High speed, low driving voltage vertical cavity germanium-silicon modulators for optical

2.1.3 Low Energy Absorption 
2.1.3.1 Shallow Impurity-Band Transition 
At lower temperatures, when impurity levels are not ionized, carriers on the energy 
levels can be excited to the band edge by a low energy photon. Fig 2.4 below shows 
the two types of low energy impurity-band transitions. 
Figure 2.4: Illustration of low energy (a) donor-band and (b) acceptor-band absorption transitions 
For this absorption the photon energy must be equal to or greater than the 
ionization energy of the impurity. This energy usually corresponds to the far infrared 
region of the optical spectrum.
+
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C
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V
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D
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-
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C
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A
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16 
2.1.3.2 Intraband Transition 
At the zone center the valence band structure of most semiconductors consists of the 
light-hole (LH), the heavy-hole (HH) bands and the split-off (SO) bands. The three 
subbands are separated by spin-orbit interaction. In a p-type semiconductor the 
valence band is filled with holes, and the occupancy of the different bands depends on 
the degree of doping and the position of the Fermi level. With the right energy of the 
incident photons, absorption between different subbands can be observed. However, 
these types of transitions are not observed in n-type materials. 
2.1.3.3 Free-Carrier Absorption
Free carrier absorption occurs when a material absorbs a photon and a carrier is 
excited from a filled region below the Fermi level to an unoccupied region above the 
Fermi level (in the same band). This is different from interband absorption in 
semiconductors because the electron being excited is a conduction electron (i.e. it can 
move freely). In interband absorption, the electron is being raised from a valence band 
to a conducting one. This transition also needs to conserve momentum. Momentum 
conservation is normally provided by optical or acoustic phonons.