Controlled growth of Si-based heterostructure nanowires and their structural and electrical properties
© Qian et al. 2015
Received: 14 October 2014
Accepted: 12 June 2015
Published: 23 June 2015
Ni-catalyzed Si-based heterostructure nanowires grown on crystal Si substrates by hot-wire chemical vapor deposition (HWCVD) were studied. The nanowires which included NiSi nanowires, NiSi/Si core-shell nanowires, and NiSi/SiC core-shell nanowires were grown by varying the filament temperature T f from 1150 to 1850 °C. At a T f of 1450 °C, the heterostructure nanowires were formed by crystalline NiSi and crystalline Si that were attributed to the core and shell of the nanowires, respectively. The morphology of the nanowires showed significant changes with the increase in the filament temperature to 1850 °C. Moreover, the effect of hydrogen heat transfer from the filament temperature demonstrated significant phase changes from NiSi to Ni2Si with increase in the filament temperature. The increased filament temperature also enhanced reactions in the gas phase thus generating more SiC clusters and consequently formed the NiSi/SiC heterostructure core-shell nanowires at T f of 1850 °C. This paper discusses the role of filament temperatures on the growth and constituted phase change of the nanowires as well as their electrical characteristics.
Semiconductor nanowires have been extensively investigated due to their potential applications in a wide range of optical and electrical applications [1–3]. The hybrid heterostructures such as core-shell nanowires tend to improve the properties of the nanowires in the application of high-temperature sensing  and high-performance field-effect transistors  and the enhancement of hydrogen generation efficiency in visible photocatalytic processes . SiC nanostructures are well known for their superior mechanical properties, high thermal conductivity, low thermal-expansion coefficient, good thermal-shock resistance, chemical stability, and electron affinity which make them excellent candidates for work in harsh environments [7, 8]. The incorporation of highly metallic properties of single-crystalline NiSi nanowires as core electrodes into the NiSi/SiC core-shell is expected to enhance their electrical and optical properties. The NiSi core electrodes could be used as 1D electrodes for enhancing the efficiency of electron transfer between the current collector supports and individual electrode materials as well as of ion transport to the electrode [9, 10]. Moreover, this metallic/semiconductor heterostructure could possibly enhance the carrier mobility which would significantly improve the performance of the existing nanowire-based devices .
Extensive work has been conducted to investigate the properties of Si-based heterostructure by various deposition techniques including chemical vapor deposition (CVD), hot-wire chemical vapor deposition (HWCVD), lithography, and laser ablation [12–15]. In this work, the Si-based heterostructure nanowires were grown by HWCVD at different filament temperatures. HWCVD is a preferred technique for the fabrication of Si-based nanowires due to its lower production costs and large-area deposition [16, 17]. Moreover, HWCVD can generate high densities of growth precursors (SiH3 and CH3) through the use of high-temperature tungsten filaments as catalyzers. In HWCVD, the filament temperature plays an important role in controlling the decomposition of the source gases and the gas phase reactions during the deposition. Since the decomposition of the SiH4 and CH4 are at different filament temperatures, the SiH4 starts to decompose at 1027 °C and the decomposition rate increases at temperatures above 1427 °C . However, moderate decomposition of CH4 only occurs at 1750 °C and above [19, 20]. Therefore, HWCVD utilizes the growth of Si-based nanostructures with different morphologies and compositions by tuning the decomposition rate of the source gases by varying the hot filament temperature. This study examines the role of the filament temperature on the growth of different morphologies of the nanowires by HWCVD. Furthermore, we investigate the structural and electrical properties of the different morphologies of the as-grown Si heterostructure nanowires. These physical and chemical properties of the nanowires such as morphological, microstructure, and compositions are characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) spectroscopy, and micro-Raman scattering spectroscopy. The electrical properties of the nanowires were analyzed by current–voltage (I–V) measurement.
The nanowires were grown on SiO2-coated p-type crystal Si (100) substrates using a home-built HWCVD system (Additional file 1: Figure S1). The SiO2 was approximately 100 nm thick and its layer used to prevent the diffusion of Ni into the c-Si substrate. The deposition conditions of HWCVD for growing the nanowires are described in details elsewhere . The crystal Si (100) substrates were cleaned by following RCA-I and II cleaning procedures before introduction into the reaction chamber . For the RCA-II, the acid hydrofluoric cleaning procedure was avoided to prevent the etching of the SiO2 layer on the substrate. The Ni films, with a measured thickness of approximately 30 nm, were deposited on the heating substrates under a vacuum environment and subsequently treated by H2 plasma for 10 min at the pressure, plasma power, and hydrogen flow rate of 0.75 mbar, 5 W, and 100 sccm, respectively, to remove contaminations and activate the Ni surface . The Ni films formed Ni nanoparticles on the surface after being treated by hydrogen plasma at a substrate temperature of 450 °C (Additional file 1: Figure S2). During the deposition, the substrate temperature and deposition pressure were fixed at 450 °C and 3 mbar, respectively, while the filament-to-substrate distance was set at 2 cm. The SiH4, CH4, and H2 flow rates were fixed at 1, 2, and 100 sccm, respectively, while the filament temperature was varied from 1150 to 1850 °C. The deposition time was set at 5 min.
The FESEM secondary electron images of the nanowires were obtained using a Hitachi SU 8000 SEM at a low electron-accelerating voltage of 2 kV and a working distance of 8 mm. A cross-section view of the backscattered electron images of the nanowires was collected by a Bruker photodiode-backscattered electron (PDBSE) detector. The TEM and HRTEM images of the nanowires were obtained using a TEM (JEOL JEM-2100F) with an accelerating voltage of 200 kV. The dispersing nanowires for TEM measurement were prepared on carbon-coated copper grids (Lacey 300 mesh Cu). The energy-dispersive X-ray spectroscopy (EDS) elemental mappings of the nanowire were performed using STEM/high-angle annular dark-field (HAADF) and Oxford EDS detectors. An XRD pattern was recorded over the 2θ range of 20 to 80° at a fixed grazing incidence angle of 1.5° using a PANalytical Empyrean X-ray diffractometer with an X-ray wavelength of 1.5406 Å. The step time and step size of the scanning were fixed at 2 s and 0.026°, respectively. The Raman spectra of the films were recorded using an InVia Raman microscope with a charge-coupled device detector and grating of 2400 lines/mm using an argon-ion laser with an excitation wavelength and laser power of 514 nm and 1 mW, respectively. A very low laser power was selected to prevent the heating effect of the laser which under normal circumstances could induce a crystallization of the Si structure . The I–V measurement of the nanowires was obtained using a Keithley Source Measure Unit 236 (Keithley Instruments, Inc.) with electrical probe station (Signatone H-100). Prior to this, drops of silver paste that were used as electrodes for the I–V measurement were placed on the nanowires and the NiSi layer near their edge with an approximately 5-mm distance between the two consecutive electrodes. The details of the measurement and electrode configuration are illustrated in Additional file 1: Figure S3.
Results and Discussion
Figure 2c demonstrates a backscattered electron signal of the nanowires detected by an external photodiode-backscattered electron (PDBSE) detector. By applying a higher electron accelerating voltage of 15 kV, the internal structure of the nanowires can be clearly observed through the image (see inset). Furthermore, the distribution of the compositions can also be illustrated by the PDBSE image which clearly reveals the heterostructure core-shell nanowires where the core and shell could be attributed to the NiSi and SiC, respectively. Moreover, the NiSi core can be clearly presented along the nanowires from the root to the tip. The formation of the NiSi nanoparticles is also clearly presented on the surface along the nanowires. On the other hand, these nanowires which grow on top of the NiSi layer and the core nanowires are strongly connected to the NiSi layer and could form a good electrical contact for energy storage applications such as lithium-ion batteries and micro-supercapacitor electrodes. The large surface area of the nanowires is expected to enhance the performance of these devices.
The experimental values of the parameters obtained from I–V fittings
Type of diode
Reverse saturation current (A/cm2)
Series resistance (Ω)
Parallel conductances (1/Ω)
Different types of Si-based heterostructure nanowires were grown by HWCVD at different filament temperatures, and their morphological, structural, and electrical properties are presented in this study. The filament temperature was a major factor in controlling the decomposition of the source gases (SiH4 and CH4) and gas phase reactions that led to the growth of different types of the nanowires. The growth of core NiSi nanowires followed the nucleation limit silicide reaction while the formation of crystalline Si and amorphous SiC shell at 1450 and 1850 °C, respectively, were attributed to surface diffusion of the growth precursors. The formation of the crystalline Si and amorphous SiC shell were dependent on the decomposition of SiH4 and CH4 at different filament temperatures. The increased filament temperature resulted in phase changes from NiSi to Ni2Si due to the increased surface temperature attributed to the effect of the hydrogen heat transfer from the filament temperature. Both NiSi/Si and NiSi/SiC heterostructure core-shell nanowires exhibited heterojunction electrical characteristics which could be used for nanoscale diode applications.
This work was supported by the Ministry of Higher Education of Malaysia, for UM/MOHE High Impact Research Grant allocation of F000006-21001, University of Malaya Global Collaborative Programme of RU022D-2014, and the University of Malaya Postgraduate Research Fund (PPP) of PO032-2014A.
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