Inverse logarithmic derivative method for determining the energy gap and the type of electron transitions as an alternative to the Tauc method

We present a method derived from Mott and Davies or Cody equation for calculating energy gap based on spectrophotometric measurements. The method has been tested on thin film TiO2 and MoS2/TiO2 bilayer produced by RF magnetron sputtering. The presented examples indicate that the Tauc and McLean methods, which are usually used to analyse data, are not justified in many cases, especially for multilayers. In the Tauc method, the problem is to determine the energy gap without knowing the nature of the optical transition. The McLean method involves fitting the power function which may cause difficulties in the proper selection of the fitting range. The presented method is based on the transformation of measurement data using inverse logarithmic derivative and performing of linear fittings. Our method does not assume value of the parameter m describing the nature of the optical transition and does not require knowledge of the thickness of the layers. The value of energy gap and parameter m can be obtained simultaneously thanks to one linear fit. In addition, this method has a practical advantages over methods of McLean and Tauc, which are described in the article.