profile may be associated with the effect of the small size of Si nanocrystals or defects. The presence of nonstoichiometric Si suboxide may also contribute to the
peak asymmetry. For comparison, the spectrum from a SiO film contains a broad
peak at 480 cm
À1 , whereas the fully-oxidized SiNWs (prepared by annealing in air)
show no Raman scattering (Figure 10.34(a)).
Si monoxide has a strong photoluminescence (PL) at 740 nm, while the oxidized
nanowire gives a weak PL peak at 600 nm (Figure 10.34(b)). The PL from the
SiNW product is weak and complicated. A typical PL spectrum from SiNWs covers
the range 600–800 nm. Clearly, the SiO and Si suboxide components in the nanowires are the main contributors to this spectrum. The SiO generated by thermal
evaporation is indeed a mixture of various oxides of Si. Si nanoparticles also coexist
with the SiO generated.
10.7.2
Field Emission from Different Si-Based Nanostructures
It is well known that nanotubes and nanowires with sharp tips are promising materials for application as cold cathode field emission devices. We have investigated
the field emission of different nanowire structures. The first is from SiNWS.
SiNWs exhibit well-behaved and robust field emission fitting a Fowler–Nordheim
(FN) plot. The turn-on field for SiNWs, which is needed to achieve a current density of 0.01 mA cm
À2 , was 15 V mm
À1 [26]. The field emission characteristics may
be improved by further optimization, such as oriented growth or reducing the oxide shell, and may be promising for applications.
The second example of field emission of Si-based nanowires is that of B-doped Si
nanochains [80]. The SiNCs were attached onto a Mo substrate by a conductive
carbon film. The anode-sample separation ranges from 120 to 220 mm. The turn-on
field was 6 V mm
À1 , and smaller than that (15 V mm
À1 ) for the SiNWs. The fieldemission characteristics of the SiNCs were analyzed according to the FN theory
[81]. All the FN curves with different anode-sample separations fall in nearly the
same region and have similar ‘‘Y ’’ intercepts, showing that the SiNCs are uniformly distributed. A stability test showed no obvious degradation of current density and the fluctuation was within G15%, indicating that the B-doped SiNCs are a
promising material for field emission applications.
The third example of field emission from Si-based nanowires is from the aligned
SiC nanowires. The field emission measurements [68] were carried out in a vacuum chamber at a pressure of @5 Â 10
À7 Torr at room temperature. An oriented
SiC nanowire array, which was used as the cathode, was stuck to a stainless steel
substrate by silver paste with the bottom end of the nanowires facing upward. A
copper plate with a diameter of 1 cm, mounted on a precision linear feedthrough,
was used as the anode. Field emission current densities of 10 mA cm
À2 were observed at applied fields of 0.7–1.5 V mm
À1 , and current densities of 10 mA cm
À2
were realized at applied fields as low as 2.5–3.5 V mm
À1 , as shown in Figure 10.35.
These results represent one of the lowest fields ever reported for any field-emitting
materials at technologically useful current densities. We attributed this emission
10 Oxide-Assisted Growth of Silicon and Related Nanowires
350
peak asymmetry. For comparison, the spectrum from a SiO film contains a broad
peak at 480 cm
À1 , whereas the fully-oxidized SiNWs (prepared by annealing in air)
show no Raman scattering (Figure 10.34(a)).
Si monoxide has a strong photoluminescence (PL) at 740 nm, while the oxidized
nanowire gives a weak PL peak at 600 nm (Figure 10.34(b)). The PL from the
SiNW product is weak and complicated. A typical PL spectrum from SiNWs covers
the range 600–800 nm. Clearly, the SiO and Si suboxide components in the nanowires are the main contributors to this spectrum. The SiO generated by thermal
evaporation is indeed a mixture of various oxides of Si. Si nanoparticles also coexist
with the SiO generated.
10.7.2
Field Emission from Different Si-Based Nanostructures
It is well known that nanotubes and nanowires with sharp tips are promising materials for application as cold cathode field emission devices. We have investigated
the field emission of different nanowire structures. The first is from SiNWS.
SiNWs exhibit well-behaved and robust field emission fitting a Fowler–Nordheim
(FN) plot. The turn-on field for SiNWs, which is needed to achieve a current density of 0.01 mA cm
À2 , was 15 V mm
À1 [26]. The field emission characteristics may
be improved by further optimization, such as oriented growth or reducing the oxide shell, and may be promising for applications.
The second example of field emission of Si-based nanowires is that of B-doped Si
nanochains [80]. The SiNCs were attached onto a Mo substrate by a conductive
carbon film. The anode-sample separation ranges from 120 to 220 mm. The turn-on
field was 6 V mm
À1 , and smaller than that (15 V mm
À1 ) for the SiNWs. The fieldemission characteristics of the SiNCs were analyzed according to the FN theory
[81]. All the FN curves with different anode-sample separations fall in nearly the
same region and have similar ‘‘Y ’’ intercepts, showing that the SiNCs are uniformly distributed. A stability test showed no obvious degradation of current density and the fluctuation was within G15%, indicating that the B-doped SiNCs are a
promising material for field emission applications.
The third example of field emission from Si-based nanowires is from the aligned
SiC nanowires. The field emission measurements [68] were carried out in a vacuum chamber at a pressure of @5 Â 10
À7 Torr at room temperature. An oriented
SiC nanowire array, which was used as the cathode, was stuck to a stainless steel
substrate by silver paste with the bottom end of the nanowires facing upward. A
copper plate with a diameter of 1 cm, mounted on a precision linear feedthrough,
was used as the anode. Field emission current densities of 10 mA cm
À2 were observed at applied fields of 0.7–1.5 V mm
À1 , and current densities of 10 mA cm
À2
were realized at applied fields as low as 2.5–3.5 V mm
À1 , as shown in Figure 10.35.
These results represent one of the lowest fields ever reported for any field-emitting
materials at technologically useful current densities. We attributed this emission
10 Oxide-Assisted Growth of Silicon and Related Nanowires
350