Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex

Letter pubs.acs.org/NanoLett

Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex Situ Doping: A Correlative Study via Atom-Probe Tomography and Scanning Tunneling Spectroscopy Zhiyuan Sun,† Ori Hazut,‡ Bo-Chao Huang,§ Ya-Ping Chiu,*,§,∥ Chia-Seng Chang,§ Roie Yerushalmi,*,‡ Lincoln J. Lauhon,*,† and David N. Seidman*,†,⊥ †

Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208-3108, United States ‡ Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel § Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan ∥ Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan ⊥ Northwestern University Center for Atom-Probe Tomography (NUCAPT), 2220 Campus Drive, Evanston, Illinois 60208-3108, United States S Supporting Information *

ABSTRACT: Dopants play a critical role in modulating the electric properties of semiconducting materials, ranging from bulk to nanoscale semiconductors, nanowires, and quantum dots. The application of traditional doping methods developed for bulk materials involves additional considerations for nanoscale semiconductors because of the influence of surfaces and stochastic fluctuations, which may become significant at the nanometer-scale level. Monolayer doping is an ex situ doping method that permits the post growth doping of nanowires. Herein, using atom-probe tomography (APT) with subnanometer spatial resolution and atomic-ppm detection limit, we study the distributions of boron and phosphorus in ex situ doped silicon nanowires with accurate control. A highly phosphorus doped outer region and a uniformly boron doped interior are observed, which are not predicted by criteria based on bulk silicon. These phenomena are explained by fast interfacial diffusion of phosphorus and enhanced bulk diffusion of boron, respectively. The APT results are compared with scanning tunneling spectroscopy data, which yields information concerning the electrically active dopants. Overall, comparing the information obtained by the two methods permits us to evaluate the diffusivities of each different dopant type at the nanowire oxide, interface, and core regions. The combined data sets permit us to evaluate the electrical activation and compensation of the dopants in different regions of the nanowires and understand the details that lead to the sharp p−i−n junctions formed across the nanowire for the ex situ doping process. KEYWORDS: Silicon nanowire, atom-probe tomography, monolayer doping, ex situ doping, diffusion

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high doping level and thus influences dramatically the electronic properties. Uniformity is one important aspect of doping control, yet nonuniform doping distributions are widely observed in many systems. In ion implantation, the dopants segregate at grain boundaries,5 Si/SiO2 interfaces,6 and dislocations.7 In the context of nanowires, ion implantation is limited due to irradiation damage of the crystal lattice and problems controlling the doping profile at the nanoscale level.8 In situ

caling down, via bottom-up or top-down approaches, is the principal method for improving the performance of many electronic devices, such as integrated circuits and hard-disk drives. Nanowires have been intensively studied as building blocks in the bottom-up approach for many electronic or optoelectronic applications, including field-effect transistors,1 sensors,2 solar cells,3 and thermoelectric devices.4 All of these applications require uniform dopant distributions and abrupt junctions. Control of the doping profile at the nanometer scale is a challenging task due to stochastic fluctuations, which make it difficult to control the dopant distribution. For example, placing 10 phosphorus atoms into a 10 nm silicon cube yields a dopant concentration of 1019 cm−3, which is considered to be a © XXXX American Chemical Society

Received: April 26, 2016 Revised: June 18, 2016

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DOI: 10.1021/acs.nanolett.6b01693 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Sample geometry and atom-probe tomography after reconstruction. (a) Cross-sectional SEM image of the sample after conformal ZnO coating by atomic layer deposition and capping with Pt using dual ion-beam induced deposition. The 75 nm diameter nanowire is doped by both P and B at 970 °C for 20 s. The white dashed line indicates the cross-section of the final specimen after annular ion-beam milling. (b) Atom-probe tomographic reconstruction of a selected region in (a). A small portion of the nanowire on the right is missing due to the limited field-of-view of the APT, which is controlled by the trajectories of the evaporated ions. (c) 30 at. % silicon isoconcentration surface viewed at different angles. (d) Hitdensity histogram of silicon displaying the 3-fold rotational symmetry of the 111 evaporation pole. (e) Spatial distribution map (SDM) for silicon in the analysis direction. In this case, the reconstruction parameters are adjusted to make the peak-to-peak distance equal to 3.14 Å, which is the lattice interplanar spacing of silicon in the [111] direction.

transmission electron microscopy (TEM) has a high spatial resolution, but a restricted detection limit (energy dispersive spectroscopy (EDS) ∼ 0.5 at. %, electron energy loss spectroscopy (EELS) of ∼0.1 at. %).18 Scanning probe microscopes measure the local electronic properties; for example, scanning tunneling spectroscopy (STS),19 and Kelvin probe force microscopy (KPFM),20 have an optimal sensitivity for electrically active dopants, but they have limited spatial resolution (5−25 nm). Additionally, because these techniques are based on electrical measurements, they are blind to electrically inactive doping atoms. Because APT detects ions (atoms) directly by time-of-flight (ToF) mass spectrometry, both electrically active and inactive dopant atoms are detected and mapped in three-dimensional (3D) space. Therefore, APT and STS are complementary methods, which, when used correlatively, can give both the chemical compositions and electronic properties of a silicon nanowire. Herein, we use APT to study the diffusion and segregation behavior of boron, phosphorus, and carbon at the nanometer length scale using ex situ doped silicon nanowires as a model system. The near-surface region is enriched in carbon and phosphorus, whereas the interior is uniformly doped with boron. The enrichment of phosphorus and carbon in a nanowire’s near-surface region and the uniform B distribution in the nanowire’s interior cannot be explained assuming only bulk diffusion of dopants and their diffusivities. The distribution of dopants is explained by assuming enhanced surface and interface diffusion of carbon and phosphorus and enhanced

doping during vapor−liquid−solid (VLS) growth of nanowires is also limited with respect to obtaining uniform dopant distributions due to vapor−solid (VS) deposition on the nanowires’ sidewalls,9,10 though this effect can be mitigated by appropriately modifying growth conditions, including the introduction of etching species.11 Previously, we showed that VLS doping yields a radially nonuniform profile, even without VS deposition, due to the faceted liquid−solid interface.12 Besides uniformity, abruptness is a very important consideration at the nanometer scale. Ho and Yerushalmi et al. introduced monolayer doping (MD) as an ex situ doping method for silicon for obtaining sharp concentration profiles at surfaces and in nanowires.13 Furthermore, vapor−solid−solid (VSS) growth was demonstrated as an alternative approach to VLS growth to obtain an abrupt axial junction by omitting the reservoir effect of the liquid catalyst in the VLS growth method.14 Mechanistic understanding and characterization of the diffusion kinetics for doping nanostructures still pose significant challenges, considering the small dimensions of current electronic devices and nanowire-based electronics, which requires ∼0.5 nm spatial resolution and 1016 cm−3 detection limit (∼0.5 atomic ppm for Si). Atom-probe tomography (APT) provides subnanometer scale spatial resolution and an atomic-ppm level detection limit (determined essentially by counting statistics)15,16 and is therefore ideal for measuring the 3D doping concentration profiles in electronic devices and doped semiconductor nanowires.17 Compared with APT, B


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Figure 2. (a) Mass spectrum of the nanowire, shown in Figure 1b, was doped by B and P at 970 °C for 20 s. The main elemental peaks for Si and Zn and the compound peaks for ZnO are labeled. (b) Peaks for B and C. (c) Phosphorus peaks. (d) Density of background counts (noise) in the range of 8.0−8.2 Da. The red circle in panel d delineates the surface of the nanowire.

bulk diffusion of boron, respectively. The APT analyses are correlated with scanning tunneling microscopy (STM) results in concert with scanning tunneling spectroscopy (STS), termed STM/S, to obtain the spatial electronic properties of the junction formed in the NWs.19 Correlating the APT and STS results permits us to identify the physical distributions of dopant atoms and activated dopant cross-sectional maps of the silicon nanowires, including the oxide, interface, and interior semiconducting regions. The boron distribution is found to be more uniform, while the phosphorus distribution is less uniform with higher concentrations near the diffusion source. The nanowires were doped by the previously reported onestep ex situ doping method.19,21 Briefly, Si nanowires synthesized by the vapor−liquid−solid (VLS) mechanism were transferred onto a silicon wafer containing an organic monolayer and then covered with another silicon wafer with a different organic monolayer.21,22 The organic monolayers containing B or P atoms serve as dopant sources. Then the sandwich structure is rapidly thermally annealed at 10−2 Torr Ar at 970 °C for different times, ranging from 3 to 80 s. In this process, the dopants are driven into the nanowire primarily from the top and bottom lines of contact; additional contributions due to evaporation and adsorption are discussed below. Different diameters of nanowires, ranging from 30 to 80 nm, were used to study the effects of NW diameter on diffusion. The standard lift-out method was used to prepare specimens for atom-probe tomography (APT). Before lift-out, nanowires lying on Si substrates were coated with a ZnO layer utilizing atomic layer deposition (ALD) to create a void-free specimen, which increases the successful yield and eliminates reconstruction artifacts.23 Next, a protective Pt capping layer was deposited by ion-beam induced deposition using a dual-beam focused-ion beam (FIB) microscope. Then lift-out and annular milling were performed to obtain APT specimens.24

APT was performed using a local-electrode atom-probe (LEAP) 4000X Si at a sample temperature of 35 K and a base pressure of 3 × 10−11 Torr in the ultrahigh vacuum chamber. The evaporation of atoms as ions from the specimen was induced by a 355 nm wavelength picosecond laser at a pulse repetition rate of 250 kHz and at a detection rate (ion detected per pulse) of 1.0−2.0%. A pulse energy of 30 pJ was chosen to achieve both high quality data and a reasonable sample yield. Data were reconstructed in three-dimensions (3D) employing the program IVAS 3.6.6 (Cameca, Madison, WI). An SEM image of each nanotip was recorded to obtain the nanotip’s profile for use in the 3D reconstruction procedure. The reconstruction parameters, including the electric field factor and the image compression factor,25 were chosen to reproduce the nanotip’s profile in its SEM images. For further parameter refinement, the hit density histogram for Si was generated near a crystallographic hkl pole, and the 3D reconstruction parameters were adjusted such that the {hkl} interplanar spacings agreed with the correct values.26 If an hkl pole was not observed, typically because the analysis direction is not sufficiently close to a low-index crystallographic direction, only the SEM profiles were used to guide the 3D reconstructions. Variations in the majority carrier concentration across the diameter of the Si nanowire were analyzed by cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy of nanowires cleaved in vacuum. Spectroscopy was conducted at different sample biases and at several lateral positions. The conduction and valence band edges and doping concentrations were extracted from differential conductance plots. Further experimental details describing the XSTM measurements are in the SI section (Figures S9−S14) and elsewhere.27 The nanowire’s structure and reconstruction are presented in Figure 1. A cross-sectional SEM image (Figure 1a) obtained before annular ion-beam milling displays the layered structure C


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Figure 3. 2D concentration plots for nanowires with different diameters (35, 55, and 75 nm) and annealing times. (a) 75 nm, 20 s; (b) 55 nm, 20s; (c) 35 nm, 20 s; (d) 35 nm, 10 s; and (e) 35 nm, 3 s. The phosphorus, C, and B atoms are indicated by the colors pink, brown, and blue, respectively. The solid yellow line indicating the surface of the nanowire is defined by the SiO+ 1.0 at. % isoconcentration surface; a distinct peak for SiO+ is found in the mass spectrum. The dashed line indicates the field-of-view of the APT data set, outside of which no atoms are detected. These plots represent the individual distributions of P, C, and B, after ex situ doping, in the interior and outside regions. Concentrations were determined utilizing 1 × 1 nm voxels; the fluctuations seen in the plots reflect the limitations of counting statistics.

The corresponding mass spectrum for these data is presented in Figure 2. The elemental peaks of silicon, zinc, and the main compound peaks of zinc oxide are labeled in Figure 2a. Figure 2b and c displays details of the mass spectrum near the boron peaks (10 and 11 Da) and the phosphorus peak (31 Da), respectively. Both the boron and the phosphorus peaks are above the level of the background counts, though the background level is higher for phosphorus as its peak lies within the thermal tails of the three silicon isotopes. The background signals correspond to 9 at. ppm for C, 7 at. ppm for B, and 40 at. ppm for P, which sets the quantification detection limits for these elements. A doubly charged phosphorus peak was not identified in this data set. Singly and doubly charged carbon ions were, however, detected and associated with decomposed organic species on the donor wafer. A plot of background noise in the region of 8.0−8.2 Da (Figure 2d) displays a significant increase in noise when the evaporating surface advances into the thermal oxide, which has a higher evaporation field threshold than does silicon. In this

from top-to-bottom: (1) Pt cap; (2) ZnO encapsulation; (3) SiO2 (thermal oxide); and (4) Si. Each silicon nanowire is embedded in a ZnO matrix, which serves as a sacrificial layer to increase the field-of-view and acquire data from most of nanowire’s surface. The sample was sharpened, utilizing ionbeam milling, into a needle-shaped specimen with a 50 nm radius end-form. After annular milling, the residual Pt coating was removed by ion-beam milling the nanotip using a 2 kV Ga+ ion beam at a current of 0.56 nA. The white dashed line in Figure 1a indicates the final nanotip’s profile for the 75 nm diameter nanowire reconstructed in Figure 1b. Figure 1c displays a silicon 30 at. % isoconcentration surface generated from the 3D reconstruction at different rotation angles about the nanowire’s long vertical axis. The hit density histogram, Figure 1d, exhibits the 3-fold symmetry characteristic of the 111 evaporation pole. The reconstruction parameters were adjusted to make the Si nearest-neighbor distance in the z-direction equal to 3.14 Å (Figure 1e), which corresponds to the {111}type interplanar spacing. D


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a

interior region (× 1018 cm−3)

diameter (nm)

time (s)

P

B

C

P

B

C

75 55 35 35 35

20 20 20 10 3

65 ± 10.9 69 ± 11.7 450 ± 41.5 49 ± 7.8 13 ± 2.3

5.4 ± 0.7 14 ± 1.6 550 ± 7.9 21 ± 1.2 ND

48 ± 0.9 32 ± 1.0 240 ± 8.5 63 ± 1.7 25 ± 1.0

6.3 ± 1.1 9.6 ± 1.7 20 ± 4.2 5 ± 1.2 2 ± 0.7

4.8 ± 0.4 11 ± 1.1 120 ± 5.4 4.2 ± 0.6 ND

ND ND ND ND ND

ND = below the detection limit.

Figure 4. Representations of doping near the surface of a 75 nm diameter nanowire, which was doped by B and P at 970 °C for 20 s. (a) Distributions of P, C, and B atoms: (i) merged; (ii) P; (iii) C; and (iv) B for a 75 nm diameter nanowire near the P contact line in a P-top-B-bottom sample. Phosphorus and C atoms segregate at the surface and at the Si/SiO2 interface, respectively. (b) 1D concentration profile in a direction normal to the nanowire’s surface near the P contact line. Gaussian fits were used to identify the peaks’ positions. (c) The same analysis as in (b) for a B-top-P-bottom sample near the B contact line, displaying evidence for B segregation in the SiO2 layer. The pale blue highlighted regions in panels b and c indicate the existence and position of the SiO2 layer.

surface of the silicon nanowire, using a 1 at. % isoconcentration surface of SiO+, which is the native oxide.28 Another 1 at. % isoconcentration surface is used below to define the Si/SiOx interface. The concentration of phosphorus and carbon is highest near the surface of the nanowire, with the highest value obtained at the upper surface. The distribution reflects the dominant role of P dopant diffusion from the top contact line. In contrast, B is distributed relatively uniformly within the nanowire. The complementary B-top-P-bottom configuration (Figure S3b) exhibits similar P and C enrichments near the surface and a uniform B distribution in the interior region, but the P concentration near the upper surface is smaller than that of the P-top-B-bottom configuration, as anticipated.

region, the control software automatically increases the voltage to maintain a constant evaporation rate, which in turn increases the rate of background field-evaporation events to approximately 300 at. ppm (1 at. ppm = 5 × 1016 cm−3 for silicon). This background noise precludes analyses of small concentrations of dopants at this interface. Hence, samples were prepared and analyzed in two distinct configurations: P-top-Bbottom and B-top-P-bottom. Variations in the dopant distribution according to dopant type and nanowire diameter are indicated in Figure 3. We focus first on the largest diameter nanowire, 75 nm (Figure 3a), in a P-top-B-bottom configuration. Doping concentrations are plotted as 2D maps with the yellow lines defining the outside E


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Figure 5. (a, b, and c) Phosphorus, B, and C diffusion pathways and angular distributions. Schematic diagrams for the dominant diffusion pathways for C (on nanowire’s surface), P (along Si/SiO2 interface), and B (bulk). (d and e) Annular distributions of C and P atoms from (d) P-top-B-bottom and (e) B-top-P-bottom samples. Both nanowires were doped at 970 °C for 20 s.

To correlate the observed distributions with possible differences in dopant diffusion, a series of different dopant drive-in (annealing) times were studied for nominally 35 nm diameter nanowires as summarized in Figure 3c−e. For a 3 s anneal, no B atoms are found driven into the nanowire as indicated by the mass spectra generated from the nanowire interior region, and no significant surface enrichment is detected. It is worth noting that, in Figure 3e, random counts (noise) in the boron mass/charge range generated an effective concentration that is below the actual detection limit. When the annealing time is increased to 10 s, the overall B concentration increases, and an enrichment near the surface is observed. With a 20 s annealing time the surface enrichment is very clear. The dopant densities in the interior of the nanowire and the surface regions for different annealing times and diameters are summarized in Table 1. The surface concentrations refer to the local dopant concentrations on a nanowire’s surface, which is discussed later, while the interior concentrations are the integral concentrations from a nanowire’s center to approximately 60% of the nanowire’s radius. In general, longer

With a decrease in diameter from 75 nm (Figure 3a) to 55 nm (Figure 3b) and then 35 nm (Figure 3c), the P and C distributions in the near-surface region become more uniform; that is, the influence of doping from the contact line is less obvious. Additionally, segregation of B at the near-surface region is observed. 1D concentration profiles near the nanowire’s surface are presented in Figure S4. First-principles calculations have predicted surface segregation of B in small diameter Si NWs, ∼2 nm,29 but the nanowires analyzed herein are an order of magnitude larger. The smaller is the radius the larger is the curvature, 1/radius, and hence the higher is the chemical potential of a segregating atom. This implies that the level of segregation will be greater for a smaller diameter nanowire as described by the Gibbs adsorption isotherm.30 Additionally, in a smaller diameter nanowire the dopant diffusion length is smaller, and the dopant can fill the nanowire faster than in the case of a larger diameter nanowire, which makes B enrichment more evident. Yet local variations in the radius of curvature, due to faceting, preclude the quantitative identification of a single coefficient of segregation. F


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role in establishing the dopant distribution within a nanowire. Diffusion perpendicular to the surface (bulk diffusion) is generally slower than the diffusion parallel to the nanowire’s surface (surface diffusion) due to the higher activation energy for bulk diffusion.37,38 We infer from the distributions of C and P atoms that their diffusivities on the nanowire’s surface is significantly larger than in the nanowire. Additionally, the quantitative analyses below focus on surface/interface diffusion. The uniform B distribution in the interior implies, however, a significantly enhanced bulk diffusivity. We note that a dopant species can also be transferred to a nanowire’s surface through the vapor phase following evaporation from the initially deposited monolayer. We first consider the distribution of P and C on the nanowire’s cylindrical surface, arising primarily from surface (C) and interface (P) diffusion from the points-of-contact, which is a line on a cylinder (a cylinder is an approximation to the shape of a nanowire), Figure 5a−c. We model the problem in polar coordinates, in which Rθ is a distance on the surface, where R is the radius of the silicon core (silicon core plus native oxide) for P (C) and θ is the polar angle. Assuming one P line source at θ = 0° and two C line sources at θ = 0° and 180°, the concentrations are given by the following Gaussian distributions:39

annealing times and smaller nanowire diameters lead to higher concentrations in both the interior and surface regions. Carbon is not, however, detected in the nanowire’s interior due to the low solubility of C31 in Si and the diffusion barrier effect of the SiOx layer. The 2D concentration profiles discussed above exhibit evidence of segregation at the near-surface region. To identify which interfaces or interfacial regions are enriched, Figure 4a presents analyses of a region-of-interest directly below the P contact line for a 75 nm diameter nanowire with a P-top-Bbottom configuration. The nanowire’s surface is enriched in C (Figure 4a, iii), while P atoms segregate at the Si/SiO2 interface (Figure 4a, ii). 1D concentration profiles (Figure 4b) in the vertical direction quantify the level of segregation. Concentration peaks for P and C were found near the SiO2/Si interface and the nanowire’s surface, respectively. A boron peak is not observed exhibiting a uniform B distribution near the P contact line. The distinction between the behavior of P and B can be attributed to the negatively charged oxygen ions formed during the nanowire’s oxidation, which draws positively charged P toward the Si/SiOx heterophase interface and pushes the negatively charged B into the nanowire.32 Similar phenomenon were observed when P and B diffuse into polycrystalline silicon, where P segregates at the negatively charged grain boundaries, while B is uniformly distributed.33 The same analysis was performed on a B-top-P-bottom configuration near the B contact (Figure 4c), in which the SiOx adjacent to the upper contact line is enriched in B relative to Si. Again, C segregates at the nanowire’s surface, but P segregation is not observed due to the remoteness of the P contact doping line from the B contact line. The slow oxidation of the nanowires during rapid thermal annealing, due to the remaining oxygen gas and water vapor partial pressures, has an effect on the distributions of dopants. The advancing silicon oxide layer was observed by performing APT experiments on naonwires utilizing different annealing times (Figure S5). The distributions of different impurities following diffusion doping of a planar Si wafer during oxidation has been treated by Sah et al.,34,35 by considering the segregation coefficient and the diffusivity ratio between Si and SiO2. The segregation coefficient is the ratio of the equilibrium concentration of an impurity atom in Si to its concentration in SiO2. For B, this value is less than unity, while for P it is greater than unity. Hence, we observe enrichment of P at the SiOx/Si interface as the P enters the Si nanowire (Figure 4b), whereas we observe enrichment of B in the SiOx layer (Figure 4c), which agrees with Sah et al.’s calculations. Besides diffusion, oxidation can also influence the drift of dopants. Oxygen ions, which are generated during high temperature oxidation via the Cabrera−Mott mechanism, can pull the phosphorus dopants out of nanowires.32 We note that SiOx behaves as a diffusion barrier for C atoms, hence the majority of the C remains outside the Si nanowire.36 The blocking effect of SiOx is an advantage for this doping method. Even though organic molecules are utilized in the doping process, the decomposed carbon atoms are blocked by the native oxide and therefore do not contaminate the silicon nanowire’s core. After the doping process, the surface carbon atoms are readily removed using a plasma treatment, an HF treatment removing the oxide and carbon contamination layer, or other methods to clean the nanowire’s surface. The diffusivities of the different elements involved, both parallel and perpendicular to the nanowire’s surface, also play a

CC =

⎧ ⎛ R2(θ )2 ⎞ ⎪ ⎜⎜ − Surf ⎟⎟ ⎨ exp ⎪ 2 πDCSurf t ⎩ ⎝ 4DC t ⎠ C0,C

⎛ R2(θ − 180)2 ⎞⎫ ⎪ ⎟⎟⎬ + exp⎜⎜ − Surf 4DC t ⎝ ⎠⎪ ⎭

(1)

⎧ ⎛ R2(θ )2 ⎞⎫ ⎪ ⎪ ⎨ ⎜⎜ − Interf ⎟⎟⎬ exp ⎪ Interf ⎪ 2 πDP t ⎩ ⎝ 4DP t ⎠⎭

(2)

and CP =

C0,P

where C0,C and C0,P are the surface concentrations of P and C, respectively, arising from the organic molecular monolayer on Interf the Si donor wafer, and DSurf are the diffusivities of C C and DP and P on the surface and in the interface, respectively. In eq 1 the range of θ in the first Gaussian is −90° to 90°, and the range of θ for the second Gaussian is 90° to 270°. In eq 2 for interface diffusion of P the range of θ is 90° to −90° (270°). Because both Si donor wafers contain C, there are two line sources of C both of which come from the organic precursor. The radial growth of the thermal oxide is neglected herein because it is not expected to influence the angular distributions of the dopants. The angular distributions of P and C for two ∼75 nm diameter nanowires with P-top-B-bottom and B-top-P-bottom configurations are displayed in Figure 5d and e, which along with their fits, are used to calculate the diffusivities employing a fitting procedure;39 DInterf is 9.59 × 10−14 cm2 s−1, and C0,P is P 2.1 × 1014 cm−2. For comparison, the diffusivity of P in bulk Si at 970 °C is 3.81 × 10−16 cm2 s−1,40 which is 250 times smaller . We note, however, that the calculated diffusivities than DInterf P should be considered upper bounds due to the contribution of vapor-phase doping originating in molecular fragmentation and evaporation, which results in deposition of P on a nanowire’s surface during annealing. This is denoted monolayer proximity doping (MLPD). In the MLPD process, the organic molecules break into small fragments, evaporate from the line-of-contact, G


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Figure 6. Comparison of P and B dopant distributions with active dopant concentrations. Phosphorus distribution maps measured by (a) APT and (b) XSTS. Boron distribution maps measured by (c) APT and (d) XSTS. All of the nanowires were doped from the top contact lines by annealing at 970 °C for 20 s. (e) Concentration profiles across the nanowire as measured using STS; the scan directions for P and B are indicated by black arrows in panels b and d; and (f) calculated band structure across the junction derived from a combination of the STS maps of P and B.

diffusion distance, L, is 1.7 nm for t = 20 s, where L = 2(Dt)1/2. A more accurate finite-element simulation considering the bulk diffusion of B in Si, leads to a similar conclusion: that is, the distribution of B should be nonuniform inside the Si nanowire, Figure S6.19 Taking the RMS diffusion distance of B to be equal to one-half the nanowire’s diameter, an approximate lower bound of the B diffusivity is 1.7 × 10−13 cm2 s−1, which is 450 times greater than the bulk diffusivity of B in silicon.40 This enhancement can be attributed to the negtive charges in the silicon oxide that push the negtively charged B into silicon as we discussed before. Another diffusion enhancement mechanism related to the interaction between B and P atoms will be discussed in a later point. Our results are in-line with previous studies where enhanced diffusivities of B and P in Si nanowires have been previously reported. For example, Chen et al. found that the diffusivity of B in a 60 nm diameter vapor−liquid−solid (VLS) silicon nanowire grown at 460 °C was enhanced by 2 orders of magnitude (4.22 × 10−16 vs 10−18 cm2 s1), which was attributed to a high density of point defects, such as interstitial silicon atoms, near the nanowire’s surface.41 A substitutional B atom, for example, can be kicked out of its site into an interstitial site (the so-called kick-out mechanism), thereby increasing the diffusivity of B in Si.42,43 Koren et al. reported that the P diffusivity in 50 nm diameter Si nanowires can be as large as 10−15 cm2 s−1 for a 460 °C chemical vapor deposition (CVD) process,20 which is considerably greater than the diffusivity

are deposited onto the nanowire’s surface, and then diffuse into the nanowire, resulting in more uniform doping in the nanowire’s outer region compared with contact doping, where the dopants only diffuse from a single point-of-contact line-source. The evaporation of molecular fragments can occur at as low as 400 °C and is completed by 600 °C, which is influenced by both pressure and temperature.21 This process is much faster than solid-state diffusion because the molecular fragments are transferred to the nanowire’s surface through the vapor phase at the earliest stages of annealing. As can be seen from the carbon distribution in Figure 3a, the organic molecular fragments are enriched in the region near the point-of-linecontact after annealing for 20 s at 970 °C. The initial distribution of the evaporated fragments cannot be more diffuse than for the case displayed in Figure 3a, because of C surface diffusion. Because of the less diffuse C distribution, we think that MLPD is playing a secondary role in determining the impurity diffusivities based on the above procedure, and hence it is not included in our calculation of the surface diffusivity. For the 75 nm diameter nanowire, the distribution of B atoms is uniform, and a significant concentration gradient is not observed. This indicates that, unlike P, bulk diffusion of B is sufficiently fast to establish a homogeneous concentration distribution within the Si nanowire. This is unanticipated because the measured bulk diffusivity of B in bulk Si at 970 °C is 3.81 × 10−16 cm2 s−1 and hence the root-mean-square (RMS) H


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Table 2. Comparisons between Atom-Probe Tomography (APT) and Scanning Tunneling Spectroscopy (STS) Instruments method atom-probe tomography

scanning tunneling spectroscopy

spatial resolution (nm) depth resolution: 0.2 lateral resolution: 0.4 ∼5

object

detection limit (at. ppm)

results

dopants

B: 7 C: 9 P: 40

nanowires with a highly doped outside region and a lightly doped interior

electrically activated dopants

B: 0.3 C: not applied P: 0.3

parallel p−n junction across nanowires

The dopant distribution maps obtained herein by STS for the single-species doping type process are used to construct a hypothetical junction that would form from a combination of P and B distribution maps; for this purpose, the P distribution map is rotated by 180°. This analysis permits us to compare the junction obtained by the summation of two one-species doping processes versus the previously reported two-species type simultaneous codoping process.19 Phosphorus and B concentration profiles obtained for the one-species type doping processes are presented in Figure 6e, with the derived band structure corresponding to the summed dopant distributions, Figure 6f; the analysis direction is indicated by arrows, Figure 6b,d. The resulting band structure of the summed maps obtained for the one-species type doping processes is similar to the band structure obtained for the codoped nanowires in our prior work with simultaneous P and B doping,19 displaying a sharp p−i−n junction bisecting the nanowire axis. The intrinsic region in the hypothetical junction results from a compensation effect, which also increases the interfacial sharpness, that is, the interfacial width, of the junction. The slight shift in the junction’s position between the two cases can be attributed to the larger diffusivities of phosphorus and boron in the twospecies doping case as discussed above. The atomic distribution maps of phosphorus and boron obtained by APT and STS contain several features that are different, owing to the different capabilities of the two methods. First, the distributions maps obtained by the two methods represent significantly different physical information. For example, while APT detects directly all dopants by mass spectrometry, STS detects only electrically activated dopants. Therefore, the high concentration levels of dopants found near the silicon oxide by APT are not detected by the STS maps as dopants are not activated at the silicon oxide layer or at the interface between the silicon oxide and semiconductor surface.46 Additionally, the STS data represent the effective doping levels considering only electrically activated and compensated dopants. Furthermore, the detection limits and dynamic detection range of the two methods are very different, resulting in quite different observed distributions. While STS is very sensitive to electrically activated doping, with detection limits as small as 1 × 1016 cm−3 (∼0.2 at. ppm) for both boron and phosphorus, while the detection limits of APT are around 2 × 1018 cm−3 (∼40 at. ppm) for P and 3.5 × 1016 cm−3 (∼7 at. ppm) for B. The significantly higher detection limitations of APT become significant in the low doping regions, specifically, at the interior region of the nanowires, where the concentration can be as small as 1016−1017 cm−3, thus resulting in qualitatively different distributions. The main differences between the two methods are summarized in Table 2. Overall, comparing the information obtained by the two methods, while carefully considering the scope and limitations of each instrument, provides vaulable information that could

extrapolated from a high-temperature (1100−1300 K) experiment, where the diffusivity is ∼10−20 cm2 s−1).40 Additional characterization of nanowires ex-situ doped with a single dopant type were performed by monolayer doping utilizing cross-sectional STS (XSTS) for obtaining the electrically activated dopant distributions (Figure 6b,d) for comparison with the chemical distributions of dopants obtained by APT (Figure 6a,c). The data shown in Figure 6 are from different nanowires doped by the same doping process. It is worth noting that, even for a single batch of NWs, the diameters, the crystal orientations facing the donor wafer and the local contact conditions between the NWs and the donor wafer can be different. Thus, the variations from sample to sample can lead to the result that active doping concentrations measured by STS sometimes are larger than chemical doping concentrations measured by APT. The dopant distribution maps obtained by APT for single-dopant type doped nanowires (P or B) are similar to the distributions obtained observed in both cases, while B atoms diffuse into the nanowire without significant surface segregation, for an 80 nm diameter nanowire. In the STS results, we find a concentration gradient from the nanowire surface to its core. Comparing with the APT results, which have much higher spatial resolution than the STS ones, we conclude that this gradient is caused by incomplete or slow diffusion of B instead of surface segregation. Notably, the calculated interfacial diffusivity of P for the single-dopant type process is 6.95 × 10−14 cm2 s−1, which is ∼27% smaller than the value obtained for a two-species type doping process. Furthermore, the B distribution is inhomogeneous when comparing it with the two-species doping process (Figure 6c), which indicates a smaller B diffusivity for the single-species doping process. A similar phenomenon has been found in bulk silicon, where the boron diffusivity is enhanced by a high phosphorus concentration.44 The faster diffusion of dopants for the two-species type doping can be attributed to the stress field produced by another dopant.45 The P and B distribution maps obtained by STS exhibit similar trends as the APT results, with greater sensitivity in the low concentration regions owing to the smaller detection limits of STS. The STS results for P exhibit higher concentrations at the monolayer doping contact line, similar to APT, with diffusion into the nanowire, resulting in a highly doped n-type region on one side of the nanowire. Once again, the B distribution is more homogeneous indicating a higher diffusivity than the diffusivity of P. Charge carrier concentrations and the distributions for P−B codoped nanowires were studied previously by Hazut et al. using scanning tunneling spectroscopy (STS) and off-axis electron holography, thereby demonstrating a p−n junction bisecting the nanowire axis.19 The junctions were formed by a one-step ex situ doping process for the transformation of undoped silicon nanowires (i-Si NWs) to a p−n junction bisecting the nanowire axis. I


Letter

Nano Letters

Program (Grant Number DMR-1121262). Additional instrumentation at NUCAPT was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN). This work also made use of the EPIC facility (NUANCE Center), which has received support from the MRSEC program (NSF DMR1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the State of Illinois, through the IIN; and support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205).

not be obtained by utilizing only one of the two methods. The combined data sets permits us to evaluate the diffusivities of each different dopant type at the nanowire oxide, interface, and core regions. The combined data sets permit us to evaluate the electrical activation and compensation of the dopants in different regions of the nanowires and understand the details that lead to the sharp p−i−n junctions formed across the nanowire for the ex situ doping process. In conclusion, atom-probe tomography results on ex situ doped silicon nanowires reveal the unique diffusion behaviors of carbon, phosphorus, and boron at the nanoscale. For ex situ doping of nanowires with both boron and phosphorus simultaneously, carbon and phosphorus are enriched in the outer region of the nanowire, while boron distributes itself uniformly in the interior region, except for the smallest nanowires (diameter = 35 nm) we have studied. These phenomena are explainable by fast surface and interfacial diffusion of carbon and phosphorus and enhanced boron bulk diffusion at the nanoscale. The source of these distinct diffusive behaviors is the small length scale and large surface area to volume ratio of nanomaterials, which provide fast diffusion paths for dopants, that is, surfaces and interfaces. This principle can be also applied to other nanomaterials, such as quantum dots and two-dimensional nanomaterials, for example, graphene and MoS2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01693. APT sample preparation method, mass spectrum, more APT data analysis, STS data acquisition, correction, and active dopant concentration calculation methods (PDF)

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REFERENCES

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS D.N.S., L.J.L., R.Y., O.H., and Z.S. acknowledge funding support from the United StatesIsrael Binational Science Foundation (grant number 2012088) and Northwestern University’s McCormick School of Engineering and Applied Science. L.J.L. acknowledges support of DMR-1308654. This work was supported in part by a starting grant from the European Research Council (ERC) under the European Community’s Seventh Framework Program Grant Agreement No. 259312 and by a joint grant between Academia Sinica and the HUJI center for nanoscience and nanotechnology. We kindly thank Dr. Michael Pellin and Mr. Sumit Bhattacharya for performing atomic layer deposition at Argonne National Laboratory, Argonne, IL. The local-electrode atom-probe tomography 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 DMR0420532) 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 J


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Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex

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