(23b) Growth Characteristics and Properties of Si and Si1-XGex Nanowires

Si and Si_1-xGe_x
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 Si_1-xGe_x nanowires
but there have been limited reports on nanowire growth kinetics. The synthesis
of Si_1-xGe_x 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 Si_1-xGe_x
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/Si_1-xGe_x
heterostructure nanowires were grown on oxidized silicon substrates coated with
1 nm thick Au film at a constant temperature of 500^oC. The
structures consisted of an initial 12 mm long Si segment followed by alternating
segments of Si and Si_1-xGe_x 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 Si_1-xGe_x
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 Si_1-xGe_x 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 Si_1-xGe_x
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 Si_1-xGe_x
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 400^oC to 500^oC
using a SiH₄ partial pressure of 0.65 Torr and a total pressure of
13 Torr. The Si_1-xGe_x 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 Si_1-xGe_x 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 500^oC to 0.034 mm/min
at 400^oC. 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 Si_1-xGe_x
nanowire growth at a temperature of 425^oC, 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 Si_1-xGe_x NW growth rate was found to be greater than
the growth rate of SiNWs for temperatures below 440^oC, but above
this temperature, the Si_1-xGe_x growth rate is less than
that of the SiNWs. These results are consistent with the observations obtained
from the studies of Si/Si_1-xGe_x heterostructured
nanowires grown at 500^oC. 
The reduction in the Si_1-xGe_x 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.