Influence of the secondary absorption and the vertical axis scale of the Tauc's plot on optical bandgap energy

The optical energy bandgap of semiconductor materials is a critical parameter for predicting optoelectronic application of materials. Because pure semiconductors usually possess single optical absorption within the interband region, the Tauc plot has been a commonly used technique in the estimation of their energy bandgaps. However, in the case where semiconductors exhibit an absorption of the interband and an additional significant absorption of a sub-band, a direct application of the Tauc method can lead to erroneous estimates of bandgap. Also pertinent to note is the rarely explored/ reported influence of the vertical axis scale of the Tauc's plot on estimated bandgap value. These errors frequently appear in many publications in which authors incorrectly determine bandgap energy by neglecting the absorption of the sub-band, or by starting the vertical axis scale of the Tauc's plot from a non-origin. Like the efforts made by a few other researchers, the present work investigates the influence of the secondary absorption and the vertical axis scale of the Tauc's plot on estimated bandgap values. It is found that the absorption of the sub-band must be considered alongside the absorption of the interband to obtain a correct bandgap estimate. Also, the vertical axis scale must be started from the origin otherwise the determined bandgap would be slightly incorrect. The optical spectra of synthesized intrinsic ZnO, Ga2O3-modified TiO2 and Zn-doped beta-Ga2O3 films are used for this study. The prepared films are characterized using field-emission scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction, and ultraviolet-visible near infrared spectroscopy.