Modified Tauc–Lorentz dispersion model leading to a more accurate representation of absorption features below the bandgap

• A modified Tauc–Lorentz dispersion model with sub-bandgap absorption is proposed.
• An approach forces the bandgap energy Eg to be unique for all Tauc–Lorentz functions.
• The model gives a way to detect the near-edge features without direct data inversion.
• It has enough flexibility to portray the dielectric functions for a variety of materials.
• It provides meaningful value for Eg and adequately describes the underlying physics.

We reviewed studies reporting the applications of the Tauc–Lorentz (TL) parameterization for the complex dielectric function in spectroscopic ellipsometry. Since this model became very popular for representation of the dielectric functions of amorphous semiconductors and dielectrics, there is a need for an improved multi-oscillator TL model with a constraint on the band-gap parameter Eg, which forces it to be common for all TL oscillators, and possibility to represent weak absorption features below the bandgap which previously very often were associated with the exponential Urbach tail. In this paper we propose an extended TL model by inclusion of additional unbounded Lorentz and/or Gaussian oscillators with transition energies located below the bandgap. We conclude that the proposed model is the most appropriate for the characterization of various materials with sub-band absorption features and gives meaningful value for the energy bandgap. A few examples to illustrate practical use of modified model have been provided.