Urbach’s rule

Further complication in semiconductor band shape analysis concerns the spectral region near the fundemental absorption onset. Ideal semiconductor crystal at 0 K should not absorb any photons with energies lower than Eg. Real systems, however, show pronounced absorption tails at energies lower than the bandgap energy (Figure 7.7). The absorption profile within the tail region can be very well approximated by the empirical Urbach s rule [23-26] ... 

Skettrup T. Urbach s rule derived from thermal fluctuations in the band-gap energy. Phys Rev B 1978 18 2622-31. 

Mahan GD. Phonon-broadened optical spectra Urbach s Rule. Phys Rev 1966 145 602-8. 

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