(23b) Growth Characteristics and Properties of Si and Si1-XGex Nanowires
AIChE Spring Meeting and Global Congress on Process Safety
2008
2008 Spring Meeting & 4th Global Congress on Process Safety
Nanoscale Science and Engineering Forum -- Jointly Co-sponsored with ACS
Chemical Processing of Nanoparticulate Materials I
Monday, April 7, 2008 - 8:20am to 8:40am
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.
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