Optical and structural characterization of CdSxTe1-x thin films for solar cell applications

The optical constants have been determined for films of CdSxTe1-x over the wavelength range 25-3200 nm. The films were prepared by vacuum evaporation from solid solutions. Rutherford backscattering spectrometry has been emplo yed to determine the thickness of the films, which is in the range 1390-243 0 nm, and x-ray diffraction has been used to determine the phase and lattic e parameters. The films were found to be cubic for x < 0.65, and hexagonal for x > 0.65. Reflectance and transmittance measurements have been made ove r the wavelength region 250-3200 nm. and a model, which includes the effect s of coherent scattering, has been used to determine the complex refractive index in the transparent region. A singly subtractive Kramers-Kronig algor ithm is derived for use with reflectance data of equal wavelength spacing. This novel Kramers-Kronig transform has been used to determine the optical constants in the opaque region. Polynomial functions are supplied which des cribe the variation of refractive index and extinction coefficient with wav elength. The photon energies required for both direct and indirect transiti ons have been found by least-squares fitting to the absorption spectra. The results are compared with previous work on 40 nm thick films.