TaSi2 Nanowires: A Potential Field Emitter and Interconnect - Nano

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NANO LETTERS

TaSi2 Nanowires: A Potential Field Emitter and Interconnect

2006 Vol. 6, No. 8 1637-1644

Yu-Lun Chueh,† Mong-Tzong Ko,† Li-Jen Chou,*,† Lih-Juann Chen,† Cen-Shawn Wu,‡,§ and Chii-Dong Chen§ Department of Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu, Taiwan, Republic of China, Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan UniVersity, Taiwan, Republic of China, and Institute of Physics, Academia Sinica, Nankang, 115, Taipei, Taiwan, Republic of China Received March 18, 2006; Revised Manuscript Received June 5, 2006

ABSTRACT TaSi2 nanowires have been synthesized on a Si substrate by annealing NiSi2 films at 950 °C in an ambient containing Ta vapor. The nanowires could be grown up to 13 µm in length. Field-emission measurements show that the turn-on field is low at 4−4.5 V/µm and the threshold field is down to 6 V/µm with the field enhancement factor as high as 1800. The metallic TaSi2 nanowires exhibit excellent electrical properties with a remarkable high failure current density of 3 × 108 A cm-2. In addition, effects of annealing temperatures and capability of metal silicide mediation layer on the growth of nanowires are addressed. This simple approach promises future applications in nanoelectronics and nanooptoelectronics.

One-dimensional (1D) systems such as nanowires (NWs), nanorods, nanobelts, and nanotubes have attracted much attention owing to their unique optical, electronic, and mechanical properties.1,2 On the other hand, the requirements for interconnect and contact in the next generation nanoelectronics are low resistivity, good ohmic contact to both p- and n-type semiconductors, high-temperature stability, low cost, and compatibility with the processing of Si complementary metal-oxide-semiconductor (CMOS) devices.3 The cathode materials, such as carbon-based materials, carbon nanotubes (CNTs), diamond films, and sp2-sp3 hybridized carbon materials, can be applied in nanodevices. The devices, including emitter for field-emission application and interconnection between individual nanodevices, ultilize mainly the metallic characteristic and high thermal stability of these materials.1,2,4 Various silicide nanowires are the other alternatives. Among them, refractory metal silicides are a group of silicides that possess satisfactory properties and may be used in nanoelectronics. The best ways for the synthesis of these silicides is by a bottom-up approach via self-assembly, but the challenges for self-assembled silicide NWs are control of aspect ratio and location. In addition, the self-assembly * To whom correspondence may be addressed. E-mail: [email protected]. Tel: +886-3-5715131, ext 3806. Fax: +886-35722366. † Department of Materials Science and Engineering, National Tsing Hua University. ‡ Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University. § Institute of Physics, Academia Sinica. 10.1021/nl060614n CCC: $33.50 Published on Web 07/12/2006

© 2006 American Chemical Society

of nanowires requires the substrate be crystalline, precluding their use for many potential applications.4 Up to the present time, only a few alternative approaches have been adopted to grow silicide nanowires without relying on the mismatch between the nanowire and the substrate. For the growth of NiSi nanowires, Ni film was deposited on Si nanowires via the chemical vapor deposition (CVD) process to form the NiSi nanowires after annealing.5 Depending on the growth conditions, single-phase Ni2Si, Ni3Si2, and NiSi nanowires were formed. Others included preparing the carbon-coated nickel silicide nanowires (C-coated NiSi NWs) via a radio frequency induction heating chemical vapor deposition (RF-CVD) reactor.6 Xiang et al. used a vapor-phase deposition method to grow TiSi2 nanowires on silicon wafers.7 Recently, TaSi2 nanowires have been synthesized by annealing FeSi2 thin film and nanodots grown on Si substrate in an ambient containing Ta vapor. Strong field-emission properties promise future electronics and optoelectronics applications.8 In addition, by using the Si nanowires as a template followed by depositing a Ni film with a thickness comparable to the average Si nanowire diameter, the NiSi nanowire can be generated after annealing at 550 °C.9 In the present study, we utilized NiSi2 film to grow TaSi2 NWs to a length up to 13 µm by annealing NiSi2 films on a Si substrate in an ambient containing Ta vapor in a vacuum that was better than 1 × 10-6 Torr. The findings in fieldemission and electrical transport behaviors demonstrate that these promising metallic nanowires can be used in a field-

Figure 1. (a) Top view SEM image of nanowires synthesized by annealing the NiSi2 film samples on Si substrate at 950 °C for 16 h in a Ta ambient. Upper insets show the corresponding side view SEM images (90° tilting). (b) A pie chart showing the distribution of diameters of TaSi2 nanowires grown by NiSi2 as a mediation layer. (c) Top view SEM image of nanowires synthesized by annealing the NiSi2 film on Si substrate at 950 °C for 32 h in a Ta ambient. The length of TaSi2 nanowires is as long as 13 µm. The long nanowires are marked by white arrows. The dark contrast regions correspond to the pinholes marked by red arrows. Lower inset shows a side view SEM image (90° tilted). (d) Top-view SEM image of nanowires synthesized by annealing the NiSi2 nanodot sample on Si substrate at 950 °C for 16 h in a Ta ambient. Upper inset shows a side view SEM image (90° tilted). (e) The corresponding GIXRD spectrum showing the presence of TaSi2 phase. (f) Top view SEM image of nanowires synthesized by annealing the FeSi2 film on Si substrate at 950 °C for 16 h in a Ta ambient. The dark contrast regions correspond to the pinholes. Upper inset shows a side view SEM image (90° tilted). The lower inset shows the side view SEM image of a pinhole.

emission device or as an interconnect and/or contact in nanoelectronic circuit. In addition, the synthesis parameters, including the annealing temperatures and the mediating metal silicide layers, are investigated and discussed to clarify the growth mechanisms. Single crystal (001) Si wafers (1-30 Ω cm) were cleaned by the standard cleaning process. Ni films 30 and 1.5 nm thick were deposited on a Si substrate with an ultrahigh vacuum e-beam deposition system at room temperature. The as-deposited samples were annealed at 850 °C for 30 min without breaking the vacuum to form the NiSi2 film and nanodot samples on the Si substrate.10,11 As-annealed samples were transferred into a Ta filament heating chamber for annealing at a pressure of lower than 1 × 10-6 Torr at 8501638

950 °C for different periods of time. The Ta atoms were vaporized constantly as the supplementary source for the growth of nanowires (Figure 1S, Supporting Information). The grazing incidence X-ray diffractometry (GIXRD) with a fixed incident angle of 0.5° was carried out to identify the phases. A field-emission transmission electron microscope (JEM-3000F, operated at 300 kV with point-to-point resolution of 0.17 nm) equipped with an energy dispersion spectrometer (EDS) and a high-angle annular dark-field (HAADF) detector was used to obtain the information of the microstructures and the chemical compositions. The surface morphology was examined with a field-emission scanning electron microscope (FESEM, JSM-6500F) operating at 15 kV. The electron field-emission properties were Nano Lett., Vol. 6, No. 8, 2006

Figure 2. (a) TEM image of a nanowire. Inset shows the corresponding diffraction pattern, revealing that the nanowire is of TaSi2 phase with [123h3] zone axis. (b) The corresponding EDS elemental line profiles show that the nanowire is composed of Ta, Si, and Ni. Upper inset shows the corresponding EDS spectrum. (c) High-resolution TEM image of a TaSi2 nanowire. The defect structures are evident, as marked by circles. (d) EELS spectra of Ni atoms in a TaSi2 nanowire and NiO film after background subtraction.

measured in a vacuum at a pressure of 1 × 10-7 Torr using a spherical stainless steel probe (1 mm in diameter) as the anode. The lowest emission current was recorded on the level of nanoamperes. The measurement distance between the anode and emitting surface was fixed at 100 µm. Electrical measurements were performed by sequential procedures including electrodes defined by electron-beam lithography, metal evaporation, and device evaluation. The 30 kV cold field emission SEM (FEI-SIRION) with nanopattern generation system (NPGS) was utilized for these purposes. A LabView program was used to control the I-V testing process. Figure 1a shows the SEM image of nanowires synthesized by annealing the NiSi2 film sample on Si substrate at 950 °C for 16 h in a Ta ambient. The diameters and lengths of the nanowires are about 20-30 nm and 3-6 µm, respectively, with aspect ratios of about 150-300. The pie chart of the distribution of diameters is shown in Figure 1b. When the annealing time was extended to 32 h, the length of nanowires could grow to over 13 µm with an aspect ratio of about 650 (Figure 1c). The density of TaSi2 nanowires with the length of 3-6 µm synthesized by NiSi2 nanodots samples was significantly increased compared to that of films, as shown in Figure 1d. Pinholes with the closest-packed {111} facets seen elsewhere indicate that the Si atoms had been vaporized or migrated out of the Si substrate to react with Ta to form TaSi2 during the growth of the nanowires (insets in parts c and f of Figure 1). All the nanowires were grown uniformly on the Si substrate, and the tip regions were semispherical in shape without the presence of metal catalysts. The GIXRD spectrum taken from NiSi2 nanodots as the mediation layer, as shown in Figure 1e, reveals that Nano Lett., Vol. 6, No. 8, 2006

the nanowires are of TaSi2 phase. The TaSi2 is hexagonal in structure (P222 (180) point group) with a lattice constant of a ) 0.48 nm and c ) 0.66 nm (JCPSD card, No-38-0483). In contrast, TaSi2 nanowires synthesized by annealing FeSi2 film in the same conditions, as shown in Figure 1f, had lengths of about 100-200 nm, considerably shorter than those with NiSi2 mediation layers. Figure 2a shows the transmission electron microscopy (TEM) image of a 20 nm TaSi2 nanowire. The tip is semispherical in shape without the trace of metal catalysts. The analysis of selected area diffraction (SAD), as shown in the inset of Figure 2a, indicates that the nanowire is singlecrystal TaSi2. The HAADF image and the corresponding EDS line-scan profiles are shown in Figure 2b. The bright image is surrounded by a thin dark layer which corresponds to the TaSi2 nanowire and the surrounding 1 nm thick amorphous oxide layer, respectively. The EDS line profiles indicate that the nanowire consists of Ta, Si, and Ni. In addition, the atomic concentrations of Ta ) 32%, Si ) 64%, and Ni ) 4% are calculated from the EDS spectrum, which is taken from the signals generated by the nanobeam incident on the nanowire, as shown in the inset. The corresponding high-resolution TEM (HRTEM) image is shown in Figure 2c. The measured lattice spacings of 0.22 and 0.25 nm are consistent with the d values of the (12h11) and (21h1h0) planes with the growth direction of the nanowire determined to be along the [21h1h0] (a axis). Point defects marked by circles are suggested to be induced by the presence of Ni atoms. The electron energy loss spectroscopy (EELS) spectra of the L2,3 edge for the Ni atoms in TaSi2 nanowires and standard NiO sample are shown in Figure 2d. A standard calibration process of the EELS spectrum was carried out. It took into 1639

Figure 3. The field-emission behaviors as a function of applied electric field at a distance of 100 µm between emitting surface and anode for NiSi2 and FeSi2 film and nanodot on Si samples annealed at 950 °C for 16 h in a Ta ambient, respectively. The data were recorded for a series of experiments, denoted as first, third, and fifth. Insets in (a) and (b) show the corresponding ln(J/E2)-1/E plots. (c) The relationships between aspect ratio and field enhancement factor for TaSi2 nanowires synthesized by annealing NiSi2 film and nanodot samples at 950 °C for 16 h. (d) The stability test with fixed voltage at 700 V over time for TaSi2 nanowires synthesized with NiSi2 nanodot sample after annealing at 950 °C for 16 h. Inset shows a similar plot with a fixed voltage at 900 V.

account the zero loss peak and removed the multiplescattering events via a deconvolution process by low loss spectrum to obtain the final spectrum caused by single scattering. In general, Ni may possess two valent states (Ni0 and Ni+) in different conditions. L3 and L2 edge peaks were found at positions of 855 and 873 eV for TaSi2 nanowires. No chemical shift in the L3 edge between a standard NiO sample and Ni atoms in TaSi2 nanowires was observed in the EELS spectra. It suggests that the Ni atom in TaSi2 nanowries is of natural type (Ni0) as the intensity ratio of the L3/L2 edges is smaller than that of the NiO standard sample.12 The Ni content in TaSi2 nanowires was found to increase with annealing time and eventually reach a saturation value. The presence of the point defects in TaSi2 nanowires may result from the Ni atoms that reside in interstitial sites (octahedral and tetrahedral coordination) of the TaSi2 lattice. Parts a and b of Figure 3 show the current density as a function of the applied voltage for the TaSi2 nanowire synthesized by FeSi2 and NiSi2 films and nanodot samples at 950 °C for 16 h in a Ta ambient, respectively. The measured distance between anode and emitting surface was fixed at 100 µm. The turn-on field and threshold field for field-emission measurements are defined as the applied voltage to produce current densities of 0.01 and 10 mA/cm2, respectively. The shorter TaSi2 nanowires synthesized with the FeSi2 mediation film exhibit a turn-on field of 6.3-7.3 V/µm. On the other hand, the turn-on field is decreased to 1640

about 5.3 V/µm for longer TaSi2 nanowires synthesized with FeSi2 mediation dots (Figure 3a). The reduction in turn-on field is associated with the increase in aspect ratio. The TaSi2 nanowires synthesized by annealing NiSi2 film and nanodot samples exhibit excellent field-emission properties. The turnon fields are down to 4.6-5.2 V/µm and 4-4.5 V/µm for film and nanodot samples, respectively. In addition, the threshold field can be found to be about 6.5 V/µm for the TaSi2 nanowires mediated by NiSi2 film while the threshold field can be down to about 6 V/µm for nanodot samples (Figure 3b). Although these values of turn-on fields are higher than the best reported values of the carbon nanotube,13,14 they are still much lower than many other types of emitters such as Si nanowires,15 In2O3,16 MoO3,17,18 TiSi2,7 W18O49 nanowires,19 CuO20 nanowires, and ZnO21 and SnO2 nanowires.22 The insets in parts a and b of Figure 3 show the plots of the ln(J/E2)-(1/E) relationship. The linear relationship is consistent with the so-called Folwer-Nordheim (F-N) plot, indicating that the field-emission behavior obeys the F-N rule for which the electrons can tunnel through the potential barrier from conduction band to vacuum sate. The F-N relationship can be simply expressed by an equation as follows23,24 J ) (Aβ2E2/Φ) exp(-BΦ3/2/βE) where J ) the current density, E ) the applied field strength, Nano Lett., Vol. 6, No. 8, 2006

and Φ ) work function. A and B are constant, corresponding to 1.56 × 10-10 (A V-2 (eV)) and 6.83 × 103 (eV-3/2 (µm-1)), respectively. The field enhancement factor, β, reflects the degree of the field-emission enhancement of the tip shape on a planar surface. The β value is dependent on the geometry of the nanowires, the crystal structure, and the density of emitting points. By determining the slopes of the ln(J/E2)-(1/E) plots in the insets of parts a and b of Figure 3 with a work function value of TaSi2 (4.7 eV),25 the β values can be calculated to be about 400 and 900 for the shorter and longer TaSi2 nanowires synthesized with FeSi2 film and nanodot samples, respectively. On the other hand, the TaSi2 nanowires synthesized with NiSi2 film and nanodot samples display the high β values of 1200 and 1800, respectively. Compared to the β values of other materials, such as Si nanowires (β ) 1000),15 NiSi2 nanorods (β ) 630),26 TiSi2 nanowires (β ) 501)7, SnO2 (β ) 1402.9),22 AlN (β ) 950),27 and ZnO (β ) 1464),21 TaSi2 nanowires are promising emitters. Furthermore, the relationships between field enhancement factor and aspect ratio were investigated for TaSi2 nanowires synthesized with FeSi2 and NiSi2 film and nanodot samples annealed at 950 °C for 16 h in a Ta ambient, respectively, as shown in Figure 3c. The field enhancement factor is proportional to the aspect ratio, consistent with a previous report.14 However, it is worth mentioning that the field enhancement factor for a NiSi2 naodot sample is higher than that of film sample but their aspect ratios are not much different. The higher β value for the nanodot samples is attributed to the highly effective emission from the highdensity TaSi2 nanowires. It is recognized that normally the field enhancement factor would be decreased as the density of the nanowires is increased due to the space charge effect.7,28,29 However, this space charge effect plays a minor role in the present study due to the fact that all the TaSi2 nanowires are well separated and aligned with the substrate.19 As shown in the inset of Figure 3d, for a fixed applied voltage of 900 V, current density dropped sharply within 20 s. By repeating the field-emission measurements at higher field (above 900 V), the current density was found to be severely degraded. The degradation of TaSi2 nanowires during the field-emission test at high electric field may be attributed to the melting, breakdown, or detachment from the substrate of nanowires. On the other hand, the degradation in current density was found to be below 5% when a fixed voltage of 700 V was applied. The current density under a fixed voltage of 700 V is high enough for applications as either emitter or display. The low turn-on voltage, low threshold voltage, and very high β value for longer TaSi2 nanowires are ascribed to the presence of a high local electrical field due to the high aspect-ratio feature. Furthermore, it is free of metal catalyst and can be considered to be an excellent candidate for the application as the fieldemission source. For I-V measurement, Cr was selected as the contact metal with a work function of 4.4 eV, which is smaller than that of TaSi2 (4.7 eV). It provides a better Ohmic contact compared to the other metals, such as Au (∼5.1 eV) or Pt (∼5.7 eV). In this Letter, the four-probe measurements were Nano Lett., Vol. 6, No. 8, 2006

performed but failed owing to the high contact resistance between electrode 1 and the nanowire, as shown in the inset of Figure 3c. Instead, the resistance of TaSi2 nanowire with 25 nm diameter synthesized by annealing NiSi2 film at 950 °C for 32 h was calculated to be 2.47 kΩ at 300 K via threeprobe measurements (2-2-3-4) minus the contact resistance of electrode 2, as shown in Figure 4a (Figure 2S, Supporting Information).30 The linear I-V behavior at 300 K shows that the electrical characteristic of TaSi2 nanowire is metallic with a low resistivity of about 114 µΩ cm, which is higher than that of its bulk counterpart.31 The higher measured resistivity is attributed to several reasons: the presence of doping Ni atoms in the nanowires; oxide coating layer; contaminations around the surface and contact regions, which are unavoidably generated during the device processing. In addition, defects and anisotropy in electron transport (along the [21h1h0] growth direction) in TaSi2 nanowires may influence the measured resistivity. The elastic scattering mean free path of electrons in the TaSi2 nanowire can be estimated to be about 8 nm at 300 K,32 indicating that the TaSi2 nanowire retains the attractive metallic transport property. As the measured temperature was reduced to 70 K, the I-V characteristic still kept the linear behavior (Figure 4b). Figure 4c shows the resistivity as a function of temperature. The resistivity was decreased with the temperature and saturated at a temperature below 30 K. The residual resistivity was found to be about 107 µΩ cm, which is also higher than that of bulk.32 Again, the oxide coating layer may be the detrimental factor. The finding suggests that TaSi2 nanowires have great potential to be used as contact and interconnect for future nanoelectronics. In addition, the negative curvature (d2F/dT2 < 0) found in Figure 4c is similar to that seen in bulk behavior. It suggests that the electron-phonon scattering is dominant for the transport mechanism (part C in Supporting Information).33 The results of durability and reliability tests of the TaSi2 nanowire were obtained by performing I-V measurements under very high applied current and voltage, as shown in Figure 4d. The nanowire can endure a current of up to 2.2 mA under high voltage stress before failure. Note that a high current density of 3 × 108 A cm-2 is estimated in this case, which is comparable with the other metallic nanowires, such as NiSi nanowires (3 × 108 A cm-2),9 CNT (109 A cm-2),34 and Pd nanowires (2 × 106 A cm-2).35 Accordingly this high failure current density give us an important feature, which is promising for the application as the interconnect in future nanodevices. The possible growth mechanism, inferred from the TEM and SEM results, consists of three growth stages, which were reported elsewhere with FeSi2 film as mediation layer.8 For the NiSi2 case, similar growth phenomena were found but the length of the nanowires is significantly enhanced (Figure 3S, Supporting Information). To clarify the growth mechanism, the effects of different annealing temperatures and capability of the metal silicide layer as the mediation layers are addressed. Figure 5a shows an SEM image of an asdeposited FeSi2 film sample as a mediation layer to synthesize the TaSi2 nanowires. The surface of the FeSi2 layer is rather uneven. After annealing at 850 °C for 16 h (Figure 1641

Figure 4. I-V curves at (a) 300 and (b) 70 K for TaSi2 nanowires synthesized by annealing NiSi2 film sample at 950 °C for 32 h, respectively. The resistances of nanowire were estimated to be about 2.47 and 2.29 kΩ, respectively. (c) The resistivity as a function of temperature. Inset shows the SEM image of the I-V measurement configuration. (d) I-V curve recorded at high applied voltages.

5b), no nanowires were found until the annealing temperature was increased to 900 °C for 16 h (Figure 5c). As the thickness of the Fe film was decreased from 30 to 1.5 nm, the nanodots feature can be formed after preannealing at 850 °C for 2 h, as shown in Figure 5d. Comparing FeSi2 film and nanodot samples annealed at 850 °C for 16 h, some TaSi2 nanowires were found to initiate (Figure 5e) at nanodots sample and lengthen for annealing at 900 °C for 16 h. The length of the nanowires is significantly longer than that of film samples (Figure 5f). The explanation is based on the size effect since FeSi2 nanodots are expected to be less stable than the continuous films.36 The FeSi2 nanodots are prone to be supersaturated with Si from the Si substrate, resulting in the segregation of more Si atoms during the annealing process. To replace the FeSi2 film by NiSi2 film, both the density and length of TaSi2 nanowires are increased in samples annealed at 850 °C for 6 h. After annealing at 900 °C for 16 h (Figure 5g), the lengths of TaSi2 nanowires are longer than 1 µm (Figure 5h). The lengthening of the nanowires in NiSi2 film samples compared to that of FeSi2 film samples is correlated to the significantly lower melting point (mp) of NiSi2 (993 °C) than that of FeSi2 (1200 °C). The NiSi2 film is prone to be supersaturated with Si from the substrate compared with FeSi2 film at the identical annealing condition, resulting in more segregated Si atoms during the annealing process. By decreasing the thickness of NiSi2 from 30 to 1.5 nm, the average density and length of TaSi2 are increased compared to that of film samples. For further probing the effects of mediation layer, 30 nm thick Co, Ti, Ta, and Pt metals were also prepared by e-beam deposition. After preannealing at 850 °C for 2 h, CoSi2, TiSi2, TaSi2, and PtSi layers were formed. Figure 5i shows an SEM 1642

image of the TaSi2 nanowires mediated by CoSi2 (mp ) 1326 °C) film sample after annealing at 950 °C for 16 h. The lengths and diameters of TaSi2 nanowires were measured to be about 100-200 nm and 20-40 nm, respectively, which is almost identical to that of FeSi2 film sample. By using the metal silicide layer with higher melting point, such as TiSi2 (mp ) 1540 °C), the length and density of TaSi2 nanowires are significantly decreased at the identical annealing condition (950 °C for 16 h). It is attributed to the degree of supersaturation with Si atoms from the substrate and the metal silicide mediation layer is decreased, resulting in the decrease in segregation for Si atoms from the metal silicide mediation layer. Accordingly, it was expected and confirmed that no nanowires would grow if very high melting point metal silicide, such as the TaSi2 (mp ) 2200 °C) mediation layer, was used after annealing at 950 °C for 16 h. An example is shown in Figure 5k. On the other hand, no nanowires were grown in samples with PtSi mediation layer after annealing at 950 °C for 16 h, although the melting point of PtSi (1229°C) is close to that of FeSi2, as shown in Figure 5l. The dominant diffusing species for the metal disilicides, such as FeSi2, NiSi2, CoSi2, TiSi2, and TaSi2, used in the present study are Si atoms, while for the PtSi, the diffusing species is dominated by Pt atoms. Thus, Si atoms diffused rapidly from the substrate, saturated, and eventually segregated from the metal disilicide mediation layers during the high-temperature annealing.25 In contrast, the segregation of Si atoms would not occur if the dominant diffusing species are metal atoms. In addition, the metal species were found to decompose from metal silicide mediation layers such as FeSi2, NiSi2, CoSi2, and TiSi2 films to distribute in the TaSi2 nanowires during the growth. Consequently, the TaSi2 Nano Lett., Vol. 6, No. 8, 2006

Figure 5. Top view SEM images of TaSi2 nanowires synthesized by annealing FeSi2 film samples at different temperatures for 16 h: (a) as-deposited, (b) at 850 °C, and (c) at 900 °C. Top view SEM images of TaSi2 nanowires synthesized by annealing FeSi2 nanodot samples annealed at different temperatures for 16 h: (d) as-deposited, (e) at 850 °C, and (f) at 900 °C. Top view SEM images of TaSi2 nanowires synthesized by annealing NiSi2 film samples at (g) 850 and (h) 900 °C. (i) Top view SEM image of TaSi2 nanowires synthesized by annealing CoSi2 film sample at 950 °C for 16 h. Inset shows the corresponding 90° tilted SEM image. Top view SEM images for (j) TiSi2, (k) TaSi2, and (l) PtSi film samples after annealing at 950 °C for 16 h. All annealings were conducted in a Ta ambient.

nanowires grow in length with the increase in metal concentration during the annealing process. In summary, TaSi2 nanowires have been synthesized successfully by annealing different silicide mediation layers at 950 °C in a Ta ambient. The length of TaSi2 can be enhanced by annealing the NiSi2 layer. Field-emission measurements show the excellent performance with the lowest turn-on field of 4-4.5 V/µm and the threshold field is down to 6 V/µm. In addition, the field enhancement factor is as high as 1800 for NiSi2 nanodot samples. Measurements of electrical transport properties indicate that the metallic TaSi2 nanowires can endure a current of up to 2.2 mA with the calculated current density of 3 × 108 A cm-2. The metal content in TaSi2 nanowires was found to increase with annealing time and eventually reach a saturation value. Detailed microstructures and compositional analysis of these unique TaSi2 nanowires are presented. The effects of different annealing temperatures and species of metal silicide mediation layers are investigated and clarified. Nano Lett., Vol. 6, No. 8, 2006

Acknowledgment. This research was supported by the National Science Council through Grant No. NSC 94-2215E-007-004, and the Ministry of Education through Grant No. 94-E-FA04-1-4. Supporting Information Available: The experimental setup for the synthesis of TaSi2 nanowires, SEM images for TaSi2 nanowires grown by annealing FeSi2 and NiSi2 mediation layers under different conditions, and growth mechanism for TaSi2 nanowires grown with the mediation of FeSi2 and NiSi2 layers. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Dean, K. A.; Chalamala, B. R. Appl. Phys. Lett. 1999, 75, 3017. (2) Zhu, W.; Kochanski, G. P.; Jin, S.; Seibles, L.; Jacobson, D.; McCormack, C. M.; White A. E. Appl. Phys. Lett. 1995, 67, 1157. (3) Chen, L. J. JOM 2005, 57 (9), 24. (4) Kiyota, H.; Araki, H.; Kobayashi, H.; Shiga, T.; Kitaguchi, K.; Lida, M.; Wang, H.; Miyo, T.; Takida, T.; Kurosu, T.; Lnoue, K.; Saito, I.; Nishitan-Gamo, M.; Sakaguchi, I.; Ando, T. Appl. Phys. Lett. 1999, 75, 2331. 1643

(5) Decker, C. A.; Solanki, R.; Freeouf, J. L.; Carruthers, J. R.; Evans, D. R. Appl. Phys. Lett. 2004, 84, 1389. (6) Lee, K. S.; Mo, Y. H.; Nahm, K. S.; Shim, H. W.; Suh, E. K.; Kim, J. R.; Kim, J. J. Chem. Phys. Lett. 2004, 384, 215. (7) Xiang, Q.; Wang, Q. X.; Wang, Z.; Zhang, X. Z.; Liu, L. Q.; Xu, J.; Yu, D. P. Appl. Phys. Lett. 2005, 86, 243103. (8) Chueh, Y. L.; Chou, L. J.; Cheng, S. L.; Chen L. J.; Tsai C. J.; Hsu C. M.; Kung S. C. Appl. Phys. Lett. 2005, 87, 223113. (9) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber C. M. Science 2004, 430, 61. (10) Chen, L. J.; Tu, K. N. Mater. Sci. Rep. 1991, 6, 53. (11) Cheng, L. W.; Cheng, S. L.; Chen, L. J.; Chien, H. C.; Lee, H. L.; Pan, F. M. J. Vac. Sci. Technol., A 2000, 18, 1176. (12) Leapamn, R. D.; Grunes, L. A.; Fejes, P. L. Phys. ReV. B 1982, 26, 26. (13) Bonard, J. M.; Maier, F.; Stockli, T.; Heer, W. A. de; Salvetat, J. P.; Forro, L. Ultramicroscopy 1998, 73, 918. (14) Liu, C.; Tong, Y.; Cheng, H. M.; Golberg, D.; Bando, Y. Appl. Phys. Lett. 2005, 86, 223114. (15) Chueh, Y. L.; Chou, L. J.; Cheng, S. L.; He, J. H.; We, W. W.; Chen, L. J. Appl. Phys. Lett. 2005, 86, 133112. (16) Li, S. Q.; Liang, Y. X.; Wang, T. H. Appl. Phys. Lett. 2005, 87, 143104. (17) Zhou, J.; Xu, N. S.; Deng, S. Z.; Chen, J.; She, J. C.; Wang, Z. L. AdV. Mater. 2003, 15, 1835 (18) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 5048. (19) Zhou, J.; Gong, L.; Deng, S. Z.; Chen, J.; She, J. C.; Xu, N. S.; Yang, R.; Wang, Z. L. Appl. Phys. Lett. 2005, 87, 223108 (20) Hsieh, C. T.; Chen, J. M.; Lin, H. H.; Shih, H. C. Appl. Phys. Lett. 2003, 83, 3383.

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(21) Tseng, Y. K.; Huang; C. J.; Cheng, H. M.; Kin, I. N.; Liu, K. S.; Chen, I. C. AdV. Funct. Mater. 2003, 87, 73109. (22) He, J. H.; Wu, T. H.; Hsin, C. L.; Li, K. M.; Chen, L. J.; Chueh, Y. L.; Chou, L. J.; Wang, Z. L. Small 2006, 2, 116 (23) Xu, C. X.; Sun, X. W. Appl. Phys. Lett. 2003, 83, 3806. (24) Chueh, Y. L.; Chou, L. J.; Hsu, C. M.; Kung, S. C. J. Phys. Chem. B 2005, 109, 21831 (25) Murarka, S. P. Silicide for VLSI Application; Academic: New York, 1983. (26) Ok, Y. W.; Seong, T. Y.; Choi, C. J.; Tu, K. N. Appl. Phys. Lett. 2006, 88, 043106. (27) He, J. H.; Yang, R. S.; Chueh, Y. L.; Chou, L. J.; Chen, L. J.; Wang, Z. L. AdV. Mater. 2006, 18, 650. (28) Barbour, J. P.; Dolan, W. W.; Trolan, J. K.; Martin, E. E.; Dyke, W. P. Phys. ReV. 1953, 92, 45. (29) Filip, V.; Nicolaesu, D.; Tanemura, M.; Okuyama, F. Ultramicroscopy 2001, 89, 39. (30) Hsiou, Y. F.; Yang, Y. J.; Stobinski, L.; Kuo, W.; Chen, C. D. Appl. Phys. Lett. 2003, 84, 984. (31) Nava, F.; Tu, K. N.; Mazzega, E.; Michelini, M.; Queirolo, G. J. Appl. Phy. 1987, 61, 1085. (32) Maex, K.; Rossum, M. V. Properties of Metal Silicides; INSPEC: London, U.K., 1995. (33) Nava, F.; Bisi, O.; Tu, K. N. Phys. ReV. B 1986, 34, 6143. (34) Yao, Z.; Kane, C. L.; Dekker: C. Phys. ReV. Lett. 2000, 84, 2941. (35) Cheng, C.; Gonela, R. K.; Gu, Q.; Haynie, D. T. Nano Lett. 2004, 5, 175. (36) Goldstein, A. N.; Echer, C. M.; Alivisatos, A. P. Science 1992, 256, 1425.

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