Si and Si1-xGex nanowires (NWs) fabricated by the vapor-liquid-solid (VLS) growth process are promising for potential applications in nanoscale electronics and sensing. However, realization of these applications requires a comprehensive understanding of the nanowire growth characteristics and properties. Prior studies have reported the synthesis of Si and Si1-xGex nanowires but there have been limited reports on nanowire growth kinetics. The synthesis of Si1-xGex nanowires using VLS growth process by gaseous reactants is complicated due to difference in thermal stabilities of SiH4 and GeH4 as well as complex interactions between the two sources. Detailed studies of the growth and properties of Si and Si1-xGex nanowires is required in order to develop controlled synthesis and intentional doping processes required for device fabrication.
The VLS technique involves a metal catalyst such as Au that forms a liquid alloy with the nanowire material. The gaseous precursors such as SiH4 and GeH4 decompose at the catalyst surface forming a eutectic alloy and the nanowire grows out of the catalyst particle. In this study, VLS growth was carried out in a low-pressure chemical vapor deposition (CVD) reactor using SiH4 and GeH4 as the precursor gases and H2 as the carrier gas. Nanowires were released from the substrate into isopropyl alcohol solution by mechanical agitation and were then dispersed onto lacey-carbon coated copper grids. The wires were characterized by transmission electron microscopy (TEM) to determine the nanowire diameter and assess the structural properties.
In an initial study, Si/Si1-xGex heterostructure nanowires were grown on oxidized silicon substrates coated with 1 nm thick Au film at a constant temperature of 500oC. The structures consisted of an initial 12 mm long Si segment followed by alternating segments of Si and Si1-xGex grown for 18 seconds each. The SiH4 and GeH4 partial pressure was held constant at 0.59 Torr and 0.024 Torr respectively and the Si1-xGex segments were grown at an inlet gas ratio (GeH4/(GeH4+SiH4)) of 0.038. The composition profiles of the heterostructures were obtained via intensity profiles from high-angle annular dark-field scanning TEM images. The growth rate of Si and Si1-xGex segments was determined by measuring the segment lengths from intensity profiles and accounting for the growth time of each segment. These results indicated that the Si1-xGex segment growth rate was unexpectedly lower than the Si segment growth rate.
To further investigate the effect of compositional effects on growth rate, a series of Si and Si1-xGex alloy nanowire samples were grown on oxidized silicon substrates coated with a 3 nm thick Au film. The Si nanowires were grown over a temperature range of 400oC to 500oC using a SiH4 partial pressure of 0.65 Torr and a total pressure of 13 Torr. The Si1-xGex nanowires were grown at a constant SiH4 flow rate of 50 sccm and introducing appropriate GeH4 to vary the inlet gas ratio (GeH4/(GeH4+SiH4)) within a range of 0.02 – 0.074. The nanowire length was measured from cross-sectional images taken from scanning electron microscopy (SEM). Chemical compositions of the Si1-xGex wires were determined via X-ray energy dispersive spectroscopy (EDS) in scanning TEM mode. At a constant SiH4 partial pressure of 0.65 Torr, the SiNW growth rate decreased exponentially from 0.53 mm/min at 500oC to 0.034 mm/min at 400oC. From the Arrhenius dependence, an activation energy of 131 KJ/mole was measured, which is within the range of values previously reported for SiNW growth from SiH4. In case of Si1-xGex nanowire growth at a temperature of 425oC, the growth rate was found to be relatively constant at 0.14 mm/min as the inlet gas ratio was varied within a range of 0.02 – 0.074 at a constant SiH4 partial pressure of 0.65 Torr. For a constant inlet partial pressure of SiH4 and GeH4, the Si1-xGex NW growth rate was found to be greater than the growth rate of SiNWs for temperatures below 440oC, but above this temperature, the Si1-xGex growth rate is less than that of the SiNWs. These results are consistent with the observations obtained from the studies of Si/Si1-xGex heterostructured nanowires grown at 500oC. The reduction in the Si1-xGex nanowire growth rate compared to that of SiNWs at higher temperatures is likely due, in part to the increasing rate of thin film deposition of Ge at higher temperatures leading to a depletion of GeH4 from the gas phase. Further studies are underway to investigate this effect.