According to this model, the width of the localized states near the mobility edges depends on the degree of disorder and defects present in the amorphous structure. In particular, it is known that unsaturated bonds together with some
saturated bonds are produced as the result of an insufficient number of atoms TGF-beta inhibitor deposited in the amorphous film . The unsaturated bonds are responsible for the formation of some defects in the films, producing localized states in the amorphous solids. The presence of high concentration of localized states in the band structure is responsible for the decrease in optical bandgap on increasing dopant (Cd) concentration in these amorphous films of (PbSe)100−x Cd x nanoparticles. This decrease in optical bandgap may also be due to the shift in the Fermi level whose position is determined by the distribution of electrons over the localized states . Figure 5 Temperature dependence of dc conductivity. It is Protease Inhibitor Library order in the range of 297 to 400 K at various concentrations of Cd in thin films of a-(PbSe)100−x Cd x nanoparticles. The values of refractive index (n) and extinction coefficient (k) have been calculated using the theory of reflectivity of light. According to this theory, the reflectance of light from a thin film can be
expressed in term of the Fresnel’s coefficient. The reflectivity [48–50] on an interface is given as follows: (5) where the value of k has been calculated by using the following formula: (6) with λ is the wavelength. Figures 6 and 7 show the spectral dependence of the extinction coefficient and refractive Ibrutinib manufacturer index for a-(PbSe)100−x Cd x thin films.
It is observed that the values of these optical constants (n and k) increases with the increase in photon energy. A similar trend has also been observed for thin films of other various amorphous semiconductors [51, 52]. The values of n and k for different concentrations of Cd are given in Table 1. It is evident from the table that, overall, the value of these optical constants increases with the increase in dopant concentration. This can be understood on the basis of density of defect states. It is well known that chalcogenide thin films contain a high concentration of unsaturated bonds or defects. These defects are responsible for the presence of localized states in the amorphous bandgap . In our case, the addition of Cd in the PbSe alloy results in the increased number of unsaturated defects. Due to this increase in the number of unsaturated defects, the density of localized states in the band structure increases, which consequently leads to the increase in values of refractive index and extinction coefficient with the addition of metal (Cd) content. Figure 6 ( α h ν ) 2 against photon energy (h ν ) for thin films of a-(PbSe) 100−x Cd x nanoparticles.