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Kinetic engineering of wurtzite and zinc blende AlSb shells on InAs nanowires Hanna Kindlund, Reza R. Zamani, Axel R. Persson, Sebastian Lehmann, Lars Reine Wallenberg, and Kimberly A. Dick Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02421 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

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Kinetic engineering of wurtzite and zinc blende AlSb shells on InAs nanowires Hanna Kindlund,1 Reza R. Zamani,1 Axel. R. Persson,2 Sebastian Lehmann,1 L. Reine Wallenberg,2 and Kimberly A. Dick1,2 1

2

Division of Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden Centre for Analysis and Synthesis, Lund University, Box 124, S-221 00 Lund, Sweden

Using AlSb as the model system, we demonstrate that kinetic limitations can lead to the preferential growth of wurtzite (WZ) AlSb shells rather than the thermodynamically stable zinc blende (ZB) AlSb, and that the WZ and ZB relative thickness can be tuned by a careful control of the deposition parameters. We report selective heteroepitaxial radial growth of AlSb deposited by metal-organic vapor phase epitaxy (MOVPE) on InAs nanowire core templates with engineered lengths of axial WZ and ZB segments. AlSb shell thickness, crystal-phase, nanostructure, and composition are investigated, as a function of the shell growth temperature Ts, using scanning electron microscopy, transmission electron microscopy, electron tomography, and energy-dispersive xray spectroscopy. We find that ZB and WZ structured AlSb shells grow heteroepitaxially around the ZB and WZ segments of the InAs core, respectively. Surprisingly, at 390 < Ts < 450 oC, the WZ-AlSb shells are thicker than the ZB-AlSb shells and their thickness increases with decreasing Ts. In comparison, the ZB-AlSb shell thicknesses increase slightly with increasing Ts. We find that the increased thickness of the WZ-AlSb shells is due to the formation and enhanced deposition on {112ത 0} facets rather than on the more commonly grown {101ത 0} sidewall facets. Overall, these results, which are in direct contrast with previous reports suggesting that heteroepitaxial radial growth of III-antimonides is always favored on the ZB-structure facets, indicate that the growth of WZ-AlSb is preferred over the thermodynamically stable ZB-AlSb at

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lower growth temperatures. We attribute this behavior to kinetic limitations of MOVPE of AlSb on ZB and WZ phases of InAs.

Keywords: core-shell nanowire, aluminium antimonide, AlSb, selective radial growth.

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The ability to tune the intrinsic properties of a material by changing the crystal structure opens new possibilities of engineering devices with potentially new applications. For example, materials such as Ge, Si1-xGex, or GaP can shift from indirect to direct band-gap when changing their crystal structure from cubic to hexagonal phase.1-3 For non-nitride based group III-V compound semiconductors,4 the zinc blende (ZB) lattice is the thermodynamically favored crystal structure. Previous studies have shown that wurtzite (WZ) structure InAs and other group III-Vs can be stable in low-dimensional structures such as nanowires.5 For instance, controlledphase synthesis of arsenide- and phosphide-based nanowires is widely reported.6-9 In cases where the realization of metastable phases cannot be achieved for normal (axial) nanowire growth, radial deposition using the nanowire side facets as templates, has been proven to be a good alternative to develop new crystal structures, for instance, hexagonal-structured Si10 and Si111 xGex

and, more recently, GaSb.12 Among the group III-V compounds, tuning the crystal

structure of III-antimonides has been shown to be more complicated, and most studies indicate that these materials crystallize in ZB even at the nano-scale.13-18 Previous studies focusing on heteroepitaxial growth of arsenide and antimonide shells around phase-engineered InAs nanowires have shown that shell formation on thermodynamically-stable ZB is always favored over WZ,19 as a result of the higher surface energies of the common {110} ZB facets compared to the {101ത 0}-type facets of the WZ phase.19-22 Here, choosing AlSb-InAs as the model system, we show that kinetic limitations can actually lead to the preferential growth of WZ-structure group III-antimonide, AlSb, (instead of the thermodynamically stable ZB AlSb structure). We report the radial growth of AlSb shells on crystal-engineered InAs nanowires. Zinc blende structure AlSb and InAs are group III-V compound semiconductors of the 6.1 Å family, with room-temperature band-gaps of 1.61 and

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0.36 eV, respectively.23 The InAs/AlSb heterojunction offers a slightly staggered valence-band offset of 0.1 eV and a large conduction band-offset around 1.3 eV, which makes possible the realization of very deep InAs/AlSb quantum wells and very high tunneling barriers,23,

24, 25

important to develop tunneling field-effect transistors and other electronic devices. Surprisingly, we find that WZ AlSb phase is favored over ZB at selected temperatures and that the relative AlSb shell thickness can be controlled with a proper selection of the deposition parameters. We suggest that these results are a consequence of kinetic limitations during growth. This result opens new pathways to explore properties of crystal phases that are previously unavailable. For instance, WZ AlSb is predicted to exhibit direct band-gap in contrast to its cubic counterpart.26 We expect that pseudomorphic growth of other metastable phases, including other WZ antimonide alloy shells, can be realized by appropriate control of growth parameters. InAs-AlSb-InAs core-double-shell nanowires are grown by metal organic vapor phase epitaxy (MOVPE) in a standard low pressure (100 mbar) showerhead reactor (Aixtron 3×2 inch) using 30 and 40 nm Au aerosol nanoparticles acting as the catalyst material. The aerosol particles are deposited on (111) B InAs substrates with a density of 1 particle/µm2, in order to minimize the effects of atomic diffusion and shadowing from neighboring nanowires. The precursors used for the InAs growth are trimethylindium (TMIn) and arsine (AsH3). Tritertiarybutylaluminum (TTBAl) and trimethylantimony (TMSb) are used for AlSb shell growth. Hydrogen acts as the carrier gas at a total flow rate of 8 l/min. Prior to growth, the InAs substrates are degassed in AsH3 atmospheres at a set temperature of 550 °C for 10 min, in order to remove surface native oxides. The InAs core structure consists of three segments: a wurtzite (WZ1) segment, followed by a zinc blende (ZB), and a second wurtzite (WZ2) on top. The InAs core is grown at a set

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temperature of 473 oC at a constant TMIn molar fraction of 1.8×10-6. The nucleation of the InAs stem is first carried out by supplying AsH3 with a molar fraction of 1.3×10-2 during 3 min. For the WZ1, ZB, and WZ2 InAs structures, the AsH3 molar fractions are changed to 2.3×10-5 (WZ1 and WZ2) and 1.6×10-5 (ZB). The growth times are 20 min (WZ1), 10 min (ZB), and 10 min (WZ2), which result in segments with lengths 1000±200 nm, 700±100 nm, and 1000±200 nm, respectively, and a total nanowire length of ~2.7 µm. All AlSb shells are grown for 30 min (unless otherwise stated later the text for one sample used for tomography, which was grown for 15 min) by switching the group III precursor from TMIn to TTBAl and supplying TMSb while having AsH3 open during the first and the last 30 s of the AlSb shell deposition. Due to the high sticking coefficient of Al, at selected growth conditions, the AlSb shell may not completely cover the InAs core, which can lead to decomposition of the InAs at high shell growth temperatures. Therefore, to avoid decomposition, arsine is flushed for 30 s in presence of TTBAl and TMSb before starting pure AlSb growth. The molar fractions of the precursors are 6.6×10−6 (TTBAl) and 4.5×10−5 (TMSb), which results in a nominal V/III ratio of 6.8. AlSb is highly hygroscopic and it rapidly oxidizes upon air exposure. Therefore, capping with another material is required to avoid decomposition. Here, we choose InAs as protective layer. The InAs shell is grown for 30 min by first flushing TMIn for 30 s and then introducing AsH3 at a molar fraction of 2.3×10-5. After growth, all samples are cooled down under AsH3. To provide uniformity the substrates are rotating at 30 rpm during growth. Nanowire morphology is investigated by scanning electron microscopy (SEM) (Zeiss LEO 1560) operated at 15 kV using a secondary-electron detector. Bird’s eye view SEM images are acquired at a tilt angle of 30°. Nanowire structure and phase composition are determined by transmission electron microscopy (TEM) techniques. For this purpose, the nanowires are

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manually broken off the substrate and transferred to lacey carbon covered Cu grids. Scanning transmission electron microscopy (STEM) combined with energy-dispersive x-ray spectroscopy (EDX) are used to assess nanowire composition. TEM and STEM-EDX analyses are carried out in a JEOL 3000F microscope equipped with a field-emission gun operated at 300 kV. TEM and STEM images are acquired along the a 〈110〉-type zone axis. Tomographic reconstructions are performed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in a JEOL JEM-2200FS TEM operated at 200 kV. The images are acquired at tilt angles ranging ±79° with 2° increments, HAADF collection angles 79-211 mrad, 1024 x 1024 pixels images, and a dwell time of 80 µs. Post-processing consists of 3 steps: (1) image alignment using cross-correlation and a geometrical correction for tilting a sample,27 (2) application of the simultaneous iterative reconstruction technique (SIRT, 40 iterations) by the ASTRA toolbox for MatLab,28-30 (3) visualization by either by MatLab for 2D slices or Chimera (RBVI, University of California, San Francisco) for 3D objects. The morphology of the as-grown InAs/AlSb/InAs core-double-shell nanowires is investigated, based upon SEM imaging, as a function of the growth temperature Ts. Results are shown in Fig. 1. Most nanowires consist of three segments growing in the vertical [111] B direction with respect to the substrate: first, a bottom wurtzite segment (WZ1), followed by a ZB segment in the center part of the nanowire, and a second wurtzite segment (WZ2) on top (a generic schematic of the heterostructure is shown in Fig. 1(a)). At high Ts, e.g., 495 oC (Fig. 1(b)) and 470 oC (Fig. 1(c)), the diameter of the ZB segment is larger than that of the WZ segments. Also, the WZ1 segment has a larger diameter than WZ2. The diameter increase from the WZ to the ZB segment is most likely due to: (1) a 30o facet rotation from one core structure to the other ({101ത 0}-type facets of WZ to {110}-type facets of

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ZB), and (2) increased AlSb deposition on ZB with respect to WZ, while the diameter difference between WZ1 and WZ2 segments is attributed to material overgrowth on the WZ1 during the growth time of the InAs core, which results in rougher side facets, following more radial overgrowth. These results are consistent with those reported for GaSb shells grown on InAs nanowires with similar WZ and ZB segments.19 However, at lower Ts, i.e., 450 oC (Fig. 1(b)), 420 oC (Fig. 1(c)) and 390 oC (Fig. 1(d)), the morphology of the heterostructure differs from that of nanowires grown at 470 oC (Fig. 1(a)) and 495 oC. At lower temperatures, the diameter of the bottom WZ segment (WZ1) is larger than that of the ZB, which indicates a preferential deposition on the WZ-structure in this temperature range (390 – 450 oC). In addition, we observe that the thickness of the WZ1 part of the nanowire is not uniform along the whole segment, being thinner on the bottom than on the top (inverse tapering), as depicted in the higher-magnification image shown as an insert in Fig. 1(c)). Also, for the majority of the nanowires grown at these temperatures, the thickness of the WZ1 segment is larger than that of the WZ2, although for some of the wires WZ1 and WZ2 have similar thickness. A combination of STEM, EDX, and TEM analyses confirm the presence of AlSb shells radially grown on both WZ- and ZB-InAs core segments at all Ts studied, and an outer InAs shell around both AlSb phases. In addition, TEM investigations show that the WZ-structure InAs grows with the c-axis [0001ത ] along the nanowire growth direction, whereas the middle segment തതതതത] direction parallel to the nanowire axis. crystallizes in the ZB structure with the [111 Representative STEM images with corresponding EDX data and TEM micrographs of an InAs/AlSb/InAs core-double-shell nanowire grown at 470 oC are presented in Fig. 2. In the Zcontrast STEM image shown in Fig. 2(a), the InAs core and the outermost shell appear brighter

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in contrast, while the AlSb shell, which is uniform along each individual segment, appears relatively darker in contrast. Fig. 2(b) and 2(c) are higher-resolution STEM images, with EDX scans, of the same wire acquired from the ZB and WZ1 areas of the nanowire shown in Fig. 2(a). The interfaces between the different materials are indicated with a white dashed line. The superimposed EDX spectra in Figs. 2(b) and 2(c), in which each element is color coded for clarity with red, yellow, green, and blue corresponding to In, As, Al, and Sb, respectively, show compositional variations across the nanowire, in agreement with the STEM image confirming the presence of an InAs core, an AlSb shell and an outermost InAs shell. The HR-TEM images in Fig. 2(e) and 2(f) show that AlSb grows epitaxially on both ZB-InAs and WZ-InAs core structures. The interfaces between the InAs core and the AlSb shell, and between the two shells, are marked by red dashed lines following contrast variations that indicate interfacial strain between the different materials (note, that the exact position of the interfaces in the ZB segment cannot be resolved in HRTEM when viewing along a 〈110〉-type zone axis). The ZB part of the nanowire (Fig. 2(d)-2(e)), is formed of irregular twinned segments that extend from the InAs core to the InAs outer shell. The thickness of the ZB-AlSb is estimated at 14 ±2 nm. The WZ part of the nanowire (Fig. 2(f)) shows an InAs core with occasional stacking defects that extend to the WZ-AlSb and WZ-InAs shells. We observe that top and bottom WZ-AlSb shells are, within the measurement uncertainties, of similar thickness. The thickness of the WZAlSb shell, ~9 ± 2 nm, is lower than that of the ZB-AlSb, which suggests that AlSb grows preferentially on ZB-InAs than on WZ-InAs at Ts = 470 oC. Similar results are obtained for AlSb shells grown at 490 oC. At lower growth temperatures (390 – 450 oC) the morphology of the heterostructure differs from that of nanowires grown at 470 and 495 oC. Fig. 3 are TEM and STEM images of an

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InAs/AlSb/InAs core-double-shell nanowire grown at 420 oC. The HR-TEM images presented in Fig. 3(a)-3(c) show that the AlSb shells grow epitaxially on WZ- and ZB-InAs and that the thickness of the WZ1-AlSb shell is higher than that of the ZB-AlSb. This is surprising and generally not expected since the surface energies of the low-index facets of WZ- and ZB- are predicted to be higher for the latter.20 A more clear view of the heterostructure is depicted in Fig. 3(d) as a lower magnification TEM image. The image shows that the thickness of the bottom WZ1 segment is not uniform, being thicker on the top than on the bottom of the segment. To determine the origin of the thickness variations within the WZ-AlSb shells, we acquire detailed STEM images and search for changes in the shell growth direction based upon STEM imaging intensity profiles obtained along the a 〈110〉-type zone axis. The profiles illustrate the cross sectional shape of the nanowire. The STEM results, presented in Fig. 3(e)-3(g), are acquired from the areas of the nanowire in Fig. 3(d) framed with yellow, green, and blue color, corresponding to the WZ1, ZB, and WZ2 segments, respectively. The bottom wurtzite segment (WZ1) is composed of two different radial cross-sections, as indicated by the curve change in the intensity profiles shown in Fig. 3(g). STEM analyses indicate that the top part of the WZ1 segment is rotated 30o with respect to the bottom part, i.e. the facets at the bottom-most of the WZ1 part of the nanowire are of the {101ത 0}-type, while the thicker part of the segment consists of {112ത 0}-type side facets. The ZB segment is terminated by {110}-type side facets (see Fig. 3(f)), while the facets of WZ2 are of the {101ത 0} family. In this particular nanowire, the WZ2 segment is made of {101ത 0}-type facets throughout the segment without any facet change, as reflected in the STEM image and the corresponding intensity profile presented in Fig. 3(e). However, other nanowires from the same sample and from samples grown at 390 and 450 oC exhibit a facet change on the WZ2 segment as well (see Figs. 1(b)-1(d)). A schematic of the

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facets and hetersostructures formed at low vs. high Ts is shown as supplemental information (Figure S1). These results provide evidence that deposition on WZ-AlSb can be enhanced by lowering the growth temperature. Essentially, similar results are obtained for nanowires grown at 450 and 390 oC. We also note that InAs grows epitaxially on both ZB- and WZ-AlSb at all growth temperatures. In order to better understand the origins of the facet changes in the heterostructure, we acquire HAADF-STEM data and simulate tomographic reconstructions of nanowires with AlSb shells grown at 420 oC for two different growth times. Results are obtained from the WZ1 part (bottom WZ segment) and are shown in Figs. 4(a)-4(c) for an AlSb shell growth time of 30 min and in Figs. 4(d)-4(f) for an AlSb shell grown for 15 min. The images indicate that the InAs core maintains its initial facets along the whole segment. The facet change occurs only on the WZAlSb shells. We also observe that after 15 min of shell growth the new {112ത 0} facets are not fully formed and well defined (see Fig. 4(b), and 4(e)), which indicates that the development of {112ത 0} facets occurs over time. The average thicknesses of the AlSb shells are depicted in Fig. 5 as a function of Ts. Here we focus only on WZ1 and ZB segments for simplicity. Shell thicknesses are determined from HRTEM analyses, aligning the nanowire to a 〈110〉-type zone axis. The thickness of ZB-AlSb increases slightly with increasing Ts. The gradual and linear increase in growth rate of ZB-AlSb can be attributed to a temperature-dependent net increase in deposition flux, more than to a temperature-dependent increase in desorption. However, the thickness of WZ-AlSb increases continuously and very dramatically with decreasing Ts. It is worth mentioning that at lower growth temperatures, most of the Al and Sb atoms contributing to the shell growth are due to direct deposition from the gas phase, since diffusion from the substrate is more limited than at

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higher growth temperatures. In Fig. 5, the AlSb shell thicknesses are separately plotted for ZBAlSb and {101ത 0}- and {112ത 0}-faceted WZ-AlSb. Between 390 and 450 oC, the thickness of the WZ shells is greater than that of the ZB shells. The thickness of {112ത 0} faceted WZ-AlSb is more than 5× that of the ZB for shells deposited at 390 oC, and ∼ 2× for shells grown at 450 oC. However, the thickness difference between ZB and {101ത 0} faceted WZ-AlSb shells is minimal, falling within the measurement uncertainties. On the contrary, at higher growth temperatures, i.e. 470 and 490 oC, the thickness of the ZB-AlSb shells are larger than that of the WZ-AlSb shells (note that at high temperatures the AlSb shells do not exhibit facet changes along the WZ segments and, therefore, Fig. 5 shows only data for ZB and {101ത 0} faceted WZ1 at 470 and 495 o

C). Selective heteroepitaxial growth of WZ phase and simultaneous suppression of deposition on

ZB phases of III-Vs is new and generally not expected. For AlSb, the {112ത 0}-type facets of the WZ structure are predicted to have the same surface energy as the corresponding {110}-type facets of the ZB structure, 1.38 J/m2,20 while the {101ത 0}-type WZ facets are predicted to have the lowest surface energy, 1.19 J/m2.20 AlSb shells grown at 390 ≤ Ts ≤ 450 oC on WZ1 segments show a 30o facet rotation during growth from {101ത 0} to {112ത 0}, even though {101ത 0} facets are predicted to be more stable. {112ത 0}-faceted WZ-AlSb is much thicker than ZB-AlSb, although their surface energies are supposedly equal. In addition, WZ-AlSb is a metastable phase (the ZB structure is the thermodynamically stable phase of AlSb). Therefore, the surface energy argument alone cannot explain our observations, i.e. the formation of thicker WZ-AlSb shells at lower

deposition

temperatures,

since

preferential

deposition

on

WZ

is

not

energetically/thermodynamically favored. Therefore, we suggest that this phenomenon of enhanced WZ shell growth is a consequence of kinetic limitations of the growth process. To

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explain the high deposition rate on the {112ത 0} WZ surfaces at low temperatures, we suggest the following mechanism: (1) the growth of {112ത 0}facets starts at the ZB-WZ interface, that is, the interface with the ZB acts as nucleation sites, (2) the reactive sticking coefficient (RSC) of Al, Sb, AlSb or the precursors on WZ {112ത 0} surfaces is higher than the RSC on zinc blende surfaces, (3) the RSC on WZ surfaces decreases more sharply with increasing temperature than on zinc blende surfaces. In conclusion, we have successfully grown InAs/AlSb/InAs core-double-shell nanowires by MOVPE at 390 ≤ Ts ≤ 495 oC. More importantly, we demonstrate control over the crystal phase and thickness of the AlSb shells. We show that both WZ and ZB structure AlSb shells grow heteroepitaxially on ZB-InAs and WZ-InAs cores, respectively. Surprisingly, WZ-AlSb shell thicknesses are temperature-dependent and increase significantly with decreasing Ts, while the thicknesses of ZB-AlSb shells increase slightly with Ts. The increased WZ-AlSb thickness corresponds to the formation of {112ത 0} facets, which form preferentially at 390 ≤ Ts ≤ 450 oC. We attribute these results to a combination of enhanced nucleation at the ZB-WZ interfaces and high reactive sticking coefficient of AlSb on {112ത 0} WZ surfaces of InAs. Overall, these results provide new insights into the MOVPE shell growth and indicate that the growth of WZ-AlSb can be controlled by carefully controlling the deposition parameters. Based upon our results we expect similar growth behavior of other III-V shells. Our result opens new pathways to explore properties of crystal phases that are previously unavailable. AUTHOR INFORMATION Corresponding Author *H. Kindlund, [email protected].

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Funding Sources The authors acknowledge the support from the Swedish Foundation for Strategic Research (SSF), the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation (KAW) and NanoLund. SUPPORTING INFORMATION A schematic of the AlSb shells crystal structure and facets is presented in Figure S1.

ACKNOWLEDGMENT The authors are grateful to S. G. Ghalamestani for fruitful insights.

FIGURE CAPTIONS Figure 1: (a) Generic schematic of the InAs/AlSb/InAs heterostructure. 30o tilted-view SEM images of InAs/AlSb/InAs core-double-shell nanowires grown at (b) 495 oC, (c) 470 oC, (d) 450 o

C, (e) 420 oC, and (f) 390 oC. A higher magnification image of a single nanowire of each sample

is shown as an insert in the upper-right corner. Figure 2: (a) STEM image of an InAs/AlSb/InAs core-double-shell nanowire grown at 470 oC. (b) higher-magnifications STEM images with superimposed EDX spectra of the nanowire in (a), acquired from (b) the ZB segment, and (c) the lower WZ segment (WZ1). (d) Low-magnification TEM image of the WZ1 and ZB segments of the nanowire shown in (a). HR-TEM images obtained from (e) the ZB segment, and (f) the lower WZ segment of a nanowire in (d). Interfaces are indicated with red dashed lines for guidance. Figure 3: HRTEM images of an InAs/AlSb/InAs core-double-shell nanowire grown at 420 oC acquired from (a) bottom wurtzite segment WZ1, (b) ZB segment, and (c) top wurtzite segment

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(WZ2). Interfaces are marked with red dashed lines. (d) Lower-magnification TEM image from the same wire than in (a)-(c). The yellow dotted lines are a guide to indicate the approximate end and start location of the ZB and WZ segments. (e), (f), and (g) are STEM images of the same wires acquired from the regions marked in (d), showing the facet change in the AlSb WZ shell. Figure 4: STEM-HAADF electron tomography reconstructions of the WZ1 segment of two InAs/AlSb/InAs core-shell nanowires with AlSb shell growth times of (a-c) 30 min and (d-f) 15 min. (b) and (c) are cross-sectional views of the wires in (a), acquired from the top and bottom regions highlighted by orange and green, respectively. Note the change in facets and increased shell thickness in the the top part of the nanowire segment, while the AlSb shell in the lower part is thinner and grows coaxially. (e) and (f) are cross-sectional views of the wires in (d), acquired from the top and bottom regions highlighted by blue and yellow, respectively. The new facets on the top part of the segments are developing but are not competely formed, while the lower part of the AlSb shell retains its original facets. The selected cross sections in (b, d) also illustrate the increased shell thickness with respect to (e, f). The red dotted line shows the shape and orientation of the InAs core. Figure 5: Average AlSb shell thickness as a function of Ts for ZB-AlSb (blue squares), {101ത 0} faceted WZ1-AlSb (filled black triangles), and {112ത 0} faceted WZ1-AlSb (open grey triangles).

REFERENCES 1. Assali, S.; Zardo, I.; Plissard, S.; Kriegner, D.; Verheijen, M.; Bauer, G.; Meijerink, A.; Belabbes, A.; Bechstedt, F.; Haverkort, J. Nano letters 2013, 13, (4), 1559-1563. 2. Cartoixa, X.; Palummo, M.; Hauge, H. I. T.; Bakkers, E. P.; Rurali, R. Nano letters 2017, 17, (8), 4753-4758. 3. Kaewmaraya, T.; Vincent, L.; Amato, M. The Journal of Physical Chemistry C 2017, 121, (10), 5820-5828. 4. Yeh, C.-Y.; Lu, Z.; Froyen, S.; Zunger, A. Physical Review B 1992, 46, (16), 10086. 5.Dick, K. A.; Thelander, C.; Samuelson, L.; Caroff, P. Nano letters 2010, 10, (9), 3494-3499. 14


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6. Caroff, P.; Dick, K. A.; Johansson, J.; Messing, M. E.; Deppert, K.; Samuelson, L. Nature nanotechnology 2009, 4, (1), 50. 7. Dick, K. A.; Caroff, P.; Bolinsson, J.; Messing, M. E.; Johansson, J.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Semiconductor Science and Technology 2010, 25, (2), 024009. 8. Joyce, H. J.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C. Nano letters 2010, 10, (3), 908-915. 9. Lehmann, S.; Wallentin, J.; Jacobsson, D.; Deppert, K.; Dick, K. A. Nano letters 2013, 13, (9), 4099-4105. 10. Hauge, H. k. I. T.; Verheijen, M. A.; Conesa-Boj, S.; Etzelstorfer, T.; Watzinger, M.; Kriegner, D.; Zardo, I.; Fasolato, C.; Capitani, F.; Postorino, P. Nano letters 2015, 15, (9), 58555860. 11. Hauge, H. I. T.; Conesa-Boj, S.; Verheijen, M. A.; Koelling, S.; Bakkers, E. P. A. M. Nano Letters 2017, 17, (1), 85-90. 12. Namazi L., e. a. http://doi.org/10.1002/adfm.201800512. 13. Caroff, P.; Wagner, J. B.; Dick, K. A.; Nilsson, H. A.; Jeppsson, M.; Deppert, K.; Samuelson, L.; Wallenberg, L.; Wernersson, L. E. Small 2008, 4, (7), 878-882. 14. Jeppsson, M.; Dick, K. A.; Nilsson, H. A.; Sköld, N.; Wagner, J. B.; Caroff, P.; Wernersson, L.-E. Journal of Crystal Growth 2008, 310, (23), 5119-5122. 15. Ercolani, D.; Rossi, F.; Li, A.; Roddaro, S.; Grillo, V.; Salviati, G.; Beltram, F.; Sorba, L. Nanotechnology 2009, 20, (50), 505605. 16. Borg, B. M.; Dick, K. A.; Eymery, J.; Wernersson, L.-E. Applied Physics Letters 2011, 98, (11), 113104. 17. Xu, T.; Dick, K. A.; Plissard, S.; Nguyen, T. H.; Makoudi, Y.; Berthe, M.; Nys, J.-P.; Wallart, X.; Grandidier, B.; Caroff, P. Nanotechnology 2012, 23, (9), 095702. 18. Gorji Ghalamestani, S.; Ek, M.; Ganjipour, B.; Thelander, C.; Johansson, J.; Caroff, P.; Dick, K. A. Nano letters 2012, 12, (9), 4914-4919. 19. Namazi, L.; Nilsson, M.; Lehmann, S.; Thelander, C.; Dick, K. A. Nanoscale 2015, 7, (23), 10472-10481. 20. Sibirev, N.; Timofeeva, M.; Bol’shakov, A.; Nazarenko, M.; Dubrovskiĭ, V. Phys. Solid State 2010, 52, (7), 1531-1538. 21. Ghalamestani, S. G.; Heurlin, M.; Wernersson, L.-E.; Lehmann, S.; Dick, K. A. Nanotechnology 2012, 23, (28), 285601. 22. Rieger, T.; Schäpers, T.; Grützmacher, D.; Lepsa, M. I. Crystal Growth & Design 2014, 14, (3), 1167-1174. 23. Kroemer, H. Physica E: Low-dimensional Systems and Nanostructures 2004, 20, (3), 196203. 24. Tuttle, G.; Kroemer, H.; English, J. H. Journal of Applied Physics 1989, 65, (12), 5239-5242. 25. Rieger, T.; Grützmacher, D.; Lepsa, M. I. Journal of crystal growth 2015, 425, 80-84. 26. De, A.; Pryor, C. E. Physical Review B 2010, 81, (15), 155210. 27. Sanders, T.; Prange, M.; Akatay, C.; Binev, P. Advanced Structural and Chemical Imaging 2015, 1, (1), 4. 28. van Aarle, W.; Palenstijn, W. J.; De Beenhouwer, J.; Altantzis, T.; Bals, S.; Batenburg, K. J.; Sijbers, J. Ultramicroscopy 2015, 157, 35-47. 29. van Aarle, W.; Palenstijn, W. J.; Cant, J.; Janssens, E.; Bleichrodt, F.; Dabravolski, A.; De Beenhouwer, J.; Batenburg, K. J.; Sijbers, J. Optics express 2016, 24, (22), 25129-25147.

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30. Palenstijn, W.; Batenburg, K.; Sijbers, J. Journal of structural biology 2011, 176, (2), 250253.

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Nano Letters

(a)

(b)

495 oC

200 nm

1 µm

(c)

470 oC

(d)

200 nm

200 nm

1 µm

(e)

1 µm 420 oC

(f)

390 oC

200 nm

200 nm

1 µm

450 oC


1 µm

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WZ2 (b)

Page (a) 19 of 22 1 2 3 4 5 6 7 8 9 10 11 InAs AlSb 12 13 14 15 16 10 nm 17 18 19 (d) 20 21 22 23 24 0.2 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

ZB (c)

Nano Letters

InAs AlSb InAs AlSb

InAs

InAs

AlSb

InAs InAs

InAs

10 nm

10 nm WZ2

WZ1

ZB

WZ1

µm

(e)

(g)


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(a)

(d) (b)

(e) 0}

10 {1-

{1-100} {11-20}

(f) 0}

1 {1-

} 00

{1 -1 0

(c)

100 nm

100 nm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Average shell thickness [nm]

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Nano Letters {10-10} WZ1 {11-20} WZ1 {110} ZB

50 40 30 20 10 0 380

ACS Paragon 400 420 Plus 440Environment 460 480 o

Ts [ C]

500

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


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Kinetic Engineering of Wurtzite and Zinc-Blende ... - ACS Publications

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