Dawei Wang'), Carolyn Jones Otten2), William E. Buhro2), Jia G. Lu'). 1) Dept. of Chemical Engineering and Materials Science & Dept. of Electrical Engineering ...
Rectifying Effect in Boron Nanowire Device Dawei Wang'), Carolyn Jones Otten2), William E. Buhro2), Jia G. Lu') 1) Dept. of Chemical Engineering and Materials Science & Dept. of Electrical Engineering & Computer Science University of California, Irvine 92697 2) Dept. of Chemistry, Washington University 63 130 Absiraci- Ni and Ti have been used as contact electrodes to the crystalline boron nanowires synthesized by catalyzed chemical vapor deposition method. Three-terminal electrical measurements showed that boron nanowire behaves as p-type semiconductor. Ni forms ohmic contact to the nanowire and Tinanowire forms Schottky junction. Rectifying effect has been observed for devices with Ni as one electrode and Ti as the other one.
and can be doped to n-type [23], and we intend to obtain both p-type and n-type boron nanowires (BNW) by doping. Consequently, both the conducting and semiconducting components necessary to the nanoelectronic devices can be provided by the B-based nanostuctures. Devices constructed fi-om them should benefit from enhanced mechanical stability, prolonged lifetime and improved performance because of the superior physical characteristics of the material. In this paper, we report the electrical property study on crystalline BNWs. Different metals, Ti and Ni, have been used as electrodes. Ni was found to form ohmic contact to the BNWs, and Ti-BNW contact forms Schottky barrier. Devices with mixed electrodes, i.e. Ti as one electrode and Ni as the other one, show rectifying effect. Break-down took place when the reverse bias is beyond a certain threshold voltage (Vtd. EXPERIMENT Boron nanowires were synthesized by catalyzed chemical vapor deposition method [24]. In brief, the B nanowires were grown upon a nickel boride (NIB) catalyst fi-om diborane (B2H6) in an argon carrier gas at 1100 "C. The nanowires possessed lengths of several micrometers, and diameters ranging fi-om 20 to 200 nm (mean diameter = 60 nm). Electron diffraction in the transmission electron microscope (TEM) established the crystallinity of the nanowires [24]. Elemental analysis by parallel electron-energy-loss spectroscopy in the TEM detected low levels of carbon, which were assigned to adventitious surface deposits [24]. The B nanowires were separated fi-om the NiB catalyst for transport measurements by the following procedure. After growth, the B nanowires were anchored to a slag of the NiB catalyst on an alumina-wafer growth substrate [24]. This NiB slag containing the B nanowire growth was removed with tweezers, and sonicated in 3 - 5 mL of 1-octanol using a 13mm high-intensity ultrasonic horn (Sonics and Materials VC600) at 60 W (10% power) for 1 h. The resulting gray suspension was allowed to stand for ca. 1 h to separate the largest particles. The remaining suspension was centrifuged for several minutes to facilitate further settling. The resulting grayish liquid dispersion was removed fkom the packed solid by pipet, and one drop was placed on a clean silicon wafer for analysis by scanning electron microscopy (SEM). This sonication-centrihgationcycle was repeated (using the liquid dispersion) until individual nanowires, free fkom catalyst and byproduct particles, were identified on the silicon wafer.
INTRODUCTION Miniaturization of electronic devices is making continuous progress. One-dimensional (1D) systems, such as nanotube and nanowire, may serve as the building blocks for the next generation devices. Field effect transistors, single electron transistors, p-n diodes, Schottky-barrier rectifiers, memory devices, and logic circuits made of nanotubes or nanowires have been achieved during the last few years [l-lo]. So far, most of these works are focused on carbon nanotubes (CNT) [l-4,7-91, group IV semiconducting nanowires such as Si [6, lo], compound semiconducting nanowires formed by group 111-V, such as InP [5], GaN [lo], etc. 1D nanostructures made of other materials, such as boron nitride nanotubes [ 111, group 11-VI [12,131 semiconducting nanowires, and metal nanowires [14,15] have also been obtained and studied. For some of these 1D structures, there are obstacles in their applications: CNT has difficulty in controlled growth to make it a specific electric type; Boron nitride nanotube is a large gap semiconductor; and pure metal nanowires lack in strength, and chemical and thermal stability. Boron (B) is a semiconductor with a band gap of about 1.4eV [16-181. Bulk boron and metal-boride phases are highly refkactory, chemical stable, strong, and hard [191. Additionally, the metal borides often exhibit lower electrical resistivities than do the corresponding pure metals [ 191. The resistivity of TiB2 (9 W c m ) and Ti (42 @cm) provide such an example. The superconductivity of MgB2 wires at T, =: 39 K is an exciting discovery for materials of this family [20]. We expect these favorable properties to be largely retained in 1D nanowires. Interestingly, theoretical studies by Boustani [2 11 and Lipscomb [22] have proposed that nanotubes made of boron, which have structures related to those of carbon nanotubes, should be stable and possibly exhibiting higher electrical conductivities than metallic CNT. These information indicate boron and metal-boride nanostructures may be unique in exhibiting excellent stability under mechanical loading and thermal stress, and while carrying significant current. Naturally, bulk boron shows p-type semiconducting behavior,
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Octanol dilutions were then made of this "stock" dispersion to produce an appropriately low nanowire density on the wafer. The nanowires were then dispersed on a p-type heavily doped Si substrate capped with 500 nm oxide. After drying for 24 hours in ambient condition, the chips were baked at 180 "C for 1 hour to further remove the solvent residue. Predefined alignment marks on the substrate are used to locate the position of the nanowires. Electron beam exposure pattern is then designed based on the nanowires' position. Two layers of electron beam resist - 380 nm methylmethacrylate and methacrylic acid (MMAMAA) on bottom layer and 170 nm polymethylmethacrylate (PMMA) on top layer - were spincoated onto the Si chip, and each was baked for 15 minutes at 180 "C. Electrical contacts to the individual nanowire are then made by electron beam (Ebeam) lithography (JEOL JBX5D11(U)) followed by metal deposition and liftoff. Evaporations were done using electron beam evaporator (CHA SEC600). Three different types of devices have been fabricated, classified by the electrodes used: type 1, both of the two electrodes contacting the BNW are made of Ni/Au, with 40 nm Ni adhesive layer covered by 150 nm Au; type 2, both electrodes are made of TVAu, with 20 nm Ti adhesive layer covered by 150 nm Au; type 3, one contact is TVAu and the other is NVAu electrode. The first two types of devices need only one time Ebeam lithography, metallization and liftoff. Type 3 devices were made by repeating the Ebeam lithography and metal evaporation twice: for the 1'' ebeam lithography and metallization, TVAu electrodes contact were made to the nanowire, then after lift off, the chip is again coated with the two-layer Ebeam resist (MMA/MAA and PMMA). Another Ebeam lithography and metal evaporation were done, this time NgAu electrodes were deposited. The thickness of corresponding material is the same as type 1 and type 2 devices. The degenerately doped Si substrate serves as the back gate. Electrical transport measurements were done using Agilent 4 156C semiconductor parametric analyzer.
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Fig. 2(a) shows the experiment setup, the TVAu electrode is grounded and NVAu electrode is connected to the voltage source. The rectifying effect can be clearly seen from I - V curve shown in Fig. 2(b). Saturation current at reversed bias is 1.8 nA. Inset is semi-log plot of I - V, the lines with arrows are drawn as guides for eyes. At V < 0.65 V, the current increases exponentially with the bias voltage (which gives the linear part in the semi-log plot, refer to the dashed arrow line). When the bias is beyond 0.65 V, the Z - V curve becomes linear (non-linear in semi-log plot, refer to the curved m o w line), the slope of the curve shows a trend to decrease at bias close to 10 V. Break-down at reverse bias around 20 V (Fig. 2(c)) has also been observed for the device. The work functions (Q) of Ni and Ti are Gi= 5.15 V, QTi = 4.33 V [26], Intrinsic P-rhombohedral boron has work function around 4.3 eV and band gap around 1.4 eV [ 16-18]. For our p-type BNW, we surmise its work function to be greater than 4.33 eV. Thus, we have the following work function relation for the three materials: %i > QBwi > QTi. A band diagram is drawn in Fig. 3(a) based on this relation to explain the behavior of the there will be no barrier at Nidevice. Because (PNi >QBwi, BNW contact for hole transportation, gives an ohmic contact. But for the BNW-Ti contact, a Schottky barrier is formed. Here QB is the Schottky barrier height and eVo is the built-in potential. When negative bias (reverse bias) is applied to the
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RESULTS AND DZSCUSSION Fig. l(a) shows three 1 - V curves of a BNW device with NVAu electrodes on both ends (gate voltages V,= 0, -20, -40 V, respectively). The inset is a scanning electron microscopy (SEM) image of BNW device with electrode contact. The Z V curves are almost linear around 0 V, indicating that an ohmic contact has been achieved. Significantly, higher conductance of the device was obtained when the gate voltage decreased fkom 0 V to - 40 V, demonstrating that the BNW behaves as ap-type semiconductor. In contrast, the I - V curve of device with Ti/Au electrodes (Fig. l(b)) shows an obvious gap around 0 V, which is a typical behavior of nanowire devices with two back-to-back Schottky barriers [25]. These results suggest that a Schottky diode can be formed using BNW. Rectifying effect was observed on type 3 devices: with one side contacted by NVAu electrode and the other side by TVAu.
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Figure 3. Band diagram for the device shown in Fig. 2(a). (a) zero bias. (b) Reverse bias (negative voltage on Ni/Au electrode). (c) forward bias (positive voltage on NVAu electrode).
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CONCLUSION Ni and Ti have been used as contact electrodes to study the electrical properties of the BNW. BNW behaves as p-type semiconductor. Rectifying effect has been observed for devices with Ni as one contact electrode and Ti as the other one.
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ACKNOWLEDGMENT We thank Pai-Chun Chang for SEM imaging. Device fabrication was done at UC Irvine Integrated Nanosystems Research Facility and the UCSB Nanofabrication Facility. This work is supported by NSF grant EEC-0210120, UC Irvine, and Washington University.
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REFERENCES
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[l] S. J. Tans, A. R. M. Verschueren, C. Dekker, “Room-temperature transistor based on a single carbon nanotube,” Nature, vol. 393, pp 49-52, 1998. [2] R. Martel, T. Schmidt, H. R. Shea, T. Hertel, Ph. Avouris, “Single- and multi-wall carbon nanotube field-effect transistors,” Appl. Phys. Lett. vol. 73, pp 2447-2449, 1998. [3] S. J. Tans, M. H. Devoret, H. Dal, A. Thess, R. E. Smalley, L. J. Geerligs, C. Dekker, “Individual single-wall carbon nanotubes as quantum wires,” Nature vol. 386, pp 474-477, 1997. [4] M. Bockrath, D. H. Cobden, P. L. McEuen, N. G. Chopra, A. Zettl, A. Thess, R. E. Smalley, “Single-electron transport in ropes of carbon nanotubes,” Science vol. 275, pp1922-1925, 1997. [SI X. Duan, Y, Huang, Y. Cui, J. Wang, C. M. Leiber, “Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices,” Nature vol. 409, pp 6649,2001. [6] Y. Cui, C. M. Leiber, “Functional nanoscale electronic devices assembled using silicon nanowire building blocks,” Science vol. 291, pp 851-853, 2001. [7] M. S . Fuhrer, J. Nygard, L. Shih, M. Forero, Y.-G. Yoon, et al, “Crossed nanotubejunctions,” Science, vol. 288, pp 494497,2000. [8] V. Derycke, R. Martel, J. Appeneller, Ph. Avouris, “ Carbon nanotube inter- and intramolecular logic gates,” Nan0 Lett. vol. 1, pp,453-456, 2001. [9] A. Bachtold, P. Hadley, T. Nakanishi. C. Dekker, “Logic circuits with carbon nanotube transistors,” Science, vol. 294, pp 1317-1320,2001. [lo] Y. Huang, X. Duan, Y. Cui, L. J. Lauhon, K.-H. Kim, C. M. Leiber, “Logic gates and computation from assembled nanowire building blocks,” Science, vo1.294, pp 1313-1317,2001.
Fig. 2 (a) Schematic of the measurement circuit. (b) I-Vcurve of a BNW device with connections as shown in (a), the BNW is 90 nm in diameter, 1.2 pn in length. Inset: semi-log plot of I - V, arrowed lines are drawn as guides for eyes. (c) Breakdown happened at reverse bias of about -20 V.
Ni side, holes from the Ti side to the BNW run into the Schottky barrier, QB (Fig. 3(b)). But for positive bias (forward bias) applied to the Ni side, holes from the BNW side to the Ti electrode only need to overcome a much smaller potential barrier than QB (Fig. 3(c)), thus giving rise to the rectifying effect. Such behavior has been observed on 4 different devices we made. The exponential to linear transition of the I - V might be due to the high resistance of the BNW [27]. When the forward bias is beyond a certain value, almost all the holes are able to contribute to the current by thermal emission. In this case, the resistance of the BNW dominates the behavior of the device, giving a linear part in the I - V. Based on this information, we can estimate that the built-in potential at TiBNW contact should be much smaller than 0.65V. Further study will be carried out to understand this thoroughly.
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[ll] M. Radosavljevic’, J. Appenzeller, V. Derycke, R. Martel, Ph. Avouris, et al, “Electrical properties and transport in boron nitride nanotubes,” Appl. Phys. Lett. vol. 82, pp 4131- 4133,2003. [ 121 X. Duan, C. M. Lieber, “General synthesis of compound semiconductor nanowires,” A h . Mat. vol. 12, pp 298-302,2000. [13] X. Duan, Y. Huang, R. Agarwal, and C.M. Lieber, “Single-nanowire electricallydriven lasers,” Nature, vol. 421, pp 241-245,2003. [ 141 C. R. Martin, “Membrane-based synthesis of nanomaterials,” Chem. Mater. vol. 8, pp 1739-1746, 1996 [IS] D. Natelson, R. L. Willet, K. W. West, L. N. Pfeiffer, “Molecular-scale metal wires,” SolidState Commun. vol. 115, pp 269-274,2000, [ 161 K. H. Hellwege Ed., Landolt-Bornstein Numerical Data and Functional Relationships in Science and technology, New Series; Vol. IIU 17e, Springer-Verlag: Berlin, 1983, pp 11-19. [I71 W. Dietz, and H. Henmann, ‘‘ Conductivity , Hall effect, optical absorption, and band gap of very pure boron,” Boron, Prepn., Properties, Appl. Papers Intern. Symp. Paris, vol. 2, pp 107-118, 1965. [IS] W. Neft, and K. Seiler, “Semiconductor properties of boron,” Boron, Prepn., Properties, Appl., Papers Intern. Symp. Paris vol. 2, pp 143-167, 1965. [19] V. I. Matkovich, Ed., Boron and Refiactory Borides, Springer Verlag, Berlin, 1977. [20] P. C. Canfield, D. K. Finnemore, S. L. Bud’ko, J. E. Ostenson, G. Lapertot, et al, “Superconductivity in dense MgBz wires,” Phys. Rev. Lett. vol. 86, pp 2423-2426,2001, [21] I. Boustani, A. Quandf E. Hemadez, A. Rubio, “New boron based nanostructured materials,”J. Chem. Phys. vol. 110, pp 3176-3185, 1999. [22] A. Gindulyte, W. N. Lipscomb, L. Massa, “Proposed boron nanotubes,” Inorg. Chem. vol. 37, pp 65446545, 1998. [23] H. Werheit, K. De. Groot, W.Malkemper, T. LundstrOm, “On doped prhombohedral boron,”J. Less-Common Met. vol. 82, pp 163-168, 1981. [24] C. J. Otten, 0. R. Lourie, M.-F. Yu, J. M. Cowley, M. J. Dyer, R. S . Ruo& and W. E. Buhro, “Crystalline boron nanowires,” J. Am. Chem. Soc. vol. 124, pp 4564465,2002. [25] S. W. Chung, J. Y. Yu, J. R. Heath, “Silicon nanowire devices,” Appl. Phys. Lett. vol. 76, pp 2068-2070,2000. [26] J. Singh, Semiconductor Devices: An Introduction, McGraw-Hill, 1994, pp 255. [27] D. Wang, C. J. Otten, W. E. Buhro, J. G. Lu, “Electrical transport in boron nanowires,” unpublished.
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