Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid

Subscriber access provided by NEW YORK UNIV

Communication

Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface Zhiyuan Sun, David N. Seidman, and Lincoln Lauhon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02071 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid-Solid Growth Interface Zhiyuan Sun a, David N. Seidman a,b*, and Lincoln J. Lauhon a* a

Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, 60208-3108, USA b Northwestern University Center for Atom-Probe Tomography (NUCAPT), 2220 Campus Drive, Evanston, 60208-3108, USA *E-mail: [email protected], *E-mail: [email protected]

Abstract Dopants modify the electronic properties of semiconductors, including their susceptibility to etching. In semiconductor nanowires doped during growth by the vapor-liquid-solid (VLS) process, it has been shown that nanofaceting of the liquid-solid growth interface influences strongly the radial distribution of dopants. Hence, the combination of facet dependent doping and dopant selective etching provides a means to tune simultaneously the electronic properties and morphologies of nanowires. Using atom-probe tomography, we investigated the boron dopant distribution in Au catalyzed VLS grown silicon nanowires, which regularly kink between equivalent directions. Segments alternate between radially uniform and nonuniform doping profiles, which we attribute to switching between a concave and convex faceted liquid-solid interface. Dopant selective etching was used to reveal and correlate the shape of the growth interface with the observed anisotropic doping.

1 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

Keywords: nanowire, dopant, liquid-solid interface, atom-probe tomography, selective etching, vapor-liquid-solid growth Introduction Doping in semiconductor nanowires is used to control electronic properties for applications, including field-effect transistors,1,

2

thermoelectrics,3,

4

and plasmonic

devices.5 Additionally, intentional variation of doping profiles has been utilized to modulate morphology employing selective etching6,

7

to permit wavelength-selective

absorption or scattering of light and to enhance mechanical interactions at nano-bio interfaces for biological applications.8 Therefore, one key to realizing diverse functionality in nanowires involves controlling the doping concentration, uniformity, and abruptness. In situ doping during vapor-liquid-solid (VLS) nanowire growth provides many opportunities for engineering functionality during synthesis but also presents challenges due to the possibility of multiple incorporation pathways, including delayed doping from the catalyst reservoir,9 vapor-solid (VS) deposition on the surface,10 segregation at the surface,11 and non-uniform enhancements of dopants due to nanofaceting of the growth interface.12 Thus, understanding growth processes at both VLS and VS interfaces is necessary to understand nanowire doping, and a substantial body of research has focused on understanding, eliminating or taking advantage of these parallel processes to achieve uniform or complex doping profiles.

In nanowire growth, by definition, the majority of matrix atoms are incorporated at the 2 ACS Paragon Plus Environment

Page 3 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

LS interface, whereas the majority of dopant13, 14 and catalyst7, 15, 16 impurity atoms may be incorporated at either the LS or VS interface. Depending on the conditions, one may seek to eliminate VS growth to reduce unintentional surface doping or to exploit VS growth to define radial junctions1 and to a grow periodic shell on a nanowire.17-19 In the present research, we focus on growth conditions in which dopant incorporation occurs predominantly at the LS interface, rather than the VS interface. In-situ transmission electron microscopy (TEM) has led to important insights into the influence of the LS interface shape on morphology, composition and crystal phase during nanowire growth.20-26 For example, the generation27, 28 and elimination29 of crystal defects, such as twin boundaries,26 stacking faults,28 and polytypes27 are related to the character of the LS interface. Additionally, the shape of the LS interface influences the crystal phase of III-V semiconductor nanowires, such as GaAs,30 where a truncated LS interface leads to the zinc blend (ZB) structure, while a flat LS interface leads to a wurtzite (WZ) structure. In Si nanowires, the shape of the LS growth interface has been shown to influence strongly the uniformity of dopant incorporation.12 Specifically, -oriented Si nanowires that grow perpendicular to a dominant (111) central facet exhibit enhanced doping near the surface of the nanowire due to nanoscale higher Miller index corner facets.12 In situ TEM measurements first revealed the presence of these corner facets, while atom-probe tomography (APT) has been used to resolve small variations in dopant distributions31 due to the relative insensitivity of electron-based spectroscopies to dilute impurity concentrations.32, 33 3 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

Herein we report APT analyses of dopant distributions in -oriented silicon nanowires whose LS growth interface assumes a concave “V” shape induced by a twin boundary bisecting the nanowire.26, 29 We find that changes in the LS interface shape from concave to convex following kinking between directions induce substantial changes in the dopant distribution as directly measured by APT and confirmed by dopant selective etching. Nonuniform doping results in nonuniform etching, providing an additional means to vary the morphology of one-dimensional nanostructures.

Kinked silicon nanowires were grown in a hot-wall chemical vapor deposition (CVD) system by a vapor-liquid-solid (VLS) mechanism using Au nanoparticles of 50-150 nm diam. The growth substrates were prepared by diluting a commercial Au colloid solution 5 times with deionizing water and then depositing the solution onto a Si (100) substrate pre-coated with commercial Poly-L-lysine. The substrate was then transferred to the CVD chamber and annealed in H2 at 460 °C for 10 min. The nanowires were grown at a total pressure of 40 Torr using SiH4 as a Si precursor, H2 as a carrier gas and B2H6 (100 ppm in He) as the doping precursor. The p-type nanowires were grown at 450 °C with SiH4, B2H6 (100 ppm in He) and H2 flow rates of 2, 10-15 and 100 sccm (cm3/min), respectively. The kinking was introduced following methods described in prior research34 by evacuating the reactor to 3.5 Torr for 10 s and then allowing the pressure to recover at the flow rates specified above (Fig. 1a inset). Pressure modulations were introduced 4 ACS Paragon Plus Environment

Page 5 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

every 2 to 4 min for a total of 10 cycles. As discussed below, the kinking probability was determined to be ~50 % for a 100 nm diam nanowire, i.e., the growth direction of half of the nanowires is changed following the pressure modulation. Growth substrates were sonicated in isopropanol (IPA) for 2 min to remove the nanowires and the solution was deposited onto a silicon wafer or TEM grid with lacy carbon for further studies. The analyses described below focus on regularly kinked nanowires with a 120° intersegmental angle (Fig. 1a). SEM images were recorded utilizing a Hitachi SU8030 operating at 5 kV. TEM images and electron diffraction patterns were acquired using a Hitachi H8100 operating at 200 kV.

APT was used to measure the dopant distribution in kinked nanowires using methods described previously.35 In brief, the nanowires were deposited onto a highly doped {111} silicon wafer and then coated with an 80 nm zinc oxide layer using atomic layer deposition (ALD). The conformal zinc oxide coating increases the field-of-view, so that the entire diameter of the nanowire can be analyzed with APT. The coating also prevents ion- beam radiation damage during sample preparation. Wedge-shaped samples were lifted out employing a FEI Helios dual-beam focused ion-beam (FIB) microscope with a micromanipulator and welded onto a commercial silicon micropost.36 Finally, ion-beam annular milling was performed to produce a needle-shaped nanotip with an ~100 nm diam. APT samples were prepared targeting both the kinks and the straight segments; regions near the growth tip were analyzed using APT to minimize the effects 5 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

of vapor-solid deposition on doping profiles. APT was performed utilizing a local-electrode atom-probe (LEAP) 4000X Si tomograph (Cameca, Madison, WI) at a specimen temperature of 35 K and a background pressure of 3.5 x 10-11 Torr. The dissection of specimens, one at a time, was controlled by a pulsed 355 nm ultraviolent focused laser at the rate of 1 to 1.5 ions every one hundred pluses. The laser energy was set at 25 pJ pulse-1 at a pulse rate of 250 to 500 kHz. The APT data were reconstructed using IVAS 3.6.12 to reproduce the shape of the nanowire in three-dimensions. For studies of dopant selective etching, nanowires were first etched with buffered oxide etch (BOE, 49 % HF: 45 % NH4F= 5 : 1) at room temperature for 20 s to remove the native oxide and passivate the surface with hydrogen. The nanowires were subsequently washed with deionized water and IPA and then etched with a freshly-made 5 wt. % KOH solution (KOH : water : IPA= 1 g : 20 mL : 20 mL) at room temperature for 2~8 min. Finally, the etched nanowires were washed with water and IPA several times and then blown dry with N2.

Figs. 1 a,b are scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of a typical kinked -oriented nanowire. Diffraction from the zone perpendicular to the kinking plane (Fig. 1b inset) exhibits evidence of just one crystalline orientation but prior studies have shown that there is a {111} twin boundary perpendicular to the viewing direction, which bisects the entire nanowire.29 Moreover, the presence of the twin boundary was found to be necessary for the 6 ACS Paragon Plus Environment

Page 7 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

observation of regular kinking of nanowires in a {111} plane.29 Accordingly, when such nanowires are deposited from solution onto a flat substrate, the twin boundary and kinking plane lie parallel to the substrate. In SEM images recorded at normal incidence to the substrate, the top facets of the nanowire are therefore {111}, while the side-facets are {110}, as shown below.

Fig. 1c is a cross-sectional TEM image of a nanowire with a bisecting {111} twin boundary grown under similar conditions.37 The surface is composed primarily of two {111} facets and four {311} facets. Two smaller {110} facets lie between the {311} facets, intersecting the twin boundary. Fig. 1d is a phase image from an atomic-force microscope (AFM) scan of a kink region. Line scans of the topography in selected regions are displayed in Fig. 1e. As demonstrated in ref [29], the region bridging the straight segments is primarily composed of four {111} facets, two of which are visible using AFM. The width of the visible facets on either side of the kink is the same within the spatial resolution of the AFM measurement. It is, however, important to note that the shape of the growth interface changes during the kinking process from concave to convex, and vice versa29. Specifically, it has been shown that -oriented nanowires with a bisecting twin boundary have initially concave “V” shaped growth interfaces, composed of two low energy {111} facets (Fig. 1f, left-hand side) when grown from a substrate. When such nanowires kink, the growth interface becomes convex (Fig. 1f, right-hand side). Subsequently, kinking between presumably equivalent directions is 7 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

accompanied by a change in the shape of the growth interface. Fig. 1f illustrates an additional hypothetical difference between the two growth interfaces involving the formation of a small corner nanofacet (displayed in black) to reduce the combined LS, VL, and VS interfacial energies. The addition of this nanofacet could be energetically favorable because it consumes the high energy {110} side facet (red, Fig. 1f) and reduces the curvature of the VLS triple junction, both of which may reduce the total interfacial energy, as described previously.20 These microfacets can lead to enhanced dopant incorporation as is discussed below.38, 39

Prior to describing the distinct radial dopant profiles produced by convex and concave growth interfaces, we first examine the impact of pressure modulations and kinking on the axial distribution (i.e., along the growth axis) of B dopants and Au catalyst atoms. For reference, a complete APT 3-D reconstruction displaying the Si nanowire and ZnO coating in cross-section is provided in the Supporting Information (Fig. S1). Fig. 2 examines the distribution of B and Au atoms along the nanowire at a pressure modulation induced kink (Fig. 2a,b,e) and at a point where the pressure modulation did not change the growth direction (Fig. 2c,d,f). Because the deposited nanowires always lie in a {111} plane, the viewing direction in this figure is along a -type direction. Fig. 2a,b display the B atom and Au impurity concentrations, respectively, in the kinked region. There is a 4X enhancement in B concentration at the end of the bridge region, where the change in growth direction is complete (Fig. 2b). As described in detail below, the 8 ACS Paragon Plus Environment

Page 9 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

enhanced doping concentration is associated with a change in the faceting of the growth interface; we presume that transient high Miller index facets are present during this change. The change in direction is also associated with a large concentration of Au on the surface of the nanowire on the inside {110} facets (parallel to the viewing direction), but not the outside {110} facets, of the kink (Fig. 2a) and the cross-sectional image in Fig. S3a. It has been demonstrated that a reduction in the silane partial pressure leads to the incorporation of Au atoms from the catalyst to the nanowire’s sidewalls.40 Indeed, enhanced surface Au concentrations on {110} facets are observed even when a pressure modulation does not result in a kink (Fig. 2c, Fig. S3 c,d). These findings are consistent with prior research on nanowires, in which the {110} facets were found to be decorated with Au atoms for growth at low silane partial pressures during a pressure modulation.7 Additionally, because the direction of growth and LS interface faceting are constant, the B dopant distribution is unperturbed (Fig. 2d, f). We, therefore, conclude that B dopant modulations arise from changes in the LS interface shape and not from pressure modulations.

Distinct LS interface shapes before and after kinking occurs produce different dopant distributions (Fig. 3). APT analyses of several straight segments enabled identification of two distinct types of radial doping profiles: non-uniform doping (Fig. 3a) associated with a convex growth interface and uniform doping (Fig. 3d) associated with a concave growth interface. The correlation of the interface shape with doping is explained 9 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

below. A comparison of doping profiles along and directions41 from the surface to the center of a nanowire (Fig. 3b, c, e, f; Fig. S1 b, c) displays a 7-fold enhancement in the region adjacent to {110} sidewalls (Fig. 3b) relative to the region adjacent to {111} sidewalls (Fig. 3c). The doping enhancement is not due to vapor-solid (VS) surface deposition because no deposition was found near the {111} facets. Fig. 3d,e,f displays doping concentration profiles of uniform segments. Uniform and non-uniform segments were found in approximately equal numbers. For both kinds of segments, no Au atoms were found within the nanowire or on the nanowire’s surface within the Au detection limit (~11 at. ppm, Fig. S2). This observation is consistent with prior observations that surfaces of -oriented nanowires are Au-free for growth at sufficiently a high silane partial pressure because they are passivated by hydrogen,29 while those of -oriented nanowires are not fully passivated by hydrogen and are more likely to be decorated by gold atoms.42 Additionally, no B or Au segregation is detected at the twin boundary; the Ʃ3(111) twin boundary is a low energy imperfection, where the level of segregation is small and therefore difficult to detect.43 Gold has been detected at grain boundaries in -oriented nanowires but has not been found at (111) stacking faults or at perfect twin boundaries in nanowires.15

As noted, we propose that the two distinct doping profiles correspond to two distinct LS interface shapes: concave (Fig. 1f, left-hand side) and convex (Fig. 1f, right-hand side). 10 ACS Paragon Plus Environment

Page 11 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Furthermore, we propose that the convex interface produces higher index nanofacets at the triple junction near the {110} facets and these nanofacets lead to enhanced doping. The correlation between the shape of the growth interface and doping uniformity was established utilizing KOH etching, which is inhibited by B-doping.44 Fig. 4a, b display SEM images near the tips of two etched nanowires, viewed along a direction, which correspond to two distinct post-etching cross-sections: wide (or belt-like) and narrow in Fig. 4c, 4,d respectively. The enhanced doping near the {110} sidewall facets of nanowires grown from convex interfaces inhibits etching, leading to a belt like cross-section (Fig. 4c,f

Fig. S4b, S5). The nanowires grown from concave interfaces are

doped uniformly, so they etch more uniformly (Fig. 4d,g Fig. S4c), leading to nanowires with more circular cross-sections. With the caveat that the Au-Si interface shape after growth may not correspond completely to the shape of the interface during growth, the convex and concave cases can be identified readily following post-growth etching (Fig. 4a,b). The red lines in Fig. 4a,b indicate the hypothesized location of the VLS triple junction and therefore the hypothesized LS interface shape consisting primarily of {111} facets. These post-growth images are not the basis for the hypothesized nanofacets displayed in black in Fig. 4c. Rather, the existence of these nanofacets during growth is proposed due to the observation of enhanced B doping regions, Fig. 3a, which was previously correlated with the existence of high Miller index nanofacets in VLS grown Si nanowires based on comparisons of APT12 with in situ TEM38 analyses.


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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

Fig. 4e-g display SEM and AFM analyses of an example of a transition from wide to narrow cross-sections associated with kinking from a convex-to-concave growth interface. As discussed above, the top of the convex nanowire is dominated by a flat {111} facet (Fig. 4f) due to the dopant-inhibited etching of the regions near {110} facets. In contrast, the narrow segment is etched more uniformly as revealed by the AFM topographical scan (Fig. 4g). Etching of the kink region is inhibited by surface Au atoms7 and dopants (Fig. 2a,b). Additional examples are provided in Fig. 5, where an n-type kinked nanowire (Fig. 5b) displays a similar etched morphology as the p-type kinked nanowire (Fig. 5a) indicating that similar changes in interface shape occur during growth with P doping. Extensive SEM imaging analyses of greater than 40 nanowires subjected to 347 pressure modulations (i.e., nanowire regions with a morphological perturbation) enabled a statistical analysis of the probability of kinking and changes in interface shape, summarized in Fig. 5c. For the conditions used, a change in growth direction is observed in 49% of the pressure modulations (Fig. 5c, i-iii), and 82% of the changes in growth direction are associated with a change in morphology and by extension the interface shape (Fig. 5c i). The number of wide-to-narrow (Fig. 5c i) and narrow-to-wide transitions (Fig. 5c ii) are very similar (69 vs 70) because one kind of transition is usually followed by another. We found no examples of a change in morphology without a change in direction (Fig. 5c vii), consistent with our expectation that the doping profile is maintained if the shape of the LS interface is preserved. The nodes in straight nanowires (Fig. 5c v) are due to pressure modulations, which induce Au decoration that protects the 12 ACS Paragon Plus Environment

Page 13 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

nanowire from etching.7

Surprisingly, 18% of kinked nanowires do not exhibit morphological evidence of a change in interface shape (Fig. 5c iii) and all of these examples correspond to the preservation of a narrow cross-section presumed to be associated with a concave LS interface (Fig. 5c iii,iv). Furthermore, no change in crystallography was observed between narrow-narrow and narrow-wide junctions (Fig. S6). To maintain consistency with the above hypotheses, one might conclude that the convex LS interface does not always lead to the formation of nanofacets, which cause enhanced doping. Although the post growth ex situ analysis described herein cannot confirm this hypothesis, a previous theoretical analysis has shown that a nanowire can grow with or without nanofacets45 for the same growth conditions (same temperature and pressure), because both configurations are stable or meta-stable and the actual growth mode depends on the growth history and initial conditions. We analyzed 217 etched segments near kinks and found 119 (55%) narrow segments (presumed concave) and 98 (45%) wide segments (convex). (Narrow segments may be over-represented in part because the initial growth from the substrate occurs with a concave interface in oriented nanowires.26) The 98 wide segments grow from convex interfaces with anisotropic doping. Of the 119 narrow segments, 107 are followed by (or follow) a wide segment after a kink, as expected. We deduce that the remaining 12 narrow segments (~10%) originate from a convex LS interface without nanofacets, enabling more uniform doping and subsequent 13 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

etching. In situ TEM would be needed to confirm this conjecture. Finally, no transitions from wide to wide cross-section were observed (Fig. c iv), indicating that the concave LS interface does not stabilize nanofacets at which doping is enhanced for the growth conditions studied here.

In conclusion, regularly kinked -oriented nanowires with bisecting twin-boundaries were subjected to etching, which revealed non-uniformities in doping. Kinking is associated with dramatic changes in the etched nanowire morphology, and examination of the Au-Si growth interface enabled correlation of a convex (or concave) LS interface with a wide (or narrow) etched nanowire cross-sectional area. In light of prior work linking the presence of nanofacets on the LS interface with an enhanced doping level, we hypothesize that the convex LS interface allows such nanofacets. Additionally, enhanced doping was detected at kinks and is attributed to transient high Miller index facets, which are present when the LS interface changes shape from convex to concave and vice versa. The newly discovered relationships between dopant distributions and LS interface faceting in -oriented Si nanowires provides a mechanism to tune of spatially dependent electronic properties.42 Furthermore, the dopant selective nature of etching provides a means to modify nanowire morphology, which may usefully modify electronic, thermal, and optical properties.

Supporting information 14 ACS Paragon Plus Environment

Page 15 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

The APT sample preparation method, mass spectra, APT data analyses, more etching results, and TEM images are found in the supporting information.

Acknowledgements DNS, LJL, and ZS acknowledge partial support of the United States–Israel Binational Science Foundation (grant number 2012088) and Northwestern University’s McCormick School of Engineering and Applied Science. LJL acknowledges support of DMR-1611341. We kindly thank Prof. Joseph T. Hupp and Mr. Will Hoffeditz for performing atomic-layer deposition. We thank Dr. Eric Hemesath for providing the high-resolution cross-sectional TEM image of the nanowire. The local-electrode atom-probe tomograph (LEAP4000X Si) at the Northwestern University Center for Atom-Probe Tomography (NUCAPT) was acquired and upgraded with equipment grants from the MRI program of the National Science Foundation (grant number DMR-0420532) and the DURIP program of the Office of Naval Research (grant numbers N00014-0400798, N00014-0610539, N00014-0910781). NUCAPT is a Research Core Facility of the Materials Research Center of Northwestern University, supported by the National Science Foundation’s MRSEC Program (Grant Number DMR-1121262). Additional instrumentation at NUCAPT was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN). Our research also made use of the EPIC facility (NUANCE Center), which receives support from the Materials Research Science and Engineering Center (MRSEC) program (NSF DMR-1121262); the International Institute for Nanotechnology (IIN); the State of Illinois and through the IIN. NUCAPT received 15 ACS Paragon Plus Environment

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

some additional support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205). We thank the anonymous reviewer for comments that helped us to improve parts of the text.

Figure 1. (a) SEM image of a regularly kinked -oriented nanowire. (b) TEM image of a kinked nanowire viewed along the [111] zone-axis. Inset: electron diffraction pattern on [111] zone-axis. (c) Cross-sectional high-resolution TEM image of a typical -oriented nanowire with a bisecting twin boundary. {111}-, {113}- and {110}-type surface facets are labeled (adapted with permission).37 (d) Atomic-force microscope phase-contrast image of nanowire kink region. The {111}- and {113}-type facets are indicated by green and blue shading, respectively. (e) Topographic line scan of nanowire surface before and after the kink; specific locations are indicated in (d). (f) Facets present in -nanowires with a bisecting twin boundary, including a concave (left) and convex (right) liquid-solid (LS) interface (the Au catalyst is not shown). The black dotted-lines indicate the twin boundaries. The stereographic projections of the leftmost crystals in each of the nanowires are shown directly below the schematic figures.


Page 17 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2 (a) Au (yellow) and (b) Boron (blue) concentration profiles near the nanowire’s kink, projected along a -type direction. (c) Au (yellow) and (d) Boron (blue) concentration profiles resulting from pressure modulations without kinking, projected along a -type direction (e) 1-D concentration profiles of Boron from the 112 segment to a 110 bridge in the direction of the red arrow in (b). (f) 1-D concentration profiles of Boron taken along the length of the nanowire. The green arrow in (d) indicates the scan direction. Due to fluctuations between the voxels used to calculate local concentrations, peak concentrations plotted in 2-D and 1-D profiles may differ.


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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

Figure 3 Boron dopant concentration maps and concentration profiles in -segments with distinct LS growth interface shapes. (a) Concentration map for a nanowire with a convex LS growth interface. Scale bar = 10 nm. (b) and (c) 1-D concentration profiles through the region in (a) generated from the blue rectangular box () and red rectangular box () . (d) Concentration map for a nanowire with a concave LS interface, scale bar = 10 nm. (e) and (f) 1-D concentration profiles of the segment in (d) generated along -type direction (blue rectangular box) and -type direction (red rectangular box). Due to fluctuations between the voxels used to calculate local concentrations, peak concentrations plotted in 2-D and 1-D profiles may differ.


Page 19 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4 (a) and (b) SEM images of two types of segments viewed from a [111]-type direction after KOH etching. The red lines indicate the approximate profiles of the Au-Si-vapor interface and the boundaries between two surfaces, {111}- and {113}-type facets before etching. (c) and (d) Top view of the convex and concave LS interfaces, respectively, before (top) and after (bottom) KOH etching. Side facets are labeled in colored fonts, blue, green and red. The dashed (convex) and dot-dash (concave) lines indicate the twin boundaries bisecting the nanowires. (e) SEM image of a kinked nanowire after 6 min etching in 5 wt. % KOH solution. (f) and (g) Atomic force microscopic topographic profiles of the nanowire in (e) before (red line) and after (blue line) kinking.


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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5

Page 20 of 26

Morphology of typical p-type (a) and n-type (b) kinked nanowires after KOH etching. (c)

Statistical analyses of the outcomes of pressure modulations based on morphologies after etching. Images are viewed along -type directions. The scale bar is 100 nm. The probability of kinking following a pressure modulation is 49% (170/347) for the 100 nm diam nanowires and the conditions investigated. Green schematics of the nanowire cross-sections follow the descriptions in Fig. 4. The number occurrences of interface morphology changes following pressure modulation are given in the text.


Page 21 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Graphical Abstract


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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

Reference 1.

Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M., Epitaxial core–shell and

core–multishell nanowire heterostructures. Nature 2002, 420, 57-61. 2.

Goldberger, J.; Hochbaum, A. I.; Fan, R.; Yang, P., Silicon Vertically Integrated

Nanowire Field Effect Transistors. Nano Letters 2006, 6, 973-977. 3.

Lee, J.; Lee, W.; Lim, J.; Yu, Y.; Kong, Q.; Urban, J. J.; Yang, P., Thermal Transport

in Silicon Nanowires at High Temperature up to 700 K. Nano Letters 2016, 16, 4133– 4140. 4.

Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.;

Majumdar, A.; Yang, P., Enhanced thermoelectric performance of rough silicon nanowires. Nature 2008, 451, 163-167. 5.

Chou, L.-W.; Boyuk, D. S.; Filler, M. A., Optically Abrupt Localized Surface

Plasmon Resonances in Si Nanowires by Mitigation of Carrier Density Gradients. ACS Nano 2015, 9, 1250-1256. 6.

Christesen, J. D.; Pinion, C. W.; Grumstrup, E. M.; Papanikolas, J. M.; Cahoon, J. F.,

Synthetically Encoding 10 nm Morphology in Silicon Nanowires. Nano Letters 2013, 13, 6281-6286. 7.

Tian, B.; Seidman, D. N.; Isheim, D., Atomic gold–enabled three-dimensional

lithography for silicon mesostructures Science 2015, 348, 1451-1455. 8.

Zhang, A.; Lieber, C. M., Nano-Bioelectronics. Chem. Rev. 2016, 116, 215-257.

9.

Christesen, J. D.; Pinion, C. W.; Zhang, X.; McBride, J. R.; Cahoon, J. F., Encoding

Abrupt and Uniform Dopant Profiles in Vapor–Liquid–Solid Nanowires by Suppressing the Reservoir Effect of the Liquid Catalyst. ACS Nano 2014, 8, 11790-11798. 10.

Allen, J. E.; Perea, D. E.; Hemesath, E. R.; Lauhon, L. J., Nonuniform Nanowire

Doping Profiles Revealed by Quantitative Scanning Photocurrent Microscopy. Advanced Materials 2009, 21, 3067-3072. 11.

Peelaers, H.; Partoens, B.; Peeters, F. M., Formation and Segregation Energies of B

and P Doped and BP Codoped Silicon Nanowires. Nano Letters 2006, 6, 2781-2784. 22 ACS Paragon Plus Environment

Page 23 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

12.

Connell, J. G.; Yoon, K.; Perea, D. E.; Schwalbach, E. J.; Voorhees, P. W.; Lauhon,

L. J., Identification of an Intrinsic Source of Doping Inhomogeneity in Vapor–Liquid– Solid-Grown Nanowires. Nano Letters 2013, 13, 199-206. 13.

Pinion, C. W.; Nenon, D. P.; Christesen, J. D.; Cahoon, J. F., Identifying

Crystallization- and Incorporation-Limited Regimes during Vapor–Liquid–Solid Growth of Si Nanowires. ACS Nano 2014, 8, 6081-6088. 14.

Schlitz, R. A.; Perea, D. E.; Lensch-Falk, J. L.; Hemesath, E. R.; Lauhon, L. J.,

Correlating dopant distributions and electrical properties of boron-doped silicon nanowires. Appl. Phys. Lett. 2009, 95, 0003-6951. 15.

Hemesath, E. R.; Schreiber, D. K.; Gulsoy, E. B.; Kisielowski, C. F.; Petford-Long,

A. K.; Voorhees, P. W.; Lauhon, L. J., Catalyst Incorporation at Defects during Nanowire Growth. Nano Letters 2012, 12, 167-171. 16.

Chen, W.; Yu, L.; Misra, S.; Fan, Z.; Pareige, P.; Patriarche, G.; Bouchoule, S.; i

Cabarrocas, P. R., Incorporation and redistribution of impurities into silicon nanowires during metal-particle-assisted growth. Nat. commun. 2014, 5, 4134. 17.

Hui, H. Y.; Filler, M. A., Solid–Liquid–Vapor Etching of Semiconductor Nanowires.

Nano Letters 2015, 15, 6939-6945. 18.

Day, R. W.; Mankin, M. N.; Lieber, C. M., Plateau–Rayleigh Crystal Growth of

Nanowire Heterostructures: Strain-Modified Surface Chemistry and Morphological Control in One, Two, and Three Dimensions. Nano Letters 2016, 16, 2830-2836. 19.

Day, R. W.; Mankin, M. N.; Gao, R.; No, Y.-S.; Kim, S.-K.; Bell, D. C.; Park,

H.-G.; Lieber, C. M., Plateau-Rayleigh crystal growth of periodic shells on one-dimensional substrates. Nat. Nanotechnol. 2015, 10, 345-352. 20.

Wen, C. Y.; Tersoff, J.; Hillerich, K.; Reuter, M. C.; Park, J. H.; Kodambaka, S.;

Stach, E. A.; Ross, F. M., Periodically Changing Morphology of the Growth Interface in Si, Ge, and GaP Nanowires. Phys. Rev. Lett. 2011, 107, 025503. 21.

Wen, C. Y.; Reuter, M. C.; Bruley, J.; Tersoff, J.; Kodambaka, S.; Stach, E. A.; Ross,


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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

F.

M.,

Formation

of

Compositionally

Abrupt

Page 24 of 26

Axial

Heterojunctions

in

Silicon-Germanium Nanowires. Science 2009, 326, 1247-1250. 22.

Wen, C. Y.; Reuter, M. C.; Tersoff, J.; Stach, E. A.; Ross, F. M., Structure, Growth

Kinetics, and Ledge Flow during Vapor−Solid−Solid Growth of Copper-Catalyzed Silicon Nanowires. Nano Letters 2010, 10, 514-519. 23.

Ross, F. M.; Tersoff, J.; Reuter, M. C., Sawtooth Faceting in Silicon Nanowires.

Phys. Rev. Lett. 2005, 95, 146104. 24.

Schwarz, K. W.; Tersoff, J.; Kodambaka, S.; Ross, F. M., Jumping-Catalyst

Dynamics in Nanowire Growth. Phys. Rev. Lett. 2014, 113, 055501. 25.

Madras, P.; Dailey, E.; Drucker, J., Kinetically Induced Kinking of

Vapor−Liquid−Solid Grown Epitaxial Si Nanowires. Nano Letters 2009, 9, 3826-3830. 26.

Gamalski, A. D.; Voorhees, P. W.; Ducati, C.; Sharma, R.; Hofmann, S., Twin Plane

Re-entrant Mechanism for Catalytic Nanowire Growth. Nano Letters 2014, 14, 1288-1292. 27.

Jeon, N.; Dayeh, S. A.; Lauhon, L. J., Origin of Polytype Formation in VLS-Grown

Ge Nanowires through Defect Generation and Nanowire Kinking. Nano Letters 2013, 13, 3947-3952. 28.

Shin, N.; Chi, M.; Howe, J. Y.; Filler, M. A., Rational Defect Introduction in Silicon

Nanowires. Nano Letters 2013, 13, 1928-1933. 29.

Shin, N.; Chi, M.; Filler, M. A., Interplay between Defect Propagation and Surface

Hydrogen in Silicon Nanowire Kinking Superstructures. ACS Nano 2014, 8, 3829-3835. 30.

Jacobsson, D.; Panciera, F.; Tersoff, J.; Reuter, M. C.; Lehmann, S.; Hofmann, S.;

Dick, K. A.; Ross, F. M., Interface dynamics and crystal phase switching in GaAs nanowires. Nature 2016, 531, 317-322. 31.

Koelling, S.; Li, A.; Cavalli, A.; Assali, S.; Car, D.; Gazibegovic, S.; Bakkers, E. P.

A. M.; Koenraad, P. M., Atom-by-atom analysis of semiconductor nanowires with parts per million sensitivity. Nano Letters 2017, 17, 599-605.


Page 25 of 26

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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

32.

Seidman, D. N., Three-Dimensional Atom-Probe Tomography: Advances and

Applications. Annu. Rev. Mater. Res. 2007, 37, 127-158. 33.

Seidman, D. N.; Stiller, K., An Atom-Probe Tomography Primer MRS Bulletin

2009, 34, 717-724. 34.

Tian, B.; Xie, P.; Kempa, T. J.; Bell, D. C.; Lieber, C. M., Single-crystalline kinked

semiconductor nanowire superstructures. Nat. Nanotechnol. 2009, 4, 824-829. 35.

Sun, Z.; Hazut, O.; Huang, B.-C.; Chiu, Y.-P.; Chang, C.-S.; Yerushalmi, R.;

Lauhon, L. J.; Seidman, D. N., Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex Situ Doping: A Correlative Study via Atom-Probe Tomography and Scanning Tunneling Spectroscopy. Nano Letters 2016, 16, 4490-4500. 36.

Thompson, K.; Lawrence, D.; Larson, D. J.; Olson, J. D.; Kelly, T. F.; Gorman, B.,

In situ site-specific specimen preparation for atom probe tomography. Ultramicroscopy 2007, 107, 131-139 37.

Hemesath, E. R., Electron Microscopy Studies of Defect Structure and Correlated

Impurity Incorporation in Si and Ge Nanowires. Ph. D. Thesis, Northwestern University, Evanston 2010, 107. 38.

Gamalski, A. D.; Ducati, C.; Hofmann, S., Cyclic Supersaturation and Triple Phase

Boundary Dynamics in Germanium Nanowire Growth. J. Phys. Chem. C 2011, 115, 4413-4417. 39.

Wang, H.; Zepeda-Ruiz, L. A.; Gilmer, G. H.; Upmanyu, M., Atomistics of

vapour-liquid-solid nanowire growth. Nat. commun. 2013, 4, 1-10. 40. den Hertog, M. I.; Rouviére, J.-L.; Dhalluin, F.; Desré, P. J.; Gentile, P.; Ferret, P.; Oehler, F.; Baron, T., Control of Gold Surface Diffusion on Si Nanowires. Nano Letters 2008, 8, 1544-1550. 41.

Hellman, O. C.; Vandenbroucke, J. A.; Rüsing, J.; Isheim, D.; Seidman, D. N.,

Analysis of three-dimensional atom-probe data by the proximity histogram. Microscopy and Microanalysis 2000, 6, 437-444.


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 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

42.

Page 26 of 26

Boyuk, D. S.; Chou, L.-W.; Filler, M. A., Strong Near-Field Coupling of Plasmonic

Resonators Embedded in Si Nanowires. ACS Photonics 2016, 3, 184–189. 43.

Dregia, S. A.; Wynblatt, P., Equilibrium segregation and interfacial energy in

multicomponent systems. Acta Metall. Mater. 1991, 39, 771-778. 44.

Christesen, J. D.; Pinion, C. W.; Hill, D. J.; Kim, S.; Cahoon, J. F., Chemically

Engraving Semiconductor Nanowires: Using Three-Dimensional Nanoscale Morphology to Encode Functionality from the Bottom Up. J. of Phys. Chem. Lett. 2016, 685-692. 45.

Schwarz, K. W.; Tersoff, J., Multiplicity of Steady Modes of Nanowire Growth.

Nano Letters 2012, 12, 1329-1332.


Nanowire Kinking Modulates Doping Profiles by Reshaping the Liquid

Recommend Documents