Silicon Nanowires: Preparation, Device Fabrication, and Transport

Nov 23, 2000 - Growth Kinetics of Needle-like Silicon Wires Formed via the Zinc Reduction Reaction of Silicon Tetrachloride ..... Evidence of electron...
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11864

J. Phys. Chem. B 2000, 104, 11864-11870

Silicon Nanowires: Preparation, Device Fabrication, and Transport Properties Jae-Young Yu,† Sung-Wook Chung,† and James R. Heath* UCLA Department of Chemistry and Biochemistry, 405 Hilgard AVenue, Los Angeles, CA 90095-1569 ReceiVed: July 20, 2000; In Final Form: October 5, 2000

The preparation of 20 ( 5 nm diameter Si nanowires and the electrical characterization of Si nanowire devices are presented. The nanowires were grown at 450-500 °C on solid substrates via the vapor-liquid-solid mechanism using Au or Zn nucleation catalysts and SiH4 as the silicon source. The wires were investigated by high-resolution transmission electron microscopy. Two types of wires were found, as characterized by different growth directions (〈111h〉 and 〈211〉). Several types of devices, including crossed nanowire devices, four- and six-terminal devices, and three-terminal (gated) devices, were fabricated. For certain devices, various electrode compositions were also studied. The measured resistivity of these nanowires was separated from the contact resistance and could be varied from >105 Ω cm to ∼10-3 Ω cm. The wide variation in resistivity was related to the nature of the electrical contact to the wires (Schottky or Ohmic) and to the doping level of the wires. Doping of the nanowires was performed by the thermal diffusion of metal catalyst into the nanowires at 750-850 °C. Au nucleated nanowires exhibited resistivity values much lower than those of similarly treated Zn nucleated nanowires. This result is attributed to the much larger relative solid solubility of gold in silicon.

1. Introduction We recently published a letter on the properties of Si nanowire (SiNW) devices,1 and this paper is intended to serve as a complete description of our approach to SiNW preparation, device fabrication, and SiNW transport characteristics. Semiconductor nanowires are most often prepared via a technique known as the vapor-liquid-solid (VLS) mechanism, and this approach has a long, well-documented history.2,3 Several chemical and materials techniques (i.e., nonlithographic approaches) for the production of various types of nanowires have been reported,4-7 including a laser-driven VLS approach, reported by the Lieber group,8 for the production of various semiconductor nanowires. Our preparation procedure is essentially a (laserless) modification of the Lieber group approach and was briefly described in our earlier letter. Two types of SiNWs are investigated heresgold-nucleated NWs (AuSiNWs) and zinc-nucleated NWs (Zn-SiNWs). Both types of wires are prepared as bulk materials using chemical vapor deposition (CVD) of SiH4 in the temperature range of 450500 °C and are characterized by an average diameter of 20 ( 5 nm. In this paper, we first present the preparation techniques and the crystallographic structure of our nanowires. Then, in some detail, we discuss experiments aimed at elucidating issues related to metal/SiNW Schottky and Ohmic contacts, SiNW resistivity values, doping SiNWs, and correlating SiNW transport characteristics with the preparation procedure for the nanowires. To separate these various properties, we have constructed a number of different SiNW devices, including four- and six-terminal devices, crossed nanowires, and gated nanowire devices. Our previous (gated-device) measurements indicated that the nucleating metal can act as a p-type dopant. In the present work, we have found that the electrical properties of the nanowires depend * Corresponding author. E-mail: [email protected]. † Contributed equally to this work.

on the elemental composition of the nucleating metal. Second, we report that thermal treatment of the nanowires leads to diffusion and ionization of the nucleating metal into the nanowire, thereby increasing the nanowire doping level. The net result is that it is possible to control the nanowire resistivity from >105 Ω cm (for “as-prepared” wires) to ∼10-3 Ω cm for the most highly doped nanowires. We evaluate various standard approaches for making electrical contact to the nanowires, and we quantify contact resistances. Finally, we explore electron transport through crossed nanowires. 2. Experimental Section 2.1. Nanowire Synthesis. Gold-nucleated silicon nanowires (Au-SiNWs) have been described previously by other workers. Morales et al.8 utilized laser synthesis as a means of growing Au-SiNWs. Our growth technique is laserless and involves the chemical vapor deposition (CVD) of silane onto a gold-coated Si wafer. A 0.5-1 nm sample of Au was electron-beam (ebeam) evaporated onto a SiO2-coated (150 nm film thickness) Si wafer substrate to form Au islands.9 The wafer was then heated at 400-450 °C for ∼1 h at 10-3 Torr. Afterward, the furnace temperature was set to ∼450 °C, and 5% SiH4 in He was introduced under constant flow at 300 SCCM and a constant pressure of 100 Torr for 20-40 min. Zinc-nucleated silicon nanowires (Zn-SiNWs) were briefly reported by us.1 For these Zn-SiNWs, particular attention was paid to substrate preparation in order to deposit Zn from the ZnCl2 solution. The backsides of boron-doped p-type Si (100) wafers (resistivity ) 1-10 Ω cm) were coated with Al metal. This metal served as an anode, and the wafer was subsequently electrochemically etched10 in a Teflon cell with a Pt cathode in a 2:3 solution of HF (45% aqueous solution, electronic grade) and absolute ethanol. The anodization current density was kept constant at 10 mA/cm2 for 3-5 min. This produces a thin porous layer on the Si substrate (average pore size of 1-3 nm).11 The porous wafer was bath sonicated (Branson Ultrasonics 1200)

10.1021/jp002595q CCC: $19.00 © 2000 American Chemical Society Published on Web 11/23/2000

Silicon Nanowires in ZnCl2/ethanol solution (0.06-0.6 mol/L) for 30-60 min, rinsed twice with 2-propanol and once with 18 MΩ cm water, and dried under flowing N2. The substrate was annealed at 450 °C in the quartz tube under vacuum (∼10-3 Torr) for 3 h and purged under 200 mTorr of a 5% H2/Ar mixture at 450 °C for 5 min prior to SiH4/He flow. Zn-SiNWs were then grown at 450-500 °C using 5% SiH4/He at a flow rate of 300 SCCM, under a total internal pressure of 50-100 Torr, and for a period of 30-60 min. Synthesized Zn-SiNWs and Au-SiNWs and devices made from these nanowires were investigated by field-emission scanning electron microscopy (FESEM; Philips model XL30) and atomic force microscopy (AFM, Thermomicroscope Autoprobe CP). The chemical compositions of the nanowires were determined by energy-dispersive X-ray spectroscopy (EDS). Crystallographic characterization of Zn-SiNWs was performed using a 200 keV Akashi high-resolution transmission electron microscopy (HRTEM). 2.2. Fabrication of Nanowire Devices. Two types of devices were fabricated. For the first type, the NWs were thermally annealed prior to any device fabrication step, while for the second type, the assembled device was thermally annealed. Thermal treatment of the devices had a profound impact on the measured device resistivity, and by exploring both types of devices, it was possible to separate issues related to wire annealing from those related to contact annealing. All annealing was carried out in a tube furnace under flowing 5% H2 in Ar at 300 SCCM and 740 Torr. Annealing temperatures were varied between 400 and 850 °C. All devices were prepared by first transferring the SiNWs onto a SiO2/Si (150 nm oxide layer) wafer that was prepatterned with alignment markers. The nanowires were located using FESEM in order to determine an electrode pattern. A ∼200 nm PMMA e-beam resist was deposited via spin coating, and electrodes were patterned by e-beam lithography (Nabity Pattern Generation System on a Philips XL 30 FESEM). The pattern was developed using a methyl-isobutyl ketone (MIBK):2propanol (1:3 by volume) solution, and Ti/Au (30/1000 Å), Cr/ Au (30/1000 Å), or Al (1000 Å) electrodes were deposited via e-beam evaporation onto the patterned substrate. By far, devices with Ti/Au contacts resulted in the highest conductance. Therefore, only the devices with Ti/Au electrodes will be discussed below. The assembled devices were then characterized by FESEM, and the diameters of the nanowires in the devices were measured using noncontact mode AFM (NC-AFM). The current-voltage (I-V) characteristics of most of the devices were then characterized using a shielded probe station (RutgerKohls) equipped with coaxial probes. For certain devices, temperature-dependent transport measurements were carried out. For those measurements, the device was placed into a chip carrier, the metal contact pads of the SiNW electrodes were wire-bonded, and the assembly was placed into an immersion cryostat. For all measurements, a 16-bit digital acquisition board (DAQ) (National Instruments) was used for the voltage source. Currents were measured using a DL Instruments Preamplifier Model 1211, and the output voltage of the preamplifier was measured using either an analog-to-digital input on the 16-bit DAQ board or with Keithley 617 Programmable Electrometer as an independent voltmeter. 3. Results and Discussion 3.1. Structural and Chemical Characterization of the Nanowires. A FESEM micrograph of Zn-SiNWs is shown in

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Figure 1. (a) FESEM image of pristine Zn-SiNWs grown on an electrochemically etched porous Si substrate. The typical diameter of the Zn-SiNWs is 20 ( 5 nm, and the lengths of the wires are between 2 and 10 µm. (b) A HRTEM image of a 23 nm diameter Zn-SiNW seen from the 〈111h〉 zone axis, which is determined by the SAED pattern shown in the inset. The growth direction of this Zn-SiNW, indicated by the arrow, is 〈211〉.

Figure 1a. Both the Au- and Zn-nucleated wires appeared to be very straight and well isolated on the surface of the substrate and were characterized by an average wire diameter of 20 ( 5 nm. A very few SiNWs were found with diameters as large as 35 nm and as small as 10 nm. For both types of wires, it is unlikely that the nucleating sites for SiNW growth were characterized by narrow size distributions. It is more likely that the temperature of SiNW growth, which was the same for both types of wires, is the critical factor in determining the diameters of the SiNWs. For the case of the Zn-SiNWs, if a low density coverage of the Zn precursor was used (0.06 mol/L ZnCl2 solution), several wires “grew” from a common nucleation site. At higher catalyst coverages (0.61 mol/L ZnCl2 solution), only one or two SiNWs grew from a single nucleation site. The elemental composition of the nanowires was investigated using EDS, and only silicon, with trace amounts of oxygen, was detected. When the SiNWs were synthesized using the laser ablation technique,8 the Au tip was clearly visible in FESEM and was detected using EDS. However, for the syntheses reported here, no metal particle was detected on any of the

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Figure 2. (a) Temperature-dependent I-V traces from two-point probe measurements of Zn-SiNW at 20, 146, and 288 K. The experimental data is shown in solid dots. Simulated I-V traces using eq 1 is shown in solid lines for each temperature. (b) Temperature-dependent I-V traces from two-point probe measurements of Au-SiNW at 56, 77, and 288 K. Only the experimental data are shown.

nanowires, except for the case of Au-SiNWs in which nanowire growth was quenched after only 3-5 min. HRTEM image of Zn-SiNWs, Figure 1b, revealed that they were coated with a rough, amorphous 1.8 nm thick native oxide layer. Crystallographic analysis of HRTEM images revealed SiNWs characterized by two distinct growth axes: 〈111h〉 and 〈211〉 . Stacking faults were visible in the TEM images of 〈111h〉 NWs, but those wires were single crystals and atomically straight. The 〈211〉 growth direction is not typical of the VLS mechanism. However, it has been observed in wires produced via laser ablation,12,13 simple physical evaporation of Si,14 and SiO2-enhanced laser ablation.15 However, for all those cases, the SiNWs were plagued with abundant defects such as microtwins, stacking faults, and grain boundaries. By contrast, our 〈211〉 growth direction SiNWs are nearly defect-free. 〈111h〉 and 〈211〉 growth direction SiNWs could not be physically separated, so it was not possible to correlate the crystallographic orientation of the NWs with their charge transport characteristics. 3.2. Transport Characteristics of SiNWs. 3.2.1. The Mechanism of Carrier Injection. We previously reported on the properties of voltage-gated SiNW devices1 contacted with Ti/ Au electrodes and investigated prior to any thermal annealing. For those devices, the measured device resistivity was very high s greater than 105 Ω cm for Zn-SiNWs and >103 Ω cm for Au-SiNWs. These particular devices were fabricated with only two electrical contacts, so it was not possible to separate contact resistance from NW resistivity. Nevertheless, the device resistivity for both types of NWs indicated that the NWs were at least lightly doped, and voltage-gated experiments indicated that the doping was p-type. Upon thermal annealing at temperatures ranging from 450 to 800 °C, the device resistivities could be decreased to ∼102 Ω cm for Zn-SiNWs and ∼1 Ω cm for Au-SiNWs. For those higher conductivity devices, the magnitude of voltage gating was reduced.1 Temperature-dependent transport measurements on annealed two-terminal SiNW devices were carried out from 17 to 300 K. Those measurements revealed large differences between the Zn-SiNW and Au-SiNW devices, as shown in Figure 2. In this section, we model the I-V traces of Figure 2 in order to ascertain information about the contact potential barrier and to differentiate between charge tunneling and thermionic emission

Yu et al. mechanisms of charge carrier injection across the electrode/ NW interface. Current flow through a metal-semiconductor (M-S) junction is a well-described process in which the transport mechanism is either thermionic emission or tunneling.16 Thermionic emission, a thermally activated current transport mechanism, is the dominant transport process for the most Schottky diodes in the forward direction, i.e., current flowing from the semiconductor to metal. However, the device configuration for our devices is the same as BARITT,17 where the device is always reversebiased. That is, the current always flows from the metal to the semiconductor. If the only mechanism for the transport is thermionic emission, then a saturation current density16 exists for the backward bias. However, as shown in Figure 2, no such behavior was observed. Even when the mechanism of an image potential barrier lowering16 is included in the calculations, the calculated I-V curves do not match the experimental data well. However, once the M-S tunneling mechanism was included in the calculation, we arrive at reasonable fits. The 1-D metal-semiconductor transport theory has been well worked out,17 and the following equations describe the current density, JMS:

(

)[

( )]

q(φBp - ∆φ) -q|V| 1 - exp kBT kBT (A*)T q(Vb - ∆φ) + FM(V)T(η)(1 - FS) dη (1) 0 kB

JMS ) (A*)T2 exp -

∫

Here, A* is the Richardson constant, T is the absolute temperature, and kB is the Boltzman constant. q is the charge, φBp is Schottky barrier height for the metal and p-type semiconductor, and ∆φ is the image potential lowering. The first part of the current density equation is the thermionic emission current density. This equation assumes that the transmission probability above the Schottky barrier is unity, which leads to the classical form of the thermionic emission. The tunneling current density is expressed in the second part of the current density equation. Fi(V) is the Fermi-Dirac distribution function of carriers, which has the form

Fi(V) )

1 µi - qV exp -1 kBT

(

)

(2)

in which the Fermi energies are represented by µi. The transmission probability, T(η), is derived using WKB approximation18

{ [

1 T(η) ) exp E00

xqVb xη + q∆φ

- (qVb - η - q∆φ)ln

xqVb - η - q∆φ xqVb - xη + q∆φ

]}

(3)

where

E00 ≡ qp

x

ND

4(m*)s

Here, p is Planck’s constant, s is the dielectric constant of the semiconductor, m* is the effective mass, and ND is the dopant concentration. η is the energy measured from the top of the barrier. Equation 3 was derived using the potential energy barrier

Silicon Nanowires typical of a metal-semiconductor junction without the contribution from the image charge. However, setting the η ) 0 point to a potential barrier maximum that has been reduced by the image potential effectively accommodates the effects of image potential lowering. In this approximation, we are only taking into account the influence of the image potential on the Schottky barrier height but not on the barrier shape. This alters the transmission probability in the vicinity of η ) 0 but should only have significant consequences at low bias.17 On the basis of these relatively simple approximations, we have fitted the temperature-dependent I-V curves for ZnSiNWs, and those fits are presented, along with the data, in Figure 2a. The fitted curve includes both the thermionic emission and the tunneling contributions. However, thermionic emission current is less than 1% of the total current. Thus, for these particular devices, current transport through the metal-semiconductor junctions is governed by tunneling. 3.2.2. Doping and Contact Resistance of Au-SiNWs. As discussed above, one possibility for the p-type dopant in these NWs is the nucleating metal itself. If that supposition is correct, then it should be possible to maximize the doping level via thermally annealing the Au-SiNWs prior to device fabrication. At 800 °C Au metal will diffuse into the NW, and the dopants should be activated via thermal ionization. Au is a p-type dopant in Si. While Au can increase the carrier density, it may also decrease the carrier mobility19 because it acts as a scattering center. Thus, even if a SiNW were highly doped with Au, it is not obvious what sort of resistivity would be expected. In this section, we explore the origin of the dopant in this type of NWs by utilizing the multiterminal NW device to quantify the intrinsic resistivity of a thermally treated Au-SiNW. A FESEM micrograph of a six-terminal device constructed from a Au-SiNW with Ti/Au contacts, and the measured I-V curves from the device are presented in Figure 3a and b. The Au-SiNW was heated to ∼800 °C in flowing 95/5 Ar/H2 at 740 Torr with a flow rate of 300 SCCM for 3 h, prior to device fabrication. In our previous letter,1 there was an ambiguity in terms of the source of the dopant. All of our devices with SiNWs without any thermal treatment have current levels at a sub-pA range. Figure 3b shows that we can achieve very high conductivity by thermally annealing the SiNW without the presence of metal electrodes, which might also dope the SiNW. This concludes that the source of the dopant is already present in the SiNW (presumably the catalytic center of the wire growth) and does not originate from the metal electrodes. Furthermore, Figure 3b shows that the current is steadily decreasing from right to left for devices with about the same length. This indicates that the source of the dopant is located at the right end of the wire and that thermal diffusion is the mechanism for doping the wire. Even though we can achieve very high conductance through the SiNWs, the I-V curves shown in Figure 3b are nonlinear, indicating that metal/NW contact is characterized by a Schottky barrier. Thus, two-point probe measurements will not directly give the “intrinsic” resistance, RSiNW, and resistivity, F, of the SiNW without the knowledge of the contact resistance, RC, resulting from the metal/NW junction. Therefore, four-point probe measurements were conducted to obtain the intrinsic resistance of the SiNW. A voltage bias was applied across contacts 2 and 5 (electrode labels are shown in Figure 3a), and the voltage drop across the contacts 3 and 4 was measured. A voltage-dependent resistance between contacts 3 and 4 can be calculated by RSiNW,3-4(Vapp) ) V3-4(Vapp)/I(Vapp), where V3-4(Vapp) is the voltage drop across the contacts 3 and 4 and I(Vapp) is

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Figure 3. (a) FESEM micrograph of the six-terminal Au-SiNW device with Ti/Au contact electrodes. The electrodes are labeled 1 through 6 from left to right. The scale bar shown in the micrograph is 5 µm. (b) I-V traces of each segment in the six-terminal devices: 2-3 (solid line), 3-4 (dashed line), 4-5 (dotted line), and 5-6 (dashed-dotted line). The current steadily increases from the rightmost device to the leftmost device for devices with the same length of SiNW. (c) Fourpoint probe measurements of the devices 2-3-4-5. RDev,2-5 (solid line) is calculated from the I-V measurements across contacts 2-5. RSiNW,3-4 (open square) is calculated from the voltage drop across contacts 3-4 and the measured current. RSiNW,2-5 (solid circle) is obtained by scaling RSiNW,3-4 to the length of exposed SiNW between contacts 3-4 to the length of exposed SiNW between contacts 2-5. RC (open circle) is obtained by subtracting RDev,2-5 by RSiNW,2-5.

the measured current when Vapp is applied to contacts 2 and 5. The four-point probe measurements also give the voltagedependent device resistance, RDev,2-5, between contacts 2 and 5, which can be calculated by RDev,2-5 ) Vapp/I(Vapp). RDev,2-5 contains both RC and RSiNW,2-5. As noted above, the SiNWs do not have uniform resistance across the entire length of the wire. However, we can still obtain rough estimates of RSiNW,2-5 by scaling the measured RSiNW,3-4 by the length, i.e., RSiNW,2-5 ) RSiNW,3-4 l2-5/l3-4. The wire lengths between contact 3 and 4 and between 2 and 5 are l3-4 and l2-5, respectively, excluding the segments buried under the metal electrodes. The exclusion of the SiNW segments that are buried under the electrodes

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Figure 4. Four-point probe measurements of the same device as that in Figure 3c after further annealing at 630 °C for 2 h. The total resistance (solid line) is obtained from the two-point probe measurements of contacts 3 and 4. RSiNW,3-4 (solid circle) is calculated from the voltage drop across contact 3-4 and the current measurements from four-point probe measurements, and RC (open circle) is obtained by subtracting the total resistance by RSiNW,3-4.

assumes that there is no voltage drop across any single electrode width; i.e., the metal electrode causes the SiNW segments that are buried under the electrode to be in a constant potential. The contact resistance, RC, is calculated by subtracting the calculated RSiNW,2-5 from RDev,2-5, measured by the four-point probe method. The measured RDev,2-5 and RSiNW,3-4 and the calculated RSiNW,2-5 and RC are shown in Figure 3c. These data show that RC is about half of RSiNW,2-5. Furthermore, the contact resistance also carries most of the nonlinear character of the I-V curve for this device. These results indicate that the metal/SiNW contacts do not limit current flow even though they are characterized by a Schottky barrier. Although RSiNW,3-4 appears to be linear in Figure 3c, it does change slightly from 18.6 MΩ at 0.2 V to 16.1 MΩ at 4 V. The Au-SiNW resistivity, F, can be estimated from the average value of RSiNW,3-4, which is 17.3 MΩ, and by assuming a cylindrical shape for the SiNW with a constant diameter. NCAFM was used for the SiNW diameter measurements. NC-AFM and FESEM images indicate that the thermal processing of the NWs leads to NW roughening. The resulting NW diameter for the six-terminal device is ∼10 nm for the segments between contacts 3 and 5, even though some sections are as large as 18 nm diameter. The large diameter sections are very rough and presumably covered with a thick SiO2 shell. The thin diameter and rough surface structure implies that we might have thinned the wire during the 800 °C annealing process. The resulting resistivity of the wire is 45 mΩ cm using 17.3 MΩ for the resistance, 10 nm for the wire diameter, and 3 µm for the length. The six-terminal device was again further annealed at 630 °C under 760 Torr of Ar/H2 (95/5) ambient for 2 h. Very nearly linear I-V curves were obtained after such a post-fabrication annealing procedure, as seen in the previous letter.1 The linear characteristic of the resulting measured I-V curves indicates that the contact resistance might be negligible compared to the wire resistance, which is the condition for the Ohmic contact. Furthermore, the conductance of the device was increased by a factor of ∼6. To quantify the RC and RSiNW of the post-annealed device, we performed two and four-point measurements again. Figure 4 shows the resulting wire and contact resistances of the device between contacts 3 and 4. The resistance of the wire

Yu et al. has a slightly negative slope of 26 KΩ/V when the resistance curve between 0.4 and 4 V is fitted to a linear equation. Using the fitted parameters, we calculate the resistances to be 2.6 and 2.5 MΩ at 1 and 4 V, respectively. The contact resistances are fitted to a linear curve from 1 to 4 V. The contact resistance have the negative slope of 24 KΩ/V. Fitted contact resistances are 0.42 and 0.34 MΩ at 1 and 4 V, respectively. The ratio between the wire resistance and contact resistance varies from 6 at 1 V to 7 at 4 V. Therefore, the contacts are very nearly Ohmic from the point of view that the contact resistance is much lower than that of the SiNW. Using the same geometric factors for the SiNW between contacts 3 and 4 as the factors above, we calculated the resistivity of the wire to be ∼6.8 mΩ cm. When this resistivity is compared to the p-type Si resistivity with Boron as its dopant, the needed boron concentration is ∼1.5 × 1019 atoms/cm3.20 The nanowire segment between contacts 4 and 5 was similarly analyzed. The wire resistance had an essentially negligible slope of 3.2 KΩ/V. The resistance of the wire is essentially flat from 1 to 4 V, remaining at 0.33 MΩ. The resulting resistivity is ∼2.6 mΩ cm, which is 2.4 times smaller than the resistivity between contacts 3 and 4, when 10 nm for the diameter of the SiNW and 1 µm for the length of the SiNW are used as the geometric factors for the device. The corresponding resistivity with boron as the dopant for bulk Si requires a boron concentration of ∼4 × 1019 atoms/cm3.20 The resistance between contacts 4 and 5 has the slope of -20 KΩ/ V, and the calculated RC values using the slope are 0.36 and 0.30 MΩ at 1 and 4 V, respectively. These contact resistances are slightly lower than those of the device between contacts 3 and 4. Even though the contact resistance is very small, the device between contacts 3 and 4 is not in the Ohmic region because the RSiNW is now comparable to RC. It must be stressed that the Ohmic contact in this paper is defined as a contact with RC much smaller than RSiNW. Because RSiNW scales with length, the shorter segment in the six-terminal device is outside of the Ohmic contact regime. When considering the resistivity of our nanowires, it is useful to consider the prototypical example of Boron-doped Si. Boron is a shallow dopant in bulk Si, so most of the boron atoms are expected to be ionized, implying that the carrier concentration in B-doped bulk Si will be approximately equal to the concentration of B atoms. The estimated carrier concentrations in our devices are in the range of 1019 carriers/cm3. This is surprisingly high, given the fact that the ionized states of Au and Zn lie close to the midpoint of the band gap. With such deep dopant levels, only small amounts of Au dopant atoms are expected to be ionized. It is known that carrier concentration in bulk Si with Au as its dopant will peak near ∼1012 carriers/ cm3, even though the Au atom concentration is significantly higher.21 One possible reason for having high carrier concentrations in our SiNW devices is that the SiNWs surface states might be supporting a large amount charge carriers. When I-V characteristics of the device shown in Figure 4 are measured several weeks after thermal treatment, the nanowire conductance decreases by as much as a factor of 6. This is reminiscent of the behavior that was observed when a H-terminated Si surface is oxidized under an ambient conditions.22,23 Further support for the surface-state-generated carriers can be inferred from measurements of RSiNW and RC. According to eq 3, the nanowire contact resistance should increase exponentially as the carrier concentration decreases. The resistivity of the SiNW segment between contacts 4 and 5 is about half that of the segment between contacts 3 and 4. Therefore, the expected reduction in RC between contacts 4 and 5 is much greater than 2, but we do

Silicon Nanowires

Figure 5. (a) NC-AFM image of the Zn-SiNW four-terminal device. The electrodes are labeled 1 through 4 from left to right. (b) The calculated resistance from the measured I-V curves of the four-terminal device shown in Figure 5a. The total resistance (solid line) was calculated from two-point measurements across electrodes 1 and 4. The calculated Zn-SiNW resistance, RSiNW, from four-point probe measurements is shown in square. The calculated contact resistance, RC, is shown in circle.

not observe this. Therefore, the conduction of the SiNW might be greatly enhanced by some mechanism involving surface states. In a very recent publication, Lieber’s group has used threeterminal (gated) devices to investigate the electrical characteristics of VLS-grown Si nanowires that were n- or p-type doped using the more traditional atoms P or B. Although those wires were substantially larger than what are described here (60150 nm diameters), the measured resistivities of their highly doped wires (∼20 mΩ cm) are very close to what is reported here.24 3.2.3. Doping and Contact Resistance of Zn-SiNWs. The second type of NWs that were investigated here were ZnSiNWs. A Zn-SiNW four-terminal device was fabricated with Ti/Au contacts for four-point probe measurements in order to separate RC and RSiNW from the total device resistance. The ZnSiNW device was heated to 750 °C in flowing Ar/H2 (95/5) for 30 min after the device fabrication. In Figure 5a, we show a NC-AFM micrograph of the four-terminal device. The diameter of Zn-SiNW was measured to be 20.3 ( 0.5 nm across the entire nanowire length. The I-V curves of this device from two-point probe measurements were highly nonlinear prior to 750 °C annealing but were reasonably linear afterward. To probe the intrinsic resistance, RSiNW, and resistivity, F, of Zn-SiNW, we performed four-point probe measurements as described previously. For these measurements, we had to assume that the Zn-SiNW resistivity was uniform along the length of the wire. In Figure 5b, we plot RSiNW, scaled by the effective length between the contacts 1 and 4. We conducted two-point probe measurements between the contacts 1 and 4, and the corre-

J. Phys. Chem. B, Vol. 104, No. 50, 2000 11869 sponding device resistance (VSiNW/ISiNW) from the two-point probe measurements was obtained. We know that the device resistance, shown as line in Figure 5b, is convoluted with the intrinsic wire resistance, RSiNW, and contact resistance, RC, so we can quantify RC (circles) with respect to RSiNW. Note that RSiNW is reasonably constant regardless of the applied bias, and most of the nonlinear character of the device resistance stemmed from the contact resistance. We found that RC is approximately 1/ of R 5 SiNW, implying that our contact is very close to Ohmic. This is consistent with the above-discussed results from the AuSiNW six-terminal device. We believe that this device behaves in a similar waysi.e., thermal annealing of the Zn-SiNW device not only facilitates nanowire doping via diffusion and ionization of the Zn metal, but it also leads to an improved metal-nanowire contact. The Zn-SiNW resistivity, FZn-SiNW, was measured to be approximately 31.9 Ω cm, of which an equivalent p-type Si (doped with the Boron) resistivity needs a dopant concentration20 of ∼4 × 1014 boron/cm3. We note that Zn-SiNW devices, after annealing, are characterized by much higher resistivity values (typically, >103) than are similarly treated Au-SiNW devices. This difference is possibly due to the larger maximum solid solubility of Au in Si than in Zn,25 although other factors, such as the role that these various atoms can play in terms of nanowire surface states, may also be important. 3.2.4. Crossed Zn-SiNWs. In Figure 6a, we present a FESEM micrograph of a cross-wire device. The device consists of two 20 nm diameter Zn-SiNWs and four electrical contacts. The measured resistance of this crossed-wire device was too large (>103 GΩ) prior to annealing for us to carry out any significant experiment. The device was annealed at 800 °C in flowing 95/5 Ar/H2 for 30 min. In Figure 6b are the resistance curves for each of the Zn-SiNWs, obtained from I-V measurements recorded after annealing. The resulting I-V traces are reasonably linear. On the bases of these measurements and our previous nanowire device experiments, we assume that the contact resistances between the metal electrodes and Zn-SiNWs are effectively Ohmic, and we do not screen the resistance of wires. The nanowire resistances were approximated by dividing the bias voltage by the measured current. Using the measured dimensions of the nanowires, we found the nanowire resistivities to be 25.2 and 22.8 Ω cm for contact (1-2) and (A-B) SiNW, respectively. In Figure 6c we present the resistance curves of the crossed-wire junctions. The resistivities at an applied voltage of 1 V are 24.3, 21.0, 27.0 Ω, and 23.7 cm for contacts (1-A), (2-A), (1-B), and (2-B), respectively. These resistivities through the crossed-wire junction are close to the resistivities of individual wires in the device, indicating that there is little or no tunneling barrier at the junction of the two crossed wires. Johansson et al.26 showed that a 10-15 Å thick native SiO2 layer may be thermally desorbed from a Si(100) surface at 840 °C. We believe that a similar thermal desorption of the native oxide covering the nanowires occurs here. Thus, these two crossed wires behave as a single conducting unit. Furthermore, in this particular device, we have two nearly identical (by diameter) SiNWs, each subjected to exactly the same thermal treatment. Nevertheless, there are differences in the two nanwires. For example, the resistance curves of the nanowire junctions exhibit different shapes, with the (1-B) and (2-B) traces characterized by curvatures lower than those of the (1A) and (2-A) traces. The common electrode for the higher and lower curvature of the resistance curve is A and B, respectively. Both the resistance and the curvature of the (A-B) resistance curve are higher than those of the (1-2) curve, even though

11870 J. Phys. Chem. B, Vol. 104, No. 50, 2000

Yu et al. ( 5 nm diameter single-crystal SiNWs, characterized by nanowire growth directions of 〈111h〉 and 〈211〉. The resistance of as-prepared Au- and Zn-SiNW devices is greater than 100 GΩ. The resistance of these devices can be lowered significantly via thermal annealing in an Ar/H2 flow. The device contacts are characterized by Schottky barriers, and temperature-dependent measurements indicate that tunneling is the dominant mechanism for carrier injection. After annealing the NW devices in the range 750-800 °C, the resistivities of annealed SiNWs are ∼30 Ω cm and ∼10 mΩ cm for Zn-SiNW and AuSiNWs, respectively, and the contacts were effectively Ohmic. This decreased resistance of the nanowire devices was attributed to doping of the nanowires by the Au or Zn catalyst used for wire nucleation and growth. The nanowire resistivities reported here for the most highly doped wires are substantially lower than would be expected based on the solid solubility of either Au or Zn in Si. The role that surface states play in electron transport through these nanowire devices is unknown, but may be responsible for this discrepancy. Acknowledgment. The authors thank Ted I. Kamins for helpful discussions. This work was funded by the ONR (Grant N00014-98-1-0422) and DARPA. References and Notes

Figure 6. (a) FESEM image of a cross-Zn-SiNWs device. There are two wires that cross each other. One wire connects electrodes 1 and 2, and the other wire connects electrodes A and B. (b) Resistance curves calculated from the I-V measurements for each of the wires shown in Figure 6a. The resistance of the devices between contacts 1-2 is shown in squares, and the resistance of the devices between contacts A-B is shown in circles. (c) Resistance curves calculated from I-V measurements of which the current goes through the SiNW-SiNW junction. Resistances between electrodes 1 and A (triangle), 1 and B (circle), 2 and A (solid line), and 2 and B (cross) are shown.

the crossed nanowire resistivities are very similar. This result may be typical of the types of fluctuations that are likely to characterize such nanoscale devicesseven in the presence of identical processing treatments. 4. Conclusions Thermal CVD of SiH4, coupled with Au- or Zn-nucleated VLS growth, was employed to fabricate high aspect ratio, 20

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