The results for the perpendicular
and in-plane
components of the electron low field mobility are presented as functions of
the layer composition
for several substrates parameterized by their Ge compositions
as well as their orientations in terms of the Miller indices.
Again the Miller indices specify only two Euler angles
and
while the third Euler angle
is kept constant equal to zero.
Fig. 5.15 compares Monte Carlo simulation results and experimental data for a
active layer grown on a relaxed
Si substrate oriented along
. As it is expected the curves have the minima at
. The electron mobility in the strained
case has Si like character over the whole range of Ge mole fractions. In the case of the perpendicular component
the mobility in the
strained
layer is higher than that in the unstrained case up to
. It follows from the fact that the two
valleys along
move up and have only
little contribution to the mobility. Thus only four in-plane valleys with transverse effective masses determine the mobility. This gives an increase in
comparison with unstrained SiGe. At very high Ge mole fractions the mobility in the unstrained case increases rapidly while for the strained SiGe it has
lower values. This is related to the increase of biaxial compressive strain which at high
makes the four in-plane valleys move strongly down
setting them equal or even lower than the
valleys. In the case of the in-plane component
the mobility in the strained layer is lower
compared to that in unstrained SiGe. This is explained by the fact that unlike in the relaxed material, where four transverse effective masses determine
the electron mobility, only two transverse effective masses are left in strained SiGe which leads to a decrease of the in-plane component.
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Fig. 5.16 to Fig. 5.19 display
and
in
grown on
and
for several substrate orientations.
Fig. 5.20 explains the behavior of the mobility components in the case of
substrates oriented along
.
It shows the populations of the
and
valleys with different orientations and the repopulation between them. The
valley oriented
along
is the most populated one up to
. Thus the contribution of the longitudinal effective masses of the
valley plays the main
role
up to this value of the Ge composition. This reduces the mobility components
and
. The increase of the in-plane component
is related to the effective mass interpolation. When the Ge mole fraction is greater than
, the repopulation between different
valleys comes into play. First, electrons scatter from the valley located along
to the valleys located along
,
and
and then from
and
to
,
. In this way the influence of the longitudinal masses decreases while the transverse masses contribute stronger, leading to the
mobility increase.
S. Smirnov: