pubs.acs.org/NanoLett
Obtaining Uniform Dopant Distributions in VLS-Grown Si Nanowires E. Koren,† J. K. Hyun,‡ U. Givan,§ E. R. Hemesath,‡ L. J. Lauhon,‡ and Y. Rosenwaks*,† †
Department of Physical Electronics, School of Electrical Engineering, and § School of Chemistry, Tel-Aviv University, Tel-Aviv, Israel, and ‡ Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States ABSTRACT Semiconducting nanowires grown by the vapor-liquid-solid method commonly develop nonuniform doping profiles both along the growth axis and radially due to unintentional surface doping and diffusion of the dopants from the nanowire surface to core during synthesis. We demonstrate two approaches to mitigate nonuniform doping in phosphorus-doped Si nanowires grown by the vapor-liquid-solid process. First, the growth conditions can be modified to suppress active surface doping. Second, thermal annealing following growth can be used to produce more uniform doping profiles. Kelvin probe force microscopy and scanning photocurrent microscopy were used to measure the radial and the longitudinal active dopant distribution, respectively. Doping concentration variations were reduced by 2 orders of magnitude in both annealed nanowires and those for which surface doping was suppressed. KEYWORDS Nanowire, VLS, doping, diffusion, Kelvin probe force microscopy
1 × 10-15 cm2 s-1) of phosphorus in Si nanowires. The observed high diffusivity under relatively low temperature conditions, i.e., 460 °C, was attributed to (a) generation of excess vacancies at the nanowires surface, (b) relatively short time and range of the dopants diffusion during growth (10-15 min and 30 nm, respectively) in nanowires compared to bulk, and (c) the very large surface to volume ratio present in nanowires. Recently, it was shown that large hydrogen (H2) partial pressures suppress surface growth for Ge nanowires grown using GeH417 and in situ etching by HCl can fully impede competitive radial growth in InP nanowires,18 suggesting general routes toward better control of doping. Here we describe two approaches to improve the uniformity of doping. First, we exploit the high diffusivity of dopants to reduce the radial nonuniformity by thermal annealing following nanowire growth without causing measurable diffusion of the Au catalyst, as probably would occur at higher temperature treatment.19 Second, we show that surface doping can be mitigated in situ by employing a high H2 partial pressure during growth. The combined effects of the H2 on gas phase and surface reaction kinetics result in a reduction of active surface dopant species. Silicon nanowires of n-type were synthesized via lowpressure chemical vapor deposition at 460 °C with solutiondeposited 50 and 80 nm Au catalyst particles for longitudinal and radial dopants profiling, respectively, using conditions comparable to those reported in the literature.20 The ratio of the precursor gases in the reactor during synthesis was 500:1 SiH4:PH3. For the longitudinal dopant profiling, samples were synthesized using 2 sccm of SiH4 and either a He (30 sccm) or H2 (200 sccm) coflow at total pressures of 40 and 170 Torr, respectively. For device fabrication, nanowires were suspended in solution by sonication in isopropyl alcohol and drop-cast onto degenerately n-doped silicon
S
emiconductor nanowires show tremendous promise in the areas of advanced optoelectronic, electronic,1-4 and mechanical5 devices. For such applications to become commercially viable, large-scale integration of nanowires into devices6-8 is required, and uniformity in electrical properties is of critical importance. In particular, it is desirable to maintain controlled and uniform dopant distribution in the growth process. This is a challenge to the widely utilized vapor-liquid-solid (VLS)9 growth method because several studies have shown evidence for surface doping in VLS-grown nanowires caused by vapor-solid (VS) deposition on the sidewalls during growth.10-13 VS deposition can lead to a highly nonuniform distribution of dopants along the nanowire because areas exposed to the gaseous dopant precursor for longer periods will exhibit a higher doping concentration if the dopant precursor is selectively incorporated. Electrical characterization10 and atom probe tomography14 on phosphorus-doped Ge nanowires and Raman spectroscopy on boron-doped Si nanowires15 have demonstrated the presence of surface doping due to VS deposition with a high degree of tapering along the nanowire. Even in the absence of tapering, Allen et al.11 showed through scanning photocurrent microscopy (SPCM) that surface doping can occur for phosphorus-doped Si nanowires, resulting in a nonuniform doping profile. Direct measurement of the surface potential along phosphorus doped Si nanowires through Kelvin Force Probe Microscopy also confirmed the presence of a nonuniform surface dopant concentration.12 Moreover, recently we reported16 on large radial nonuniform dopant distribution (100-fold difference between nanowire surface and its core) and enhanced diffusivity (D ≈
* Corresponding author. Received for review: 09/23/2010 Published on Web: 12/03/2010 © 2011 American Chemical Society
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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. Thermal annealing of the nanowires on the growth substrates (i.e., prior to device fabrication) was carried out at 460 °C in forming gas (90% N2 and 10% H2) for one hour using a commercial AS-MICRO system of ANNEALSYS Company. SPCM measurements were performed on a Witec confocal microscope equipped with a piezoelectric scanning stage. The induced photocurrent from 532 nm light modulated at 1837 Hz was measured using a current preamplifier and lock-in amplifier. The laser power was approximately 15 kW-cm-2. The spatial resolution was set by the diffraction limit at approximately ∼360 nm. The KPFM measurements were conducted using Dimension 3100 (Veeco Inc.) AFM system in a controlled nitrogen environment glovebox (less than ∼5 ppm H2O). During the surface potential measurements, all the electrical contacts where grounded (source, drain and back gate electrodes). Auger spectroscopy was used to measure the surface dopant concentration of Si nanowires deposited onto a GaAs wafer employing a PHI 700Xi Scanning Auger nanoprobe. For the radial dopants profiling, the active dopant concentration at the nanowire surface was determined to be ∼2.5 × 1020 cm-3 as reported previously.12,21 The radial dopant distribution for both as-grown and annealed nanowires was measured by etching a section of the nanowire surface (Figure 1a), followed by the measurement of the potential difference between the etched and the unetched areas using KPFM. This process was repeated several times to gradually remove material, and the surface potential difference (between the etched and unetched parts) was measured for a number of nanowire radii. The radial dopant distribution was then obtained by fitting the measured potentials with a 3D solution of the Poisson equation as described previously.21 Figure 1a shows a schematic illustration of an etched device following 4 etching steps. The nanowire selective etching was carried out by coating the sample with a thick layer of PMMA and defining windows by electron-beam lithography in the center of the contacted wire device as reported earlier.21 Figure 1b presents the simulated (solid lines) and measured (symbols) surface potential profiles for both as-grown and annealed nanowires. The plotted profiles are averages of measurements from five different wires, measured at the end that is more highly doped; the error bars represent the standard deviation in the measurement for all the averaged profiles. The simulated potential profiles in Figure 1b are produced after first calculating the dopant profile. As mentioned above, during the VLS growth dopants are incorporated into the wire both through the gold catalyst (causing a © 2011 American Chemical Society
FIGURE 1. (a) Schematic illustration of the measured device after four etching steps. (b) Simulated (solid lines) and measured (symbols) radial potential profiles for the as-grown (initial profile) and annealed nanowires. (c) Calculated radial active dopants concentration profiles (left scale) of the as-grown (initial profile) and annealed nanowires and Auger spectroscopy measurements (right scale) of the nanowire’s P surface concentration (symbol) before and after the anneal. The insets are the 2D potential (b) and concentration (c) profiles for the as-grown (right sides) and for the annealed (left sides) nanowires.
uniform dopant concentration, C (t ) 0)) and through the surface by uncatalyzed decomposition. The time-dependent diffusion equation below was solved to obtain the radial dopant profile inside the as-grown nanowires:21,22
(
C(r, t) ) k t -
)
a2 - r2 + 4DGrowth ∞
J0(rαn) 2k + C| t)0 exp(-DGrowthαn2t) 3 aDGrowth n)1 αn J1(aαn)
∑
(1)
where C is the dopant concentration, r is the radial distance from the middle of the nanowire, t is time, Rn’s are the roots 184
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of J0(aRn) ) 0, a is the nanowire radius, DGrowth is the diffusion coefficient during nanowire growth, k is the surface doping rate, and Jn is the Bessel function of the first kind of order n. The diffusion equation was solved according to the parameters used for the growth,21 for different diffusion coefficients. The resulting dopant concentration profiles were used to calculate a surface potential profile for comparison with the measured surface potential profile. The calculated profile assuming DGrowth ) 1 × 10-16 cm2 s-1 (green solid line in Figure 1c) is in good correspondence with the measured surface potential profile (empty symbols in Figure 1b). In the fitting, the variation in dopant distribution along the wire within the etch window can be neglected because 2-fold change within the 1 µm etch window is much less than the 10-fold change per ∼6 nm of radial etching. The same procedure was used to simulate the radial surface potential profile for the annealed nanowires following a calculation of the dopant diffusion. Thus, we have used the initial dopant distribution, f(r), (established by eq 1 for the as-grown nanowires) to solve the diffusion equation for impermeable cylinder22 (to simulate the annealing treatment):
C(r, t) )
2 a2
{∫
a
0
surface segregation of P atoms, which has been predicted to occur in Si nanowires23,24 and has been extensively studied in thin films.25,26 Specifically, the segregation of P to the Si-SiO interface,27 and the stability of this interfacial excess upon annealing, has been observed before in Si thin films.28 The high dopant concentration measured in the annealed nanowires is consistent with the expectation of surface segregation. The redistribution of dopants during annealing was confirmed by current-voltage measurements of two-terminal devices. To highlight the influence of the surface dopants, 20 nm was removed from the surface of both as-grown and annealed nanowires in a single step, and the characteristics were compared with unetched nanowires (Figure 2). Etched, as-grown nanowire devices showed rectifying behavior in agreement with previous work,11,13 and their conductivity was strongly influenced by a back-gate voltage. However, the conductivity of the annealed nanowires was only weakly affected by the back gate voltage both before and after the etching, as expected for higher doped nanowires. This behavior further confirms that our annealing treatment encouraged the diffusion of P atoms from the nanowires surface to core, resulting in a uniform dopant distribution with ND ≈ 1 × 1019 [cm-3]. Importantly, TEM images show no evidence of Au diffusion after the annealing (see the Supporting Information, Figure S1c,d), indicating the appropriateness of the low temperature annealing. While gentle annealing was sufficient to create a much more uniform doping profile, it is also desirable to grow nanowires with uniform doping from the outset. Figure 3 compares SPCM measurements and calculated phosphorus concentration profiles for phosphorus doped silicon nanowires grown in He (Figure 3b) and H2 atmospheres (Figure 3c). An increasingly large positive photocurrent is observed toward the nanowire tip (right contact) for a nanowire grown in a He background (Figure 3b), whereas in the case of a H2 background, the nanowire shows much more uniform photocurrent along the channel (Figure 3c).
rf(r) dr +
∞
∑ exp(-D
2
annealαn
n)1
FIGURE 2. ID-VG current-voltage measurements for as-grown and annealed nanowires before (solid lines) and after (symbols) 20 nm etch of the Si surface. The inset is a schematic illustration of the measured etched device.
t)
J0(rαn) J02(aαn)
}
∫ rf(r)J (α r) dr a
0
0
n
(2)
Here, C is the dopant concentration, r is the radial distance from the middle of the nanowire, f(r) is the initial dopant distribution, Rn’s are the roots of J1(aRn) ) 0, a is the nanowire radius, and Danneal is the diffusion coefficient during the annealing. Figure 1b,c present surface potential and dopant concentration profiles, respectively, for two different diffusion coefficients after 1 h of annealing. Using the calculated profile assuming Danneal ) 5 × 10-16 cm2 s-1 (purple solid line in Figure 1c), we get good agreement with the measured surface potential profile (filled symbols in Figure 1b). The insets in Figure 1b,c are the 2D potential and concentration profiles, respectively, for the as-grown (right side) and for the annealed (left side) nanowires. We note the relatively constant, high measured surface potential of the nanowires implies that a high surface dopant concentration remained after annealing. In order to explore how the annealing process affected the surface phosphorus concentration and the fraction of active dopants, we conducted nanoscale Auger measurements to evaluate the P concentration. Comparing the Auger transitions of Si and P obtained in locations comparable to where the KPFM measurements were done revealed a similar surface dopant concentration (symbol in Figure 1c) of 0.51% ( 0.03% (or 2.55 × 1020 ( 0.15 × 1020, taking the Si solid concentration to be 5 × 1022) before and after annealing. The high surface concentration may result not only from direct surface doping, but also from © 2011 American Chemical Society
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nanowire, grown by the VLS process. In particular, we have shown that low temperature annealing induced the diffusion of P atoms from the enriched nanowire surface to core, with a diffusion coefficient of ∼5 × 10-16 cm2 s-1, resulting in a more uniform dopant distribution with ND ≈ 1 × 1019 cm-3, and with no sign of Au diffusion into the nanowires or on the surface. Furthermore, the use of high H2 partial pressures during nanowire growth greatly reduced the doping concentration gradient to a variation of a factor of 2 in effective carrier concentration along a 20 µm nanowire. Acknowledgment. The authors would like to thank Dr. John Hammond and Dennis Paul from PHI electronics for their help with scanning Auger measurements. This research was generously supported by Grant No. 2008140 from the United States-Israel Binational Science Foundation [BSF] and DOE Basic Energy Sciences DE-FG02-07ER46401. Supporting Information Available. TEM images of the asgrown and annealed nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.
FIGURE 3. (a) Schematic illustration of the measured device. (b and c) Photocurrent maps of devices prepared from nanowires grown in a He (b) and H2 (c) environment, respectively. A 2 V bias was applied to the right contacts and the left contacts were grounded. Scale bars are 3 µm. (d) The averaged carrier concentrations for nanowires grown in He (red circles) and H2 (black squares). Results from six devices were averaged for the He-grown nanowires, and four devices were averaged for the H2-grown nanowires.
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Figure 3d compares the free carrier concentration profiles of nanowires grown in He and H2 atmospheres; estimation of effective free carrier concentration based on analysis of the SPCM profiles has been described previously.11 The profiles were averaged over 4 and 6 individual nanowires for H2 and He, respectively, for devices under 1 V bias. Nanowires grown in an H2 atmosphere show a significant improvement in doping uniformity relative to those grown in a He atmosphere. There are multiple mechanisms by which the H2 may reduce the surface doping and improve uniformity. Perhaps the simplest hypothesis is that H2 limits the dissociative chemisorption of the precursor gases, though additional work is necessary for confirmation. If this is the case, one expects the mechanism to be general to nanowire growth from hydride precurors. Alternatively, the ambient gas may influence the activity of the dopants that form the surface layer. Intriguingly, preliminary Auger analysis suggests that the nanowires grown in H2 still show surface enrichment of P; it is possible that preferential incorporation of P at the VLS triple junction is responsible for this surface doping. Given the desirability of forming axial p-n junctions in situ during growth, however, the improved radial uniformity achieved with modified synthesis conditions represents an important advance in control of doping profiles. In summary, we have demonstrated two promising approaches to obtain uniform longitudinal and radial dopant concentration profiles within phosphorus doped silicon © 2011 American Chemical Society
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