Metallic conduction and large electron-phonon-impurity interference effect in single TiSi nanowires
© Hsu et al.; licensee Springer. 2012
Received: 24 July 2012
Accepted: 27 August 2012
Published: 5 September 2012
We report on the first electrical characterizations of single-crystalline TiSi nanowires (NWs) synthesized by chemical vapor deposition reactions. By utilizing the focused-ion-beam-induced deposition technique, we have delicately made four-probe contacts onto individual NWs. The NW resistivities have been measured between 2 and 300 K, which reveal overall metallic conduction with small residual resistivity ratios in the NWs. Surprisingly, we find that the effect due to the interference processes between the elastic electron scattering and the electron-phonon scattering largely dominates over the usual Boltzmann transport even at room temperature. Such prominent electron-phonon-impurity interference effect is ascribed to the presence of large amounts of disorder and high Debye temperatures in TiSi NWs.
Physical properties of transition metal silicides are intensively investigated for their potential usefulness in many device applications. Among them, titanium silicides constitute a valuable material family that is widely utilized as gate electrodes and interconnects in ultra-large-scale integrated circuits, owing to their relatively low electrical resistivities and good thermal and chemical stability that is highly compatible with present-day silicon processes . Among all the six phases of titanium silicides reported to date, titanium monosilicide (TiSi)  has the highest mechanical hardness which, combined with its low resistivity  (≈ 60 μΩ cm in bulk form at 300 K), can be of interest for the possible engineering of micro- and nano-electromechanical systems . However, as compared to other titanium silicides, TiSi has not been much studied either in bulk form or at the nanoscale level due to the difficulties of either preparing quantities sufficient for bulk characterizations  or preparing single-phased nanostructures .
Recently, some of us have successfully synthesized single-crystalline TiSi nanowires (NWs) . The material belongs to one of the eight transition metal silicide NWs currently known . In this work, we aim to study the intrinsic electrical transport properties of this nanoscale material by measuring four-probe individual NW samples in a wide temperature range of 2 to 300 K. We demonstrate that these NWs are indeed metallic, revealing decreasing resistivity with reducing temperature. Furthermore, we observe a large electron-phonon-impurity (EPI) interference effect, which is theoretically predicted to exist in disordered conductors . Surprisingly, we find that this EPI interference effect strongly dominates over the usual Boltzmann transport even at temperatures as high as room temperature. This unique property renders the TiSi NWs useful for the investigations of the interplay among a variety of electron-scattering processes at the nanoscale level. Previously, the EPI interference effect has been found to be important in normal metals only at considerably lower temperatures [8–12].
Our TiSi NWs were synthesized via a low-pressure chemical vapor deposition process using TiCl4 and titanium powder as precursors; no templates or catalysts were needed . The growth mechanism and detailed structure and composition characterizations by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and other techniques have been reported by Lin et al. in . Since these NWs can be readily coated by a thick (approximately 10 nm) oxide layer over the surfaces [5, 13], we choose the ‘invasive’ focused-ion-beam-induced deposition (FIBID) technique to fabricate the electrical contacts onto individual NWs. It should be stressed that the existence of the thick oxide layers prevented us from making good ohmic contacts when employing the electron-beam lithographic technique. After transferring the NWs onto the silicon substrate capped with a ≈ 500-nm thick SiO2 layer, the positions of individual NWs were first located by the scanning electron microscopy (SEM) capability of the FIB system (Model FEI Nova 200, FEI Co., Hillsboro, OR, USA). Platinum electrodes were then deposited onto the NWs using a 30-KeV, 10-pA Ga+ FIB. The Pt leads connected the individual NWs to the microelectrodes which were photolithographically pre-patterned on the SiO2/Si substrate. The substrate was thermally anchored to the sample holder mounted with a calibrated silicon diode thermometer on a standard 4He cryostat. A standard four-probe current-reversal method was applied for resistance measurements using a Keithley K-220 current source and a K-182 nanovoltmeter (Keithley Instruments Inc., Cleveland, OH, USA).
Results and discussion
Values of relevant parameters for three TiSi NW samples
ρ(300 K)/ρ 0
(μΩ cm K-1)
(The quantum interference weak localization and electron-electron interaction effects  that could arise at lower temperatures are ignored here. They contribute negligibly to the measured resistivity in the temperature range of interest in the present work.)
Since the EPI interference effect is governed by the various interference processes generated between the elastic electron scattering and the electron-phonon scattering, our observation may be understood as follows. For a given material characterized by the same βBG, θD, and βint values, when the amount of disorder (ρ0) contained in the sample increases, the number of electronic waves generated from the elastic scattering off defects increases correspondingly. This can consequently enhance the strength of the EPI interference effect, giving rise to an increasingly pronounced ρint(T), as dictated by Equation 2. Furthermore, a metal with a high θDvalue means that the number of phonons that can participate in the electron-phonon scattering is relatively large. (Note that the upper limits of the integrals in Equations 1 and 2 are given by θD.) This can also lead to an enhanced EPI interference effect. As both the ρ0 and θD values in our TiSi NWs are higher than those in normal metals, a significant contribution due to such an effect hence can result.
Inspection of Table 1 indicates that our extracted βBG,θD, and βint parameters vary among samples. Such variations cannot be explained in terms of current theoretical concepts. According to the current theoretical understanding, the two electron-phonon coupling parameters βBG and βint are expected to be independent of disorder for a given material in the weakly disordered regime . On the other hand, whether the value of θD should vary with disorder (or sound velocity which could be disorder dependent) is less clear . These issues deserve further theoretical and experimental investigations.
In summary, we have measured the temperature-dependent resistivity of single TiSi NWs between 2 and 300 K. We demonstrated that as-grown TiSi NWs are metallic. Furthermore, we found a large electron-phonon-impurity interference effect, which strongly dominates the total resistivity up to temperatures as high as room temperature. This is ascribed to originating from large Debye temperatures and high levels of disorder (e.g., point defects) in as-grown TiSi NWs. Our observations suggest that TiSi NWs can serve as a useful system for studying the rich electron scattering processes at the nanoscale level.
This work was supported by the Taiwan National Science Council through grant nos. NSC 100-2120-M-009-008 (JJL) and NSC 100-2113-M-009-003-MY3 (HTC), and by the MOE ATU Program.
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