Environmental Engineering Reference
In-Depth Information
Figure 5.7
Variation of calculated longitudinal thermal conductivity (
κ
)of
nitrogenated SiNWs as a function of surface nitrogenation ratio, compared
tothermalconductivityofH-SiNW(
κ
0
).Thecross-sectionalareais2.36nm
2
.
Reprintedwithpermissionfrom[43].Copyright c
2012,CityUniversityof
Hong Kong.
films on Si, that is, 25% nitrogenation of an NCD film results in a
50% reduction in thermal conductivity at room temperature [54].
Inoursimulations,theaverageN-Sibondlengthisapproximatly
0.176 nm in N-SiNWs, which is slightly larger than the N-Si
bond length (0.173 nm [38]) in the N(SiH
3
)
3
molecule but much
smaller than the standard Si-Si bond length (0.235 nm) in bulk
silicon. Therefore, surface nitrogenation changes the surface geom-
etry of studied SiNWs and induces surface lattice deformations.
Additionally nitrogenation favors the formation of
surface defects
in SiNWs, which leads to enhanced thermal diffuse scattering
[55]. The defects near the surface with higher concentrations will
scatter phonons more effectively when the surface nitrogenation
is increased, thereby further decreasing the thermal conductivity
of the SiNWs. In semiconductor nanowires, charge carriers are
naturally pushed away from the surface [44]. Thus, surface
scattering by nitrogenation can reduce the thermal conductivity at
a higher degree than the electrical conductivity, which is beneficial
to obtaininghigher TE performance [4].
