Parametric Optical Property Database for CdSe1-xSx Alloys

A comprehensive database of the optical response in the form of complex dielectric function (epsilon) spectra of magnetron co-sputtered CdSe1-xSx alloy thin films is developed spanning the full 0x1 range of compositions. A parametric model is presented and used to determine epsilon describing each film while in both as-deposited and thermally annealed states. This model combines a critical point electronic transition lineshape and an Urbach tail in the above- and below-bandgap portions of the measured spectrum, respectively, while maintaining first derivative continuity. Additionally, this hybrid parametric description of epsilon automatically determines the Urbach energy (E-U) describing the width of the sub-bandgap absorption tail, thereby providing a relative measure of the defect density of the modeled material. These as-deposited CdSe1-xSx films are generally found to be the most defective at intermediate compositions with some E-U reaching energies >200meV. Annealing reduces E-U in all films to a relatively uniform value <100meV for all compositions. Leveraging the full epsilon database in modeling CdS-CdSe bi-layer stacks is demonstrated to be effective in detecting the composition gradients resulting from inter-diffusion upon annealing. The accuracy of this technique is verified through excellent agreement with cross sectional energy dispersive X-ray spectroscopy measurements. [GRAPHICS] .