Optical properties of CdTe1-xSx (0 <= x <= 1): Experiment and modeling

Spectral ellipsometry at 300 K, in the range 0.75-5.4 eV, has been used to determine the optical constants epsilon(E)[= epsilon(1) (E) + i epsilon(2) (E)] of a series of CdTe1-xSx (0 less than or equal to x less than or equal to 1) films fabricated by a laser-deposition process. The measured epsilon (E) data reveal distinct structures associated with critical points (CPs) a t E-0 (direct gap), spin-orbit split E-1, E-1 + Delta(1) doublet and E-2. T he experimental data over the entire measured spectral range (after oxide r emoval) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near thes e CPs (also E-0 + Delta(0) CP) plus excitonic and band-to-band Coulomb enha ncement (BBCE) effects. In addition to evaluating the energies of these var ious band-to-band CPs, our analysis also makes it possible to obtain inform ation about the binding energies of not only the three-dimensional exciton associated with E-0 but also the two-dimensional exciton related to the E-1 , E-1 + Delta(1) CPs. Our results will be compared to previous experiments and modeling (which neglect the BBCE terms) of epsilon(E) of CdTe and CdS a s well as optical absorption measurements of E-0 of CdTe1-xSx (0 less than or equal to x less than or equal to 1). The results of this experiment demo nstrate conclusively that the band-to-band line shape at E-0 is BBCE even i f the exciton is not resolved. (C) 1999 American Institute of Physics. [S00 21-8979(99)02910-2].