Figure 5 shows the optical transmission spectra recorded in the 200 to 800 nm range for ZnCoO films synthesized at different Co concentrations. the undoped and the weakly doped samples show high visible range transmission and the high cobalt concentration samples show three Co absorption peaks at 567 (2.18eV), 611 (2.02eV) and 656 nm (1.89eV). Many group such as Ramachandran et al. 45, Koidl et al. 46 and Abdel-Galil et al. 47 have shown by mean of variety of technique that these peaks are correlated to characteristic features of d-d transition of the tetrahedrally coordinated Co2+ ions. For high concentrations of Co, we observe a deformation of the fundamental absorption front.
In order to study the effect of Co concentration on ZnO:Co thin films, the optical band-gap energy (Eg) values for the samples were calculated from the transmission spectra using the following relations:
(Ah?)^2=B(h?-E_g ) (7)
where A is the absorbance, d is the film thickness; T is the transmittance spectra of thin films.
Figure 6 which shows the typical variation of (Ah?)2 as a function of photon energy (h?) is used to extrapolate band gap energy. This last is determined by extrapolation of the linear region to (Ah?)2 = 0. The inset shows the plot of lnA as a function of (h?) for deduce the Urbach energy from the absorption coefficient ? given by 48.
?=?_0 e^(h?/E_u ) (8)
where ?0 is a constant and Eu is the Urbach energy.
As clearly seen in the Figure 7, the optical gap energy decreased from 3.20 to 2.98 eV with increasing Co doping from 0 to 6 wt% respectively. This red shift of the band gap energy with incorporation of Co is closely related to increasing of Urbach energy as seen in Figure 7. This may be explained by the introduced disorder and defect resulting of difference size between Zn and Co ionic radius. Similar trend has also observed in transition metal (TM) doped ZnO