NANO LETTERS
Doping Limits of Grown in situ Doped Silicon Nanowires Using Phosphine
2009 Vol. 9, No. 1 173-177
Heinz Schmid,* Mikael T. Bjo¨rk, Joachim Knoch, Siegfried Karg, Heike Riel, and Walter Riess IBM Research GmbH, Zurich Research Laboratory, Sa¨umerstrasse 4, 8803 Ru¨schlikon, Switzerland Received September 10, 2008; Revised Manuscript Received November 19, 2008
ABSTRACT Structural characterization and electrical measurements of silicon nanowires (SiNWs) synthesized by Au catalyzed vapor-liquid-solid growth using silane and axially doped in situ with phosphine are reported. We demonstrate that highly n-doped SiNWs can be grown without structural defects and high selectivity and find that addition of the dopant reduces the growth rate by less than 8% irrespective of the radius. This indicates that also the dopant incorporation is radius-independent. On the basis of electrical measurements on individual wires, contact resistivities as low as 1.2 × 10-7 Ω cm-2 were extracted. Resistivity measurements reveal a reproducible donor incorporation of up to 1.5 × 1020 cm-3 using a gas phase ratios of Si/P ) 1.5 × 10-2. Higher dopant gas concentrations did not lead to an increase of the doping concentration beyond 1.5 × 1020 cm-3.
Doping of semiconductors in a well-controlled manner and ideally over a wide concentration range is a prerequisite for the fabrication of any solid-state device. For high-performance field-effect transistors, doping the source and drain contacts as high as possible is crucial1 because it will reduce the contact and series resistances. This enables a further shrinking of the device dimensions, faster switching or, alternatively, higher drive currents. In conventional planar devices, ion implantation followed by annealing is the preferred doping technique. However, in three-dimensional nanowire structures, implantation damage can lead to crystalline defects or even complete amorphization, which often can no longer be completely recovered by recrystallization anneals.2 Other work3 reported that implantation-induced defects in silicon nanowires (SiNWs) can to a large extent be recovered after an 800 °C anneal. Ion implantation was employed by ref 4 for the fabrication of a SiNW-based field effect transistor. In situ doping of SiNWs could circumvent defects from implantation altogether and facilitate device fabrication.Previousworkoninsitudopingofvapor-liquid-solid (VLS)-grown SiNWs around 500 °C showed that phosphorus from phosphine (PH3) can be used as an n-type dopant and that the incorporation can be controlled by the ratio of dopant to silicon precursor gas flow.5,6 Moreover, the effects of dopants on the epitaxial growth of SiNWs were reported.7 Apart from these initial results, important properties of VLSdoped SiNWs, for example, the maximum attainable doping concentration, the abruptness of the doping profile within a SiNW, and whether the doping concentration is diameter* To whom correspondence should be addressed. E-mail:
[email protected]. 10.1021/nl802739v CCC: $40.75 Published on Web 12/17/2008
2009 American Chemical Society
independent, are still unknown. Here we report on the structural and electrical characterization of n-type doped SiNWs and axially doped segments in otherwise nominally undoped SiNWs. The SiNWs were synthesized at a temperature of 440 °C and a total pressure of 25 or 50 Torr in a cold-wall chemical vapor deposition (CVD) reactor. Silane (SiH4) was diluted by coflowing He and Ar and was adjusted to a partial pressure between 100 to 480 mTorr. Phosphine (0.3% in He) was added as doping gas. Gold nanoparticles of 20, 40, and 60 nm diameter were used as catalytic seeds and were deposited from a colloidal solution (BBI International) on a hydrogen-terminated Si (111) substrate by adding buffered hydrofluoric acid (BHF) for 30 s. Typically, a short intrinsic segment (∼100 nm long) was grown initially, followed by the doped segment. To investigate the incorporation of the dopant into the nanowires, a series of n-type SiNWs were grown using PH3/SiH4 gas-phase ratios ranging from 3.8 × 10-2 down to 1.2 × 10-4. The growth time was adjusted from 45 to 90 min to yield SiNWs having a length of 4-6 µm. For structural characterization, both scanning electron microscopy (SEM) and high-resolution scanning transmission electron microscopy (TEM/STEM) were carried out. For electrical characterization, devices were fabricated by mechanically removing the wires from the growth substrate and transferring them onto a highly doped Si wafer with a 100 nm thick top insulating SiO2 layer. Electron-beam lithography, metal evaporation, and lift-off were used to electrically connect four individual 400 nm wide leads to each SiNW. The electrodes were made from thermally evaporated layers
Figure 1. TEM images of phosphorus-doped SiNW grown at 440 °C with PH3/SiH4 ) 1.2 × 10-2, corresponding to ND ) 9 × 1019 cm-3. (a) Highly doped SiNW can be grown with high selectivity. The arrows point to detected Au particles deposited along the wire. (b) The SiNW has smooth sidewalls that are free from poly silicon deposits. (c) The high-resolution image reveals the single-crystalline structure of the SiNW.
of 10 nm Ni/70 nm Au or 10 nm Ti/70 nm Au deposited on the SiNWs after having removed the native oxide in buffered hydrofluoric acid. Ni/Au and Ti/Au contacts were found to be equally good. No postannealing treatments were made to prevent any changes of nanowire or contact properties. The structural properties of highly doped SiNWs were investigated using TEM. Similar results of n-doped SiNWs grown at 500 °C were reported6 although for much lower doping concentrations of about 5 × 1018 cm-3. Figure 1 shows TEM images of a SiNW synthesized using a PH3/ SiH4 ratio of 1.2 × 10-2, corresponding to a donor concentration ND value of 9 × 1019 cm-3, as will be shown later. SiNWs grown at 440 °C are single-crystalline even at high doping concentrations. Most importantly, the highly doped SiNWs are free from polysilicon deposition (no taper) as is demonstrated in the lower right inset of Figure 1. The presence of a doped polysilicon shell around the SiNWs would severely obstruct electrical characterization of the nanowire core because of the high dopant incorporation. For comparison, the phosphorus concentration of in situ doped polysilicon layers deposited by conventional CVD using a gas-phase ratio of PH3/SiH4 ∼1 × 10-2 reaches values of up to ∼1 × 1021 cm-3.8-10 TEM and high-angle annular darkfield scanning TEM (HAADF-STEM) measurements of the tip region [Supporting Information, Figure S1] revealed the presence of nanometer-sized Au droplets and silicon deposits that are not detected in control samples of nominally undoped (further referred to as intrinsic) SiNWs. Furthermore, SEM investigations of intrinsic SiNWs that contain axial segments with high doping concentration show enhanced contrast on the doped segments as shown in Figure 2. A close inspection of the SEM images of 55 nm radii wires (Figure 2, bottom panel) reveals a speckled contrast in brighter areas that mostly occurs on the doped sections. In addition, the extension of these areas is sometimes bound by specific surface facets of 174
Figure 2. SEM images of SiNWs containing doped segments. The wires were grown at 440 °C at a silane partial pressure of 200 mTorr. The segments were doped by coflowing 1 part PH3 in 66 parts of silane for 60 s. The radii are (top to bottom) 11, 16, 55, and 55 nm. The scale bar is 100 nm. Areas of extended bright contrast correspond to the doped segments, as is indicated in the schematic at the very bottom. The locations of the doped segments are additionally indicated by bars.
the SiNWs. On the basis of the TEM and SEM observations, we assign the origin of the contrast to Au traces left on the surface of the nanowires that diffused from the catalyst predominantly during growth of the highly doped segments. To confirm this, we performed chemical etching of the SiNWs [see Supporting Information, Figure S2] that revealed the selective dissolution of silicon below areas of high contrast indicating the presence of metal (Au) particles. The contrast difference obtained in the SEM images provides a convenient method to measure the length of each segment and thus to determine the growth rate of both the intrinsic (νi) and doped (νn) sections. In Figure 3a, the growth rates of the intrinsic segments are plotted versus the radius of the SiNWs. A diameter-dependent reduction in growth rate is observed that is attributed to the Gibbs-Thomson effect.11 Similar results were obtained for the growth rates of the highly doped (PH3/SiH4 ∼1.5 × 10-2) sections (Figure 3b). A comparison of the two data sets is shown in Figure 3c and reveals that the growth rates of the doped segments are lower than that of the intrinsic ones by less than 8% within the experimental error, independent of radius. This means that even with high concentrations of phosphine growth is still incorporation limited (Gibbs Thompson effect) with a slight radius independent reduction in growth rate. The growth rate does not affect the incorporation of dopants as is confirmed with electrical data from wires grown with the same PH3/SiH4 ratio but at different partial pressure (proportional to growth rate) resulting in identical resistivity. These observations are a strong indication that the doping incorporation is radius-independent in the size range explored Nano Lett., Vol. 9, No. 1, 2009
Figure 4. SEM images of SiNWs with five axially doped segments of various length and dopant concentrations. (a) The length of the doped segments are (from left to right) 30, 65, 110, 65, and 110 nm. (b) SEM contrast of a 20 nm radius SiNW with segments of variable dopant concentration. The PH3/SiH4 ratios during segment growth were (left to right) 3.7 × 10-2, 1.5 × 10-2, 7.5 × 10-3, 3.7 × 10-3, and 1.5 × 10-2. The locations of the doped segments are additionally indicated by bars.
Figure 3. Growth rate as function of doping and radius. (a) Growth rate of nominally undoped (νi) and (b) highly doped (νn) segments in SiNWs versus radius. (c) Radial dependence of the normalized growth rates of undoped and doped segments, showing growth rates that differ by less that 8% independently of the radius.
here. The decrease in growth rate for doped segments is also observed in conventional CVD deposition of silicon using high phosphorus concentrations.8,12,13 In contrast to our observation, the reduction in growth rate is much more severe in conventional CVD and is attributed to the high affinity of n-type dopants to the silicon surface, leading to a blocking of free surface sites for the adsorption of silane molecules. Another distinct effect related to high phosphine concentrations during VLS growth is the reduction in nucleation yield of SiNWs at PH3/SiH4 ratios above ∼2 × 10-3, which also affects the epitaxial growth on the substrate. A further increase of the phosphine concentration resulted in complete inhibition of SiNW nucleation and growth. A similar observation was made using arsine (AsH3)7 as n-type doping gas, and suggests a dopant-induced lowering of the surface tension of the Au melt. Importantly, the poisoning effects of phosphorus on SiNW nucleation can be circumvented by first growing a short segment with PH3/SiH4 < 2 × 10-3 and subsequently increasing the PH3 flow to higher values. Understanding the influence of phosphine on SiNW growth is clearly more complex than in CVD or MBE thin-film growth because of the additional presence of the eutectic Au-Si-P melt and the additional vapor-liquid and liquid-solid interfaces that need further investigations. Obviously, the axial length of the segments can be controlled by timing the dopant flow into the reaction chamber. An example is shown in Figure 4a where PH3 was introduced for 60, 30, 60, 30, and 15 s during the VLS growth, resulting in detectable segments as short as 30 nm. Equally, the doping level can be varied by changing the PH3/SiH4 ratio as shown Nano Lett., Vol. 9, No. 1, 2009
in Figure 4b. Here it can be seen that the contrast difference between doped and undoped segments depends on the doping level and starts to disappear at doping levels below 1 × 1019 cm-3. This contrast difference does not necessarily reflect the actual dopant concentration in the SiNW. Electrical measurements are therefore required to verify the active dopant concentration. The active dopant incorporation during growth was extracted by measuring the resistivity, Fwire, using either a four-probe technique or by measuring the two-probe total resistance, RT, as a function of the contact spacing, l, on individual wires. The two methods yield the same extracted Fwire, but because of ease of measuring RT in our setup, we preferentially used the two-probe technique. The total resistance of a wire can then be described by the following equation: RT ) 2RC +
Fwire πr2
l
(1)
where r and l are the wire radius and length, respectively, and RC is the contact resistance given by the expression RC )
FwireLT 2
πr
[ ]
coth
L LT
(2)
using a transmission line model.14 Here, L is the contact width, and LT is the transfer length. If it is furthermore assumed that the metal contacts wrap around 75% of the wire circumference, the transfer length is LT )
2rFc 3Fwire
(3)
In eq 3, Fc is the specific contact resistivity. In our case LT ranged from 150 to 250 nm, depending on doping concentration. Note that this model is only valid as long as the contact resistance is larger than the resistance of the wire segment under the contact. All data presented in this study showed linear I-V characteristics. Devices displaying the slightest deviation from Ohmic behavior were discarded from further analysis to prevent erroneous extraction of Fwire. Typically, for each doping concentration more than six devices were measured. 175
Figure 6. Surface depletion for doped SiNWs with an interface state density of 2 × 1012 eV-1 cm-2. The inset shows a schematic cross section of a SiNW with an undepleted region determined by the electronic radius, relec, and a depleted area represented by the cross-hatched region. The electronic radius is plotted versus ND for seven different physical radii ranging from 10 to 70 nm. Figure 5. Electrical characterization of in situ doped SiNWs. (a) SEM image of a SiNW contacted with four Ni/Au contacts having a width L ) 400 nm. (b) Total resistance versus contact spacing for a SiNW of 30 nm radius grown at a PH3/SiH4 ratio of 1.2 × 10-2. A resistivity of 0.7 mΩcm is extracted from the plot, corresponding to a donor concentration of 9 × 1019 cm-3.
Figure 5a shows an SEM image of the electrical connections made to an n-type nanowire grown with a PH3/SiH4 ratio of 1.2 × 10-2. The corresponding electrical results are displayed in Figure 5b and show RT as a function of l. Using eq 1, we extract a resistivity of 0.7 mΩcm. Since the electron mobility is independent of the crystalline direction at the dimensions used here, we can use the bulk mobility value to obtain an electrically active donor density of 9 × 1019 cm-3. The lowest-doped SiNWs that still exhibited Ohmic behavior were obtained using a PH3:SiH4 ratio of 6 × 10-4, which corresponds to a donor concentration of 1 × 1018 cm-3. Lower doping concentrations resulted in non-Ohmic contacts that prevented the determination of Fwire, thereby also setting a lower bound on the required contact doping concentration. To assess the validity of the measurements at the lowest doping concentration used, the influence of surface charges that may deplete the charge carriers within the nanowire has to be taken into account. The depletion width was calculated following ref 15. For this calculation, an interface state density, Dit, of 2 × 1012 eV-1 cm-2 was used.16 In Figure 6, the electronic radius (relec), which corresponds to the undepleted cross section of the wire, is plotted as a function of the doping density for wires with physical radius (r) ranging from 10 to 70 nm. Here it can be seen that for the nominal 30 nm radii wires used in this work a minimum chargecarrier concentration of ∼2 × 1018 cm-3 is required for an accurate determination of the resistivity. Therefore, surfacedepletion effects can be comfortably neglected for all but the lowest doping level used. A reduction of the interface charges can be achieved by replacing the native oxide coating of the SiNWs with a high-quality thermal oxide, but the effect of dopant diffusion during the thermal oxidation must be taken into account, especially for structures containing doping profiles. As the dimensions of high-performance transistors are decreased, parasitic series resistances start to limit device 176
performance. Of particular importance is the source and drain contact resistance. Ultralow contact resistivities below 10-8 Ωcm2 have recently been demonstrated17 using silicides, and to SiNWs contact resistivities as low as 2 × 10-7 Ωcm2 were achieved by using extended epitaxially grown source and drain contacts18 and implantation. In our case, the specific contact resistance of the device in Figure 5b was extracted by first calculating the transfer length of the electrical contact using eq 2 iteratively, with 2RC given by the value of RT at l ) 0. Once LT had been obtained, the specific contact resistivity was calculated using eq 3, yielding Fc ) 1.2 × 10-7 Ωcm2. This is the lowest contact resistivity to a nanowire reported so far. Finally, the resistivity of the SiNWs is plotted versus PH3/ SiH4 gas phase ratio in Figure 7a. The plot shows a linear relationship of resistivity with doping ratio for low-doped samples, whereas above 6 × 10-4 Ωcm the resistivity saturates and a further decrease of the SiNW resistance was not observed, even after a 3-fold increase of the dopant concentration up to PH3/SiH4 ) 0.038 during growth. Moreover, an activation anneal at 1000 °C for 10 s of the SiNWs did not lead to a reduction of the resistivity, which indicates that all of the incorporated phosphorus donors were already electrically active. This also suggests that the total phosphorus concentration saturates beyond a PH3/SiH4 ratio of 1.5 × 10-2. To further support this finding, a compositional analysis using SIMS measurements, ideally on a single SiNW, would be required, which unfortunately is technically not feasible yet. Assuming bulk electron mobility, the resistivity values were then converted to a donor concentration19 and plotted versus the PH3/SiH4 gas phase ratio. Figure 7b shows that the donor concentration increases with decreasing resistivity and finally reaching a limit at approximately 1.5 × 1020 cm-3. Interestingly, the ND of 1.5 × 1020 cm-3 obtained from SiNWs grown at 440 °C is close to the reported phosphorus equilibrium solid solubility (∼2 × 1020 cm-3 at 700 °C).20,21 This indicates that for in situ doping in VLS growth the equilibrium solid solubility condition might apply. In popular growth techniques, like CVD and MBE, high doping concentrations are achieved by operating far away from equilibrium, exploiting kinetically limited dopant solubility. An increase of the doping conNano Lett., Vol. 9, No. 1, 2009
concentrations, a saturation of ND at 1.5 × 1020 cm-3 is observed, which is attributed to the equilibrium solubility limit of P in Si at the growth temperature. Whether this observation is general and also holds for p-type doping remains to be verified. Annealing of doped SiNWs did not lead to a reduced resistivity, indicating that for in situ doping most donors are electrically active. From electrical measurements on highly doped SiNWs using Ni-Au contacts a specific contact resistance of 1.2·10-7 Ωcm is extracted which is the lowest value reported so far. Overall, the structural and electrical information presented here will assist the controlled fabrication of novel devices based on SiNWs. An important parameter that remains to be quantified is the abruptness of the doping profiles in in situ doped nanowires.
Figure 7. Resistivity and donor concentration in SiNWs grown at 440 °C using silane and phosphine. (a) Measured resistivity of n-doped SiNWs versus PH3/SiH4 gas-phase ratio. Saturation is observed above a PH3/SiH4 ratio of 1.5 × 10-2 corresponding to a resistivity of 6 × 10-4 Ωcm. (b) Measured resistivity of n-doped SiNWs versus donor concentration assuming bulk mobility. Saturation (dashed vertical line) is observed above a PH3/SiH4 ratio of 1.5 × 10-2 corresponding to a donor concentration of 1.5 × 1020 cm-3.
centration for in situ doped VLS-grown SiNWs might nevertheless be possible using conditions different than used here. We reported results on the structural and electrical characterization of in situ phosphorus-doped SiNWs. Highly doped n-type SiNWs were grown with high selectivity several micrometers in length as well as sub 50 nm segments sandwiched within intrinsic (undoped) SiNW segments. Structural analysis revealed a single crystalline core and a surface free from polysilicon deposits up to the highest dopant concentration of PH3/SiH4 ) 3.8 × 10-2 used. Compared to intrinsic SiNWs, doping with phosphine can lead to a deposition of an Au-rich surface layer which varies with the doping concentration used. Importantly, the growth rate of doped SiNWs decrease only by 8% as compared to intrinsic wires. In addition, the change in growth rates is found to be independent of wire radius in the range of 7 to 55 nm. Together, this shows that the presence of phosphorus is not significantly affecting the growth kinetics and suggests that in situ doping of SiNWs is radius-independent. Electrical measurements on individual SiNWs that were grown with varying doping gas concentrations show that phosphorus is incorporated in the SiNW. The validity of the electrical measurements was assessed by calculating surface depletion effects which are found to limit the resistivity determination at the lowest doping concentrations used. The resistivity data of the doped SiNWs show a linear incorporation of donors with increasing doping gas concentration. At the highest PH3
Nano Lett., Vol. 9, No. 1, 2009
Acknowledgment. The authors thank F. Gramm, ETH Zurich, for the TEM images and M. Lörtscher, D. Webb, and M. Tschudy for their support. This work was partially supported by the EU program NODE 015783. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References (1) ITRS, International Technology Roadmap for Semiconductors 2007, www.itrs.net. (2) Duffy, R.; Van Dal, M. J. H.; Pawlak, B. J.; Kaiser, M.; Weemaes, R. G. R.; Degroote, B.; Kunnen, E.; Altamirano, E. Appl. Phys. Lett. 2007, 90, 241912. (3) Colli, A.; Fasoli, A.; Ronning, C.; Pisana, S.; Piscanec, S.; Ferrari, C. Nano Lett. 2008, 8, 2188. (4) Hayden, O.; Bjo¨rk, M. T.; Schmid, H.; Riel, H; Drechsler, U.; Karg, S. F.; Lo¨rtscher, E.; Riess, W. Small 2007, 2, 230. (5) Zheng, G.; Lu, W.; Jin, S.; Lieber, C. M. AdV. Mater. 2004, 16, 1890. (6) Wang, Y.; Lew, K.; Ho, T.; Pan, L.; Novak, S. W.; Dickey, E. C.; Redwing, J. M.; Mayer, T. S. Nano Lett. 2005, 5, 2139. (7) Schmid, H.; Bjo¨rk, M. T.; Knoch, J.; Riel, H.; Rice, P.; Topuria, T.; Riess, W. J. Appl. Phys. 2008, 103, 024304. (8) Learn, J.; Foster, D. W. J. Appl. Phys. 1987, 61, 1898. (9) Sarret, M.; Liba, A.; Bonnaud, O. Appl. Phys. Lett. 1991, 59, 1438. (10) Kurokawa, H. J. Electrochem. Soc. 1982, 129, 2620. (11) Givargizov, E. I. Highly Anisotropic Crystals: Kluwer Academic Press: Norwell, MA, 1987. (12) Meyerson, B.; Yu, M. J. Electrochem. Soc. 1984, 131, 2366. (13) Ahmed, W.; Ahmed, E.; Hitchman, M. L. J. Mater. Sci. 1995, 30, 4115. (14) Mohney, S. E.; Wang, Y.; Cabassi, M. A.; Lew, K.; Dey, S.; Redwing, J. M.; Mayer, T. S. Solid-State Electron. 2005, 49, 227. (15) Schmidt, V.; Senz, S.; Go¨sele, U. Appl. Phys. A 2006, 86, 187. (16) Seo, K.; Sharma, S.; Yasseri, A. A.; Stewart, D. R.; Kamins, T. I. Electrochem. and Solid-State Lett. 2006, 9, G69. (17) Kazuya, O.; Lavoie, Ch.; Murray, C. E.; D’Emic, Ch. P.; Lauer, I.; Chu, J. O; Bin Y.; Besser, P.; Gignac, L. M.; Bruley, J.; Singco, G. U.; Pagette, F.; Topol, A. W.; Rooks, M. J.; Bucchignano, J. J.; Narayanan, V.; Khare, M.; Mariko, T.; Kazunari, I.; Park, D.-G.; Shahidi, G.; Solomon, P. M. 8th Intl. Workshop on Junction Technology, Shanghai, May 15–16, 2008; Extended Abstracts, p 150. (18) Cohen, G. M.; Solomon, Laux, S. E.; Chu, J. O.; P. M.; Rooks, M. J.; Haensch, W. DeVice Research Conference, South Bend, 2007; 65th DRC, Conference Digest; 2007, p 175. (19) Sze, S. M. Physics of Semiconductor DeVices, 2nd edition; Wiley: New York, 1981. (20) Schwettmann, F. N.; Kendall, D. H. Appl. Phys. Lett. 1972, 21, 2. (21) Equilibrium solubility concentrations below 700 °C are not available for direct comparison
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