Fabrication and Gas-Sensing Properties of Ni-Silicide/Si Nanowires
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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.
KeywordsNanowire Sensor Si Silicide Schottky barrier Scanning probe lithography
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, 7, 8] and in-plane-orientated SiNWs [9, 10, 11, 12, 13, 14, 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, 10, 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, 13, 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, 17, 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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