Mapping Free-Carriers in Multijunction Silicon Nanowires Using

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Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy Earl T. Ritchie, David J. Hill, Tucker M. Mastin, Panfilo C Deguzman, James F. Cahoon, and Joanna M. Atkin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02340 • Publication Date (Web): 15 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy Earl T. Ritchie,† David J. Hill,† Tucker M. Mastin,† Panfilo C. Deguzman,‡ James F. Cahoon,∗,† and Joanna M. Atkin∗,† †Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, United States ‡Center for Optoelectronics and Optical Communications, University of North Carolina at Charlotte, Charlotte, NC 28223, United States E-mail: [email protected]; [email protected]

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Abstract We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a non-destructive method to map free-carriers in axially modulationdoped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically-active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor-liquid-solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type freecarriers, for carrier densities above ∼ 1 × 1019 cm−3 . We also resolve subtle changes in local conductivity properties, which can be correlated with growth conditions and surface effects. The use of s-SNOM is especially valuable in low mobility materials such as boron-doped p-type SiNWs, where optimization of growth has been difficult to achieve due to the lack of information on dopant distribution and junction properties. s-SNOM can be widely employed for the non-destructive characterization of nanostructured material synthesis and local electronic properties, without the need for contacts or inert atmosphere.

Keywords VLS, silicon, nanowire, doping, near-field microscopy, infrared s-SNOM


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Semiconducting nanowires (NWs) have the potential to act as building blocks for a variety of electronic, 1–3 photonic, 4–9 photoelectrochemical, 10,11 and photovoltaic devices. 12–14 Silicon nanowires (SiNWs) are particularly interesting due to the extensive literature on the electronic and optical properties of bulk silicon and compatibility with existing processes and architectures. NWs are made by both top-down 11,13 and bottom-up 1,3,6–9,14 approaches, but bottom-up approaches such as the vapor-liquid-solid (VLS) method allow for a greater degree of compositional control on the nanometer scale. 2,15,16 Dopants can be introduced during VLS growth by changing the vapor-phase composition, modifying the local electronic properties and encoding a wide range of functionality. 4,17,18 However, a reservoir effect, in which dopant atoms remain dissolved in the liquid catalyst after the gas-phase dopant is removed, can occur under certain growth conditions. 19–21 This leads to a broadening of the junctions, specifically when the dopant flow is turned off, that can negatively impact device performance. Additionally, discrepancies between reaction stoichiometry, dopant incorporation and activation at high doping levels, 21–24 inhomogeneous dopant distribution 25 and diameter-dependent mobility effects 26 complicate NW design and synthesis. Because highquality semiconductor devices require precise characterization and control of local electronic properties, 2,27 these challenges hinder the widespread implementation of VLS-grown NWs. While the dopant transition lengths of modulation-doped n-type SiNWs have been reduced to sub-10 nm lengths and confirmed with existing methods, 24 the impact of the reservoir effect on p-i junctions is not as well understood due to the difficulty in detecting activated p-type dopants in SiNWs. Scanning probe methods are well suited for this, as they can acquire correlated topographical and conductivity information on nanometer length scales. Kelvin Probe Force Microscopy (KPFM) studies have suggested that residual p-type doping from B atoms in the catalyst is an issue in p-i-n SiNW diodes, 28 but the presence of a native oxide layer can obscure the measured surface potential difference. A non-destructive and spatially-resolved method to measure the electrically-active doping concentration with minimal processing is needed to accurately assess NWs for future electronic applications.


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Infrared (IR) scattering-type near-field optical microscopy (s-SNOM) has emerged as a powerful method to measure the free carrier response in metallic 29 and doped-semiconducting 30–32 NWs, but has been largely limited to high-mobility III-V semiconductors. In contrast, free carrier measurements in low-mobility materials such as silicon have been difficult to achieve with bench-top mid-infrared sources, and the use of IR s-SNOM to characterize extrinsic variations such as changes in dopant activation has not been explored. SiNW samples provide an ideal platform for these studies since bulk silicon and n-type SiNWs have been extensively characterized, allowing us to access local electronic properties through the IR dielectric function. We study NWs with multiple junctions to determine the reproducibility of the catalyst reservoir effect, the uniformity of dopant incorporation and NW growth, and the effects of nanowire diameter, with 20 nm spatial resolution. The near-field microscopy experiments, as illustrated in Figure 1A, are performed using a home-built infrared s-SNOM system based on a commercial atomic force microscope (AFM) (Bruker Innova) modified for optical access. 33 A CO2 laser (Access Lasers) with wavelength λ = 10.6 µm is used as an excitation source, horizontally polarized with respect to the sample plane. The laser is focused with an off-axis parabolic mirror onto a Pt/Ir-coated AFM tip oscillating at a frequency Ω ∼ 250 kHz. The back-scattered light is collected by the parabolic mirror and detected with a mercury-cadmium-telluride (MCT) detector. Background suppression and near-field amplification are achieved by interference with a reference signal of controllable phase, and subsequent demodulation of the detected signal at higher harmonics of the tapping frequency (nΩ). 34 In these experiments, demodulation at the third harmonic (n = 3) of the tapping frequency was used to construct a near-field infrared image simultaneously with the AFM topography. To allow for comparison of the near-field signal across samples, measurements were normalized to the high-intensity signal from a Au substrate (I3 (Au)). Electrostatic simulations of the tip-sample interaction are used in semiquantitative conductivity calculations 35–37 of the near-field amplitude (Figure S1), and explained in detail in the Supporting Information.


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Figure 1: s-SNOM imaging of modulation-doped SiNWs. (A) Schematic of infrared s-SNOM. Demodulation at higher harmonics of the cantilever oscillation frequency Ω is performed to extract the near-field signal of interest. (B) Illustration of VLS NW growth, with nucleation and growth from a Au nanoparticle catalyst. The flow of dopant precursor was modulated during growth, leading to an axially repeating n-type/intrinsic or p-type/intrinsic (illustrated) doping pattern. (C) Schematic of SiNW doping profile (top), AFM topography (middle), and near-field infrared image normalized to Au (bottom) of a representative SiNW sample. Scale bar: 1 µm.


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SiNWs were grown from Au nanoparticles dispersed on silicon wafers using VLS growth (Figure 1B). 24 The SiNWs were synthesized in a hot-wall chemical vapor deposition (CVD) reactor at 420◦ C or 650◦ C and 20 Torr using SiH4 , PH3 , and B2 H6 as the Si, P, and B precursors respectively, H2 as the carrier gas, and HCl for suppression of sidewall deposition 38 at 650◦ C (see Supporting Information for further details). 39 SiNWs ranging from 50−200 nm in diameter and ∼ 30 µm in length were suspended in solution and transferred onto Au-coated Si3 N4 substrates for s-SNOM analysis. In order to minimize the effect of dopant compensation and gain a better understanding of the reservoir effect on junction properties, we study SiNWs with only one type of precursor (P for n-i; B for p-i). The samples studied are optimized to minimize the growth of a vapor-solid (VS) shell on the samples, which could screen the s-SNOM signal from the VLS-grown core and complicate interpretation. 18,24,39 Three axially-modulated doping schemes were analyzed in this experiment: samples with alternating n-type and intrinsic regions (n-i), 24 alternating p-type and intrinsic regions (pi), and alternating n-type sections with different dopant concentrations (n-n+ ). Values for electrically-active doping concentration in both phosphorus and boron-doped SiNWs were obtained from single NW four-point probe measurements, assuming the empirical relationship between resistivity and doping level from bulk silicon. 24 For comparison to near-field measurements, a KOH wet-chemical etch was used to selectively etch intrinsic sections along the SiNW (Figure S2), 18 followed by scanning electron microscopy (SEM) imaging to estimate the dopant profile. Figure 1C shows the simultaneously acquired AFM topography and near-field signal from a NW encoded with alternating sections of boron-doped (∼ 2 × 1019 cm−3 active) and intrinsic silicon, demonstrating that IR s-SNOM has the sensitivity to differentiate regions with large doping variations. Figures 2 and 3 demonstrate the high spatial resolution and sensitivity of IR s-SNOM for n-i and p-i SiNWs respectively. We are able to differentiate highly doped n-type regions (∼ 3.3 × 1019 cm−3 ) from intrinsic regions in the n-i SiNW sample (Figure 2A,B). The near-field


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Figure 2: Near-field response of n-type/intrinsic SiNWs. (A, B) Encoded doping profile (top), AFM topography (middle), and IR s-SNOM signal normalized to the signal from an Au substrate (bottom) for the modulation-doped n-i SiNWs grown at 420◦ C. Section lengths at half-maximum (Dn ) and half-minimum (Di ) are illustrated in (B). Scale bars are 400 nm. (C) SiNW doping profile (top) and IR s-SNOM image (bottom) for modulation-doped n-i SiNWs grown at 650◦ C. The long intrinsic section on the left exhibits a much smaller nearfield signal than the sections of SiNW grown after the introduction of dopant. Scale bar: 2 µm. (D) AFM topography (top) and IR s-SNOM signal (bottom) of NW in (C) along the dashed line.


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signal closely follows the encoded PH3 dopant flow graph (Figure 2B) with no significant topographic variation associated with dopant flow rate along the length of the NW, indicating that we are measuring the incorporation of the dopants in the NW. The contrast arises due to free-carrier absorption in the mid-infrared, with more highly-conductive regions showing a stronger near-field signal at a wavelength of 10.6 µm. 40 We compare the results for n-i SiNWs grown at low temperatures (420◦ C) to similar modulation-doped n-i NWs grown at higher reaction temperatures (650◦ C) with the same PH3 stoichiometry. As seen in Figure 2C and D, we observe a greater near-field signal for n-type regions grown at 650◦ C (I3 (Si)/I3 (Au) = 1.2 ± 0.10), compared to those grown at 420◦ C (I3 (Si)/I3 (Au) = 0.80 ± 0.12). This is consistent with a greater dopant incorporation due to a higher P solubility at 650◦ C, with some contribution from a greater activation of dopants due to the possible interruption of P dimer formation. 41,42 Where the modulated flow of PH3 gas produced alternating intrinsic and highly-doped regions at 420◦ C, no such domains were observed in NWs grown at 650◦ C. We resolve a low intensity intrinsic region until the initial introduction of PH3 , after which a high intensity signal persists for the length of the NW, suggesting a long P retention time. These observations are consistent with a substantial reservoir effect in the high-temperature reaction. This is corroborated by wet-chemical etching, where very little silicon removal in the “intrinsic” sections is observed until more than a micron after the flow of PH3 was turned off. 39 The liquid catalyst acts as a reservoir, with an increase of the Si crystallization rate, kSiC , due to the elevated temperature producing a broadened dopant distribution after turning off the dopant gas. 43 However, the non-negligible concentration of dopants incorporated in the NW even after several minutes suggest the presence of an additional reservoir leading to reincorporation over time. One possible cause is the difference in evaporation rates between PH3 and the PCl3 species that evolve due to the introduction of HCl in higher temperature growths, 44 which would inhibit P clearance from the reaction system.


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Figure 3: Near-field response of p-type/intrinsic SiNWs. (A, B) Encoded doping profile (top), AFM topography (middle), and IR s-SNOM signal normalized to the signal from an Au substrate (bottom) for the modulation-doped p-i SiNWs grown at 650◦ C. Section lengths at half-maximum (Dp ) and half-minimum (Di ) are illustrated in (B). Scale bars are 400 nm. (C) Top: Schematic of SiNW doping profile for an p-i alternately doped SiNW. Bottom: Near-field infrared image of a section of wire described above. Scale bar: 500 nm. (D) Top: Encoded dopant flow profile of section of NW shown in D (left axis, sccm), and corresponding total reactor pressure (right axis, Torr) with respect to growth time (s). Bottom: Near-field (left axis, normalized) and NW diameter (right axis, nm) profile along dashed line in (D), showing an initial increase in near-field signal correlated with the change in reactor pressure.


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Figure 3A shows boron-doped (p-type) SiNWs grown at 650◦ C with dopant modulation patterns similar to those in the n-i sample. The doping schematic of this sample can be seen in Figure S3. We are able to differentiate highly doped p-type regions from intrinsic regions (Figure 3A,B), which has proved challenging using e.g. energy dispersive x-ray spectroscopy (EDS) due to low sensitivity to boron (B) dopants. While in the n-i NWs the near-field signal was relatively constant within a single doping region, for p-type doped regions we observe variations that can be traced back to unexpected fluctuations in the reaction conditions (Figure 3C,D). For sections greater than 400 nm long, we observe an initial near-field intensity spike during the introduction of dopant flow (“on”), followed by a gradual fall to a steady-state signal. This behavior mirrors an approximately 10 second spike in total reactor pressure at the introduction of dopant flow, followed by a slow decrease in reactor pressure until the dopant flow is removed. For p-type sections shorter than 400 nm, no intensity spike was resolved, since the growth time was similar to the length of the pressure spike. Although the pressure fluctuations are relatively small, they could cause instability in the catalyst and affect the incorporation of dopants or crystallization. The measured change in NW diameter associated with the initial pressure fluctuation is insignificant, but the NW diameter then gradually increases as the s-SNOM signal decreases, suggesting a slow change in the ratio of Si and B incorporation. The near-field intensity decreases by approximately 20% over the p-type section, which may be attributed to a 20% decrease in dopant concentration or strain modifying the local mobility, 45 according to the tip-dipole model for tip-sample interaction. 46 Table 1: Growth Rates of p-type/intrinsic SiNWs Location Near-field Etch

Gp (nm/min) 641 ± 15 601 ± 20 658 ± 16 662 ± 14

Middle End Middle End

Gi (nm/min) 506 ± 11 553 ± 10 454 ± 18 426 ± 14


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We use line-profiles of the near-field intensity along the SiNWs to define section distances, D, over which the near-field signal is above 50% of its maximum value (Dn , Dp ), and the distance over which the signal is below 50% of its maximum value (Di ). By comparing the section distances to the NW growth pattern, we are able to calculate growth rates, G, and compare our results from s-SNOM measurements to etch measurements. These results are summarized in Table 1 for the p-i SiNWs, and in the supporting information for the n-i SiNWs. The p-i SiNWs have a higher growth rate than the n-i SiNWs grown at 420◦ C due to a higher crystallization rate and the boron precursor acting as a growth catalyst. 47 By comparing calculated growth rates from the end of the NW to those obtained earlier in the growth, we observe that while the growth rate was consistent along the length of the low-temperature n-i SiNWs, there is a significant change in the p-i SiNW growth rate as the growth progresses. Similar to the high-temperature n-i sample discussed above, boron evaporation could be slowed in the p-i SiNW growth due to the HCl used to prevent VS shell deposition. As B precursor accumulates in the reaction system, it would act as a growth catalyst 47 and increase the intrinsic growth rate towards the end of the growth. We present these data as evidence of a secondary B reservoir in the reaction vessel in addition to the catalyst reservoir effect shown in Hill et al. 39 with etch measurements. Regardless of the position along the p-i SiNW, we find near-field derived values of Dp to be shorter and Di to be longer than expected compared to values extracted from etch studies. This is in contrast to values found in the n-i SiNW sample, where Dn is longer and Di is shorter than expected. Since etching rate is also related to local conductivity, 48 this suggests that the IR s-SNOM signal is sensitive to an additional factor that leads to the apparent broadening in the n-type doped region and shortening in the p-type doped region, such as the in-plane electric fields 33,49–51 at the space-charge region at junctions. 52 We performed COMSOL modeling to simulate the electrostatic properties of a multijunction doped silicon device, in order to investigate the behavior of the electric field at junctions (Figure S4). While the carrier concentration falls off quickly at junctions, the electric field


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can extend tens of nanometers away from the junction, and acts in opposite directions for n vs p-type doping. While there may be some inadvertent doping in the intrinsic sections of the NWs, for the large carrier concentration differences expected between highly doped and intrinsic NW sections, the presence of a space-charge region may contribute to the differences in section lengths Dn , Dp , and Di . In order to quantify the sensitivity range of IR s-SNOM to free-carrier concentration and diameter-dependent effects, we analyze a NW sample with different phosphorus concentrations along the length of the wires (n-n+ ). These wires, as seen in Figure 4, were grown while modulating the gas-phase concentration of phosphorus, leading to alternating n-type sections with stoichiometric concentrations of phosphorus between 0 and 5 × 1020 cm−3 . By averaging across multiple wires and normalizing to the Au substrate, we obtain near-field intensity values for each doping concentration to compare across different wires and samples. We study two groups of nanowires, with diameters d of 40 − 60 nm and 90 − 110 nm. The highest normalized intensity regions correspond to the most highly-doped regions (5 × 1020 cm−3 encoded, 3.3 × 1019 cm−3 active concentration), while the intrinsic sections exhibit the lowest normalized intensity. The normalized intensities for each concentration can be found in Table 2. Table 2: Normalized Near-field Intensities With Respect to Encoded and Active Doping Concentration

n+ n4 n3 n2 n1 i

Encoded Doping Concentration 5 × 1020 2.5 × 1020 1.25 × 1020 5 × 1019 2.5 × 1019 undoped

Active Doping Concentration 24 3.3 × 1019 2.1 × 1019 1.5 × 1019 1.1 × 1019 1.0 × 1019

Normalized Near-Field Intensity d ∼ 100 nm d ∼ 50 nm 0.80 ± 0.12 0.63 ± 0.14 0.66 ± 0.07 0.54 ± 0.15 0.56 ± 0.05 0.49 ± 0.08 0.52 ± 0.03 0.48 ± 0.08 0.48 ± 0.06 0.46 ± 0.07 0.41 ± 0.09 0.45 ± 0.09

We observe a monotonic increase in normalized intensity for this regime of carrier concentrations, with an approximately linear response in the range of ∼ 1−4 × 1019 cm−3 . Based on the Drude model in highly doped semiconductors, 53 the plasma wavelength of n-type silicon 12 ACS Paragon Plus Environment

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Figure 4: Near-field variations with free-carrier concentration. (A) Schematic (top) and near-field infrared image (bottom) of n-n+ SiNW sample. (B) Topography and near-field line profile of d = 100 nm for dashed section in A. In this doping regime, the near-field intensity monotonically increases with respect to the active-doping concentration. (C) Plot of near-field intensity normalized to the Au substrate vs active-doping concentration 24 for NWs with diameter d ∼ 100 nm (blue) and d ∼ 50 nm (red). The top axis shows the stoichiometric doping concentration.


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lies at 10.6 µm for carrier concentrations of ∼ 3 × 1019 cm−3 . At lower carrier concentrations, the plasma wavelength will be shifted towards longer wavelengths. We therefore expect the IR s-SNOM signal at 10.6 µm to have maximum sensitivity to variations in the doping for carrier concentrations in the range of 1019 cm−3 . Our experimental results are consistent with this expectation, with a normalized intensity difference of ∼ 0.07, comparable to the standard deviation, observed between the intrinsic sections and the lowest n-type doped section in the large diameter wires. This difference is even less in the small diameter wires, with the normalized intensity difference falling within the standard deviation of the intrinsic intensity at doping concentrations below 1.5 × 1019 cm−3 . The near-field intensity for the smaller diameter NWs is consistently reduced in comparison to the large diameter NWs at higher doping concentrations, as seen in Table 2. The reduced signal does not appear to be associated with the reduced polarizable volume, since the intrinsic segments display the same near-field intensity, within uncertainty. Instead, we propose that in smaller diameter NWs, surface effects due to the dielectric mismatch lead to a decreased carrier activation. 26 We model this using the tip-dipole model with a Drude model description of the SiNW, finding an estimated 30% decrease in effective carrier concentration would account for the observed changed in near-field intensity (Figure S5). This suggests a 3-5 nm reduction of the electronic radius in the smaller nanowires, as described in Bjork et al. 26 There may however be other contributions to the diameter-dependent effect, such as changes in dopant incorporation and carrier mobility. s-SNOM analysis compares favorably to existing techniques used to measure the chemical and electrical properties of multijunction nanowire structures. Several methods can measure the chemical dopant concentration with high spatial resolution, including energy dispersive xray spectroscopy (EDS) and atom probe tomography (APT). EDS has been used to determine the chemical dopant concentration in phosphorus (P) doped (n-type) SiNWs, as well as the transition abruptness between n-type and intrinsic regions in axially-doped NWs. 24,54 However, the limit of detection for this technique is fairly high (> 1018 dopants/cm3 ), and


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it has not been demonstrated to be sensitive to boron (B) doping in p-type SiNWs. APT can detect both n-type and p-type doping profiles with high sensitivity, 25,55,56 but it requires precise and time-consuming sample preparation, limiting the breadth of samples that can be studied. Moreover, neither EDS nor APT are sensitive to the free-carrier concentration, but rather detect the presence of dopant atoms whether activated or unactivated. KPFM can characterize single wires and devices with sub-50 nm resolution, 21,28,57 but the presence of a native oxide layer hinders its effectiveness, often requiring an inert, humiditycontrolled atmosphere. Additionally, KPFM has been primarily limited to studying materials after they have already been incorporated into a device. In contrast, s-SNOM has been demonstrated to be useful in ambient conditions for both unprocessed semiconducting structures 30,31,58 and those that have been incorporated into devices. 32,59 While we focus on axially-modulated NWs in this work, s-SNOM has also been demonstrated for the study radial doping in vertically oriented NWs. 59 With an excitation wavelength of 10.6 µm, we are able to probe a relatively high doping range for SiNWs, but are sensitive to small changes within this range of carrier concentrations. The results we report demonstrate that we are able to distinguish electrically-active doping in SiNWs for concentrations above 1 × 1019 cm−3 , and detect changes in relative doping concentration as small as 13% with sub-20 nm spatial resolution. By choosing a wavelength in the THz range we can shift the sensitivity range of IR s-SNOM to carrier concentrations as low as ∼ 1 × 1016 cm−3 (Figure S6). 37 In summary, we demonstrate that near-field optical microscopy in the mid-IR can be used to collect detailed information about the distribution of charge carriers in both n-type and p-type doped SiNWs. For SiNWs in the doping range of this study, we are able to identify subtle variations in the signal, related to carrier incorporation and activation, and investigate the sensitivity of s-SNOM to space charge regions and the reservoir effect in boron-doped SiNWs. All measurements were acquired in the presence of the native oxide layer with minimal sample preparation, demonstrating the ability of s-SNOM to penetrate


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through surface layers and eliminating the need for destructive etching agents. We show that s-SNOM has the potential to measure functional, multijunction nanomaterials during development, opening the door to more efficient device manufacturing. This approach can potentially enable the characterization and optimization of optoelectronic properties in a wide range of semiconducting materials and nanostructured devices.

Acknowledgement The authors thank Glenn Boreman (University of North Carolina - Charlotte) for use of a near-field optical microscope for initial measurements, and Brendan Wells for assistance throughout these experiments. J.F.C and D.J.H acknowledge support from the National Science Foundation (NSF) through grant DMR-1555001. D.J.H. acknowledges an NSF graduate research fellowship. SEM analysis was performed at the Chapel Hill Analytical and Nanofabrication Laboratory, CHANL, a member of the North Carolina Research Triangle Nanotechnology Network, RTNN, which is supported by the National Science Foundation, Grant ECCS-1542015, as part of the National Nanotechnology Coordinated Infrastructure, NNCI.

References (1) Chung, S.-W.; Yu, J.-Y.; Heath, J. R. Silicon nanowire devices. Applied Physics Letters 2000, 76, 2068–2070. (2) Lu, W.; Lieber, C. M. Semiconductor nanowires. Journal of Physics D-Applied Physics 2006, 39, R387–R406. (3) Vallett, A. L.; Minassian, S.; Kaszuba, P.; Datta, S.; Redwing, J. M.; Mayer, T. S. Fabrication and characterization of axially doped silicon nanowire tunnel field-effect transistors. Nano Letters 2010, 10, 4813–4818. 16 ACS Paragon Plus Environment

Page 16 of 24

Page 17 of 24

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(4) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Growth of nanowire superlattice structures for nanoscale photonics and electronics. Nature 2002, 415, 617–620. (5) Yang, C.; Barrelet, C. J.; Capasso, F.; Lieber, C. M. Single p-type/intrinsic/n-type silicon nanowires as nanoscale avalanche photodetectors. Nano Letters 2006, 6, 2929– 2934. (6) Chou, L. W.; Shin, N.; Sivaram, S. V.; Filler, M. A. Tunable mid-infrared localized surface plasmon resonances in silicon nanowires. Journal of the American Chemical Society 2012, 134, 16155–16158. (7) Chou, L.-W.; Filler, M. A. Engineering Multimodal Localized Surface Plasmon Resonances in Silicon Nanowires. Angewandte Chemie International Edition 2013, 52, 8079–8083. (8) Hill, D. J.; Pinion, C. W.; Christesen, J. D.; Cahoon, J. F. Waveguide Scattering Microscopy for Dark-Field Imaging and Spectroscopy of Photonic Nanostructures. ACS Photonics 2014, 1, 725–731. (9) Fang, H. et al. Visible Light-Assisted High-Performance Mid-Infrared Photodetectors Based on Single InAs Nanowire. Nano Letters 2016, 16, 6416–6424. (10) Kibria, M. G.; Zhao, S.; Chowdhury, F. A.; Wang, Q.; Nguyen, H. P. T.; Trudeau, M. L.; Guo, H.; Mi, Z. Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting. Nature Communications 2014, 5, 3825. (11) Chandrasekaran, S.; Nann, T.; Voelcker, N. Silicon Nanowire Photocathodes for Photoelectrochemical Hydrogen Production. Nanomaterials 2016, 6, 144. (12) Tian, B.; Kempa, T. J.; Lieber, C. M. Single nanowire photovoltaics. Chemical Society Reviews 2009, 38, 165–84. 17 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

(13) Garnett, E.; Yang, P. Light trapping in silicon nanowire solar cells. Nano Letters 2010, 10, 1082–1087. (14) Kim, S. K.; Zhang, X.; Hill, D. J.; Song, K. D.; Park, J. S.; Park, H. G.; Cahoon, J. F. Doubling absorption in nanowire solar cells with dielectric shell optical antennas. Nano Letters 2015, 15, 753–758. (15) Schmidt, V.; Wittemann, J. V.; Senz, S.; Gósele, U. Silicon nanowires: A review on aspects of their growth and their electrical properties. Advanced Materials 2009, 21, 2681–2702. (16) Pinion, C. W.; Christesen, J. D.; Cahoon, J. F. Understanding the Vapor–Liquid– Solid Mechanism of Si Nanowire Growth and Doping to Synthetically Encode Precise Nanoscale Morphology. J. Mater. Chem. C 2016, 4, 3890–3897. (17) Yang, C.; Zhong, Z.; Lieber, C. M. C. Encoding electronic properties by synthesis of axial modulation-doped silicon nanowires. Science 2005, 310, 1304–1307. (18) 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. (19) Clark, T. E.; Nimmatoori, P.; Lew, K.-k.; Pan, L.; Redwing, J. M.; Dickey, E. C. Diameter Dependent Growth Rate and Interfacial Abruptness in VaporLiquidSolid Si/Si 1 x Ge x Heterostructure Nanowires. Nano Letters 2008, 8, 1246–1252. (20) Dick, K. A.; Bolinsson, J.; Borg, B. M.; Johansson, J. Controlling the abruptness of axial heterojunctions in III-V nanowires: Beyond the reservoir effect. Nano Letters 2012, 12, 3200–3206. (21) Amit, I.; Givan, U.; Connell, J. G.; Paul, D. F.; Hammond, J. S.; Lauhon, L. J.;


Page 18 of 24

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Rosenwaks, Y. Spatially resolved correlation of active and total doping concentrations in VLS grown nanowires. Nano Letters 2013, 13, 2598–2604. (22) Mueller, D. C.; Fichtner, W. Highly n-doped silicon: Deactivating defects of donors. Physical Review B - Condensed Matter and Materials Physics 2004, 70, 1–8. (23) Kawashima, T.; Imamura, G.; Saitoh, T.; Komori, K.; Fujii, M.; Hayashi, S. Raman Scattering Studies of Electrically Active Impurities in in Situ B-Doped Silicon Nanowires: Effects of Annealing and Oxidation. The Journal of Physical Chemistry C 2007, 111, 15160–15165. (24) 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. (25) 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. (26) Björk, M. T.; Schmid, H.; Knoch, J.; Riel, H.; Riess, W. Donor deactivation in silicon nanostructures. Nature Nanotechnology 2009, 4, 103–107. (27) Léonard, F.; Talin, A. A. Electrical contacts to one- and two-dimensional nanomaterials. Nature Nanotechnology 2011, 6, 773–783. (28) Amit, I.; Jeon, N.; Lauhon, L. J.; Rosenwaks, Y. Impact of Dopant Compensation on Graded p-n Junctions in Si Nanowires. ACS Applied Materials and Interfaces 2016, 8, 128–134. (29) Jones, A. C.; Olmon, R. L.; Skrabalak, S. E.; Wiley, B. J.; Xia, Y. N.; Raschke, M. B.


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Mid-IR Plasmonics:

Near-Field Imaging of Coherent Plasmon Modes of Silver

Nanowires. Nano Letters 2009, 9, 2553–2558. (30) Stiegler, J. M.; Huber, A. J.; Diedenhofen, S. L.; Gómez Rivas, J.; Algra, R. E.; Bakkers, E. P. A. M.; Hillenbrand, R. Nanoscale free-carrier profiling of individual semiconductor nanowires by infrared near-field nanoscopy. Nano Letters 2010, 10, 1387–1392. (31) Choi, W.; Seabron, E.; Mohseni, P. K.; Kim, J. D.; Gokus, T.; Cernescu, A.; Pochet, P.; Johnson, H. T.; Wilson, W. L.; Li, X. Direct Electrical Probing of Periodic Modulation of Zinc-Dopant Distributions in Planar Gallium Arsenide Nanowires. ACS Nano 2017, 11, 1530–1539. (32) Arcangeli, A.; Rossella, F.; Tomadin, A.; Xu, J.; Ercolani, D.; Sorba, L.; Beltram, F.; Tredicucci, A.; Polini, M.; Roddaro, S. Gate-Tunable Spatial Modulation of Localized Plasmon Resonances. Nano Letters 2016, 16, 5688–5693. (33) Kinzel, E. C.; Ginn, J. C.; Olmon, R. L.; Shelton, D. J.; Lail, B. A.; Brener, I.; Sinclair, M. B.; Raschke, M. B.; Boreman, G. D. Phase resolved near-field mode imaging for the design of frequency-selective surfaces. Optics Express 2012, 20, 11986–11993. (34) Atkin, J. M.; Berweger, S.; Jones, A. C.; Raschke, M. B. Nano-optical imaging and spectroscopy of order, phases, and domains in complex solids. Advances in Physics 2012, 61, 745–842. (35) Knoll, B.; Keilmann, F. Enhanced dielectric contrast in scattering-type scanning nearfield optical microscopy. Optics Communications 2000, 182, 321–328. (36) Keilmann, F.; Hillenbrand, R. Near-field microscopy by elastic light scattering from a tip. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences 2004, 362, 787–805.


Page 20 of 24

Page 21 of 24

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

(37) Huber, A. J.; Keilmann, F.; Wittborn, J.; Aizpurua, J.; Hillenbrand, R. Terahertz Near-Field Nanoscopy of Mobile Carriers in Single Semiconductor Nanodevices. Nano Letters 2008, 8, 3766–3770. (38) Gentile, P.; Solanki, A.; Pauc, N.; Oehler, F.; Salem, B.; Rosaz, G.; Baron, T.; Den Hertog, M.; Calvo, V. Effect of HCl on the doping and shape control of silicon nanowires. Nanotechnology 2012, 23, 215702. (39) Hill, D. J.; Teitsworth, T. S.; Kim, S.; Christesen, J. D.; Cahoon, J. F. Encoding Highly Non-Equilibrium Boron Concentrations and Abrupt Morphology in P-Type/N-Type Silicon Nanowire Superlattices. ACS Applied Materials & Interfaces 2017, (40) Schroder, D. K.; Thomas, R. N.; Swartz, J. C. Free carrier absorption in silicon. IEEE Transactions on Electron Devices 1978, 25, 254–261. (41) Schmid, H.; Björk, M. T.; Knoch, J.; Karg, S.; Riel, H.; Riess, W. Doping limits of grown in situ doped silicon nanowires using phosphine. Nano Letters 2009, 9, 173–177. (42) Celle, C.; Mouchet, C.; Rouvière, E.; Simonato, J. P.; Mariolle, D.; Chevalier, N.; Brioude, A. Controlled in situ n-doping of silicon nanowires during VLS growth and their characterization by scanning spreading resistance microscopy. Journal of Physical Chemistry C 2010, 114, 760–765. (43) 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, 6, 6081–6088. (44) Stull, D. R. Vapor Pressure of Pure Substances. Organic and Inorganic Compounds. Industrial & Engineering Chemistry 1947, 39, 517–540. (45) Sun, G.; Sun, Y.; Nishida, T.; Thompson, S. E. Hole mobility in silicon inversion layers: Stress and surface orientation. Journal of Applied Physics 2007, 102 . 21 ACS Paragon Plus Environment

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(46) Huber, a. J.; Ziegler, A.; Köck, T.; Hillenbrand, R. Infrared nanoscopy of strained semiconductors. Nature nanotechnology 2009, 4, 153–157. (47) Islam, M. S.; Kawashima, T.; Sawada, K.; Ishida, M. High-yield growth of p-Si microprobe arrays by selective vapor-liquid-solid method using in situ doping and their properties. Journal of Crystal Growth 2007, 306, 276–282. (48) Seidel, H.; Csepregi, L.; Heuberger, A.; Baumgartel, H. Anisotropic Etching of Crystalline Silicon in Alkaline Solutions. J. Electrochem. Soc. 1990, 137, 3612–3626. (49) D’ Archangel, J.; Tucker, E.; Kinzel, E.; Muller, E. A.; Bechtel, H. A.; Martin, M. C.; Raschke, M. B.; Boreman, G. Near- and far-field spectroscopic imaging investigation of resonant square-loop infrared metasurfaces. Optics Express 2013, 21, 17150. (50) Tucker, E.; D’Archangel, J.; Raschke, M. B.; Boreman, G. Near- and far-field measurements of phase-ramped frequency selective surfaces at infrared wavelengths. Journal of Applied Physics 2014, 116, 044903. (51) Xu, Y.; Tucker, E.; Boreman, G.; Raschke, M. B.; Lail, B. A. Optical Nanoantenna Input Impedance. ACS Photonics 2016, 3, 881–885. (52) Bassani, F.; Periwal, P.; Salem, B.; Chevalier, N.; Mariolle, D.; Audoit, G.; Gentile, P.; Baron, T. Dopant profiling in silicon nanowires measured by scanning capacitance microscopy. Physica Status Solidi - Rapid Research Letters 2014, 8, 312–316. (53) Van Exter, M.; Grischkowsky, D. Carrier dynamics of electrons and holes in moderately doped silicon. Physical Review B 1990, 41, 12140–12149. (54) Cohen-Karni, T.; Casanova, D.; Cahoon, J. F.; Qing, Q.; Bell, D. C.; Lieber, C. M. Synthetically encoded ultrashort-channel nanowire transistors for fast, pointlike cellular signal detection. Nano Letters 2012, 12, 2639–2644.


Page 22 of 24

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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

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(55) Xu, T.; Nys, J. P.; Grandidier, B.; Stievenard, D.; Coffinier, Y.; Boukherroub, R.; Larde, R.; Cadel, E.; Pareige, P. Growth of Si nanowires on micropillars for the study of their dopant distribution by atom probe tomography. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 2008, 26, 1960–1963. (56) Chen, W.; Dubrovskii, V. G.; Liu, X.; Xu, T.; Lardé, R.; Philippe Nys, J.; Grandidier, B.; Stiévenard, D.; Patriarche, G.; Pareige, P. Boron distribution in the core of Si nanowire grown by chemical vapor deposition. Journal of Applied Physics 2012, 111, 094909. (57) Koren, E.; Berkovitch, N.; Rosenwaks, Y. Measurement of active dopant distribution and diffusion in individual silicon nanowires. Nano Letters 2010, 10, 1163–1167. (58) La Rosa, A. H.; Yakobson, B. I.; Hallen, H. D. Optical imaging of carrier dynamics in silicon with subwavelength resolution. Applied Physics Letters 1997, 70, 1656. (59) Stiegler, J.; Tena-Zaera, R.; Idigoras, O.; Chuvilin, A.; Hillenbrand, R. Correlative infraredelectron nanoscopy reveals the local structureconductivity relationship in zinc oxide nanowires. Nature Communications 2012, 3, 1131.

Supporting Information Available Additional information and figures related to NW synthesis, characterization methods, etching procedures, and supplementary near-field measurements. This material is available free of charge via the Internet at http://pubs.acs.org/.


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IR

1 μm

Figure 5: For Table of Contents Only


Mapping Free-Carriers in Multijunction Silicon Nanowires Using

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