Fabrication and Gas-Sensing Properties of Ni-Silicide/Si Nanowires
© The Author(s). 2017
Received: 2 December 2016
Accepted: 24 February 2017
Published: 9 March 2017
Ni-silicide/Si nanowires were fabricated by atomic force microscope nano-oxidation on silicon-on-insulator substrates, selective wet etching, and reactive deposition epitaxy. Ni-silicide nanocrystal-modified Si nanowire and Ni-silicide/Si heterostructure multi-stacked nanowire were formed by low- and high-coverage depositions of Ni, respectively. The Ni-silicide/Si Schottky junction and Ni-silicide region were attributed high- and low-resistance parts of nanowire, respectively, causing the resistance of the Ni-silicide nanocrystal-modified Si nanowire and the Ni-silicide/Si heterostructure multi-stacked nanowire to be a little higher and much lower than that of Si nanowire. An O2 sensing device was formed from a nanowire that was mounted on Pt electrodes. When the nanowires exposed to O2, the increase in current in the Ni-silicide/Si heterostructure multi-stacked nanowire was much larger than that in the other nanowires. The Ni-silicide nanocrystal-modified Si nanowire device had the highest sensitivity. The phenomenon can be explained by the formation of a Schottky junction at the Ni-silicide/Si interface in these two types of Ni-Silicide/Si nanowire and the formation of a hole channel at the silicon nanowire/native oxide interface after exposing the nanowires to O2.
Gas sensors that are based on Si nanostructures are emerging as very powerful because they are readily compatible with existing semiconductor processing technologies. In previous studies, Si nanostructures have been used to detect different gas molecules (such as NH3, NO, H2, O2, and NO2) . The gas-sensing SiNW-based devices consist of both vertically standing [6–8] and in-plane-orientated SiNWs [9–15]. In-plane-orientated SiNW-based devices are more easily integrated into multifunction devices than are vertically standing SiNW-based ones. In-plane-orientated SiNW-based devices can be fabricated using two well-known methods. In the first, nanowires are grown by bottom-up fabrication or etching methods and then transferred onto a substrate; contact pads are then fabricated [9–11]. The second method is a top-down method in which large-scale patterns are formed on a substrate and the lateral dimensions reduced to the nanoscale by nanoimprinting [12–14] or electron-beam lithography ; this method has the advantages of both reproducibility and reliability. In this work, the nanowires were fabricated by atomic force lithography and selective wet etching on a silicon-on-insulator (SOI) wafer, before the contact pads were fabricated.
With respect to in-plane-orientated nanowire-based devices, the sensing performance of metal oxide semiconductor nanowire is enhanced by functionalization with metal or semiconductor nanoparticles, because of the formation of nanosized Schottky or p-n junctions, which forms electron depletion regions within the nanowire, shrinking the effective conduction channel and effectively manipulating the local charge carrier concentration [16–18].
Silicide/Si heterostructures are widely used for source/drain contacts and the low-resistivity interconnects of conventional silicon devices. They have been incorporated into Si nanowires. Nickel silicide/silicon contacts in silicon nanowires are Schottky junctions [19, 20] and can be used to fabricate multifunctional devices [21, 22].
In this work, a Ni-silicide nanocrystal was used to functionalize the Si nanowires, and the O2 sensing performance of the Si nanowires with Ni-silicide nanocrystals was compared with that of those without.
For O2 sensing experiments, nanowire device was placed in a vacuum furnace tube with the base pressure of 1 × 10−3 torr, in which the flux of the gases could be controlled. The sensitivity (S) was calculated as ΔI/I 0, where ΔI is the change in current that is induced by exposure of the device to the target gas atmosphere and I 0 is the initial current.
Results and Discussion
Then, Ni-silicide nanocrystals were formed on Si nanowires by reactive epitaxy to deposit Ni atoms at 400 °C with various Ni/Si atomic ratios. The Ni/Si atomic ratio was estimated based on only Ni film on top of the Si nanowire reacting with Si . Figure 7b, c shows SEM images of Si nanowires after their reaction with Ni atoms with Ni/Si atomic ratios of 1/80 and 1/8, respectively. A nanocrystal-modified Si nanowire (NMSiNW) was formed when the Ni/Si atomic ratio was 1/80, as shown in Fig. 7b. The size of Ni-silicide nanocrystals was smaller than 100 nm. When the Ni/Si atomic ratio was increased to 1/8, the size of the Ni-silicide crystals were increased, forming Ni-silicide/Si multi-stacked heterostructure nanowire (MSHNW), as shown in Fig. 7c.
Three devices that were based on Ni-silicide nanocrystal-modified Si nanowire, Ni-silicide/Si multi-stacked heterostructure nanowire, and Si nanowire were carried out in a vacuum chamber with a base pressure of 1 × 10−3 torr. The current was measured using a bias voltage of 10 V. When the temperature was increased to 250 °C, the current increased gradually, reaching a stable value after about 1 h of holding at that temperature. This phenomenon indicates that when the nanowire was in air, water condensed on its surface revealing in turn that the p-type Si nanowire exhibited high resistance . When the nanowire was placed in the vacuum and heated, the water that condensed on its surface was desorbed. Thus, the resistance of the nanowire decreased, causing the current to increase. After 1 h, all of the water was desorbed and the current reached a stable value.
The resistances of Si nanowire, Ni-silicide nanocrystal-modified Si nanowire, and Ni-silicide/Si multi-stacked heterostructure nanowire at 250 °C in vacuum were 2 × 1010, 2.5 × 1010, and 3.3 × 109 Ω, respectively. The resistances of the Ni-silicide nanocrystal-modified Si nanowire and Ni-silicide multi-stacked heterostructure nanowire were a little higher and much lower than that of the Si nanowire, for the following reason. Since the Ni-silicide grew into Si nanowire, two factors affect the resistance of that nanowire. One is the formation of Ni-silicide/Si interface with a Schottky characteristic, which can increase the resistance of nanowire. Furthermore, the formation of the nanoscopic depletion region in the Schottky junction reduces the cross-sectional area for carrier transmission in the Si region, increasing the resistance of nanowire. The other factor is that nanowire contains a low resistance Ni-silicide phase. Since the resistivity of Ni-silicide is much lower than that of Si, according to the resistivity-mixture rule, the resistance of the nanowire can be reduced. Thus, in the Ni-silicide nanocrystal-modified Si nanowire, the former factor has a greater effect on the resistance of nanowire than the latter factor. However, the amount of the Ni-silicide phase in the multi-stacked heterostructure nanowire is much more than that in the Ni-silicide nanocrystal-modified Si nanowire. The latter factor dominates.
The change in current (∆I) and sensitivity (S) of Si nanowire (SiNW), Ni-silicide nanocrystal modified Si nanowire (NMSiNW), and Ni-silicide/Si multi-stacked heterostructure nanowire (MSHNW) devices exposed to 750 ppm O2
0.9 × 10−9
1.4 × 10−9
7.5 × 10−9
However, the sensitivity of the nanowires may be influenced when exposed in O2 that mix with water. Previous reports show that the resistance of Si nanowire change when Si nanowire was operated in ambient air with various relative humidity  or with various pH values . Furthermore, the influence decreases with the increase of temperature . In this work, O2 sensing is operated at 250 °C. Therefore, we infer that the influence of H2O on the sensitivity when operating in 250 °C may be slighter than that when operating in room temperature.
Si nanowires were fabricated by AFM nano-oxidation on silicon-on-insulator substrates and selective wet etching. Ni-silicide nanocrystal-modified Si nanowire and Ni-silicide/Si heterostructure multi-stacked nanowire were formed by depositing Ni with Ni/Si atomic ratio of 1/80 and 1/8, respectively, at 400 °C on Si nanowires. Since the Ni-silicide grew into Si nanowire, the Ni-silicide/Si Schottky junction and Ni-silicide region were attributed high- and low-resistance parts of nanowire, respectively, causing the resistance of the Ni-silicide nanocrystal-modified Si nanowire and the Ni-silicide/Si heterostructure multi-stacked nanowire to be a little higher and much lower than that of the Si nanowire. An O2 sensing device was formed from a nanowire that was mounted on Pt electrodes. The change in current in Ni-silicide/Si nanowire increases with the amount of Ni-silicide nanocrystal in the Si nanowire after the exposure of the nanowire to O2. The Ni-silicide nanocrystal-modified Si nanowire device had the highest sensitivity. The phenomenon can be explained by the formation of a Schottky junction at the Ni-silicide/Si interface in the Ni-Silicide/Si nanowires and the formation of a hole channel at the silicon nanowire/native oxide interface after exposing nanowires to O2.
To the author’s knowledge, no prior work has been reported on the gas-sensing properties of Ni-silicide/Si nanowires. More work is required to guarantee selectivity, long-time reliability, and stability in the Ni-silicide/Si nanowires.
We thank Prof. Ping-Hung Yeh (Tamkang Univ., Dept. Phys., Taiwan) for fruitful discussions.
This work was supported financially by funding from the Republic of China Ministry of Science and Technology Grants 105-2221-E-005-024-
HFH was involved in the design, data analyzing, and manuscript writing. CAC and SWL carried out the nanowire formation experiments. CAC and CKT performed the gas-sensing property measurements. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Yu P, Wu J, Liu S, Xiong J, Jagadish C, Wang ZM (2016) Design and fabrication of silicon nanowires towards efficient solar cells. Nano Today 11:704–737View ArticleGoogle Scholar
- Chen X, Wong Cell KY, Yuan CA, Zhang G (2013) Nanowire-based gas sensors. Sen Actuators B 177:178–195View ArticleGoogle Scholar
- Huan XJ, Choi YK (2007) Chemical sensors based on nanostructured materials. Sens Actuators B 122:659–671View ArticleGoogle Scholar
- Niranjan RS, Yang Y, Margit Z (2010) Nanowire-based sensors. Small 6:1705–1722View ArticleGoogle Scholar
- Serdar O, Gole JL (2007) The potential of porous silicon gas sensors. Curr Opin Solid State Mater Sci 11:92–100View ArticleGoogle Scholar
- Peng KQ, Wang X, Lee ST (2009) Gas sensing properties of single crystalline porous silicon nanowires. Appl Phys Lett 95:243112View ArticleGoogle Scholar
- Noh JS, Kim H, Kim BS, Lee E, Cho HH, Lee W (2011) High-performance vertical hydrogen sensors using Pd-coated rough Si nanowires. J Mater Chem 21:15935–15939View ArticleGoogle Scholar
- Naama S, Hadjersi T, Keffous A, Nezzal G (2015) CO2 gas sensor based on silicon nanowires modified with metal nanoparticles. Mater Sci Semi Process 38:367–372View ArticleGoogle Scholar
- Mcalpine CM, Ahmad H, Wang D, Heath JR (2007) Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors. Nature Mater 6:379–384View ArticleGoogle Scholar
- Chen ZH, Jie JS, Luo LB, Wang H, Lee CS, Lee ST (2007) Applications of silicon nanowires functionalized with palladium nanoparticles in hydrogen sensors. Nanotechnology 18:345502View ArticleGoogle Scholar
- Skucha K, Fan Z, Jeon K, Javey A, Boser B (2010) Palladium/silicon nanowire Schottky barrier-based hydrogen sensors. Sen Actuators B 145:232–238View ArticleGoogle Scholar
- Wana J, Shao-Ren Deng SR, Yang R, Shu Z, Lu BR, Xie SQ, Chen Y, Huq E, Liu R, Qu XP (2009) Silicon nanowire sensor for gas detection fabricated by nanoimprint on SU8/SiO2/PMMA trilayer. Microelectron Eng 86:1238–1242View ArticleGoogle Scholar
- Gao C, Deng SR, Wana J, Lu BR, Liu R, Huq E, Xin-Ping Qu XP, Chen Y (2010) 22 nm silicon nanowire gas sensor fabricated by trilayer nanoimprint and wet etching. Microelectron Eng 87:927–930View ArticleGoogle Scholar
- Gao C, Xu ZC, Deng SR, Wana J, Chen Y, Liu R, Huq E, Qu XP (2011) Silicon nanowires by combined nanoimprint and angle deposition for gas sensing applications microelectron. Eng 88:2100–2104Google Scholar
- Yun J, Jin CY, Ahn JH, Jeon S, Park I (2013) A self-heated silicon nanowire array: selective surface modification with catalytic nanoparticles by nanoscale Joule heating and its gas sensing applications. Nanoscale 5:6851–6856View ArticleGoogle Scholar
- Kolmakov A, Klenov DO, Lilach Y, Stemmer S, Moskovits M (2005) Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles. Nano Lett 5:667–673View ArticleGoogle Scholar
- Qian LH, Wanga K, Lia Y, Fang HT, Lua QH, Ma XL (2006) CO sensor based on Au-decorated SnO2 nanobelt. Mater Chem Phys 100:82–84View ArticleGoogle Scholar
- Mashock M, Yu K, Cui S, Mao S, Lu G, Chen J (2012) Modulating gas sensing properties of CuO nanowires through creation of discrete nanosized p-n junctions on their surfaces. Appl Mater Interfaces 4:4192–4199View ArticleGoogle Scholar
- Woodruff SM, Dellas NS, Liu BZ, Eichfeld SM, Mayer TS, Redwing JM, Mohney SE (2008) Nickel and nickel silicide Schottky barrier contacts to n-type silicon nanowires. J Vac Sci Technol B 26:1592–1596View ArticleGoogle Scholar
- Hong SH, Kang MG, Kim BS, Kim DS, Ahn JH, Whang D, Sull SH, Hwang SW (2001) Electrical characteristics of nickel silicide–silicon heterojunction in suspended silicon nanowires. Solid-State Electro 56:130–134View ArticleGoogle Scholar
- Mongillo M, Spathis P, Katsaros G, Gentile P, De Franceschi S (2012) Multifunctional devices and logic gates with undoped silicon nanowires. Nano Lett 12:3074–3079View ArticleGoogle Scholar
- Glassner S, Zeiner C, Periwal P, Baron T, Bertagnolli E, Lugstein A (2014) Multimode silicon nanowire transistors. Nano Lett 14:6699–6703View ArticleGoogle Scholar
- Kern W, Puotinen DA (1970) Cleaning solutions based on hydrogen. RCA Rev 31:187–206Google Scholar
- Seidel H, Csepregi L, Heuberger A, Baumgartel H (1990) Anisotropic etching of crystalline silicon in alkaline solutions: I. Orientation dependence and behavior of passivation layers. J Electrochem Soc 137:3612–3626View ArticleGoogle Scholar
- Bassous E (1978) Fabrecation of novel three-dimensional microstructures by the anisotropic etching of (100) and (110) silion. IEEE Trans Electron Devices ED-25:1178–1184View ArticleGoogle Scholar
- Chen TH, Hsu HF, Wu HY (2012) Formation of Ni-silicide nanowires on slicon-on-insulator substrates by atomic force microscope lithography and solid phase reaction. ECS J Solid State Sci Technol 1:P90–P93View ArticleGoogle Scholar
- Hsueh HT, Hsueh TJ, Chang SJ, Hung FY, Weng WY, Hsu CL, Dai BT (2011) Si nanowire-based humidity sensors prepared on glass substrate. IEEE Sens J 11:3036–3041View ArticleGoogle Scholar
- Yuan GD, Zhou YB, Guo CS, Zhang WJ, Tang YB, Li YQ, Chen ZH, He ZB, Zhang XJ, Wang PF, Bello I, Zhang RQ, Lee CS, Lee ST (2010) Tunable electrical properties of silicon nanowires via surface-ambient chemistry. ACS Nano 4:3045–3052View ArticleGoogle Scholar