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Growth of single-crystalline cobalt silicide nanowires and their field emission property


In this work, cobalt silicide nanowires were synthesized by chemical vapor deposition processes on Si (100) substrates with anhydrous cobalt chloride (CoCl2) as precursors. Processing parameters, including the temperature of Si (100) substrates, the gas flow rate, and the pressure of reactions were varied and studied; additionally, the physical properties of the cobalt silicide nanowires were measured. It was found that single-crystal CoSi nanowires were grown at 850°C ~ 880°C and at a lower gas flow rate, while single-crystal Co2Si nanowires were grown at 880°C ~ 900°C. The crystal structure and growth direction were identified, and the growth mechanism was proposed as well. This study with field emission measurements demonstrates that CoSi nanowires are attractive choices for future applications in field emitters.


Possessing low resistivity and excellent compatibility with conventional silicon device processing, transition metal silicide nanowires have been widely studied [15]. Compared with silicon nanowires (NWs), fabricating free-standing silicide NWs is more complicated since metal silicides have lots of phases. In terms of methods, the synthesis of free-standing silicide NWs can be divided into four classifications, which are silicidation of silicon nanowires [611], delivery of silicon to metal films [1216], reactions between transition metal sources and silicon substrates [1722], and simultaneous metal and silicon delivery [2325]. Cobalt silicide nanowires have many relatively good characteristics, including low resistivity, good thermal stability, appropriate work function, and compatibility with current processing of Si devices. There are three main methods for synthesizing CoSi NWs, including reactions of CoCl2 with silicon substrates by chemical vapor deposition (CVD) processes [2628], cobalt silicide nanocables grown on Co films [29], and CVD with single-source precursors [30]. In this work, we synthesized cobalt silicide nanowires through CVD processes and changed and studied the effects of several critical processing parameters. Additionally, we conducted scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses for identifying the structure and composition of the resultant products and investigating their growth mechanisms. Also, the electrical properties of the nanosilicides were measured and discussed for potential applications.


In our study, we synthesized cobalt silicide nanowires by CVD processes using single-crystal Si (100) wafers of native oxide as substrates, anhydrous cobalt chloride powders (97%) as precursors, and Ar gas (99.99%) with H2 gas (15%) as carrier gases. The metal sources were put in the upstream zone where the temperature was 610°C, while the silicon (100) substrates were put in the downstream zone, the temperature range of which was 750°C ~ 900°C. To understand the factors that influence the growth of cobalt silicide nanowires, we conducted experiments with different substrate temperatures, vapor pressures, and gas flow rates. SEM was utilized for the morphology of the nanowires, and TEM analysis was conducted for structure identification and atomic resolution imaging of the nanowires.

Results and discussion

In our experiments, we varied some processing parameters to investigate how they affect the growth and morphology of the cobalt silicide nanowires. Firstly, we focused on the effect of different substrate temperatures as shown in the SEM images of Figure 1a,b,c,d. Figure 1a shows the case with the substrate temperature of 750°C ~ 800°C, where many nanoparticles and few nanowires were found on silicon substrates. Figure 1b shows the case with the substrate temperature of 800°C ~ 850°C, where there were many nanoparticles larger in size than those found in Figure 1a and few nanowires on silicon substrates. When we increased the substrate temperature to 850°C ~ 880°C as shown in Figure 1c, lots of nanowires of about 15 ~ 20 μm in length and few larger nanoparticles appeared. Figure 1d shows the case with the substrate temperature of 880°C ~ 900°C, where on silicon substrates, we can see many nanowires as well but they are of different morphologies as compared in Figure 1c. For further investigation on the atomic structures of the nanowires, we conducted TEM analysis as shown in Figure 2. It has been confirmed that the nanowires on 850°C ~ 880°C substrates are single-crystal CoSi nanowires with 10 ~ 20 nm SiOx as an outer layer as shown in Figure 2a. The high-resolution TEM image in Figure 2b and the corresponding selected area diffraction pattern in its inset show that the single-crystal CoSi nanowire has a cubic B20-type structure with a lattice constant of 0.4446 nm; also, the growth direction is [211], and the interplanar distance of (211) is 0.1816 nm. Figure 2c is an energy-dispersive X-ray spectroscopy (EDS) spectrum for the nanowires showing that in addition to cobalt and silicon, there is also oxygen and that the atomic percentage ratio for Co/Si/O = 5:8:12. Since the core structure has been identified to be CoSi, all these results reasonably indicate that the shell material is amorphous silicon oxide. On 880°C ~ 900°C substrates, Figure 2d shows a single-crystal Co2Si nanowire without surface oxide. The high-resolution TEM image in Figure 2e and the corresponding selected area diffraction pattern in its inset show that the single-crystal Co2Si nanowire has an orthorhombic structure with [002] growth direction and lattice constants of a = 0.4918 nm, b = 0.7109 nm, and c = 0.3738 nm and that the interplanar distances of plane (002) and plane (310) are 0.187 and 0.213 nm, respectively. Figure 2f shows an EDS spectrum indicating that the ratio of Co and Si is close to 2:1.

Figure 1
figure 1

SEM images of as-synthesized nanowires. At silicon substrate temperatures of (a) 750°C ~ 800°C, (b) 800°C ~ 850°C, (c) 850°C ~ 880°C, and (d) 880°C ~ 900°C, respectively.

Figure 2
figure 2

TEM images and EDS spectra of cobalt silicide nanowires. (a) Low-magnification, (b) high-resolution TEM images and (c) EDS spectrum of CoSi nanowires grown at 850°C ~ 880°C. The inset in (b) shows the corresponding selected area diffraction pattern with a zone axis of [0-11]. (d) Low-magnification, (e) high-resolution TEM images and (f) EDS spectrum of Co2Si nanowires grown at 880°C ~ 900°C. The inset in (e) shows the corresponding selected area diffraction pattern with a zone axis of [130].

The second processing parameter we investigated was the vapor pressure. Figure 3a,b,c show our SEM studies for 100, 300, and 500 Torr, respectively. It turns out that CoSi nanowires grew particularly well at the reaction pressure of 500 Torr. In this experiment, the higher the vapor pressure, the longer the nanowires grown. Additionally, with the increasing vapor pressure, the number of nanoparticles reduces, but the size of the nanoparticles increases.

Figure 3
figure 3

SEM images of CoSi nanowires. At vapor pressures of (a) 100, (b) 300, and (c) 500 Torr, respectively.

For the synthesis of cobalt silicide nanowires, the third and final processing parameter we studied was the gas flow rate. We conducted experiments at the gas flow rate of 200, 250, 300, and 350 sccm, obtaining the corresponding results shown in Figure 4a,b,c,d, respectively. It can be found in the SEM images of Figure 4 that at 850°C ~ 880°C, the number of CoSi nanowires reduced with the increasing gas flow rate; thus, more CoSi nanowires appeared as the gas flow rate was lower.

Figure 4
figure 4

SEM images of CoSi nanowires. At gas flow rates of (a) 200, (b) 250, (c) 300, and (d) 350 sccm, respectively.

The growth mechanism of the cobalt silicide nanowires in this work is of interest. Figure 5 is the schematic illustration of the growth mechanism, showing the proposed growth steps of CoSi nanowires with a SiOx outer layer. When the system temperature did not reach the reaction temperature, CoCl2 reacted with H2 (g) to form Co following step (1) of Figure 5:

CoCl 2 g + H 2 g Co s + 2 HCl g
Figure 5
figure 5

The schematic illustration of the growth mechanism. (1) CoCl2(g) + H2(g) → Co(s) + 2HCl(g), (2) 2CoCl2(g) + 3Si(s) → 2CoSi(s) + SiCl4(g), (3) SiCl4(g) + 2H2(g) → Si(g) + 4HCl(g), (4) 2Si(g) + O2(g) → 2SiO(g), and (5) Co(solid or vapor) + 2SiO(g) → CoSi(s) + SiO2(s).

The Co atoms agglomerated to form Co nanoparticles on the silicon substrate. When the system temperature reached the reaction temperatures, 850°C ~ 880°C, CoCl2 reacted with the silicon substrate to form a CoSi thin film and SiCl4 based on step (2) of Figure 5:

2 CoCl 2 g + 3 Si s 2 CoSi s + SiCl 4 g , T = 850 C ~ 880 C

The SiCl4 product then reacted with H2(g) to form Si(g) following step (3) of Figure 5:

SiCl 4 g + 2 H 2 g Si g + 4 HCl g

The Si here reacted with either residual oxygen or the exposed SiO2 surface to form SiO vapor from step (4) of Figure 5[30]:

2 Si g + O 2 g 2 SiO g or Si g + SiO 2 g 2 SiO g

The SiO vapor reacted with Co nanoparticles via vapor-liquid–solid mechanism. Consequently, CoSi nanowires with a SiOx outer layer were grown through step (5) of Figure 5[30]:

Co solid or vapor + 2 SiO g CoSi s + SiO 2 s

When the substrate temperature was at 880°C ~ 900°C, CoCl2 reacted with the silicon substrate to form Co2Si nanoparticles and SiCl4:

2 CoCl 2 g + 3 Si s 2 Co 2 Si s + SiCl 4 g , T = 880 C ~ 900 C

The SiCl4 also reacted with CoCl2 to form Co2Si, transforming Co2Si nanoparticles to Co2Si nanowires through self-catalysis:

2 CoC l 2 g + SiC l 4 s 2 C o 2 Si s + 4 C l 2 g , T = 88 0 C ~ 90 0 C

In addition to understanding the growth behaviors of the cobalt silicide nanowires, we explored their physical properties and etched away the oxide shell before measurements. Figure 6 shows the field emission measurements for CoSi NWs. Figure 6a is the plot of the current density (J) as a function of the applied field (E) with the inset of the ln(J/E2) − 1/E plot. The sample was measured in a vacuum chamber pump to approximately 10−6 Torr. According to the Fowler-Nordheim plot and the Fowler-Nordheim equation:

J = A ß 2 E 2 / φ exp B φ 3 / 2 / ßE ,

where J is the current density, E is the applied electric field, and φ is the work function; for CoSi, φ is 4.7 eV. A and B are constants, corresponding to 1.56 × 10−10 (A (eV)/V−2) and 6.83 × 109 (V (eV)−3/2 m−1), respectively. The field enhancement ß has been calculated to be 1,384 from the slope of ln(J/E2) = ln(2/φ) − 3/2/ßE, proving that CoSi NWs are promising emitters. Also, the higher the density of CoSi NWs, the better the field emission property as shown in Figure 6b. The outstanding field emission properties of CoSi NWs are attributed to their metallic property and special one-dimensional geometry.

Figure 6
figure 6

Field emission analysis. (a) The field emission plot of CoSi NWs. The inset in (a) shows the corresponding ln(J/E2) − 1/E plot. (b) The field emission plot of CoSi NWs with different densities.


In this study, using a CVD method, we have synthesized cobalt silicide nanowires of two different phases, which are CoSi NWs and Co2Si NWs, respectively. Effects of some processing parameters, including the temperature, gas flow rate, and pressure, were investigated; for example, the number of CoSi nanowires shows a decreasing trend with the increasing gas flow rate. Also, the growth mechanism has been proposed. Electrical measurements demonstrate that the CoSi nanowires are potential field-emitting materials.


  1. Zhang SL, Ostling M: Metal silicides in CMOS technology: past, present, and future trends. Crit Rev Solid State Mat Sci 2003, 28: 1–129. 10.1080/10408430390802431

    Article  Google Scholar 

  2. Chen LJ: Silicide Technology for Integrated Circuits. London: The Institution of Electrical Engineers; 2004.

    Book  Google Scholar 

  3. Zhang SL, Smith U: Self-aligned silicides for ohmic contacts in complementary metal–oxide–semiconductor technology. Vac J Sci Technol A 2004, 22: 1361–1370. 10.1116/1.1688364

    Article  Google Scholar 

  4. Maszara WP: Fully silicided metal gates for high-performance CMOS technology: a review. J Electrochem Soc 2005, 152: G550-G555. 10.1149/1.1924307

    Article  Google Scholar 

  5. Schmitt AL, Higgins JM, Szczech JR, Jin S: Synthesis and applications of metal silicide nanowires. J Mater Chem 2010, 20: 223–235. 10.1039/b910968d

    Article  Google Scholar 

  6. Yamamoto K, Kohno H, Takeda S, Ichikawa S: Fabrication of iron silicide nanowires from nanowire templates. Appl Phys Lett 2006, 89: 083107. 10.1063/1.2338018

    Article  Google Scholar 

  7. Lu KC, Wu WW, Wu HW, Tanner CM, Chang JP, Chen LJ, Tu KN: In-situ control of atomic-scale Si layer with huge strain in the nano-heterostructure NiSi/Si/NiSi through point contact reaction. Nano Lett 2007, 7: 2389–2394. 10.1021/nl071046u

    Article  Google Scholar 

  8. Wu WW, Lu KC, Wang CW, Hsieh HY, Chen SY, Chou YC, Yu SY, Chen LJ, Tu KN: Growth of multiple metal/semiconductor nanoheterostructures through point and line contact reactions. Nano Lett 2010, 10: 3984–3989. 10.1021/nl101842w

    Article  Google Scholar 

  9. Lu KC, Wu WW, Ouyang H, Lin YC, Huang Y, Wang CW, Wu ZW, Huang CW, Chen LJ, Tu KN: The influence of surface oxide on the growth of metal/semiconductor nanowires. Nano Lett 2011, 11: 2753–2758. 10.1021/nl201037m

    Article  Google Scholar 

  10. Hsu SC, Hsin CL, Yu SY, Huang CW, Wang CW, Lu CM, Lu KC, Wu WW: Single-crystalline Ge nanowires and Cu3Ge/Ge nano-heterostructures. Cryst Eng Comm 2012, 14: 4570–4574. 10.1039/c2ce25316j

    Article  Google Scholar 

  11. Wu WW, Lu KC, Chen KN, Yeh PH, Wang CW, Lin YC, Huang Y: Controlled large strain of Ni silicide/Si/Ni silicide nanowire heterostructures and their electron transport properties. Appl Phys Lett 2010, 97: 203110. 10.1063/1.3515421

    Article  Google Scholar 

  12. Kim J, Lee ES, Han CS, Kang Y, Kim D, Anderson WA: Observation of Ni silicide formation and field emission properties of Ni silicide nanowires. Microelectron Eng 2008, 85: 1709–1712. 10.1016/j.mee.2008.04.034

    Article  Google Scholar 

  13. Kim J, Anderson WA: Spontaneous nickel monosilicide nanowire formation by metal induced growth. Thin Solid Films 2005, 483: 60–65. 10.1016/j.tsf.2004.12.025

    Article  Google Scholar 

  14. Kim CJ, Kang K, Woo YS, Ryu KG, Moon H, Kim JM, Zang DS, Jo MH: Spontaneous chemical vapor growth of NiSi nanowires and their metallic properties. Adv Mater 2007, 19: 3637–3642. 10.1002/adma.200700609

    Article  Google Scholar 

  15. Kim J, Shin DH, Lee ES, Han CS, Park YC: Electrical characteristics of single and doubly connected Ni silicide nanowire grown by plasma-enhanced chemical vapor deposition. Appl Phys Lett 2007, 90: 253103. 10.1063/1.2749430

    Article  Google Scholar 

  16. Yan XQ, Yuan HJ, Wang JX, Liu DF, Zhou ZP, Gao Y, Song L, Liu LF, Zhou WY, Wang G, Xie SS: Synthesis and characterization of a large amount of branched Ni2Si nanowires. Appl Phys A 2004, 79: 1853–1856.

    Article  Google Scholar 

  17. Kang K, Kim SK, Kim CJ, Jo MH: The role of NiOx overlayers on spontaneous growth of NiSix nanowires from Ni seed layers. Nano Lett 2008, 8: 431–436. 10.1021/nl072326c

    Article  Google Scholar 

  18. Chueh YL, Chou LJ, Cheng SL, Chen LJ, Tsai CJ, Hsu CM, Kung SC: Synthesis and characterization of metallic TaSi2 nanowires. Appl Phys Lett 2005, 87: 223113. 10.1063/1.2132523

    Article  Google Scholar 

  19. Chueh YL, Ko MT, Chou LJ, Chen LJ, Wu CS, Chen CD: TaSi2 nanowires: a potential field emitter and interconnect. Nano Lett 2006, 6: 1637–1644. 10.1021/nl060614n

    Article  Google Scholar 

  20. Xiang B, Wang QX, Wang Z, Zhang XZ, Liu LQ, Xu J, Yu DP: Synthesis and field emission properties of TiSi2 nanowires. Appl Phys Lett 2005, 86: 243103. 10.1063/1.1948515

    Article  Google Scholar 

  21. Ouyang L, Thrall ES, Deshmukh MM, Park H: Vapor phase synthesis and characterization of ϵ-FeSi nanowires. Adv Mater 2006, 18: 1437–1440. 10.1002/adma.200502721

    Article  Google Scholar 

  22. Varadwaj KSK, Seo K, In J, Mohanty P, Park J, Kim B: Phase-controlled growth of metastable Fe5Si3 nanowires by a vapor transport method. J Am Chem Soc 2007, 129: 8594–8599. 10.1021/ja071439v

    Article  Google Scholar 

  23. Szczech JR, Schmitt AL, Bierman MJ, Jin S: Single-crystal semiconducting chromium disilicide nanowires synthesized via chemical vapor transport. Chem Mater 2007, 19: 3238–3243. 10.1021/cm0707307

    Article  Google Scholar 

  24. Song Y, Schmitt AL, Jin S: Ultralong single-crystal metallic Ni2Si nanowires with low resistivity. Nano Lett 2007, 7: 965–969. 10.1021/nl0630687

    Article  Google Scholar 

  25. Schmitt AL, Bierman MJ, Schmeisser D, Himpsel FJ, Jin S: Synthesis and properties of single-crystal FeSi nanowires. Nano Lett 2006, 6: 1617–1621. 10.1021/nl060550g

    Article  Google Scholar 

  26. Seo K, Lee S, Yoon H, In J, Varadwaj KSK, Jo Y, Jung MH, Kim J, Kim B: Composition-tuned Co(n)Si nanowires: location-selective simultaneous growth along temperature gradient. ACS Nano 2009, 3: 1145–1150. 10.1021/nn900191g

    Article  Google Scholar 

  27. Liang YH, Yu SY, Hsin CL, Huang CW, Wu WW: Growth of single-crystalline cobalt silicide nanowires with excellent physical properties. J Appl Phys 2011, 110: 074302. 10.1063/1.3643007

    Article  Google Scholar 

  28. Tsai CI, Yeh PH, Wang CY, Wu HW, Chen US, Lu MY, Wu WW, Chen LJ, Wang ZL: Cobalt silicide nanostructures: synthesis, electron transport, and field emission properties. Cryst Growth Des 2009, 9: 4514–4518. 10.1021/cg900531x

    Article  Google Scholar 

  29. Hsin CL, Yu SY, Wu WW: Cobalt silicide nanocables grown on Co films: synthesis and physical properties. Nanotechnology 2010, 21: 485602. 10.1088/0957-4484/21/48/485602

    Article  Google Scholar 

  30. Schmitt AL, Lei Z, Schmeiβer D, Himpsel FJ, Jin S: Metallic single-crystal CoSi nanowires via chemical vapor deposition of single-source precursor. J Phys Chem B 2006, 110: 18142–18146. 10.1021/jp064646a

    Article  Google Scholar 

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KCL acknowledges the support from the National Science Council through grant 100-2628-E-006-025-MY2.

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Correspondence to Kuo-Chang Lu.

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The authors declare that they have no competing interests.

Authors’ contributions

CML and KCL conceived the study and designed the research. CML conducted the experiments. CML, HFH, and KCL wrote the manuscript. All authors read and approved the final manuscript.

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Lu, CM., Hsu, HF. & Lu, KC. Growth of single-crystalline cobalt silicide nanowires and their field emission property. Nanoscale Res Lett 8, 308 (2013).

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  • CVD
  • Cobalt silicide
  • Nanowires
  • Single crystalline
  • Field emission