Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system (Phys. Status Solidi B 8/2015)

Abstract

ZnO was used by Viezbicke et al. (pp. 1700?1710) for assessing the accuracy of the Tauc method for band-gap determination. This method is based on optical absorption data: longer wavelengths are barely absorbed, but wavelengths shorter than the band-gap cut-off are absorbed progressively more strongly, as illustrated schematically for three particular photon energies (upper left). These photons excite gradually wider direct transitions within the material's electronic structure (upper right) yielding larger absorption coefficient values. Direct-gap materials like ZnO show a linear region in the (αh?)2 vs. Ephoton plot, which is extrapolated back to the x-axis to find the threshold band-gap value (lower right). Viezbicke et al. examined a large population of these bandgap determinations leading to an improved understanding of the accuracy of the method. Finally, an easily-evaluated figure-of-merit was defined that may help guide more accurate Tauc fitting in the future. For samples with relatively sharper Tauc plot shapes the data found Eg(ZnO) to be 3.276 ± 0.033 eV, in good agreement with data for single crystalline material (lower left).