Tauc plot

Tauc representation illustrating the optical bandgap (E_Tauc~ 0.8 eV) of a film of amorphous Si. The insert shows their corresponding transmittance/reflectance spectra.

A Tauc plot is used to determine the optical bandgap, or Tauc bandgap, of either disordered or amorphous semiconductors.

In his original work Jan Tauc () showed that the optical absorption spectrum of amorphous germanium resembles the spectrum of the indirect transitions in crystalline germanium (plus a tail due to localized states at lower energies), and proposed an extrapolation to find the optical bandgap of these crystalline-like states. Typically, a Tauc plot shows the photon energy E (= hν) on the abscissa (x-coordinate) and the quantity (αE)^1/2 on the ordinate (y-coordinate), where α is the absorption coefficient of the material. Thus, extrapolating this linear region to the abscissa yields the energy of the optical bandgap of the amorphous material.

A similar procedure is adopted to determine the optical bandgap of crystalline semiconductors. In this case, however, the ordinate is given by (α)^1/r, in which the exponent ^1/r denotes the nature of the transition:^,,

• r = 1/2 for direct allowed transitions
• r = 3/2 for direct forbidden transitions.
• r = 2 for indirect allowed transitions
• r = 3 for indirect forbidden transitions

See also

• Band gap
• Urbach energy