Letter pubs.acs.org/NanoLett
Coaxial Metal-Silicide Ni2Si/C54-TiSi2 Nanowires Chih-Yen Chen,† Yu-Kai Lin,† Chia-Wei Hsu,† Chiu-Yen Wang,† Yu-Lun Chueh,† Lih-Juann Chen,† Shen-Chuan Lo,‡ and Li-Jen Chou*,† †
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Material and Chemical Research Laboratories, ITRI, Hsinchu, Taiwan
‡
S Supporting Information *
ABSTRACT: One-dimensional metal silicide nanowires are excellent candidates for interconnect and contact materials in future integrated circuits devices. Novel core−shell Ni2Si/C54TiSi2 nanowires, 2 μm in length, were grown controllably via a solid−liquid−solid growth mechanism. Their interesting ferromagnetic behaviors and excellent electrical properties have been studied in detail. The coercivities (Hcs) of the core−shell Ni2Si/ C54-TiSi2 nanowires was determined to be 200 and 50 Oe at 4 and 300 K, respectively, and the resistivity was measured to be as low as 31 μΩ-cm. The shift of the hysteresis loop with the temperature in zero field cooled (ZFC) and field cooled (FC) studies was found. ZFC and FC curves converge near room temperature at 314 K. The favorable ferromagnetic and electrical properties indicate that the unique core−shell nanowires can be used in penetrative ferromagnetic devices at room temperature simultaneously as a future interconnection in integrated circuits. KEYWORDS: Core−shell, nanowire, Ni2Si, C54-TiSi2, silicide, interconnect
A
concerns about radial silicide nanowire heterostructures have only recently emerged.20,26−28 It is worth mentioning that one dimensional (1D) core−shell Ge/Si nanowires have been used as high-performance coaxial FET devices26 and core−shell Cr5Si3/Si nanopillars can improve the carrier transport between the metal and semiconductor layers.28 As a result, core−shell metal silicide nanostructures should be explored as nanodevices due to their interesting and unique properties. Herein, we report the formation of core−shell Ni2Si/C54-TiSi2 nanowire heterostructures as well as their magnetic and electrical properties. Results and Discussion. Core−shell Ni2Si/C54-TiSi2 nanowires were fabricated by annealing NiSi2 films on Si substrate at an ambient containing Ti vapor in an ultrahigh vacuum (UHV) chamber. Silicon (100) wafers (1−5 Ω-cm) were cleaned using a standard RCA cleaning process. Next, a 30 nm Ni thin film was deposited on the Si substrate using a UHV electron beam evaporator. The samples were then annealed at 600 °C for 30 min without breaking the vacuum to develop the NiSi2 film into a nanodot sample.29,30 The chamber pressure was below 10−9 Torr during both processes. The annealed samples were quickly transferred into a heating chamber for annealing at 850 °C in a vacuum below 10−7 Torr and heated with a Ti filament at 800 °C during the designated time
s the scaling trend of the microelectronics continues, onedimensional nanowires are attractive materials for nanoelectronics owing to their peculiar morphology as well as unique electronic, magnetic and piezotronic properties.1−3 Nanostructured-metal-oxide-semiconductor field-effect transistors (MOSFET) devices have been made in integrated circuits comparable to the macroscale devices fabricated from the same materials.4,5 Accordingly, there have been numerous studies in the literature dealing with the interface between semiconductors and metallic electrodes,6−8 in which metal silicides have attracted much attention for their low resistivity serving as device-to-device interconnects and nanocontacts in silicon complementary metal-oxide-semiconductor (CMOS) devices.6−10 Furthermore, nickel silicides9−12 and titanium silicides are the two most extensively investigated group of silicides.9,10,13 NiSi has the best lattice match to silicon, TiSi2 also possessed low resistivity, and both silicides have been utilized often in the electronic devices. Ni2Si has been used as a gate material for 45 nm CMOS devices and p-MOS technology due to their low process temperature, higher conductivity, and more suitable work function compared to other nickel silicides.14,15 Besides, Ni2Si has achieved great importance in recent years due to its ferromagnetic properties and significant catalytic activity.16−18 On the other hand, C54-TiSi2 is also a well-known semiconductor material, which has a lower electrical resistivity and is commonly used in small scale devices.19,20 While considerable attention has been paid in the past to axial metal silicide nanowire heterostructures,6,21−25 © 2012 American Chemical Society
Received: December 18, 2011 Revised: March 27, 2012 Published: March 27, 2012 2254
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Figure 1. The cross-section SEM image (a) showing the core−shell Ni2Si/TiSi2 nanowires synthesized on the silicon substrate. The inset shows the top view of the sample with the average length of nanowires being close to 2 μm. The high-magnification TEM image (b) revealing the diameter of core−shell Ni2Si/TiSi2 nanowire to be about 200 nm. (c) HAADF image and the corresponding EDS line-scan profile. (d) High-resolution TEM image of the core−shell nanowire in (a). The two inset images show the diffraction patterns of inner Ni2Si core structure and outer TiSi2 shell layer.
morphologies of NiSi2 thin films formed with different reaction times are shown in Figure 2. Figure 2a is an SEM image of the
periods. Ti vapor was deposited and reacted with the Si, segrating from the NiSi2, to form core−shell Ni2Si/C54-TiSi2 nanowires. Figure 1 shows the core−shell Ni2Si/C54-TiSi2 nanowires synthesized in our unconventional chamber (see Figure S1 in the Supporting Information). Figure 1a and the inset are tilted and top-view SEM images of our sample, respectively, displaying a high density of 1−2 μm nanowires. Figure 1b is a typical low-magnification TEM image of a core−shell Ni2Si/ C54-TiSi2 nanowire. The diameter of the whole nanowire is close to 200 nm and a distinct contrast between the 100 nm dark core matrix and 50 nm brighter shell layer is observed. Figure 1c shows a line scan profile and HAADF image of an individual nanowire demonstrating an elemental distribution consistent with the structure of a core−shell Ni2Si/TiSi2 nanowire. Figure 1d is a high-resolution TEM image taken at the corresponding region in Figure 1b. The two selected-area electron diffraction (SAED) images in the upper and lower insets of Figure 1d indicate the presence of different compound structures in the core and shell regions. The upper inset image can be assigned to the (011̅) and (102) planes of single crystal orthorhombic Ni2Si with a [21̅1̅] zone axis; two sets of fringes spaced 0.331 and 0.252 nm apart are assigned to the lattice spacing of the (011)̅ and (102) planes of Ni2Si phase, respectively. The SAED pattern in the lower insert is of the shell layer and was determined to correspond to the two (400) and (004̅) planes of orthorhombic C54-TiSi2 with [010] zone axis. Two sets of fringes spaced 0.229 and 0.214 nm apart are assigned to the lattice spacing of the (400) and (004̅) planes of C54-TiSi2. All the data revealed that the nanowires are core− shell Ni2Si/C54-TiSi2 nanostructures. In the current investigation, a Ti filament was heated to 800 °C in a UHV chamber, and NiSi2 thin films on silicon were placed in the center of the chamber at 850 °C. The
Figure 2. The tilted angle SEM micrographs of NiSi2 films after annealing for (a) 0, (b) 1, (c) 6, and (d) 12 h. (e,f) Two cross-section TEM images corresponding to those of panels a and d, respectively. (g,h) The growth mechanism model of core−shell NiSi2−TiSi2 nanowires. Schematics in panels g and h are magnified views of the consonant region of (g) and (h), respectively.
as-grown NiSi2 films in which NiSi2 grains can be seen. The morphological evolution during the annealing process is displayed in Figure 2b−d. After 1 h of reaction, nanocrystals were segregated on top of the substrate, as shown in Figure 2b. When the reaction time was increased to 6 h, the nanorods, about 500 nm in diameters, grew from the interface of the NiSi2 2255
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nanocrystals (Figure 2c). The nanowires would become longer and wider as the reaction time was prolonged to 12 h (Figure 2d). Panels e and f in Figure 2 are cross-sectional TEM images corresponding to Figure 2a,b, respectively. Figure 2e displays a perpendicular morphology with micro NiSi2 grains on the silicon substrate postannealing in a UHV system at 600 °C. Its inset image shows the micro NiSi2 grains of low misorientation angles (θ) formed on the substrate. Figure 2g illustrates the formation of core−shell nanowires. Recent research showed that a solid−liquid−solid (S−L−S) process can be used to explain the growth of TaSi2 nanowires due to supersaturation.30 In the first solid−liquid process, Ni films deposited on a Si (100) crystal substrate, the sequential formation of a metal-rich Ni−Si alloy liquid phase of from a solid Ni/Si interface is usually observed at low temperature (∼350 °C).9,10 The reaction follows the sequence of NiSi at (350−750 °C) to NiSi2 above (750 °C). Meanwhile, Si atom would segregate on the surface of the NiSi2 grains, which formed with a specific angle (θ) and diameter (R) during heating (Figure 2g).29−31 The formation mechanism is suggested as follows: according the Ni−Si phase diagram,32 spherical NiSi2 grains with isotropic surface energy grew via Volmer−Weber mode.33 In the second liquid−solid reacting process, a continuous supply of Ti vapor reaches the NiSi2 grains in which Ti atoms reacted with the segregated Si atoms, forming TiSi2 at the interface between Ti and NiSi2. Through an S−L−S reaction process, core−shell Ni2Si/TiSi2 nanowires can be grown epitaxially on NiSi2 grains. As seen in Figure 2f and the inset, core−shell nanocrystals with a diameter of 100 nm were grown at the interface. Growth begins at the interface of the silicide crystals. In the later stages (Figure 2h), the Si atoms separate from the NiSi2 layer.29−31 Comparing the two samples, the Ni/Si ratio increases with increasing Ti concentration, which leads to the transformation of the Si-rich silicide to Ni-rich silicide by dissolution of Si atoms into the metallic Ti lattice (Figure S2 in Supporting Information). As shown in the Supporting Information, in situ TEM was used to demonstrate the evolution of an individual core−shell Ni2Si/TiSi2 nanowire upon heating from 600 to 950 °C. The in situ TEM images show that the core−shell nanowire would break due to the difference in thermal expansion between the interface of Ni2Si and C54-TiSi2 upon heating above 900 °C. The results are consistent with the temperature range for synthesis of core−shell Ni2Si/TiSi2 nanowires (see Figure S3 in Supporting Information). In order to understand the growth mechanism, two different silicided films, 30 nm FeSi2 and TiSi2, were prepared as catalytic layers. Figure 3a,b reveals TiSi2 nanowires synthesized by annealing FeSi2 film samples at 850 °C for 6 and 12 h, respectively, in Ti vapor-containing ambient. No long nanowires were observed after annealing for 6 h at 850 °C (Figure 3a). However, the short nanowires were grown after annealing for 12 h at the same condition (Figure 3b). With TiSi2 as the catalytic layer, only TiSi2 precipitates were observed as shown in Figure 3c,d. Longer nanowires were grown in NiSi2 film substrate compared to FeSi2. This is due to the lower melting point of NiSi2 (993 °C) than FeSi2 (1220 °C) and TiSi2 (1773 °C).29,30 This demonstrates that NiSi2 precipitates are more likely to form in a supersaturated Ni−Si system from Si substrate compared to FeSi2 and TiSi2 films at the same experimental conditions.29−31 As shown in Figure 4, the magnetic properties of the core− shell Ni2Si/TiSi2 nanowires were characterized using vibrating
Figure 3. Top-view SEM images showing annealed FeSi2 films for (a) 6 and (b) 12 h at 850 °C. Top view SEM images of annealed TiSi2 films for (c) 6 and (d) 12 h at 850 °C. All annealing processes were conducted in Ti ambient within a UHV chamber.
sample magnetometer (VSM) and superconducting quantum interference device (SQUID) magnetometry. The influence from the Si substrate was deducted from the M−H data. Figure 4a shows the VSM hysteresis loops of radial silicide nanowires where in-plane (parallel to substrate) and out-of-plane (perpendicular to substrate) magnetic fields are applied at room temperature. Since TiSi2 is nonmagnetic, the hysteresis loop along the applied field axis at room temperature (Figure 4a), indicates that Ni2Si plays an important role on the devices ferromagnetic features.15−17 According the magnetic experimental results, Ni2Si nanowires, which has ferromagnetic properties is very distinct from NiSix for x > 1. In general, most nickel silicides are not ferromagnetic. Some recent literature34,35 reported that Ni2Si thin film or nanoparticles have been investigated their magnetic moments, which are established unequivocally by experimental verification or theoretical calculations due to its significative local spin moment at the transition metal Ni atoms.34 The magnetic data revealed that core−shell Ni2Si/TiSi2 nanowires exhibit ferromagnetism with a coercive field of about 50 Oe and saturation magnetization of 4000 Oe. In order to deduct background M−H strength of the substrate at 300 K, the M−H curves of the substrate before reacting was taken as a control. An extremely little hysteresis was found as shown in Figure 4b. The lower inset SEM image is of the initial substrate. Besides, we also measured the other control sample, which the TiSi2 shell layer have been etched away by buffered hydrofluoric etching (BHE) method, as shown in Supporting Information Figure S4. The SEM image (Supporting Information Figure S4a) shows a low density of short nanopillars and nanoparticles and its small magnetization has been recorded by VSM measurement in Supporting Information Figure S4b. Low-temperature magnetic measurements were carried out to minimize thermal interference. Magnetic hysteresis loops were observed in SQUID magnetometry (Figure 4c). The applied out-of-plane magnetic field contains a higher residual magnetization with a coercive field of 200 Oe. Moreover, the applied in-plane magnetic field parallel to the substrate exhibits a coercive field of about 100 Oe. The hard remanences and coercive fields originate from the shape anisotropy owing to the vertical orientation of the nanowires on the substrate, which forces the magnetic moments to align mainly along the orientation of the nanowires. In theory, similar saturation magnetizations should be obtained by applying the magnetic 2256
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Figure 4. (a) The VSM hysteresis loop curves of core−shell Ni2Si/TiSi2 nanowires where the magnetic fields are applied in parallel and perpendicular directions to the substrate at room temperature. The inset shows a magnified image taken from the zero field area. (b) The VSM hysteresis loop obtained from the initial substrate. (c) The hysteresis loop curves of core−shell Ni2Si/TiSi2 nanowires in two different orientations measured by SQUID magnetometry at 4 K. The inset shows the magnified images taken from the zero field area. (d) Low-temperature magnetic results of the FC and ZFC measurements of the core−shell Ni2Si/TiSi2 nanowires perpendicular to the substrate.
Figure 5. (a) The I−V curve of core−shell Ni2Si/TiSi2 nanowires obtained by two-terminal measurement in FIB. The inset illustrates in situ electrical measurements of an individual core−shell Ni2Si/TiSi2 nanowire inside the FIB equipment. (b) A plot of the R versus the L/S for the core− shell Ni2Si/TiSi2 nanowires. The red dot corresponding to contact resistance (Rc), is obtained by direct contact between the tip and the electrode. The plot fits well to the formula R = Rc + (ρL/S). The retrieved nominal resistivity (ρ) for this core−shell Ni2Si/TiSi2 nanowires is 30.6 μΩ-cm and the contact resistance (Rc) is 24.4 Ω.
fields with parallel and perpendicular directions.35 In addition, zero-field-cooled (ZFC) and field-cooled (FC) curves are different for these dilute magnetic materials, as seen in Figure 4d, showing variation of magnetization in perpendicular direction as a function of temperature in the range of 4 to 354 K. The other parallel ZFC and FC curves in parallel direction are given in Supporting Information Figure S5. In the FC magnetization measurements, the sample was cooled from 354 to 4 K under an external magnetic field of 100 Oe during the process. Furthermore, in the ZFC measurements, the sample was also cooled from 354 to 4 K in the zero field and after stabilization at 4 K, the data was obtained immediately while heating in a desired applied field of 100 Oe.
In the ZFC curve, the moment increased with the temperature and then decreased, while the moment decreased in the FC curve. The in-plane ZFC and FC thermal magnetization curves reveal that the blocking temperature (Tb) of the core−shell Ni2Si/TiSi2 nanowires is nearly 314 K, below which the magnetization direction of each core−shell Ni2Si/TiSi2 nanowires aligns with its easy axis. However, recent research shows that the blocking temperature of nickel nanoparticles is near room temperature, such as fcc-Ni (300 K),36 Ni/NiO (300 K),37 and Ni/C (350 K).38 Compared to other nickel silicides, core−shell Ni2Si/TiSi2 nanowires were adopted as the candidate for magnetic application due to its ferromagnetism near room temperature. 2257
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I−V measurements were carried out for core−shell Ni2Si/ C54 TiSi2 nanowires using in situ multisite measurements in the FIB. Because of the lack of the suitable electrodes, the electrical properties were measured using two-probe measurement. The actual parallel resistance of nanowire 200 nm in diameter and 1.5 μm in length was calculated to be 39.4 Ω. The linear I−V behavior shows that the nanowire has a higher resistivity around 82.5 μΩ-cm, which is worse than Ni2Si nanowire (21 μΩ-cm)12 or TiSi2 (∼ 30 μΩ-cm).13 Generally speaking, two-probe measurement tests are normally not desirable because of the high contact resistance (Rc) between the tungsten probe and the nanowire. However, a novel extrapolation has been demonstrated to directly deduct Rc from the interface connecting the electrode and nanowire.39,40 Figure 5b shows a plot of resistances obtained from a series of samples with different aspect ratios using the same operation procedures with the FIB instrument. The measured resistance (R) can be expressed as R = Rc + Rwire, where Rwire is the resistance of the Ni2Si/C54-TiSi2 nanowire. The resistance (R) versus length to cross area ratio (L/S) relation is plotted for the nanowire and the linear formula can be presented as R = Rc + ρL/S, where ρ is the resistivity, S is the cross-sectional area, and L is the length. Rc can be calculated as the resistance of the nanowire with 0 length. Since Rc is almost 24.4 Ω, we can obtain an average resistivity of 30.6 μΩ-cm for core−shell Ni2Si/C54-TiSi2 nanowires (see Table S1 in the Supporting Information), close to that of single crystal C54-TiSi2 nanowires.13 Comparing a nearby theoretical (∼20 μΩ-cm) and our actual experimental (30.4 μΩ-cm), a differential value 10.4 μΩ-cm could be obtain in our core−shell nanowire (see Figure S6 in Supporting Information). The differential value would be a cause the material defect such as dislocations between core and shell layer (see Figure 1) and the effect of electron scattering41 due to the surface roughness in nanowire. Conclusions. In summary, core−shell Ni2Si/C54-TiSi2 nanowires were grown using the S−L−S process. Single crystal core−shell Ni2Si/C54-TiSi2 nanowires with 200 nm diameter and 2 μm length have been synthesized. These nanowires display a coercive field of 50 Oe at room temperature. Residual magnetization with the coercive field of 100 and 200 Oe were found along the directions perpendicular and parallel to the substrate, respectively. The ZFC and FC thermal magnetization curve measurements have been carried out over the temperature range of 4−354 K and a blocking temperature (Tb) of the core−shell Ni2Si/TiSi2 nanowires was found to be close to room temperature, 314 K (∼41 °C). An FC hysteresis loop shift is described for these nanowires as a function of temperature. This is ascribed to an exchange interaction between the ferromagnetic Ni2Si core and its surface TiSi2 shell layer with disordered spins. These nanowires also reveal good electrical characteristics by two-terminal measurement. Through subtracting contact resistance, the core−shell Ni2Si/ C54-TiSi2 nanowires exhibit characteristics meeting the criterion for interconnect application. The results indicate that the core−shell structures may find electronic interconnect or magneto-resistance application in future microelectronic devices. Experimental Section. The morphologies and crystal structures of the as-synthesized nanowires were analyzed using field-emission scanning electron microscope (FE-SEM, JSM-6500F) and X-ray diffraction (XRD, SRD-600). The highresolution lattice images and chemical composition information were obtained with a field-emission high-resolution trans-
mission electron microscope (FE-HRTEM, JEM-3000F), operating at 300 kV with a point-to-point resolution of 0.17 nm, and equipped with a high-angle angular dark field (HAADF) detector and energy dispersive spectrometer (EDS). The magnetic properties of these nanowires were measured in a superconducting quantum interference device (SQUID, MPMS-XL) and vibrating sample magnetometer (VSM, Model-4). For magnetic measurements, all of our samples measured 4 × 6 mm. The magnetic hysteresis loop was recorded at room temperature (300 K) and very low temperature (4 K). Field-cooled and zero-field-cooled magnetization measurements at low magnetic fields were also carried out to determine the ferromagnetic properties of core−shell Ni2Si/C54-TiSi2 nanowires. Finally, the electrical characteristics were measured with contacts deposited using a dual-beam focused ion beam (FIB, Nova-200).
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ASSOCIATED CONTENT
S Supporting Information *
The contents of Supporting Information includes the following: (1) photographs of the reaction chamber, (2) HAADF images and EDS-mapping results of core−shell nanodots, (3) in situ TEM heating experiments of the core−shell nanowires, (4) VSM hysteresis loop of Ni2Si nanopillars and nanoparticles, (5) FC and ZFC measurements in parallel direction, (6) a diagram of parallel circuit with a voltage source, and (7) in situ electrical characterizations of individual core−shell nanowires. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: author:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors gratefully acknowledge Ken C. Pradel for helpful discussions. We also acknowledge National Science Council and the Foundation Grants 100-2221-E-007-027 and 99-2923E-007-002-MY3 for financial assistance.
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REFERENCES
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