Abstract
The authors report on formation of high current carrying Ni-Si nanowires between Ni/heavily doped polycrstaliine Si contacts using substrate sourced growth without any supply of Si source gas. The conductance (G) of a 18 nm diameter nanowire bridge in the temperature range of 13-293 K showed a power function of temperature Tα, with a typical exponent α≈0.62±0.06, and the dI/dV were fitted to the V α with α≈0.55±0.15. Although the power law dependence of the G and dI/dV, which provides evidence for the interaction of the electrons in a low-dimensional infinite conductor, corresponds to the featured behavior of the interacting electrons within an error tolerance, the strength of the interaction is very weak due to the highly metallic characteristics with finite length and lack of purity of the channel. As a result, the in situ high current carrying Ni-Si nanowire junction can be utilized not only as a nanointerconnector, but also as a tool to study low-dimensional electrical transport properties.
| Original language | English |
|---|---|
| Article number | 253115 |
| Journal | Applied Physics Letters |
| Volume | 90 |
| Issue number | 25 |
| DOIs | |
| Publication status | Published - 2007 |
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)
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In situ Ni-Si nanowire junction based on substrate sourced growth and its electrical transport behavior. / Lee, Yun Hi; Kwon, Hyuk Sang.
In: Applied Physics Letters, Vol. 90, No. 25, 253115, 2007.Research output: Contribution to journal › Article › peer-review
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TY - JOUR
T1 - In situ Ni-Si nanowire junction based on substrate sourced growth and its electrical transport behavior
AU - Lee, Yun Hi
AU - Kwon, Hyuk Sang
N1 - Funding Information: Lee Yun-Hi a) Kwon Hyuk-Sang National Research Laboratory for Nano Device Physics, Department of Physics, Korea University , Seoul 136-713, Korea a) Author to whom correspondence should be addressed; electronic mail: yh-lee@korea.ac.kr 18 06 2007 90 25 253115 13 03 2007 30 05 2007 22 06 2007 2007-06-22T15:18:33 2007 American Institute of Physics 0003-6951/2007/90(25)/253115/3/ $23.00 The authors report on formation of high current carrying Ni–Si nanowires between Ni/heavily doped polycrstaliine Si contacts using substrate sourced growth without any supply of Si source gas. The conductance ( G ) of a 18 nm diameter nanowire bridge in the temperature range of 13 – 293 K showed a power function of temperature T α , with a typical exponent α ≈ 0.62 ± 0.06 , and the d I ∕ d V were fitted to the V α with α ≈ 0.55 ± 0.15 . Although the power law dependence of the G and d I ∕ d V , which provides evidence for the interaction of the electrons in a low-dimensional infinite conductor, corresponds to the featured behavior of the interacting electrons within an error tolerance, the strength of the interaction is very weak due to the highly metallic characteristics with finite length and lack of purity of the channel. As a result, the in situ high current carrying Ni-Si nanowire junction can be utilized not only as a nanointerconnector, but also as a tool to study low-dimensional electrical transport properties. KMST KOSEF C00054 Silicon-based nanowires (Si NWs) have been intensively studied as materials for the active region of field effect transistors and as nanointerconnectors for integrated nanosystems. 1–20 The reason why Si NWs are of interest in active channels and nanointerconnectors 5–7 is the expectation that they will exhibit the same low-dimensional transport behavior as carbon nanotubes while being an industry friendly material. There is a clear need to investigate multiple and parallel processing methods for fabricating Si-based NW bridges or junctions, in order to achieve an integrated system without an Au catalyst. In semiconductor physics, it is well known that the Au catalyst forms a deep trap level within the band gap of Si. Therefore, other alternatives must be pursued if nanowire-based electronics is to become a reality. Of course, the best method of fabricating NW bridges might be selective lateral growth at a predefined position in the nanocircuit. In conventional top-down semiconductor technology, as the device dimensions become smaller and smaller, the low resistivity of NiSi is expected to provide the low resistivity required for the fabrication of shallow junctions for use in future complementary metal-oxide semiconductor technology. In the physics of one-dimensional systems, one of the main concerns is the effect of electron-electron interactions. 18–23 Therefore, the systematic investigation of the temperature and bias dependent transport is of importance when studying the features of NWs for their expected applications. In this work, we describe the in situ formation of highly conducting Ni–Si nanowires between patterned Ni/heavily doped poly-Si electrodes, without any gaseous or liquid Si source, and their characterization. One of the key ideas behind this method, which was derived from our previous work on the substrate sourced growth of W nanowires and Cr nanowires with the SLS mechanism, 12,19,20 was the use of a layered structure consisting of Ni ∕ p -type polycrystalline Si(PPSi) for the growth of conductive Si NW bridges and low resistive contact electrodes. The Ni ( 10 – 50 nm ) ∕ highly p-type poly- Si ( 100 – 1000 nm ) structure was prepared by conventional sputtering of the various layers in sequence. As a gate insulator, a 500 - nm -thick Si O 2 layer was grown on a n -type heavily doped Si wafer with a resistance of about 0.01 Ω cm by a conventional dry oxidation process followed by the deposition of an approximately 100 - nm -thick PPSi layer using an Ar plasma. The formation of NWs was possible in the temperature and pressure ranges of 900 – 1200 ° C and 40 mtorr – 1 torr , respectively, for 10 – 60 min . Figure 1(a) shows a schematic drawing of the whole junction structure adopted in this work. Figure 1(b) and 1(c) show the HRTEM image of an individual Ni–Si nanowire grown on the unpatterned Ni/heavily doped conducting poly-Si substrate and its compositional analysis by energy dispersive x-ray analysis, respectively, showing that Ni and Si atoms are dominant. As shown in Fig. 1(c) , the nanowires show a highly crystalline structure with a lattice spacing of 0.3 nm . Next, we examine the results obtained for the lateral nanowires grown between the prepatterned contact electrodes, which both act as the source for the growth of the nanowires. Figure 2(a) shows the x-ray diffraction patterns for the patterned electrodes immediately after their deposition and after their subsequent annealing (i.e., the role of self-sourcing). This result indicates that the crystallinity of the layer was enhanced during the transformation of the thin film into one-dimensional nanowires. We conducted experiments in the high temperature range from 970 to 1200 ° C , while the phase transformation of NiSi to NiSi2 occurred at a temperature of 750 ° C . As a consequence, it is natural that more of the Si layer would be consumed during this transformation. The high-resolution transmission electron microscopy (HRTEM) measurements demonstrated that the nanowires, whose average diameter was about 20 nm and whose length was up to a few millimeters, were composed mainly of Si, as shown in Fig. 2(b) , which shows a low-magnification scanning electron microscopy (SEM) image. Figure 2(c) , which was obtained from the HRTEM observations, shows an image of the patterned Ni ∕ P P Si catalyst with a suspended NW bridge which was grown laterally between the electrodes, and Fig. 2(d) shows the corresponding in-depth compositional profiles from the self-catalyst to the NW bridge when scanned along the growth direction of the bridge. The spectra indicate that the NW bridge did not start from an individual droplet of Ni–Si, but from the bulky Ni-rich patterned electrodes. At a process temperature of about 970 ° C , an Ni Si x eutectic alloy was formed over the patterned electrodes. The Si atoms dissolved more and more because of their high solubility. Finally, the eutectic alloy became saturated, resulting in the lateral growth of the NWs. Supporting evidence for the growth mechanism of the NWs was obtained from Fig. 2(d) , where an increase of the Si content was observed at the very location of the junction, whereas, on the left electrode, the Si content was decreased. Just beyond the junction, between the self-catalyst and the NW, the mainly Si-based nanowire started to grow laterally toward the right electrodes. The elemental concentration of Si was homogeneous along the growth axis of the NW. The existence of 5%–15% oxygen was attributed to surface oxidation during the handling process of the specimen in air after the growth of the NWs. Normally, the oxide layer would be etched out by reduction treatment. From the experimental observations, the growth process can be explained as follows: normally, the vapor-liquid-solid growth of Si NWs is based on a mechanism in which a catalyst such as Au acts as a liquid-forming agent which reacts with the vapor of the Si powder or Si-containing gas, resulting in an Si-rich NiSi eutectic layer. In the present case of the Ni thin film ∕ p + -poly-Si film, however, the source of Si comes from the heavily doped p + -poly-Si films, because no Si source was externally provided for the formation of the eutectic. As shown in Figs. 1(c) and 1(d) , the Ni films reacted with the Si at a temperature of < 973 ° C to form Ni–Si NWs. Due to the high solubility of Si atoms in the Si–Ni eutectic, an eutectic-solid interface was formed and, then, the Ni–Si NWs which grew as the liquid phase became supersaturated because of compositional fluctuations. Because almost all of the devices showed a high current, we measured the transport properties in the rather low bias regimes for both V sd and V g in order to lessen the risk of occurrence of a gate leakage current. From only 10 2 change of the current as a function of the gate bias ( V g ) as shown in Fig. 3(b) , we confirmed that the nanowire bridge was a metallic or highly doped semiconducting channel. The current passing through an individual nanobridge with a diameter of 18 nm at V sd = 100 mV reached a very high level of about 100 μ A , indicating that the NW channel is high quality single crystalline. The channel current-voltage characteristics ( I ds − V sd ) with temperature dependences of the electrical conductance in the low bias and high bias regimes were obtained as presented in Figs. 3(c) and 3(d) . Figure 4(a) shows the increase of G with increasing temperature. Two possible explanations for this behavior are the thermally activated conduction model of e V ⪡ k B T or the one-dimensional hopping model of ln ( G ) ∼ ( − 1 ∕ T s ) . However, neither of these conduction mechanisms fits well with the measured characteristics. The characteristic feature of Fig. 4(a) is the linear slope of the G - T (K) plot in the log-log scheme, indicating a power law function of G ∼ T 0.62 ± 0.06 . For a system of N -identical pure Luttinger-liquid quantum wires with infinite length, each with M weak links to contacts, G ( T ) under lower bias and I ( V ) of high bias regime can be rewritten as G ( T ) = C T N e 2 ∕ h [ ∑ j = 1 M ( ε ∕ t j ) 2 ] − 1 ( k T ∕ ε ) α , and I ( V ) = C I N e 2 ∕ h ( ε ∕ e ) [ ∑ j = 1 M ( ε ∕ t j ) 2 ∕ b ] − 1 ( e V ∕ ε ) β , where C T and C I are about 1, t j is the measure of the tunneling transparency of the j th link, ε ∼ E F , and α = 2 g − 2 and β = 2 ∕ g − 1 , 17,18 where g is a dimensionless constant that measures the interaction strength between electrons. 22,23 The power law fitting of G allowed us to estimate α to be about 0.62 ± 0.06 in the small bias regimes (i.e., e V ⪡ k B T ). In the measured curve of Fig. 4(b) , d I ∕ d V was constant in the lower bias regime and started to increase with increasing V ds in the higher bias regime in the log ( d I ∕ d V ) - log ( V ds ) plot. In the high bias regime, d I ∕ d V , at different temperatures, collapsed into a single plot. The linearity in the log-log scale plot implies that d I ∕ d V can be described by a power law, i.e., d I ∕ d V ∼ V α , where α = 0.55 ± 0.15 . As a final verification to confirm the agreement of the theoretical one-dimensional interacting electron system, 16,17 d I ∕ d V , measured at different temperatures, was replotted against e V ∕ k B T instead of V or T , as presented in Fig. 4(c) . The value of ( d I ∕ d V ) ∕ T α was nearly constant when e V ∕ k B T approached zero, but when the applied energy ratio of the thermal energy e V ∕ k B T exceeded 2, ( d I ∕ d V ) ∕ T α started to increase, showing the collapse of all of the curves into a universal single curve over the measured V ds range. This collapsing into a universal curve provides a final confirmation of the presence of electron-electron interactions in this nanowire. The observed values of 0.62 ± 0.06 and 0.55 ± 0.15 for α and β , respectively, resulted in g = 0.7 – 0.8 . According to the interacting electron theory, the obtained g value indicates that the strength of the interactions between electrons within this NW junction is not strong. Considering the fact that the strength of the electron-electron interactions depends on the carrier density and electron-electron separation distance, we explained the decreased electron interactions by the existence of numerous conducting channels in these large diameter ( ∼ 18 nm ) metallic NWs. In summary, we described the in situ growth of highly conductive Ni–Si nanowire interconnectors between Ni/heavily doped poly-Si electrodes without any gaseous or liquid Si source. The electrical transport of the highly conducting Ni–Si NW bridge followed a power law behavior of G - T (K) and d I ∕ d V - V ds (V). The results obtained herein indicate that the in situ grown Ni–Si nanowire junction based on substrate sourced growth provides a promising tool to study electronic transport in nanostructures and high current carrying bridge for integrated nanointerconnector electronics based on familiar semiconductor materials and processes. This work was supported by National Research Laboratory Program of MOST (KOSEF), Nano Core Technology of KOSEF, and partially by Pure Basic Research Project No. C00054 of KRF. FIG. 1. (Color online) (a) Schematic illustration for the whole Ni–Si NW junction structure brought into contact with Ni/heavily doped poly-Si electrodes using a substrate sourcing mechanism. The upper figure is a real image of a typical Si NW junction with a gap spacing of 5 μ m . (b) A representative TEM image of an individual Ni–Si nanowire, whose diameter is about 18 nm . (c) Typical energy dispersive x-ray analysis spectrum of the nanowire shown in (a). FIG. 2. (a) X-ray diffraction patterns for the as-deposited and annealed Ni/heavily doped poly-Si electrodes, showing the formation of an NiSi phase. (b) A typical SEM image of the wool-like Ni–Si NWs grown on a large area over the Ni/heavily doped poly-Si film by the self-source mechanism. (c) Enlarged image of a robust Si N W – Ni ∕ P + -poly-Si electrode junction. (d) In-depth elemental mapping of Ni, Si, O, and C atoms for an individual N W – Ni ∕ p + -poly-Si junction. FIG. 3. SEM image of the tested junction with gap spacing of 3 μ m . (b) Gate bias dependence of the channel current, indicating nearly metallic nature. [(c) and (d)] temperature dependent current vs bias characteristics in the low and high bias regimes, respectively. FIG. 4. (Color online) Conductance ( G ) and differential conductance ( d I ∕ d V ) of the junction. (a) Log (channel conductance) ( I sd ) - log (temperature) (K) plot in the regime of low bias. The plot shows a linear slope. (b) Bias dependent d I ∕ d V curves measured at different temperatures. The curve on the log-log scale was fitted with a straight line to indicate power law behavior. (c) A replot made using universal scaling relations: ( d I ∕ d V ) ∕ T a - eV ∕ k T .
PY - 2007
Y1 - 2007
N2 - The authors report on formation of high current carrying Ni-Si nanowires between Ni/heavily doped polycrstaliine Si contacts using substrate sourced growth without any supply of Si source gas. The conductance (G) of a 18 nm diameter nanowire bridge in the temperature range of 13-293 K showed a power function of temperature Tα, with a typical exponent α≈0.62±0.06, and the dI/dV were fitted to the V α with α≈0.55±0.15. Although the power law dependence of the G and dI/dV, which provides evidence for the interaction of the electrons in a low-dimensional infinite conductor, corresponds to the featured behavior of the interacting electrons within an error tolerance, the strength of the interaction is very weak due to the highly metallic characteristics with finite length and lack of purity of the channel. As a result, the in situ high current carrying Ni-Si nanowire junction can be utilized not only as a nanointerconnector, but also as a tool to study low-dimensional electrical transport properties.
AB - The authors report on formation of high current carrying Ni-Si nanowires between Ni/heavily doped polycrstaliine Si contacts using substrate sourced growth without any supply of Si source gas. The conductance (G) of a 18 nm diameter nanowire bridge in the temperature range of 13-293 K showed a power function of temperature Tα, with a typical exponent α≈0.62±0.06, and the dI/dV were fitted to the V α with α≈0.55±0.15. Although the power law dependence of the G and dI/dV, which provides evidence for the interaction of the electrons in a low-dimensional infinite conductor, corresponds to the featured behavior of the interacting electrons within an error tolerance, the strength of the interaction is very weak due to the highly metallic characteristics with finite length and lack of purity of the channel. As a result, the in situ high current carrying Ni-Si nanowire junction can be utilized not only as a nanointerconnector, but also as a tool to study low-dimensional electrical transport properties.
UR - http://www.scopus.com/inward/record.url?scp=34547279697&partnerID=8YFLogxK
U2 - 10.1063/1.2750522
DO - 10.1063/1.2750522
M3 - Article
AN - SCOPUS:34547279697
SN - 0003-6951
VL - 90
JO - Applied Physics Letters
JF - Applied Physics Letters
IS - 25
M1 - 253115
ER -