Optical and structural characterization of CdS_xTe_1-x thin films for solar cell applications

The optical constants have been determined for films of CdS_xTe_1-x over the wavelength range 250-3200 nm. The films were prepared by vacuum evaporation from solid solutions. Rutherford backscattering spectrometry has been employed to determine the thickness of the films, which is in the range 1390-2430 nm, and x-ray diffraction has been used to determine the phase and lattice 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 over the wavelength region 250-3200 nm, and a model, which includes the effects of coherent scattering, has been used to determine the complex refractive index in the transparent region. A singly subtractive Kramers-Kronig algorithm 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 describe the variation of refractive index and extinction coefficient with wavelength. The photon energies required for both direct and indirect transitions have been found by least-squares fitting to the absorption spectra. The results are compared with previous work on 40 nm thick films.