Fig. 5 depicts the variation of (ahm)2 versus hm, which is a straight line in the higher energies domain, showing a direct optical transition. The band gap energy Eg can be determined by the extrapolation of the linear part of the (ahm)2
versus hm variation to ahm = 0. As can be seen from Fig. 5, the optical Eg gradually decreased from 3.27 to 3.10 eV with increasing Fe-doping concentration from 0 to 0.1. The similar results have been also reported by Chen et al. [25] and Wang et al. [32] for thin films prepared by the radio-frequency and facing-target magnetron sputtering technique and Alver et al. [33] by ultrasonic spray pyrolysis.