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Previous: 6.3 Electron Inversion Layer Mobility of Strained Si Up: 6.3 Electron Inversion Layer Mobility of Strained Si Next: 6.3.2 Degeneracy Effects on Inversion Layer Mobility |
The influence of stress on the effective electron mobility enhancement in UTB
MOSFETs is analyzed. For (001) oriented substrate it was shown before that a
stress in the [110] direction induces two beneficial effects for the electron
mobility in the [110] direction, namely (i) stress-induced valley splitting
and (ii) stress-induced effective mass change. At relatively large body
thicknesses like
20 nm, the mobility enhancement
can be understood from a combination of these two
effects yielding an anisotropic
as compared to the
unstressed system (see Figure 6.21). In ultra-thin Si bodies,
however, the strong geometrical quantum confinement induces a large intrinsic
valley splitting, thus the additional stress-induced valley shifts have a
negligible effect on the mobility. At
=2.4 nm, the
larger component
parallel and the smaller component
perpendicular to the stress direction
result from the effective mass change only, which is in good agreement with
experimental data [Uchida05].
[stress along
[110]]
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In Figure 6.21 the effect of uniaxial stress along channels in
[100] direction is shown. Stress in direction [100] lifts the degeneracy of
the fourfold degenerate, primed ladder. Since no effective mass change occurs,
occurring at large
is a result of
subband ladder repopulation only. As the body thickness is decreased, the
population of the higher subband ladders inevitably decreases, and strain
cannot further decrease the population. Therefore, in Si films with a
thickness of
nm
even stress at a level of
1 GPa does not induce a mobility enhancement.
Stress-induced
for (110) oriented substrates can be
understood from similar arguments. Tensile stress along [001], which
increases the bulk- and inversion layer mobility parallel to stress (see
Figure 6.16 and Figure 6.20), does not alter the
mobilities at small
, because it does not change the effective
masses, but merely increases the splitting between the primed and unprimed
subband ladders. Figure 6.22 shows how the stress-induced mobility
enhancement, that can be observed at
nm, vanishes at
nm.
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Previous: 6.3 Electron Inversion Layer Mobility of Strained Si Up: 6.3 Electron Inversion Layer Mobility of Strained Si Next: 6.3.2 Degeneracy Effects on Inversion Layer Mobility |