Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling

Abstract

Spectral ellipsometry at 300 K, in the range 0.75–5.4 eV, has been used to determine the optical constants ε(E)[=ε1(E)+iε2(E)] of a series of CdTe1−xSx (0⩽x⩽1) films fabricated by a laser-deposition process. The measured ε(E) data reveal distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E1, E1+Δ1 doublet and E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs (also E0+Δ0 CP) plus excitonic and band-to-band Coulomb enhancement (BBCE) effects. In addition to evaluating the energies of these various band-to-band CPs, our analysis also makes it possible to obtain information about the binding energies of not only the three-dimensional exciton associated with E0 but also the two-dimensional exciton related to the E1, E1+Δ1 CPs. Our results will be compared to previous experiments and modeling (which neglect the BBCE terms) of ε(E) of CdTe and CdS as well as optical absorption measurements of E0 of CdTe1−xSx (0⩽x⩽1). The results of this experiment demonstrate conclusively that the band-to-band line shape at E0 is BBCE even if the exciton is not resolved.