- Nano Express
- Open Access
Fabrication of Straight Silicon Nanowires and Their Conductive Properties
© Wu et al. 2015
- Received: 5 June 2015
- Accepted: 27 July 2015
- Published: 14 August 2015
Straight Si nanowires (Si NWs) with tens to hundreds of micrometers in length and 40–200 nm in diameter are achieved by annealing a Si substrate coated with metallic Fe. The influences of annealing gas and temperature on the formation of Si NWs are investigated. It is found that the annealing gas has significant impacts on the microstructure of the NWs. By increasing the hydrogen ratio in the forming gas, straight and crystal Si NWs with thin oxide shells are obtained. Both the conductive properties along and perpendicular to the NW are investigated by conductive atomic force microscopy (CAFM) on single NWs. The conductance perpendicular to the NW is too poor to be detected, while a weak conductance can be measured along the NW. The results indicate that the measured resistance mainly comes from the contact(s), and the Si NWs exhibit typical semiconductive conductance themselves, which should have potential applications in nanoelectronics.
- Si nanowires
- Annealing conditions
- Conductive atomic force microscopy
Silicon nanowires (Si NWs) have many unique electronic, optoelectronic, and mechanical properties owing to their anisotropic morphology, high surface-to-volume ratio, tailorable bandgap, and quantum confinement [1, 2]. Due to these unique properties and compatibility with traditional silicon technology, Si NWs have been demonstrated for a variety of applications, such as field-effect transistors, solar cells, integrated logic circuits, lithium-ion batteries, thermoelectric devices, biosensors, and many others [3–10]. In the past decades, considerable efforts have been devoted to nanowire synthesis and a variety of methods have been established for growing Si and its oxide NWs [7–19]. Among these growth methods, solid-liquid-solid (SLS) growth is a relatively straightforward technique for producing Si NWs [11, 12], because in SLS the silicon substrate itself serves as the silicon source, with no additional Si source needed like that in vapor-liquid-solid (VLS) growth [13, 14]. However, the SLS-grown NWs are usually in a mess of curved wires and typically have thick oxide shells or incorporate nonstoichiometric amounts of oxygen [11, 12, 15–19]. Therefore, the growth of straight and crystal Si NWs with thin or no oxide shell is still a challenge and well demanded for nanoelectronic applications.
On the other hand, in order to realize the applications, it is extremely important to get a good understanding of their electrical properties. However, compared to the intensive studies on the growth of NWs, studies on their electrical property are few, especially on single Si NWs. Scanning probe microscopy (SPM)-based electrical measurements reveal themselves as powerful techniques for electrical characterizations at nanoscale [20, 21]. Among these SPM techniques, conductive atomic force microscopy (CAFM) has been most often applied to study the conductive properties of individual nanostructures such as films, heterostructures, dots, or nanoparticles [22–25]. The CAFM studies on one-dimensional nanostructures have also been attempted, including GaN , CdTe , FeSi2 , CuO  and Bi2S3  NWs, ZnO nanoneedles , and carbon nanotubes [32, 33]. There are also a few studies reported for Si NWs, such as the conductance measurements on horizontal and vertical NWs prepared by different methods , and the dielectric property studies on single Si nanowire oxide . Despite all of these efforts, the CAFM studies on single NWs still need to be improved. It is mainly due to the electrical contact problem between the NW and the tip as well as that between the NW and substrate (or electrode) , and/or the easy distortion or damage of the NW by the CAFM tip, particularly for the NWs separated from the substrate.
In this letter, we demonstrate an approach to synthesize straight Si NWs with thin oxide shells and crystal structures by increasing the hydrogen ratio in the annealing gas. Their conductive properties perpendicular to and along the NWs are investigated by CAFM on single NWs. Due to the poor electrical contact(s), the conductance of Si NWs perpendicular to the NW cannot be detected, whereas a weak conductance is measured along the NW with the contact between the NW and the electrode improved.
P-type Si (100) wafers with a resistivity of 5~10 Ω cm were chemically cleaned with the Shiraki method  and then loaded into the ultrahigh vacuum growth chamber of the molecular beam epitaxy (MBE) system. After the thermal treatment at 950 °C for 10 min to remove the protected silicon dioxide layer, a 1-nm-thick Fe film was deposited on the Si wafer at room temperature by MBE. The choice of Fe as catalyst is because Fe would not form a solid solution with Si according to the Si-Fe phase diagram, so that the possibility for Fe contaminating Si NWs can be avoided . An annealing furnace equipped with a temperature controller and a quartz tube was used for the NW synthesis. Si NWs were synthesized at different annealing temperatures in forming gas (with a flow rate of 150 L/h) at atmospheric pressure. Before annealing, the forming gas was flowed through the tube for more than 20 min to expel out the oxygen gas in the tube. After the completion of the NW synthesis, the furnace was cooled naturally to room temperature in flowing forming gas.
Fabrication of Si NWs
On the other hand, much more straight NWs are observed on all the samples annealed in the forming gas with the hydrogen ratio increased to 10 % (termed as 10 % H2 forming gas). The topography images of the NWs fabricated at different temperatures of 1200, 1250, and 1300 °C are shown in Fig. 2c–e, respectively. At the annealing temperature of 1200 °C, straight Si NWs with the length up to several tens of micrometers are formed with their diameters largely scattered from 50 to 500 nm. When the annealing temperature increases, the density of the NWs increases, whereas the average diameter of the NWs decreases. Meanwhile, the fluctuation of NWs’ diameter also decreases, which is about 40~200 nm and 30~150 nm for 1250 and 1300 °C annealed samples, respectively. At the annealing temperature of 1300 °C, much dense, long and relatively fine and uniform NWs are formed (Fig. 2e), compared to those annealed at 1200 and 1250 °C, except the NWs are a little curved due to the decreased diameter. From the above results, it can be demonstrated that the annealing gas primary determines the NWs’ shape, while the annealing temperature mainly changes the NWs’ diameter, length, and density.
To check the oxidation status of the NWs, SEM images before and after HF etching are measured. The results found that the NWs annealed in 5 % H2 forming gas have little left after HF etching (not shown here), indicating the formation of a thick oxide shell. It is well consistent with the TEM results reported in our previous paper , which found that the NWs formed in 5 % H2 forming gas exhibited a core-shell structure with a thin Si core (5~7 nm) and a thick Si oxide shell (~40 nm). On the contrary, for the NWs formed by annealing in the 10 % H2 forming gas, the etching result is significantly different. Figure 2f presents the SEM image of the NWs as shown in Fig. 2e after a 10 % HF etching for 2 min, and no obvious decrease of the NWs’ diameter is observed. Due to the large scattering of the NWs’ diameter, the exact oxide thickness cannot be deduced from the SEM images, but it can still be declared that the decrease of the NWs’ diameter after HF is small, indicating that these NWs have very thin oxide shells compared to their silicon cores. So, the above results suggest that the annealing gas has a great impact on the formed NWs that would not only change the NWs’ shape, but also vary their chemical constitution.
The reasons why 10 % H2 forming gas can greatly inhibit the formation of oxide shell and why straight NWs can be obtained in this case are not very clear at present. Here only a rough explanation is supposed. Due to the contamination, the oxygen in the annealing gas and/or in the annealing tube would react with Fe to form Fe oxide, and the latter will catalyze Si to form SiOx shell in the presence of oxygen contamination, as reported in previous literatures [11, 12]. As the hydrogen ratio in the annealing gas is increased to 10 %, the formation of the Fe oxide could be largely reduced, and hence, the growth of the Si oxide shell catalyzed by Fe oxide is greatly inhibited. Maybe because the oxygen contamination in the annealing gas is low, and most of the oxygen gas in the annealing tube has been expelled out by flowing forming gas through the tube for more than 20 min before annealing, the 10 % hydrogen in the annealing gas seems to be enough to react with the residual oxygen. Therefore, the formation of the Si oxide shell is much decreased. On the other hand, the decrease in the SiO2 shell thickness and increase in crystalline Si core diameter may result in the formation of straight Si NWs. According to the points as reported in references [39, 40], when the NWs mainly consist of amorphous oxide, there is no crystalline phase to stabilize the growth direction anymore, resulting in curved NWs. On the contrary, when the NWs mainly consist of no-oxide crystal, straight NWs would be preferentially formed. As a result, by annealing in 10 % H2 forming gas, straight Si NWs with thin oxide shells can be formed.
Characteristic of Si NWs
Electrical Measurements on Si NWs
By comparing the conductance measurements perpendicular to and along the NW, it can be suggested that the poor measured conductance is mainly attributed to the electrical contact(s), since the current is largely increased when decreasing the number of small-area contacts from double to single. The results further suggest that the contact between the NW and the substrate should be the dominating factor, and current can be measured by improving this contact. On the other hand, the Si NWs exhibit typical semiconductive conductance themselves. So from all the above results, it can be demonstrated that straight, conductive, and crystalline Si NWs are fabricated by using forming gas with increased hydrogen ratio, which should have potential applications in nanoelectronics. Our results also suggest, after solving the contact problems, CAFM should be an effective approach to measure the conductive properties on single Si NW along the two directions without the need of nanoelectrode fabrications.
By increasing the hydrogen ratio in the forming gas, straight and crystal Si NWs with a very thin oxide shell are fabricated. Both the conductive properties along and perpendicular to the NW are investigated by CAFM. Due to the poor electrical contact between the NW and the substrate as well as that between the NW and the tip, together with the native oxide layer, the conductive measurement should be further improved. Nevertheless, our results still present that the Si NWs exhibit typical semiconductive conductance, which may have potential applications in nanoelectronics.
This work was supported by the Major State Basic Research Project of China (No. 2011CB925601), National Natural Science Foundation of China (No. 11274072), Natural Science Foundation of Shanghai (No. 12ZR1401300), and National Science Fund for Talent Training in Basic Science (J1103204).
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