechanism for minimizing the system strain energy caused by the lattice mismatch between the core and shell. In this study, no misfit dislocations were experimentally found at the core-shell interface along the nanowire, as evidenced by the side-view TEM results (TEM image shown in Figure 3a and high-resolution TEM image shown in Figure 3c). Additionally, geometric phase analyses (GPA) derived from high-resolution TEM images on nanowire cross-sections were carried out (refer to the supporting information) that confirm the strain level in 112A facets is similar to that in the core, while the strain level in 112B facets is slightly higher (refer to the strain map shown in Figure S3). These findings strongly support the proposed polarity-driven effect. It is of interest to note other polarity-driven effects have been observed in III-V lateral nanowire heterostructures.29-31 For example, Zheng et al.31 observed six phase-segregated Al-rich bands (with identical composition) along directions alternating in thickness in the AlGaAs shell, and significant morphological difference in 112A and 112B facets, which alters the hexagonal core-multishell cross-section to a 3-fold symmetrical shape.
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However, such finding is distinct from our observation, where different compositional zones are formed along the entire prominent facets of nanowire shells in different polar directions. We believe that the polarity-driven compositional variations we observed could be applicable to core-shell nanowire systems with zinc-blende structure that have small lattice mismatch between core and shell, e.g. A/A1-xBx binary/ternary core-shell nanowires with low value of x, where the lattice parameters of core and shell are slightly different and core-shell interfaces remain coherent (no misfit dislocations at the interface for strain relief). The consideration of lattice polarity may hold the key in understanding the selective sites of quantum dots formed on the nanowire sidewalls29,30 or at core-shell interface.23 Additionally, the discovery of this effect demonstrates the uniqueness of the nanowire geometry for studying the fundamental physics of epitaxy as opposite polar surfaces may be available as vicinal surfaces, which is impossible in the case of standard 2D epitaxy. As for understanding the growth of ternary III-V nanowires, our study shows that, apart from the commonly observed formation of core-shell structure, VLS-grown ternary nanowires might develop complex shells that are chemically non-uniform along the radial direction. Understanding of such phenomenon may provide new opportunities in utilizing crystal polarity for fabricating nanowire devices. With appropriate surface passivation, these complex nanowire structures may be used for more controlled fabrication of hierarchical nanowire heterostructures23, 32, 33 and nanowire networks34, 35 for complex nanowire devices.
Acknowledgements The authors acknowledge the financial support from the Australian Research Council. Facilities used in this work are supported by the Australian Microscopy & Microanalysis Research Facility
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at the University of Queensland and Australian National University, and the Australian National Fabrication Facility, ACT Node. Dr Jennifer Wong-Leung from Australian National University is thanked for helping with the GPA included in the supporting information.
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Cantoni, M.; Marzari, N.; Arbiol, J.; Zunger, A.; Warburton, R. J.; Fontcuberta I Morral, A. Nature Mat. 2013, 12, (5). 24. Rudolph, D.; Funk, S.; Döblinger, M.; Morkötter, S.; Hertenberger, S.; Schweickert, L.; Becker, J.; Matich, S.; Bichler, M.; Spirkoska, D.; Zardo, I.; Finley, J. J.; Abstreiter, G.; Koblmüller, G. Nano Lett. 2013, 13, (4), 1522-1527. 25. Skold, N.; Wagner, J. B.; Karlsson, G.; Hernan, T.; Seifert, W.; Pistol, M. E.; Samuelson, L. Nano Lett. 2006, 6, (12), 2743-2747. 26. Wagner, J. B.; Sköld, N.; Reine Wallenberg, L.; Samuelson, L. J. Cryst. Growth 2010, 312, (10), 1755-1760. 27. Kley, A.; Ruggerone, P.; Scheffler, M. Physical Review Letters 1997, 79, (26), 52785281. 28. Biasiol, G.; Gustafsson, A.; Leifer, K.; Kapon, E. Physical Review B 2002, 65, (20). 29. Tambe, M. J.; Allard, L. F.; Gradecak, S., Characterization of core-shell GaAs/AlGaAs nanowire heterostructures using advanced electron microscopy. In 16th International Conference on Microscopy of Semiconducting Materials, Walther, T.; Nellist, P. D.; Hutchison, J. L.; Cullis, A. G., Eds. 2010; Vol. 209. 30. Rudolph, D.; Funk, S.; Doblinger, M.; Morkotter, S.; Hertenberger, S.; Schweickert, L.; Becker, J.; Matich, S.; Bichler, M.; Spirkoska, D.; Zardo, I.; Finley, J. J.; Abstreiter, G.; Koblmuller, G. Nano Lett. 2013, 13, (4), 1522-1527. 31. Zheng, C.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Etheridge, J. Nano Lett. 2013, 13, (8), 3742-3748. 32. Paladugu, M.; Zou, J.; Guo, Y. N.; Zhang, X.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y. Angew. Chem.-Int. Edit. 2009, 48, (4), 780-783. 33. Paladugu, M.; Zou, J.; Guo, Y. N.; Zhang, X.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y. Appl. Phys. Lett. 2008, 93, (20). 34. Dick, K. A.; Deppert, K.; Karlsson, L. S.; Larsson, M. W.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Mrs Bulletin 2007, 32, (2), 127-133. 35. Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nature Mat. 2004, 3, (6), 380-384.
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Figure 1. (a) TEM bright-field (BF) image of a typical InGaAs nanowire. The insets show magnified TEM phase contrast images of the three areas marked by colored rectangles in (a). ] zone axis, showing Moiré fringes along the The phase-contrast images were taken along [ nanowire. (b) The corresponding XEDS spectra from the top (point 1), middle (point 2) and bottom (point 3) areas of the nanowire as indicated by the circles and numbers in (a).
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Figure 2. (a) TEM BF image of a typical cross-section of an InGaAs nanowire obtained from the bottom region of the nanowire, taken along [111] zone axis, showing some contrasts of the coreshell structure, where the shell is divided into 6 zones by straight lines. (b) The corresponding SAED pattern taken along [111] zone axis on the nanowire cross-section shown in (a). (c) Typical EDS spectra taken on the areas of the core (point 1) and two different areas in the shell (points 2 and 3 in (a)). (d) and (e) are the STEM BF and HAADF Z-contrast images,
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respectively, taken on another cross-section of an InGaAs nanowire obtained from the middle region of the nanowire. (f) The corresponding cross-sectional In map of the nanowire.
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1] zone axis at the bottom region of a Figure 3. (a) Side-view TEM BF image taken along [ nanowire, showing the clear boundaries that demarcate the core and shell of the nanowire. The nanowire growth direction [111]B and the boundaries between core and shell are indicated in the 1] on the area shown in (a) and figure. (b) SAED pattern taken with the beam direction of [ indexed based on the growth direction being [111]. All directions can be assigned with polarities as indicated in the figure. The polarities of the two 211 sidewall surfaces shown in (a) can be determined, with the left one being (211)B and the right one being (211)A. (c) High-resolution 1] zone axis showing TEM image of the highlighted rectangular area in (a) taken along the [ the dislocation-free core-shell interface.
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Figure 4. (a) Cross-sectional illustration of the core-shell nanowire, with the polarities of all six sidewall surfaces being indicated. Three In-enriched and three Ga-rich zones within the shell surround the Ga-rich hexagonal core in the middle of the nanowire. (b) A 3D crystal model of ] direction (side view of nanowire lattice), zinc-blende GaAs lattice projected along the [ ] zone axis indicated in both (a) and (b). Type 1 (green with the viewing direction of [ circles) and type 2 (yellow circles) atomic positions are highlighted as possible sites for group-III atoms to be incorporated on A and B surfaces.
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TABLE OF CONTENTS GRAPHIC TEM image of a typical nanowire cross-section and the illustration of its compositional profile
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Figure S2. (a) High-resolution TEM image taken along [111] zone axis, focusing on one of the {112}A corners of the nanowire cross-section, corresponding to the square-marked area in Figure S1. (b) Diffractogram generated by the GPA software package through FFT used for phase calculation. Two {220} spots (marked with circles) were used for phase calculation. (c) Magnified high-resolution TEM image of the square-marked area in (a) highlighting the {220} lattice fringes. 434x654mm (72 x 72 DPI)
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