Electrical conductivity measurement of silicon wire prepared by CVD

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Electrical conductivity measurement of silicon wire prepared by CVD Hiroshi Suzuki, Hiroshi Araki, Masahiro Tosa, Tetsuji Noda * National Institute for Materials Science, Materials Reliability Center, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan article info Article history: Received 11 October 2008 In final form 27 November 2008 Available online 6 December 2008 abstract The electrical resistivity of a silicon nanowire formed from Si 2 H 6 by CVD was measured using micro- probes equipped with SEM. The resistivity of 6.58 Â 10 5 X cm at room temperature was obtained from the current–voltage (I–V) curve for the wire with both ends fused to the probes. The non-linear I–V curve measured only by contacting the wire with the probes could be explained by the resistivity in a series of silicon and dielectric thin oxide films formed on the silicon nanowires. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Silicon nanowires (SiNWs) are potential materials for nanoscale electronic and chemical devices [1–4] due to their quantum con- finement effects and peculiar structures with a high aspect ratio in nano size. Most SiNWs have been formed by bottom-up pro- cesses, such as laser ablation [5], thermal evaporation [6–8], solid reaction [9], and chemical vapor deposition (CVD) [10,11]. In each case, SiNWs are synthesized by essentially utilizing a vapor– liquid–solid (VLS) mechanism [12]. SiNWs are single crystalline and grow along the h111i or h112i direction with sizes of 3–1000 nm in diameter and 10 l m – several mm in length [5,7]. Among several physical and chemical properties of SiNWs, there have been several reports on electrical conductance measure- ments, in which the single wire was placed on the patterned elec- tric circuit fabricated by lithography and 2- and 4-terminal methods were applied to measure the resistances [1,2,4]. In gen- eral, SiNWs are covered with a thin silicon oxide film formed dur- ing the growth process or after exposure to air after the formation [5–11]. Oxide films have been removed so that the silicon crystals directly contact the electrodes [13]. However, the effect of such thin oxide film on the electrical resistance measurement of SiNWs has not been clarified. In the present work, the direct measurement of electrical resis- tance of a silicon nanowire was attempted using micro probes manipulated in a scanning electron microscope. 2. Experimental procedures 2.1. Preparation of materials Silicon nanowires were grown on {100} silicon wafers with a thin gold film. The silicon wafer had a rectangular shape (10 mm long, 4 mm wide, and 0.5 mm thick) and was covered with gold film of approximately 2 nm in thickness. The gold film as a catalyst growing SiNWs [3,11] was formed by sputtering. The substrate was put into a stainless tube and placed inside a vacuum chamber so that the flow of the reactant gas could be controlled. After evacuat- ing the chamber up to 1 Â 10 À5 Pa, the substrate was heated at 569 K. A 10% Si 2 H 6 gas diluted with H 2 and argon gas were then introduced into the chamber. The disilane and argon gases were run at constant rates of 0.0167 cc/s and 0.33 cc/s, respectively. The total pressure was set at 0.667 kPa. The disilane gas was chem- ically decomposed to form SiNWs on the substrate through form- ing a liquid Au–Si eutectic, followed by silicon nanowire growth underneath the liquid eutectic droplet [12]. After the deposition of SiNWs for 1200 s, the structure of SiNWs was examined with a scanning electron microscope (JEOL JSM-6700F) equipped with an energy dispersive X-ray spectrometer (EDS) and high-resolution transmission electron microscope (JEOL JEM-3000F TEM) with a Gatan GIF Tridiem energy-filter system. Plasmon-loss images were taken using the Gatan GIF system. 2.2. Electrical resistance measurements The SiNWs formed on the substrate were manipulated with a Zyvex S100 system installed in LEO 1550 FE-SEM. The manipula- tion system consisted of four tungsten probes. One nanowire was picked up with the probes, and the current–voltage relations were measured at room temperature using 4-terminal and 2-terminal methods. 3. Results 3.1. Silicon nanowires Fig. 1 shows the morphology of SiNWs formed on a silicon wa- fer at 569 K. The size of the wires ranged from 5 to 100 nm in diam- eter and 10 – several hundred l m in length. The EDS analysis 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.11.090 * Corresponding author. Fax: +81 29 859 2100. E-mail address: noda.tetsuji@nims.go.jp (T. Noda). Chemical Physics Letters 468 (2009) 211–215 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett indicated the presence of oxygen along with a strong Si peak in the spectra of SiNWs. The typical microstructure of a SiNW taken by TEM is shown in Fig. 2. The SiNWs are crystalline and grow parallel to the h110i direction. A high-resolution lattice image shows that the SiNWs consist of a crystalline core and an amorphous sheath. The diffrac- tion pattern indicates that the [1 1 1] zone axis of silicon single crystal and additional 1/3{422} spots appeared. The 1/3{422} reflection has been reported elsewhere as well [14]. The appear- ance of these spots is reported due to the effect of the infinite structure of the nanowire [14]. The surface amorphous layer is con- sidered as thin silicon oxide formed by oxidation in a small amount of residual oxygen in the reaction chamber during the growth of Fig. 1. Morphology of SiNWs formed for 1200 s at 569 K. Fig. 2. TEM micrograph and diffraction pattern of a SiNW. The diffraction pattern was taken for a square region of the lattice image of the nanowire. Fig. 3. Plasmon-loss images for (a) Si and (b) SiO 2 for the SiNW shown in Fig. 2; (c) and (d) are the intensity profiles of the boxed areas in (a) and (b), respectively. 212 H. Suzuki et al. / Chemical Physics Letters 468 (2009) 211–215 SiNWs [2,5,7,15] and after exposure to air [13] when the samples were taken out of the chamber. The chemical state of the SiNWs was examined using plasmon- loss images. According to the electron energy loss spectra (EELS), a peak at 16.7 eV corresponding to the first plasmon-loss peak of Si appears for silicon crystal [16]. The peak for the first plasmon-loss of SiO 2 is 22.4 eV [16]. Then, the plasmon-loss images were ob- tained by placing the slit at 16.7 ± 2.5 eV and 22.4 ± 2.5 eV for the SiNW shown in Fig. 2. The Si and SiO 2 plasmon-loss images for the nanowire are shown in Fig. 3a and b, respectively. The corresponding intensity profiles from the boxed areas perpendicular to the wire axis, as seen in Fig. 3a and b, were indicated in Fig. 3c and d, respectively. The full-width at half-maximum (FWHM) value of SiO 2 is 3.5 nm wider than that of Si. Consequently, the SiNW of around 22 nm in diameter in Fig. 2 was estimated to consist of the core silicon crystal of 19 nm covered with silicon oxide of around 1.8 nm in thickness. Other SiNWs with different diameters were also exam- ined by TEM. Most SiNWs were found to be covered with oxide film with a thickness ranging from around 1 nm to several tens nm. 3.2. Electrical resistance measurements One SiNW with length longer than 100 l m was pulled out from bundled SiNWs formed on a silicon wafer by manipulating four tungsten probes in a vacuum. Fig. 4 shows how to measure the electrical resistance of a SiNW by the probes. The SiNWs had enough stiffness to be held by the probes. The wire was pushed with four probes that were alternately positioned so that the contact between the wire and tungsten probes could be sufficiently obtained (see Fig. 4a). A con- stant electrical current was applied between the outer two probes. The voltage between the inner two probes was then measured with a voltmeter. The diameter of the SiNW examined in this experiment was 850 nm. The distance between the two inner probes was 2.82 l m. Fig. 5 shows the result of a 4-terminal measurement for the sample shown in Fig. 4. The current increases with the measured voltage, but no linear relationship in current versus voltage (I–V) behavior was observed. The slope of the I–V curve gradually in- creased with the voltage. The linear I–V relation can be assumed at a certain voltage range. The slope gives an approximate resistiv- ity of 3.1 Â 10 6 X cm at 0–10 V, 1.7 Â 10 6 X cm at 10–15 V, and 6.9 Â 10 5 X cm at 30–38 V. The result of Fig. 5 seems to be caused by the effect of surface oxide surrounding the silicon core. Then, the applied current in- creased, resulting in the melting of the end of the SiNW contacting the tungsten probe. In Fig. 4b, the features of the contacts of the fused SiNW on the two tungsten probes are shown. It is evident that the end of the SiNW contacting the probe was melted and cov- ered the probe. According to the binary phase diagram for Si–W system, W 2 Si phase appears at temperatures lower than the melt- ing point of silicon [17].This means that the tungsten silicide could be formed at the interface between the fused silicon wire and the tungsten tip. The silicon wire therefore firmly adhered to the tung- sten probe and the contact resistivity between the silicon wire and the tungsten tip is considered to be negligibly small. The length of the SiNW between the two probes was 38.6 l m. The I–V relation was then measured. The linear relation was obtained as shown in Fig. 6. The slope gives a resistivity of 6.58 Â 10 5 X cm for the SiNW of 850 nm in diameter and 38.6 l m in length. The present result is in good agreement with an intrinsic resistivity in the order of 10 5 X cm for silicon [18]. This value corresponds to p- or n-type impurity concentration of 10 9 –10 10 /cm 3 [19]. Assuming the num- ber of silicon atoms in a cubic centimeter as 5 Â 10 22 , the purity of the present SiNW is to be better than 11 N. Furthermore, the resistivity value for the 4-terminal method is close to this value at high voltage. After the measurement of the electrical resistance by the 2-terminal method shown in Fig. 6, the silicon wire was fractured by bending. The fractured surface indicated the cleavage showing the clear single crystalline feature. The silicon wire was found to be surrounded by a thin oxide film of 5–20 nm in thickness. 4. Discussion As seen in Figs. 2 and 3, the SiNW consists of a silicon core and silicon oxide on the surface. The resistivity measurement by the 4-terminal method (see Fig. 5) indicated a non-linear relation between the current and the voltage. This means that the surface silicon oxide might affect the I–V curve. On the other hand, the I–V measurement after fusing both ends of the SiNW on the tips of the tungsten probes showed a clear linear relation. The resistiv- ity obtained from Fig. 6 was 6.58 Â 10 5 X cm, which corresponds to the resistivity of silicon crystal with a purity better than 11 N. In Fig. 5, the resistivity decreases with increasing voltage and came close to the value of that of Fig. 6. The value of 6.9 Â 10 5 X cm at 30–38 V was described in the previous section. The surface silicon oxide is essentially an insulator and has ionic conductive property. By applying voltage, the ionic current gradually increases with increasing the electric field, and finally, a dielectric breakdown phenomenon occurs at high electric fields. The ionic current, I, through a dielectric material is given as Eq. (1) [20]: Fig. 4. SEM micrographs of a SiNW contacted with 4 tungsten probes (a), and the same SiNW of which both ends were fused to the probes (b). The electrical measurements were conducted with the 4-terminal method (a) and the 2-terminal method (b). H. Suzuki et al. / Chemical Physics Letters 468 (2009) 211–215 213 I ¼ K sinhðk q E=2kTÞ; ð1Þ where k is the ionic jump distance, q, the charge, E, the electric field, k, the Boltzmann constant, and T, the temperature. K is a constant at the measured temperature. Fig. 7 is a schematic drawing of the electrical measurement of the SiNW covered with the silicon oxide layer. The total resistance, R, is the sum of those of SiNW and silicon oxide as R ¼ R 1 þ 2 Â R 2 ðEÞð2Þ where R 1 is 6.58 Â 10 5 X cm and is taken from the present experiment. R 2 is given as a function of electric field, E, as expected from Eq. (1). The total voltage, V T , is then V T ¼ V 1 þ 2 Â V 2 ¼ I Â R ð3Þ V 1 and V 2 are applied voltages to the silicon wire and the surface oxide layer, respectively. Assuming T = 298 K, k = 0.491 nm of the lattice parameter for the (1 0 0) plane of quartz [21], q = 3.2 Â 10 À19 C for the oxygen ion, and the average thickness of oxide of 12.5 nm as a result of the observation of the fractured surface of the SiNW described in the previous section, the I–V relationship can be obtained by giving proper K and V 2 values. In the present case, the ratio of V 2 /V T , r, was assumed to be constant independently of V T . Fig. 8 is the result of curve fitting using Eq. (3), where the K va- lue and r were assumed to be 1.1 Â 10 À11 A and 0.055. A fairly good coincidence between the experimental and simulation results is observed. The electric field, E, estimated from the r value, was changed from 4 kV/mm at VT of 1 V and 44 kV/mm at 15 V. The reported breakdown electrical field for silicon oxide film is around 40 kV/mm [22]. As described in the previous section, the resistivity measured by the 4-terminal method started to decrease at around 10–15 V of the applied voltage. The dielectric breakdown was as- sumed to be completed at 30 V since the resistivity value measured with the 4-terminal method agrees with that with the 2-terminal method at around the applied voltage. That means that the break- down of the surface oxide film was considered to gradually occur at lower voltage than 30 V. Fig. 5. I–V curve for the SiNW measured with the 4-terminal method. Fig. 6. I–V curve for the SiNW measured with the 2-terminal method. Fig. 7. Schematic illustration of the electrical circuit of SiNW. Fig. 8. Fitting the estimated I–V curve using Eq. (3) to the experimental data measured with the 4-terminal method. 214 H. Suzuki et al. / Chemical Physics Letters 468 (2009) 211–215 In the present study, the surface oxide film surrounding the sil- icon wire was assumed to be SiO 2 only from the result of EELS anal- ysis. In addition, the removal of the oxide film has not been tried by such a way heating the wire in vacuum. It will be necessary to fur- ther examine the precise structure of the surface oxide and its thickness effect on the electrical resistivity of the SiNW. 5. Conclusion The electrical resistivity of SiNW formed by thermal decompo- sition of Si 2 H 6 was measured with both 4-termial and the 2-termi- nal methods using tungsten microprobes equipped with SEM at room temperature. In the case of the 2-terminal method, the end contact points of the SiNW were fused to the tungsten probes. The following conclusions were derived. (1) The prepared SiNWs were single crystalline and grow paral- lel to the h110i direction. The SiNWs were covered with thin silicon oxide film. (2) The I–V curve for the SiNW of 850 nm in diameter measured with the 4-terminal method using micro probes showed a non-linear relationship that might have been caused by the effect of surface silicon oxide on the SiNWs. (3) The 2-terminal measurement, in which both ends of a SiNW were fused to the probes, indicated a linear I–V curve giving the resistivity of 6.58 Â 10 5 X cm. This value corresponds to that of bulk silicon with purity better than 11 N. (4) The I–V curve obtained with the 4-terminal method could be explained by the resistivity in a series of silicon and thin oxide film. Acknowledgements The authors are grateful to Zyvex Corp. for their cooperation in the measurement of the electrical conductivity and Dr. K. Mitsuishi for the TEM analyses. References [1] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [2] A. Chung, J. Yu, J.R. 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