Fundamental absorption

Under the fundamental absorption refers to the phenomenon that a material at a particular wavelength of light most strongly absorbed . This wavelength is given by the size of the band gap (in the band model ) of the material, more precisely the energy of a photon of this wavelength is exactly equal to the energy gap (the band gap) between the valence band and the conduction band .

This effect can be observed particularly well in semiconductors if a spectrum of the absorption factor of the material is measured with an optical spectrometer : At (photon) energies around the band gap, the absorption increases sharply and shows increased values ​​even with further increasing energies.

For the fundamental absorption to be clearly reflected in absorption, the responsible interband transition must also be probable. A counterexample is silicon : This is an indirect band transition with a very low probability. Correspondingly, the fundamental absorption in the absorption spectrum can only be recognized as a tiny peak with good resolution (cf. spectrum of the extinction coefficient for the band gap energy of 1.1 eV under physical properties of silicon ).

The fundamental absorption does not only play a role in semiconductors. Even with metals , where, due to the conduction band, which is already partially filled, there is no great difference in the charge carrier density , the interband transitions that are additionally possible from this energy on can become noticeable in the optical spectra. With silver, for example, two closely adjacent possible transitions lead to the effect of the plasma edge in its reflection spectrum .