Fabrication and Physical Properties of Single-Crystalline Βeta-FeSi2 Nanowires

In this study, self-catalyzed β-FeSi2 nanowires, having been wanted but seldom achieved in a furnace, were synthesized via chemical vapor deposition method where the fabrication of β-FeSi2 nanowires occurred on Si (100) substrates through the decomposition of the single-source precursor of anhydrous FeCl3 powders at 750–950 °C. We carefully varied temperatures, duration time, and the flow rates of carrier gases to control and investigate the growth of the nanowires. The morphology of the β-FeSi2 nanowires was observed with scanning electron microscopy (SEM), while the structure of them was analyzed with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The growth mechanism has been proposed and the physical properties of the iron disilicide nanowires were measured as well. In terms of the magnetization of β-FeSi2, nanowires were found to be different from bulk and thin film; additionally, longer β-FeSi2 nanowires possessed better magnetic properties, showing the room-temperature ferromagnetic behavior. Field emission measurements demonstrate that β-FeSi2 nanowires can be applied in field emitters.


Introduction
As the dimension of CMOS devices is down to the nanoscale, metal silicide technology will be even more significant; the substrate of many photonics and microelectronics devices has been silicon. Transition-metal silicides have been studied extensively owing to their outstanding properties, including low resistivity, and great stability [1][2][3][4][5]. For instance, CrSi 2 , β-FeSi 2 , and MnSi are suitable as thermoelectric materials due to their narrow energy gap and great thermostability [6]; NiSi, CoSi 2 , and TiSi 2 are frequently utilized as materials of the metal gate for decreasing the resistance [7].
With excellent properties, such as high compatibility and low defect density, one-dimensional nanostructures are promising for current and future microelectronic devices [8], drawing widespread attention not only from academic studies but also from industry applications [9].
There are different phases for iron disilicides [15][16][17][18][19], among which, the unusual characteristics of β-FeSi 2 is particularly fascinating. As previously reported, β-FeSi 2 nanowires were demonstrated to have important applications in the field of communication [20]; unfortunately, over the many years, few have been able to successfully repeat the fabrication of β-FeSi 2 nanowires with chemical vapor deposition. For β-FeSi 2 , the room temperature equilibrium phase, the potential applications in light emitters, and infrared detectors for silicon-based optoelectronics are attributed to its direct bandgap. In this work, we report direct growth and structural characterization of the single crystalline β-FeSi 2 nanowires via a chemical vapor deposition method. The as-synthesized β-FeSi 2 nanowires exhibited the room-temperature ferromagnetic behavior. Field emission measurements show that the β-FeSi 2 nanowires are great field emission materials.

Methods
In this study, we synthesized β-iron disilicide nanowires using chemical vapor deposition with anhydrous FeCl 3 powder as a precursor, silicon (100) substrates, and Ar carrier gas but without any catalysts. Silicon substrates were cleaned with 3 %-buffered HF and put in the downstream zone of the furnace; anhydrous FeCl 3 powder was placed in an alumina boat upstream from the substrates, the temperature range of which was 750 950°C. We carefully varied temperatures, duration time, and the flow rates of carrier gases in order to realize the factors that influenced the growth of β-iron disilicide nanowires. We utilized scanning electron microscopy (SEM) to investigate the morphology of the β-FeSi 2 nanowires; X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies were conducted for structural identification. In addition, characteristics such as magnetism and field emission property were measured. The magnetic property measurements of the β-FeSi 2 nanowires were conducted by the Superconducting Quantum Interference Device (SQUID) with the VSM option, while field emission property was measured by Kiethly-237.

Results and Discussion
We explored the parameters that could affect the growth of the β-iron disilicide nanowires. Firstly, different gas flow rates were investigated from 50 to 200 sccm as shown in the SEM images of Fig. 1a-c. Figure 1a reveals the gas flow rate at 50 sccm, where we found lots of nanowires with the diameters of 40 nm and lengths of 10 μm. Figure 1b shows the gas flow rate at 80 sccm, where there were some nanowires but the amount was reduced. In Fig. 1c showing the gas flow rate at 120 sccm, there were even fewer nanowires formed. According to the results, the amount of nanowires decreased with the increase of the gas flow rates. When the nanowires grow, the precursor, FeCl 3 , should be carried to the downstream zone of the tube furnace and react with the Si substrate by a carrier gas. At higher gas flow rates, it may be difficult for the nanowires to grow. Based on the chemical vapor deposition mechanisms, there were generally five steps in the deposition process, namely, (1) diffusion of reactants to the surface, (2) absorption of reactants at the surface, (3) chemical reaction at the surface, (4) desorption of products from the surface, and (5) diffusion of products from the surface. The slowest step determines the rate of the CVD reaction. If (1) or (5) is the slowest step, it is mass transfer-controlled. If (2), (3), or (4) is the slowest step, it can be called "surface reaction-controlled." At low temperatures and slow gas flow rates, the surface chemical reaction is slower than reactant diffusion; thus, it is surface reaction-controlled. When it is surface reaction-controlled, variation in the film thickness across the wafer in the chamber will depend on the distribution of temperature, and thin film tends to form. However, our purpose is to grow nanowires; therefore, we should avoid surface reaction-controlled reaction. On the other hand, it is mass transfer-controlled at high temperatures and low gas flow rates. When it is mass transfer-controlled, the rate of the top obtained reactants is faster than that at the sidewall since the axial growth is faster than the radial growth; as a result, nanowires tend to form, and thus, we obtained many nanowires with decreasing gas flow rates. Therefore, mass transfercontrolled reaction is necessary for nanowire growth.
The second parameter we investigated was different growth temperatures as shown in the SEM images of Fig. 1d-f. Figure 1d reveals the growth temperature at 750°C, where there were some nanowires but their lengths and diameters were short and small. Figure 1e shows the growth temperature at 850°C, where we found lots of nanowires with the diameters of 40 nm and lengths of 10 μm. When we increased the growth temperature to 950°C as shown in Fig. 1f, nanowires became nanorods due to more deposition of precursors. The third parameter we investigated was the duration time; Fig. 1g-i shows the SEM images for 1 h, 2 h, and 5 h. Generally, we found longer nanowires with the increasing duration time. After more than 5 h, the morphology of the nanowires would not change significantly, which may be attributed to the fact that the precursor had been completely consumed.
To identify the structure of the nanowires, we conducted X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis as shown in Fig. 2. All the peaks in the corresponding XRD spectrum could be indexed to the structure of orthorhombic β-FeSi 2 phase as shown in Fig. 2a. Figure 2b  The growth mechanism in our experiment may involve two reactions to produce β-FeSi 2 nanowires as shown in Fig. 3. In the first reaction, evaporative FeCl 3 was carried to the furnace downstream, reacting with the Si substrate to form β-FeSi 2 nanoparticles and byproducts of SiCl 4 with β-FeSi 2 nanoparticles appearing increasingly. In the second reaction, SiCl 4 from the first reaction would also react with the precursor of evaporative FeCl 3 and form β-FeSi 2 and Cl 2 . With Cl 2 carried out by Ar gas, we gradually obtained β-FeSi 2 nanowires from both the first and second reactions. The growth mechanism was VS because we did not observe catalystlike metal droplets at the front end of the nanowire. The synthesis via VLS mechanism requires a catalyst; however, no catalyst was used in the experiment. To further investigate the growth mechanism, we tried hydrogen, which may have a reduction effect; still, no metal catalytic droplet was formed. Therefore, we demonstrate that the growth mechanism was VS.
The magnetization of β-FeSi 2 was interesting with different dimensions. It has been found to exhibit superparamagnetism in nanoparticles, even though no magnetic ordering occurs in bulk [21], while in the case of β-FeSi 2 thin film, ferromagnetism was found only at temperatures below 100 K [22]. The ferromagnetic behavior of β-FeSi 2 nanowires may be due to the large specific surface area of the nanowire, leading to many unpaired iron atoms on the surface. Additionally, some strain and defects generated during the growth process could be another factor contributing to the ferromagnetism. To examine the magnetic properties of the grown β-FeSi 2 nanowires, the magnetic properties were measured using the Superconducting Quantum Interference Device (SQUID) with VSM option. Figure 4a is the magnetic response only from the silicon substrate, which clearly shows diamagnetic behavior; we subtracted the magnetism of the silicon substrate for all the following magnetism of β-FeSi 2 nanowires. The magnetization curve of the β-FeSi 2 nanowires was growing in 2 h as shown in Fig. 4b. The nonlinear hysteresis loop curve shows that the β-FeSi 2 nanowires exhibited ferromagnetic behavior at room temperature. The coercivity was about 264 Oe. Larger saturation magnetization was found at 2 K because of the decreasing thermal fluctuation. Due to the reduced coordination of the surface iron atoms, or the strain and structural defects in the crystal, β-FeSi 2 nanowires grown here were found to be ferromagnetic [23]. Figure 4c shows the magnetization curve of the longer β-FeSi 2 nanowires growing in 5 h. From shorter to longer nanowires, the coercivity increased from 264 to 345 Oe at 300 K, and even to 575 Fig. 3 Schematic illustration of the growth mechanism 1 FeCl 3 (s) → FeCl 3 (g); 2 4FeCl 3 (g) + 11Si(s) → 4β-FeSi 2 + 3SiCl 4 (g); 3 2FeCl 3 (g) + 4SiCl 4 (g) → 2β-FeSi 2 + 11Cl 2 Fig. 2 a XRD pattern for β-FeSi 2 NWs, b a low-magnification TEM image of a β-FeSi 2 NW nanowire, and c HRTEM of a β-FeSi 2 NW. The inset in c is the corresponding diffraction pattern with [011] zone axis Oe at 2 K; saturation magnetization was raised more as well. It was confirmed that the longer nanowires possessed better magnetic properties. Temperaturedependent field cooling (FC) and zero-field cooling (ZFC) magnetization measurements are shown in Fig.  4d, where the magnetization curve did not drop to zero, showing that the curie temperature of β-FeSi 2 NWs was higher than room temperature. The ZFC and FC curves of β-FeSi 2 NWs did not overlap; the temperature of curve separation is called blocking temperature (T b ), indicating that a large magnetic anisotropy energy barrier distribution existed [24]. When the temperature was lower than T b , the magnetic anisotropy energy was larger than the thermal fluctuation. As a result, grains were blocked and not impacted by the applied magnetic field; thus, the magnetization was observed.
To explore the field emission properties, we conducted the field emission measurements for the β-FeSi 2 nanowires. The sample was measured in a vacuum chamber at~10 -6 torr. Figure 5 shows the current density (J) -field (E) plot with β-FeSi 2 nanowires of different lengths. According to the Fowler-Nordheim (F-N) plot and the Fowler-Nordheim equation: where J is the current density, E is the applied electric field, and φ is the work function; the inset reveals the ln(J/E 2 )-1/E plot. The field enhancement ß was calculated to be 1060 from the slope of ln(J/E 2 ) = ln(Aß 2 /φ)-Bφ 3/2 /ßE, and ß increased from 1060 to 2367 with the increasing length of nanowires, demonstrating that longer β-FeSi 2 NWs had better field emission properties than shorter ones, and that β-FeSi 2 NWs could be great field emission materials. Conclusions β-FeSi 2 nanowires were successfully synthesized with a CVD method. Processing parameters, including temperature, gas flow rate, and duration time were investigated for their effect on the nanowire growth. The growth mechanism has been proposed. Unlike bulk and thin-film β-FeSi 2, the as-synthesized β-FeSi 2 nanowires exhibited room-temperature ferromagnetic behavior. Field emission measurements demonstrate the β-FeSi 2 nanowires as potential field emitting materials.