XRD and optical studies of CdI2 thin films with varying grain size and thickness for the control of bandgaps

Cadmium iodide (CdI₂) thin films with varying grain size and thickness were studied for exploring the strategy of controlling bandgap (E_g) in the thin film-materials. The grain size (D) was calculated from the X-ray diffraction (XRD) and was found to increase with increasing the thickness (L) of the film. A variation of the refractive index (n) with the variation of L was observed, which might be due to the variation of both density and electronic structure. However, n was found to decrease with increasing the wavelength of light. The optical absorption spectra showed both allowed direct and indirect interband transitions across a fundamental gap. It was found that both indirect and direct Eg decrease with increasing L and that the indirect E_g was lower than the direct E_g, confirming the results of the earlier band structure calculations. The direct E_g was also found to decrease with increasing D. The results demonstrate the possibility of controlling and manipulating E_g by adjusting D and L.