混合相多結晶半導体のバンドギャップエネルギーと組成の抽出：可能な代替方法

The optical band gap energy, Eg, of semiconductors is routinely determined using UV-Vis absorption measurements followed by Tauc-plot analysis. This method requires knowledge of the Tauc exponent n, which corresponds to the nature of the electronic transition. While Tauc analysis is effective for single-phase semiconductors, it may not be applicable to mixed-phase materials that exhibit two different types of transitions. In this work, the absorbance of mixed-phase TiO2 (anatase and rutile) with varying compositions is examined. Anatase has an indirect band gap (n=2), whereas rutile has a direct band gap (n=1/2). It is shown that the first derivative of the absorbance with respect to the wavelength, dA/dλ, yields phase-composition information consistent with that obtained from XRD analysis. Furthermore, the positive peak of the second derivative, d2A/dλ2, provides a band-gap value that does not shift (within < 0.05 eV) with changes in TiO2 phase composition. The method was also tested on two additional semiconductors (ZnO and C3N4), yielding band-gap energies comparable to those obtained using established techniques. These results demonstrate that the band gap energy can be determined without prior knowledge of the transition type by using the second derivative of the absorbance.