The Urbach tail refers to a particular feature of the optical absorption edge in disordered or amorphous semiconductor materials and insulators. It describes the exponential tail of the absorption spectrum that appears just below the bandgap energy of a material. In ideal crystalline semiconductors, the absorption edge is typically sharp and well-defined due to the periodic lattice structure. However, in disordered materials, defects, impurities, and localized states within the bandgap can lead to a broadening of the absorption spectrum.
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