The second type is called Palik, which is an optical measurement method.
The third plot is calculated via density functional theory (DFT). In this theory it is very simple to
calculate things like dispersion relationships or the density of states. Its possible to get software pack-
ages which calculate all these things more or less exactly.
Figure 110: Real (right) and imaginary (left) part of the Dielectric function of different materials.Dielectric function of semiconductors:
A common approximation for the real part of the dielectric function of a semiconductor is:
′(ω) = 1 +ωp^2
ω^2 g−ω^2(236)
withωgthe gap frequency (Eg=~ωg).
Fig. 111 shows the real part of the dielectric function for a semiconductor. The imaginary part would
be a peak around this resident. Whenωggets smaller and smaller, in the end the band gap gets
smaller and smaller and as a result a metal observes.
Fig. 112 and 113 show some experimental functions of semiconductors. As discussed above, it is very
simple to see the real part (has negative components) and the imaginary part (peaks around the res-
idents). It is very interesting that the peaks of the real part aren’t very strict, which is common for
semiconductors. One explanation for that property is, that semiconductors do have two atoms per
unit cell, so they have two coupled modes which splits the peaks.