Controlled in Situ n-Doping of Silicon Nanowires ... - ACS Publications

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J. Phys. Chem. C 2010, 114, 760–765

Controlled in Situ n-Doping of Silicon Nanowires during VLS Growth and Their Characterization by Scanning Spreading Resistance Microscopy Caroline Celle, Ce´line Mouchet, Emmanuelle Rouvie`re, and Jean-Pierre Simonato* CEA, LITEN, DTNM/LCRE, 17, rue des Martyrs, 38054 Grenoble Cedex 9, France

Denis Mariolle and Nicolas Chevalier CEA, LETI, MINATEC 17, rue des Martyrs, 38054 Grenoble Cedex 9, France

Arnaud Brioude Laboratoire des Multimate´riaux et Interfaces, UMR 5615, CNRS-UniVersite´ Lyon 1 43 bd du 11 NoVembre 1918, F-69622 Villeurbanne, France ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: NoVember 27, 2009

Silicon nanowires fabricated by the VLS method were n-doped with phosphine during their growth. The doping level was controlled by tuning the P/Si ratio in the CVD reactor during the synthesis. We observed that the growth rate of silicon nanowires was dopant dependent and that even the doping level of the substrate could play a poisoning role. The nanowires were characterized before and after thermal activation by scanning spreading resistance microscopy (SSRM) in a vertical structure directly on their growth substrate, showing that thermal activation led to a significant decrease of the resistance. SSRM is a simple AFM-mode-based technique that allows us to characterize in a short time a large number of individual nanowires and to correlate their dimensions to their electrical properties. Complementary techniques including scanning electron microscopy (SEM) and Raman spectroscopy were also used for the characterization of the n-doped silicon nanowires. Introduction Research into 1D nanomaterials has been rapidly growing in the past few years due to their outstanding properties. Semiconducting nanowires are promising building blocks for the development of nanotechnology-based applications like electronics,1-4 biological diagnostics,5-7 thermoelectrics,8-11 photovoltaics,12-14 and so forth. More specifically, silicon nanowires (Si NWs) are among the most studied semiconducting nanowires, probably due to their relative ease of fabrication, their ability to be equally n- or p-doped, their straightforward chemical functionalization, and the easiness to integrate them with conventional bulk-silicon technology.15-19 Despite recent reports, a complete study of the doping is still needed to achieve good control of the synthesis of doped silicon nanowires and to determine the direct relationship between synthesis parameters and resulting electrical properties.20-27 In particular, an interesting recent work reported by Schmid et al. drew our attention and will be discussed in part in this article.28 It is mandatory to control the overall process chain from Si NWs synthesis to electrical properties of Si NWs-based electronic devices to enable device fabrication with predictable and reproducible characteristics. In this article, we report growth of n-doped Si nanowires, with controlled doping level, via the VLS (vapor-liquid-solid) method. The synthesis parameters were chosen to cover a wide range of doping, leading to Si NWs that present a large amplitude of resistive properties. An analysis of the growth rate as a function of the doping level was carried out. The Si NWs * To whom correspondence [email protected].

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were characterized by scanning electron microscopy (SEM) and by Raman spectroscopy. Electrical resistances of the nanoobjects were studied by scanning spreading resistance microscopy (SSRM), before and after thermal dopant activation, and for several doping levels. Synthesis of n-Doped Silicon Nanowires and Dopant Concentration Effect on the Growth Rate Thin gold films of 2 nm were deposited on (111) oriented Si substrates in a PVD reactor. Dewetting was then performed at 550 °C for 10 min under dihydrogen atmosphere, leading to catalytic gold nanoparticles. Catalytic growth of Si NWs was performed in a 200 mm CVD Centura reactor (Applied Materials), using silane (SiH4) diluted in hydrogen. The growth temperature was set at 500 °C, the silane gas flow was maintained at 150 sccm, and the pressure was fixed at 10 Torr (1.33 × 103 Pa). A flow of H2 was kept constant during the experiment at 10 slm.29,30 The dopant addition during the synthesis was realized by adjusting the flow rate of phosphine (1% PH3 in hydrogen), with a P/Si ratio ranging from 0 to 2 × 10-2. The effect of phosphine addition on the growth rate of silicon films by CVD has been reported by several groups.31-33 It was observed that the deposition rate was significantly lowered (up to 42%) by the addition of phosphine, which was mainly attributed to the tendency of PH3 to block the surface sites and inhibit silicon deposition. In a recent report, Schmid et al. observed no changes of the growth rate when using PH3 for doping Si NWs.34 We realized several syntheses of silicon nanowires under identical conditions, using undoped silicon wafers as growth substrates, except for the dopant content in

10.1021/jp9094326  2010 American Chemical Society Published on Web 12/23/2009

Controlled in Situ n-Doping of Silicon Nanowires

J. Phys. Chem. C, Vol. 114, No. 2, 2010 761 (0.01 Ω.cm, 5.0 × 1019 at.cm-3). As shown in Figure S2 of the Supporting Information, the growth rates are consistently lowered, whereas doped wafers are used in comparison with undoped substrates. The substantial loss in activity differs between the species initially present in the silicon wafer and their concentrations, probably because each substance interacts differently with the catalyst. For instance, it was recently shown that antimony addition has a dramatic effect on VLS growth of Si NWs, which was attributed to Sb segregation in the gold tip.36 This deactivation effect can tentatively be ascribed to the diffusion of dopant species in the catalyst nanoparticle during the incubation time at the very beginning of the VLS mechanism, when the Au-Si alloy is formed. Further experiments are ongoing to clear this issue, a study focused on this specific topic will be published elsewhere.

Figure 1. Growth rate of silicon nanowires versus phosphine concentration (P/Si ratio) in the CVD reactor.

the gas phase, which was set first to zero and gradually raised up to a P/Si ratio of 2 × 10-2. The change in growth rate as a function of the doping level is presented in Figure 1. Without any dopant, we measured a growth rate of 95 nm.min-1. The first doping level reachable in our CVD reactor was rather low, the P/Si ratio being ca. 4 × 10-6. Even at such a low content of phosphine, a significant 16% decrease of the growth rate was observed. When the doping level was raised, a decrease of Vg following a logarithmic trend was observed, with a minimum value of 65 nm.min-1 when P/Si equals 2 × 10-2. Therefore, more than a 30% decrease of the growth rate was measured between undoped and highly doped Si NWs. The phosphorus atoms are actually inserted in the nanowires (vide infra), which means that the catalytic process in the presence of phosphine does not prevent dopant addition, it simply leads to an overall decrease of the kinetics. The addition of phosphine has a poisoning effect because it lowers the catalytic activity of the gold-based catalyst. Interestingly, we did not observe any morphological difference of the nanowires with or without dopant besides a slight decrease of the Si NWs density (Figure 2 and Figure S1 of the Supporting Information). The nanowires were straight and the diameters ranged approximately from 40 to 100 nm. No significant shift in diameter was observed between the two ends of the Si NWs, indicating that uncatalyzed deposition of atoms along the nanowires does not occur, and thus the phosphorus atoms are fully incorporated via the goldbased catalyst at the tip of the nanowires. The poisoning effect can hardly be ascribed to those of the epitaxy CVD technique.31 Indeed, the latter was processed without any catalyst, whereas decomposition of gases happens at the metal catalyst in our Si NWs growth experiments. The bond dissociation energy of P-H (from PH3) is slightly lower than the Si-H bond (from SiH4), with respective values of 351.0 and 383.7 kJ.mol-1,35 hence it is possible that phosphine molecules are more easily dissociated by gold. The preferential dissociation of phosphine molecules could change the droplet composition and thus modify the overall activity of the catalytic system, leading to a variable growth rate. Further experiments were carried out using six silicon wafers variously doped. The reference was the undoped one (14-22 Ω.cm, 6-8 × 1014 at.cm-3), two were p-doped with boron at different doping levels (0.017-0.018 and 0.002 et 0.005 Ω.cm, i.e. respectively ∼5 × 1018 and 2.5-6.0 × 1019 at.cm-3) and three were n-doped, either highly doped with phosphorus or arsenic atoms (0.002-0.005 Ω.cm, 1.5-4.0 × 1019 at.cm-3) or moderately (“n+ type”) doped with arsenic

Electrical Characterization by Scanning Spreading Resistance Microscopy (SSRM) 1. Dopant Activation by Thermal Annealing. A vertical structure was chosen for electrical measurement of Si NWs. After synthesis, the Si NWs were dipped in a 0.6% BOE (buffered oxide etch) solution for 60 s and rinsed with deionized water to remove native silicon oxide. The gold catalyst, as well as gold traces along the walls of the Si NWs, were etched by placing the nanowires in a IKI solution (VWR ProlaboTM, mixture of 6.34 g of I2 and 75.00 g of KI in 1000 mL H2O) for 10 min. The nanowires, still on the growth substrate, were rinsed with deionized water and dried under argon. When the dopant activation step was carried out, the nanowires were placed in a RTA (rapid thermal annealing) system under vacuum, and were heated for 5 min at 750 °C, the highest temperature achievable with our apparatus. The Si NWs were then encapsulated using a combination of methyl siloxane-based SOG (spin-on glass) process and CMP (chemical mechanical polishing). First, a solution of SOG material (Honeywell Accuglass 512 B) was spin-coated on the substrate, and annealed for 2 min at 150 °C, then 1 min at 250 °C and finally 60 min at 400 °C. The Si NWs protruding from the SOG surface were polished away using Supreme Rodel finishing pads and SiO2 nanoparticles (Grace Davison Ludox-PW30 aqueous solution of colloidal silica) for 1 min at 50 rpm. The final length of Si nanowires was adjusted to 1 µm for all samples. A final etch was realized for 20 s with BOE 0.6% to ensure a good digging out of the nanowires. The resistivity of individual Si NWs was evaluated by SSRM using a Dimension 3100 microscope from Veeco instruments. SSRM is an AFM-mode mapping simultaneously the topography and the local resistivity of the sample. This technique was widely used on silicon for the dopant profiling,37 and recently used for the study of the silicidation in Si NW.47 If we consider a constriction of given radius a, separating two semi-infinite bodies of resistivity F, the spreading resistance is dominated by a diffuse scattering mechanism, and is given by the Maxwell formula:38

Rmaxwell )

F 2a

(1)

This formula is currently used in the literature to link the measured resistance by SSRM to the resistivity of the sample. In fact, this formula is only valid if the electron mean free path λ is much smaller than the radius a. If it is not the case, that is if a , λ, then the resistance is dominated by the Sharvin

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Figure 2. SEM images showing Si nanowires grown at 500 °C, 10 Torr (1.33 × 103 Pa) for 6600 s (a) without PH3 and (b) with PH3, P/Si ) 2 × 10-2.

Figure 3. Illustration of the SSRM principle for the electrical measurement of the Si NWs.

mechanism,39 in which electrons are projected ballistically through the contact without being scattered:

RSharvin )

4Fλ 3πa2

(2)

The transition between the Maxwell Regime and the Sharvin was treated by Wexler who proposed a solution in the following form:40

Rwexler )

F 4F Γ(Kn) + K 2a 3πa n

(3)

where Kn ) λ/a and Γ(Kn) is a function that takes the value 1 at Kn ) 0, and decreases slowly to 9π2/128 as Kn increases to large values. In this study, a conductive diamond coated tip (Nanoworld, reference CDT-FMR, force constant in the range 1-5 N/m), was scanned in a contact mode across the sample, whereas a dc bias Vdc was applied between the tip and the back side of the sample (Figure 3). In all of the results presented below, the applied dc bias was 1 V and the interaction forces between the tip and the sample were on the order of few micronewtons. A logarithmic current amplifier covering a range of 10 pA to 0.1 mA measured the current flowing through the sample. The resistance R is calculated from Ohm’s law. Measurements were performed at room temperature under ambient atmosphere. Figure 4 shows a simultaneous topographic and electrical image of a 5 × 5 µm2 scanned area obtained on an encapsulated n-doped Si NWs sample. A lot of Si NWs are clearly visible, extending 1 to 10 nm from the surface after the encapsulation process and CMP (the roughness is around 1 nm). The protuberances observed on the topographic image are also visible on the SSRM image, confirming that practically all of the Si NWs extending from the surface are conductive.

The SSRM technique appears to be a relatively fast method well suited to evaluate the doping level in individual Si NWs. It is worth noting that the goal of this assessment is not to obtain an exact quantitative value but to show that the SSRM technique provide a good estimate for the controlled n-doping measurement for various P/Si ratio. For each experiment, resistance characterizations were carried out on approximately 30 Si NWs having measured diameters of less than 200 nm. We took into account the different contributions of resistance: the probe resistance (about 4.2 kΩ for diamond coated Si tip), the intrinsic Si NW resistance by a classical approach of transport (R ) Fl/ πr2) with l the length and r the radius of the Si NW, the tip-Si NW as well as the Si NW-substrate contact resistance. The resistivity of the Si(111) substrate is 5.10-5 Ω.cm (p-doped at 1019 at.cm-3). The Wexler resistance equation (eq 3) is used for the two contact resistances where the electron mean free path is estimated from bulk values. As a first example, Figure 5 represents the decrease of the resistance of Si NWs after the thermal dopant activation. Thermal dopant treatment has clearly a marked effect on the electrical properties of the nanoobjects since it induces a significant decrease of the resistance values. The average measured resistance (0.2 MΩ for a machine ratio of P/Si ) 2 × 10-5) was 80% lower after thermal activation. This result is in contradiction with a recent study,28 but in our case the activation anneal was performed at lower temperature (750 °C vs 1000 °C) and for several minutes, whereas a short 10 s process was used by Schmid et al. The effect of thermal annealing indicates that all of the incorporated phosphorus donors were not already active after synthesis. The activation anneal was necessary to activate them for bulk silicon technology. It is worth noting that passivation of some defects at the Si/SiOx interface41 cannot be ruled out and might contribute to the observed lowering of resistivities like for boron-doped Si nanowires.42 Following this experimental result, further studies presented thereafter were all realized with the thermal activation step. 2. Characterization of Si Nanowires n-Doped at Different Levels. Silicon nanowires, prepared without dopant and at two doping levels (P/Si ) 2 × 10-5 and 2 × 10-3), were characterized by SSRM. The resistances of highly doped Si NWs (P/Si ratio of 2 × 10-2) could not be measured by this technique because the values were beyond the accessible range of our SSRM equipment. The results are summarized in a histogram (Figure 6). Very high resistances were measured for undoped Si NWs, in the range of few GΩ. A tiny addition of phosphine during in situ doping of nanowires (P/Si ) 2 × 10-5) decreases drastically the resistance by 4 orders of magnitude. At high doping level (P/Si ) 2 × 10-3), the resistances were reduced to few tens of kΩ. Experimental measurements for each P/Si ratio were fitted

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Figure 4. Topographic (a) and SSRM (b) images of a 5 × 5 µm2 scanned area. The color scales are respectively in nanometers and in ohms. The darkest colors in the SSRM image indicate low resistances. (c) SEM image of a Si NW extending from the surface.

Figure 5. Bar graph resistance exhibits a decrease by a factor of 5 of the mean value resistance after thermal activation of the dopants (P/Si during synthesis: 2 × 10-5).

Figure 7. Experimental data fitted by the contribution of each resistance considering two contact radius a ) 1 nm (solid lines) and a ) 10 nm (dashed lines). No dopant in blue, moderately doped in red (P/Si ) 2 × 10-5), and heavily doped in green (P/Si ) 2 × 10-3). The contribution of the intrinsic resistance of 1 µm length Si NW is also represented (dotted lines).

Figure 6. Bar graph resistance of Si nanowires measured by SSRM after thermal activation. P/Si ratios are from left to right 2 × 10-3 (green), 2 × 10-5 (red), and 0 (blue).

from the total contribution to extract the mean resistivity of the Si NWs. Calculated resistances obtained respectively for no, moderately, and heavily doped samples are presented in Figure 7 considering two tip-Si NW contact radii: a ) 1 nm (solid lines) and a ) 10 nm (dashed lines). A good agreement with the experimental data respectively for no, moderately, and heavily doped samples was found for resistivities equal to 44.4, 4.29 × 10-2, and 3.56 × 10-3 Ω.cm, corresponding respectively to a doping level (extracted from silicon bulk values) of 1 × 1014, 3 × 1017, and 2 × 1019 at.cm-3. For comparison, the contribution of the intrinsic resistance of Si NW (calculated from the relation Fl/S) for the obtained resistivities is also represented in Figure 7 (dotted lines), showing values always smaller than the total contribution. Micro-Raman scattering measurements were performed with an ARAMIS apparatus at room temperature using a 100X objective operating at 633 nm with a numerical aperture of 0.95 resulting in a spot size between 0.5 and 1 µm. To avoid temperature heating of the sample, the laser power was remained constant at only few 0.08 mW.43 In Figure 8, we have reported the experimental Raman curve (dots) obtained for a gas ratio PH3/SiH4 ) 2 × 10-3 during the

Figure 8. Experimental Raman curves (dots) obtained for undoped (blue) and doped (black, P/Si ) 2 × 10-3 during the synthesis), and the best fit (line) obtained using eq 4.

synthesis, and the best fit (line) obtained using the eq 4. Raman curve of undoped Si NWs is also presented for comparison. The experimental curve shows a first peak centered at 498 cm-1, which is currently interpreted by the strained Si44 resulting of the growth process. Indeed, structural defects are inherent to the VLS synthesis and were recently clearly identified by TEM and HRTEM.30 Contrary to previous studies, where the Raman band centered around 520 cm-1 downshifts to 511 cm-1 for high P concentrations,45 in our case this band remains constant whatever the doping level. On the other hand, this resonance seems asymmetric when the nanowire is doped with phosphine. Different mechanisms have been proposed to explain the deviation and distortion of the LO-TO phonon vibrational modes in the case of sufficiently high doping levels: thermal effect involving

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tensile stress due to the laser-induced crystallization process, large concentrations of doping (P, B) and Fano effects. This last phenomenon appears when electrons and holes participate in the light scattering process and excite the free carriers. The overlapping of energies between the electronic continuum and the phonons induced an asymmetric Fano line shape.44 In our experiment, the laser power has been reduced to the lower value available to limit the influence of the first mechanism. This is confirmed by the unchanged position of the Raman peak at 520 cm-1 in the spectra taken for both undoped and doped Si NWs (PH3/SiH4 ) 2 × 10-3). Indeed, the downshift generally observed in many experiments depends certainly more on the effect of laser heating than on the doping species present in high concentration.46 This is not the case of the profile, which seems directly related to the n-doping levels. The Raman scattering cross section can be described by:

I(w) ) C + I0

(q + ∈)2 (1 + ∈2)

(4)

where I is the Raman intensity versus the frequency, C and I0 prefactors, q the symmetry parameter, and ε is given by (w w0)/Γ, where w0 is the phonon wavenumber and Γ is the full width at the half maximum. The q parameter was determined around -8.9 for doped Si NWs, whereas it is around -33 for undoped nanowires. It is well-known that in the case of n-doped Si nanowires this parameter starts to become important for a minimum doping of few 1019 cm-3, which corresponds to the minimum interference threshold for n-type Si.45 Thus Raman spectroscopy results are in perfect agreement with those obtained by the SSRM technique, confirming that SSRM can be useful for Si NWs electrical characterization. Conclusions We have performed studies addressing the in situ n-doping of silicon nanowires during VLS growth with phosphine. We observed that PH3 has a poisoning effect on the catalytic activity of gold, leading to a significant decrease of the growth rate (30% less at high P/Si ratio). For the first time, we carried out SSRM experiments on silicon nanowires. This technique allowed us to measure a significant effect of thermal activation on silicon dopants, with 80% lowering of the resistance. An estimate of the dopant concentration in the Si NWs was also realized thanks to the SSRM technique, which was also confirmed by Raman spectroscopy. Acknowledgment. This research was supported partly by the European integrated project Hydromel NMP2-CT-2006-026622. The authors would like to acknowledge Severine Poncet for her help and to acknowledge the Cecomo (Centre Commun de Microscopie Optique) for access to the Raman spectrophotometer. Authors also thank François Bertin for fruitful discussion about SSRM technique. Supporting Information Available: SEM images of silicon nanowires and bar chart showing effect of substrate doping on Si NWs growth rate. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Huang, Y.; Duan, X. F.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Logic gates and computation from assembled nanowire building blocks. Science 2001, 294 (5545), 1313–1317.

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