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direction is widely used to enhance
the electron mobility for {001} Si substrate. Hence, in this work special
emphasis was put on this strain configuration.
The effect of strain on the band structure of Si was investigated
systematically using the linear deformation potential theory and the
kp method. Shear strain lifts the degeneracy of the two lowest conduction
bands 
and
at the zone boundary X points. The change of
the shape of the lowest conduction band was quantified in terms of (i) an
effective mass change, (ii) a shear-strain-induced valley splitting, and (iii)
a change in position of the valley minimum in
-space.
Additionally, the empirical pseudopotential method was adapted to incorporate
strain effects. The results from numerical band structure calculations were
compared to the analytical expressions derived using the kp theory and good
agreement was observed.
Furthermore, the effect of strain on the subband structure of Si inversion
layers formed at the surface between one or two Si-SiO
interfaces with
{001}, {110}, and {111} substrate orientation was shown. The
transport masses and the degeneracy of the subband ladders depend on the
substrate orientation and can be modified by strain. The strain configurations
that enhance carrier mobility were identified for each substrate orientation.
Fullband MC simulations were performed using VMC to analyze the effect of
strain on the electron mobility. MC simulations using an analytical description
of the electron bands were shown to be valid in a limited range of shear strain
(
). At larger shear strain the band deformation is so pronounced,
that fullband modeling is required. Hence, for modeling of transport in
strained Si a fullband description is of particular importance. MC
simulations and a rigorous modeling of the strain effect on the electron band
structure reproduce experimentally observed mobility data for bulk Si and Si
inversion layers on different substrate orientations.
A method for the inclusion of the Pauli principle in a Monte Carlo algorithm is presented to study the effect of degeneracy both on the phonon-limited mobility and the effective mobility including surface-roughness scattering in Si inversion layers. It is shown that at room temperature and for {001} substrate orientation incidentally degeneracy has a minor effect on the effective mobility, despite non-degenerate statistics yields unphysical subband populations and an underestimation of the mean electron energy. In general a correct treatment of the degenerate carrier statistics of the 2DEG is important.
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| Previous: 6.3.2 Degeneracy Effects on Inversion Layer Mobility Up: Dissertation Enzo Ungersboeck Next: Bibliography | ||||