pubs.acs.org/NanoLett
Measurement of Active Dopant Distribution and Diffusion in Individual Silicon Nanowires Elad Koren, Noel Berkovitch, and Yossi Rosenwaks* School of Electrical Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel ABSTRACT We have measured the radial distribution and diffusion of active dopant atoms in individual silicon nanowires grown by the vapor-liquid-solid (VLS) method. Our method is based on successive surface etching of a portion of a contacted nanowire, followed by measurement of the potential difference between the etched and unetched areas using Kelvin probe force microscopy (KPFM). The radial dopant distribution is obtained by fitting the measured potentials with a three-dimensional solution of Poisson equation. We find that the radial active dopant distribution decreases by almost 2 orders of magnitude from the wire surface to its core even when there is no indication for tapering. In addition, the dopant profile is consistent with a very large diffusion coefficient of D ∼ 1 × 10-19 m2 s-1. This implies that phosphorus (P) diffusion during the VLS growth is remarkably high and subsequent thermal annealing must be used when a homogeneous dopant distribution is required. KEYWORDS Nanowires, doping, diffusion, Kelvin probe force microscopy
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emiconductor nanowires are one of the most promising building blocks for near future nanoelectronics since they provide a new route to continuing miniaturization as well as a wealth of opportunities in nanoscale science and technology.1–4 The understanding and control of doping concentration, distribution, and activation is critical for the fabrication of high-quality electrical devices for various future applications. There are many nanowire systems in which the majority carrier type is controlled by intentional doping, but in all cases the actual dopant concentration, its spatial distribution, and the fraction of active dopants are unknown.5–7 There are very few methods that can be used to measure the dopant concentration and distribution in a single nanowire. Garnett et al.8 have recently measured the radial dopant profile in silicon nanowires using capacitancevoltage measurements where the nanowire doping was controlled by postgrowth BCl3 gas diffusion. They have found that the dopant concentration decreased by almost 2 orders of magnitude over a radial distance of 25 nanometers from the wire circumference to its center. Perea et al.9 have used three-dimensional (3D) atom probe tomography to directly measure the dopant concentration in individual tapered Ge nanowires. They have found that differences in precursor decomposition rates between the liquid catalyst and the solid nanowire surface result in a radial dopant decrease of one and a half orders of magnitude for a radial distance of 10 nanometers. Here, we quantitatively demonstrate the important role of dopants diffusion taking place during VLS10 growth of individual silicon nanowires. We use KPFM11 to measure the
active radial dopants distribution within single untapered n-type silicon nanowires. We find, in agreement with previous works, that the dopant concentration is decreasing by almost 2 orders of magnitude from the wire surface to its core. Moreover the profile is consistent with dopant diffusion during the growth with a diffusion coefficient of D ) 1 × 10-19 m2 s-1 that is much larger than the expected value for this temperature in bulk silicon. This implies that P diffusion during the VLS growth of silicon nanowires is very significant and subsequent thermal annealing must be used when a homogeneous dopant distribution is required. Silicon nanowires of n-type were synthesized via lowpressure chemical vapor deposition at 460 °C with solutiondeposited monodispersed 50 nm Au catalyst particles, comparable to conditions reported in the literature.12,13 The ratio of the precursor gases in the reactor during synthesis was 500:1 SiH4/PH3. For device fabrication, nanowires were suspended in solution by sonication in isopropyl alcohol and drop-cast onto degenerately n-doped silicon substrates with 200 nm of Si3N4 on top. Contact regions were defined by electron beam lithography using a PMMA resist. Thirty second oxygen plasma cleaning of the electrode regions was followed by a 3 s etch in buffered hydrofluoric acid, after which the substrates were immediately loaded into an electron beam evaporator for Ni contact evaporation. Samples were lifted off in acetone and rinsed with isopropyl alcohol. Before etching the wires, we have determined the active dopants concentration at the nanowire surface to be ∼1 × 1020 cm-3 by measuring the potential drop along a biased nanowire using KPFM and assuming electron mobility of ∼100 [cm2 V-1 s-1]14 as described elswhere.15 We note that the above surface concentration is a reasonable approximation to extract the radial dopants distribution, because as shown below their radial concentration drops by almost 2 orders of magnitude from the wire surface to its core.
* To whom correspondence should be addressed. E-mail:
[email protected]. Received for review: 10/5/2009 Published on Web: 03/02/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl9033158 | Nano Lett. 2010, 10, 1163–1167
in the center of the contacted wire device. The PMMA is protecting the metal contacts and ensuring that the nanowire is etched only in the exposed region defined by the window as demonstrated in Figure 1. Wet chemical etching was carried out by the following procedure: After a short oxygen plasma clean, the native oxide was removed using a 2 s buffered HF etch, and a few nanometers (5-7 nm) of the silicon surface were subsequently removed by a 20 s NH4F etch. Four sequential wet chemical etching steps were applied, where in each step a smaller window was defined and opened; the result was a staircase-like structure of the nanowire, as schematically depicted in Figure 1. An HRSEM image of three etching steps (Figure 1c) and a magnified image of a single step (Figure 1d) show parts of the etched nanowire. We note that, following each etching step, the nanowire surface becomes rougher resulting in a larger distribution of nanowire diameters; having a larger number of nanowire radii enables the determination of the radial dopant distribution with a better depth resolution, as explained in the next sections. Figure 2 shows the measured topography (a), and -CPD (b) of a single silicon nanowire following four etching steps. For convenience we present -CPD, because it expresses the surface potential of the nanowire; -CPD is defined as -CPD ) (φtip - φsample)/q, where φtip and φsample are the work functions of the tip and the sample, respectively, and q is the elementary charge. The observed change in -CPD as a function of the etching-depth indicates a change in the Fermi level (EF) relative to the sample local vacuum level (ELVL) as shown in Figure 2c. We attribute this change to a decreasing doping level from the nanowire shell to its core. The observed correlation between the topography and -CPD cannot be due to the changes in the scanning tip height as the nanowire is etched. Such tip convolution effects were analyzed by us in the past19 and have shown that such small (order of magnitude) close to the nanowire surface in agreement with the doping profile. Calculated 2D (b) and 3D (c) of the dopants distribution in the nanowire for D ) 1 × 10-19 m2 s-1 following several etching steps (a matched 3D distribution of the potential was used to extract the radial surface potential profile).
FIGURE 5. (a) Simulated (solid lines) surface potential profiles with negative surface charge of 0, 5 × 1011 cm-2, and 1 × 1012 cm-2 and the averaged measured profile (symbols). The error bars in the measured profile represent the standard deviation in the KPFM measurement for all the three nanowires. The averaged measured profile shows a relatively good agreement with the simulated profile for NSC ) 5 × 1011 cm-2. (b) Simulated surface potential band-bending of the etched nanowire with the presents of negative surface charge of NSC ) 5 × 1011 cm-2.
density is larger by more than an order of magnitude at the nanowire circumference compared to its interior part. Following etching of more than 15 nm, the measured surface potential profile deviates (downward bending) from the calculated dopant diffusion profile indicating a lower Fermi energy with respect to the conduction band minimum, Ec. This deviation can be due to negatively charged surface states that induce upward band bending at the wire circumference. This was simulated by including a negative surface charge (NSC) of 5 × 1011 cm-2, which has been estimated for Si nanowires by several groups.8,22 Figure 5a shows the simulated (solid lines) surface potentials for a diffusion constant of D ) 1 × 10-19 m2 s-1 including negative surface charge (NSC) of 0, 5 × 1011, and 1 × 1012 cm-2 together with the average-measured potential profile (symbols). As the nanowire is etched with a concurrent decrease in its surface doping concentration, the existence of a © 2010 American Chemical Society
negative surface charge results in larger surface bandbending (Figure 5b). Mikael et al.22 have recently demonstrated that donor deactivation in Si nanowires induce a decrease in the free charged carrier concentration as a function of the wire diameter. They observed a sharp decrease in the free charge carrier concentration when the nanowire radius was smaller than ∼ 15 nm. We assume that this effect is small in our measured wires, which are etched down to a radius of around 15 nm even following the fourth etch (see Figure 2a). In addition, it should be mentioned that although the nanowires etching was not totally conformal (especially under the nanowire) this did not show significant affect on the calculated potential profile as a function of etching depth (see Supporting Information). Our extracted value for the diffusion coefficient, D ∼ 1 × 10-19 m2 s-1, is considerably larger from any value extrapo1166
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lated from high temperature data,23 and to the best of our knowledge there are no reports of P diffusion in bulk silicon with conditions resembling the VLS growth. However, many works have shown that P diffusion is remarkably enhanced under several conditions. For example, the diffusion is enhanced when the surface concentration of P exceeds a value of ns ∼ 1 × 1020 cm-324–26 (as in the present study). This high surface concentration leads to an apparent “kink” in the dopants profile which is a result of dissociation of P+V) ion pairs, causing an enhanced diffusion in the form of a “tail”.27 Fair et al.,28 have shown that this effect increases the diffusivity at 600 °C by 4 orders of magnitude relative to the value extrapolated from high temperatures; they have also shown that this enhancement will be even larger at lower temperatures. H. Shibayam et al.29 have reported on boron and arsenic diffusivities reaching values of ∼ 1 × 10-20 m2 s-1 in the temperature range of 500-800 °C and with no sign for temperature dependence. They associated this enhancement to a generation of excess vacancies at low temperatures. Figure S3 (in the Supporting Information) summarizes most of the literature reported data of P low temperature diffusion for the case of high surface concentration in comparison with our result. The figure shows that the maximum predicted diffusivity, based on the above, for our growth temperature of 460 °C is ∼ 1 × 10-21[m2 s-1] (see our linear fit in the Supporting Information) which is still smaller than our extracted value. This may be attributed to (a) the relatively short time and range of the dopants diffusion (10-15 min and 30 nm, respectively) in nanowires compared to bulk, and (b) to the very large surface to volume ratio present in nanowires. Furthermore, the kink observed in our KPFM measured surface potential profiles (see individual wire profiles in the Supporting Information) is in agreement with P diffusion in bulk silicon and may imply that similar diffusion mechanisms exist in nanowires.30 However more dopant distribution measurements are required in order to obtain the P diffusion mechanism in silicon nanowires. In summary, we have used KPFM to map the radial active dopants distribution within a single n-type silicon nanowire grown by the VLS method. Our results show a radial decrease in the dopant concentration from the surface toward the wire core with a change of almost 2 orders of magnitude even when there is no indication of taper. Furthermore, we estimate the diffusion coefficient of the dopant P atoms in the silicon nanowires, D ∼ 1 × 10-19 m2 s-1. The latter does confirm the diffusion of P from the vapor phase into the silicon nanowire during its growth, in the VLS method. Finally, our simulated nonuniform current density through the nanowire demonstrates the implications of such non homogeneous dopant distribution on the performance of nanowire based electrical devices.
Science Foundation [BSF]. We greatly acknowledge the nanowires growth and discussions with the group of L. L. Lauhon, Northwestern University. Supporting Information Available. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3)
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Acknowledgment. This research was supported by Grant No. 2008140 from the United States - Israel Binational © 2010 American Chemical Society
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