10.3
Control of SiNW Nanostructures in OAG
10.3.1
Morphology Control by Substrate Temperature
One of the most important issues of nanomaterial growth is the control of the
morphology. This can be done by varying different process parameters. The effect
of the growth temperature on the structure of the Si nanowires has been studied
systematically [32, 33].
The substrate temperature substantially affects the SiNW growth in several ways:
(1) determination of the growth process (metal catalyst VLS growth or OAG), (2)
determination of the SiNW shape in the growth process itself (e.g. the SiNW
diameter), (3) annealing effects that change the structure and morphology of the
SiNW after its formation. Examples of these effects were given in our study of
SiNWs deposited by laser ablation of a mixture of Si, SiO x and metals (the metallic
constituent introduced either intentionally or as impurities or contamination of the
system). The study was focused on the nature of the nanostructures produced on
the substrate at temperatures ranging from 850 to 1200
C.
The Si nanostructures produced in this temperature region can be divided to two
groups: (1) Region I (1200@1100
C) forming nanostructures by the metal catalyst
VLS process (indicated by the presence ofmetallic caps), (2) Region II (1100@
850
C) where the OAG process is dominant (the nanostructures do not have
metallic caps). HRTEM analysis of samples prepared in region I (temperatures
of 1190, 1160, and 1130
C denoted I1, I2 and I3) and region II (temperatures
1050, 950, and 900
C denoted II1, II2 and II3) revealed the structure evolution
at different temperatures.
In region I the diameter of the SiNWs (all single-crystalline SiNWs) decreases
with temperature (200, 80 and 50 nm for 1190, 1160 and 1130
C respectively) as
shown in Figure 10.8. While the first two types of nanowires are straight, continuous ones, the third one has a tadpole-like shape and appears to be broken into
short Si rods. The head of the tadpoles is crystalline Si, while the tail of the tadpoles is amorphous SiO x .
In region II the diameter of the Si nanostructures is constant (@20 nm), but
their shape changes from tadpole-like through chain-like to wire-like as the temperature decreased from 1050, through 950, to 900
C respectively. The wires have
SiO x caps rather than metallic caps, and are encapsulated in a SiO 2 sheath, all indicative of an OAG process.
We will now explain these results as a combination of: (1) the dominant growth
process (metal-assisted VLS or OAG) and (2) extended annealing of Si nanowires at
high temperatures.
Region I is characterized by a metal catalyst VLS growth, as indicated by the
metal caps on top of the nanostructures. The diameter of the nanowires in this
growth process is determined by the diameter of the liquid alloy droplet at their
tips. Metal silicide clusters of different sizes are present in the flowing gas above
10.3 Control of SiNW Nanostructures in OAG 319
Control of SiNW Nanostructures in OAG
10.3.1
Morphology Control by Substrate Temperature
One of the most important issues of nanomaterial growth is the control of the
morphology. This can be done by varying different process parameters. The effect
of the growth temperature on the structure of the Si nanowires has been studied
systematically [32, 33].
The substrate temperature substantially affects the SiNW growth in several ways:
(1) determination of the growth process (metal catalyst VLS growth or OAG), (2)
determination of the SiNW shape in the growth process itself (e.g. the SiNW
diameter), (3) annealing effects that change the structure and morphology of the
SiNW after its formation. Examples of these effects were given in our study of
SiNWs deposited by laser ablation of a mixture of Si, SiO x and metals (the metallic
constituent introduced either intentionally or as impurities or contamination of the
system). The study was focused on the nature of the nanostructures produced on
the substrate at temperatures ranging from 850 to 1200
C.
The Si nanostructures produced in this temperature region can be divided to two
groups: (1) Region I (1200@1100
C) forming nanostructures by the metal catalyst
VLS process (indicated by the presence ofmetallic caps), (2) Region II (1100@
850
C) where the OAG process is dominant (the nanostructures do not have
metallic caps). HRTEM analysis of samples prepared in region I (temperatures
of 1190, 1160, and 1130
C denoted I1, I2 and I3) and region II (temperatures
1050, 950, and 900
C denoted II1, II2 and II3) revealed the structure evolution
at different temperatures.
In region I the diameter of the SiNWs (all single-crystalline SiNWs) decreases
with temperature (200, 80 and 50 nm for 1190, 1160 and 1130
C respectively) as
shown in Figure 10.8. While the first two types of nanowires are straight, continuous ones, the third one has a tadpole-like shape and appears to be broken into
short Si rods. The head of the tadpoles is crystalline Si, while the tail of the tadpoles is amorphous SiO x .
In region II the diameter of the Si nanostructures is constant (@20 nm), but
their shape changes from tadpole-like through chain-like to wire-like as the temperature decreased from 1050, through 950, to 900
C respectively. The wires have
SiO x caps rather than metallic caps, and are encapsulated in a SiO 2 sheath, all indicative of an OAG process.
We will now explain these results as a combination of: (1) the dominant growth
process (metal-assisted VLS or OAG) and (2) extended annealing of Si nanowires at
high temperatures.
Region I is characterized by a metal catalyst VLS growth, as indicated by the
metal caps on top of the nanostructures. The diameter of the nanowires in this
growth process is determined by the diameter of the liquid alloy droplet at their
tips. Metal silicide clusters of different sizes are present in the flowing gas above
10.3 Control of SiNW Nanostructures in OAG 319