95
Electronic Properties of Strain-Engineered Semiconductors
with m x , m y , and m z being the effective masses, according to the wave vectors
k x , k y , and k z , respectively. This equation describes in the space of k with constant energy an ellipsoid of mass.
4.6 Silicon Valence Band
The valence band filled up with holes; its maximum is centred in Γ point.
Figure 4.5 shows the detail of the structure of bands in the directions <110>
and <100>. Three bands coexist of (1) heavy holes (HHs), (2) light holes (LHs),
and (3) spin-orbit (SO) holes.
We notice that the bands HH and LH are degenerated into their maximum
but do not have the same ray of curve, and for the spin-orbit band are one; it
is located at the lower part of the two others. Contrary to the bands of conduction, the valence bands are strongly anisotropic (Figure 4.6). In particular,
heavy holes that occupy most of the valence band have higher mass. Thus,
the choice of the direction of transport becomes important for p-MOSFETs.
K Z
K Y
K X
FIGURE 4.4
Ellipsoids of mass valleys Δ along crystallographic principal directions.
HH
LH
SO
<10>
<100>
Γ
FIGURE 4.5
Valence bands in the directions <100> and <110>.