Energy Gap and Fundamental Absorption

Electromagnetic radiation can be absorbed by an intrinsic semiconductor or insulator crystal to promote an electron from the valence band (VB) 

This representation takes into account the s-o coupling, and is necessary for the study of the optical properties related to the VB. This is usually not the case for those related to the CB and Table 3.3 gives the correspondence between the IRs of the double group used above and those of the standard group used for instance by Cohen and Bergstresser [25] to label the electronic 

Double group Standard group Double group Standard group 

In germanium and in other indirect-gap semiconductors, the direct T8+ — r7 transitions from the VB to CB can be detected in the fundamental absorption region by an increase of the absorption cross-section, as shown in Fig. 3.5. 

There are a few cubic crystals for which an optical transition between the maximum of the VB and the absolute minimum of the CB is symmetry-forbidden. This is notably the case for CU2O, where the absolute minimum of the CB is about 0.8 eV above the VB (direct thermal gap) while the direct optical gap is 2.2 eV [23]. 

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