Single-crystalline chromium silicide nanowires and their physical properties
© Hsu et al.; licensee Springer. 2015
Received: 10 December 2014
Accepted: 21 January 2015
Published: 6 February 2015
In this work, chromium disilicide nanowires were synthesized by chemical vapor deposition (CVD) processes on Si (100) substrates with hydrous chromium chloride (CrCl3 · 6H2O) as precursors. Processing parameters, including the temperature of Si (100) substrates and precursors, the gas flow rate, the heating time, and the different flow gas of reactions were varied and studied; additionally, the physical properties of the chromium disilicide nanowires were measured. It was found that single-crystal CrSi2 nanowires with a unique morphology were grown at 700°C, while single-crystal Cr5Si3 nanowires were grown at 750°C in reducing gas atmosphere. The crystal structure and growth direction were identified, and the growth mechanism was proposed as well. This study with magnetism, photoluminescence, and field emission measurements demonstrates that CrSi2 nanowires are attractive choices for future applications in magnetic storage, photovoltaic, and field emitters.
Recently, transition metal silicide nanowires have been widely studied [1-9] for their utilization in semiconductor device technologies. Low-resistivity silicides, such as TiSi2, CoSi2, and NiSi, have been applied for interconnection in CMOS devices . The group of refractory semiconducting silicides, composed of silicon and metals, have different physical properties that are useful and importantly meaningful. Among them, semiconducting silicides, such as CrSi2 and ß-FeSi2, with a narrow energy gap (0.1 to 0.9 eV) have been extensively investigated for their potential use in silicon-integrated optoelectronic devices  such as LEDs [12,13] and IR detectors . In particular, CrSi2 is a narrow bandgap (0.35 eV) semiconductor [15-17], offering applications in the Schottky barrier solar cell technology . Hexagonal CrSi2 with a C40-type structure has a high melting point and excellent resistance to oxidation, deformation, and stretching, being considered to be a potential structural material for aerospace and energy generation industries . Additionally, it is a thermoelectric conversion component that could be applied to generate electric power at high temperatures ; the figure of merit (ZT) of CrSi2 has been measured to be 0.25 at 900 K . CrSi2 also has good field emission with relatively low work function (3.9 eV)  as compared with generally studied field emission materials such as CNTs (5 eV)  and ZnO (5.3 eV) . With excellent intrinsic properties of CrSi2, one-dimensional CrSi2 nanowires are expected to improve field emission performances by bulk and thin film CrSi2. Though there have been some previous studies on CrSi2 nanowires [25-28], two special aspects can be found in this research. Firstly, we conducted a more systematic study on the influences of each processing parameter on growth. Secondly, we provided a low-cost and simple method to synthesize high-quality CrSi2 nanowires with very good physical properties.
In our experiments, we synthesized chromium disilicide nanowires with chemical vapor deposition (CVD) processes. Single-crystal Si (001) wafers, the native oxide of which was etched by BOE solution, were substrates. The metal source was from hydrous chromium chloride (CrCl3 · 6H2O) powders, and the flow gas is Ar gas (99.99%). The CrCl3 · 6H2O powders were put in the upstream zone of the furnace, where the temperature ranged from 700°C to 800°C, while the silicon (001) substrates were put in the downstream zone with the same temperature range. During the growth process, with oxygen environment, CrSi2 nanowires may transform to be CrSi2(core)/SiO2(shell) nanowires due to oxidation. To understand what factors influence the growth of chromium disilicide nanowires, we varied reaction time and temperatures of substrates and the metal source. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) studies were conducted for morphology observation and structure identification of the nanowires. Additionally, physical properties, including magnetism (SQUID), photoluminescence (PL), and field emission (Keithley-237), were measured.
Results and discussion
The SiCl4 also reacted with CrCl3 to form CrSi2, which is the reason why the XRD analysis shows both CrSi2 and Cr5Si3 phases.
The CVD synthesis system can be divided into three sub-systems, which are momentum control system, mass transfer control system, and surface reaction control system. At a lower gas flow rate, mass transfer control system would be the main reaction mechanism, with which gas adsorption and desorption occurred on the Si wafer and fabrication of chromium silicide nanowires was preferred. On the other hand, at a higher gas flow rate, surface reaction control system would be the main reaction mechanism, with which CrCl3 reacted on the Si wafer surface by chemical vapor deposition; thus, chromium silicide films appeared.
where J is the current density, E is the applied electric field, φ is the work function, and A, B are constants, respectively. We put +1,000 V on the sample with a 100-μm spacing between the anode and cathode, and we defined the turn-on field could obtain a current density of 10 μA/cm2 and the turn-on field we measured for CrSi2 nanowires was 7.5 V/μm. The field enhancement factor ß has been calculated to be 1,366 from the slope of ln(J/E 2) = ln(Aß 2/φ) − Bφ 3/2/ßE (for CrSi2, φ = 3.9 eV ), demonstrating that CrSi2 NWs are promising emitters. The outstanding field emission properties of CrSi2 NWs are attributed to their metallic property and special one-dimensional geometry with a high aspect ratio as compared with those of many other materials.
In this study, using a CVD method, we have successfully synthesized chromium silicide nanowires of two phases with unique morphologies. Effects of some processing parameters, including the temperature, gas flow rate, and heating time, were investigated; for example, the growth of chromium disilicide nanowires were influenced by CrSi2 vapor supersaturation, CrSi2 vapor formation rate, and CVD control system. Also, the growth mechanism has been proposed. Field emission and photoluminescence measurements demonstrate that the CrSi2 nanowires are potential field-emitting and photovoltaic materials with a low turn-on field. Additionally, the magnetic property measurements for the CrSi2/SiOx nanowires, showing a ferromagnetic characteristic, demonstrate promising applications for magnetic storage and biological cell separation.
KCL acknowledges the support from the National Science Council through grants 100-2628-E-006-025-MY2 and 102-2221-E-006-077-MY3.
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