Limitations of the Tauc Plot Method

The Tauc plot is a method originally developed to derive the optical gap of amorphous semiconductors such as amorphous germanium or silicon. By measuring the absorption coefficient α(hν) and plotting (Formula presented.) versus photon energy hν, a value for the optical gap (Tauc gap) is determined. In this way non-direct optical transitions between approximately parabolic bands can be examined. In the last decades, a modification of this method for (poly-) crystalline semiconductors has become popular to study direct and indirect interband transitions. For this purpose, (ahν)ⁿ (n = (Formula presented.), 2) is plotted against hν to determine a value of the electronic bandgap. Due to the ease of performing UV–vis measurements, this method has nowadays become a standard to analyze various (poly-) crystalline solids, regardless of their different electronic structure. Although this leads partially to widely varying values of the respective bandgap of nominally identical materials, there is still no study that critically questions which peculiarities in the electronic structure prevent a use of the Tauc plot for (poly-) crystalline solids and to which material classes this applies. This study aims to close this gap by discussing the Tauc plot and its limiting factors for exemplary (poly-) crystalline solids with different electronic structures.