Fig. 5.6 illustrates the behavior of the perpendicular component
of the electron mobility,
while Fig. 5.7 shows the same dependence for
. The two components
and
for
reach their maximum values at
because at this value of
the population of the
valley with orientation
is the highest
and due to the orientation the influence of the longitudinal masses
of the
valleys oriented along
and
is minimal.
However, the influence of the
valleys of these orientations is significant at
and
where the two components have
their minima.
The analogous results for
and
for a substrate composition of
are depicted in Fig. 5.8 and
Fig. 5.9. The main difference from the case of
substrate is that here the
valley comes into play which
causes an additional intervalley scattering process between
and
valleys.
The repopulation processes are most clearly seen in Fig. 5.10 and Fig. 5.11, which display the populations of different orientations of
both the and
valleys.
As can be seen from Fig. 5.10, the
valleys with the orientation
are the most populated ones while all the
valleys remain empty. In
the case of substrates with higher Ge mole fraction (see Fig. 5.11) the
valley oriented along
becomes important.
The next three figures Fig. 5.12, Fig. 5.13, and Fig. 5.14 show the dependence of the in-plane component of the electron mobility
on the in-plane angle, that is the third Euler angle
, in polar coordinates for three substrate orientations.
The mobility on these figures is obviously anisotropic. This means that the in-plane transport turns out to be dependent on the orientation of
devices grown on the substrate.
Thus in order to reach their optimal characteristics devices, or more specifically, their active strained regions (base, channel, or other parts) can be properly oriented on the surface of the substrate. Where it is necessary this can be used to increase output currents. Additionally, this effect can also be used to reduce leakages by orienting some parts of the device so as to reduce the mobility along the possible leakage directions.
S. Smirnov: