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.

Modeling the Optical Constants of Diamond- and Zincblende-Type Semiconductors: Discrete and Continuum Exciton Effects at E0 and E1

“…The ability to determine R 1 has considerable implications for recent first-principles band structure calculations which include exciton effects [10]. An analysis of the GaAs SPS data (measured in this work) and the CdTe SE [8] results near E 0 show conclusively that even if the exciton at this CP is not resolved the lineshape is BBCE and not band-to-band single particle (BBSP) [4,5].…”

Response to “Comment on ‘Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling’ ” [J. Appl. Phys. 88, 2172 (2000)]

“…Djurišić and Li's main point in their comment is that the results of Wei et al 1 do not represent conclusive proof of the advantages of their approach over other models in the literature. I strongly disagree with this statement.…”

“…3 deduced values of R 1 are in reasonable agreement with a simple effective mass/k"p calculation. 1,12,[13][14][15] The approach of Djurišić and Li is particularly suspect. [8][9][10] They have completely ignored excitonic effects at both the E 0 /E 0 ϩ⌬ 0 and E 1 /E 1 ϩ⌬ 1 CPs.…”

Optimization of Processing and Modeling Issues for Thin Film Solar Cell Devices Including Concepts for the Development of Polycrystalline Multijunctions Annual Subcontract Report, 24 August 1999 - 23 August 2000

“…For mixed crystal films formed at lower temperatures, from 120°C to 200°C, a continuous range of alloys can be obtained [31,33,35,36,37]. For the mixed films, heating above ~300°C induces phase segregation in accordance with the existence of the equilibrium miscibility gap found in mixed CdTe-CdS crystals at higher temperature, indicating that single phase films with composition lying within the miscibility gap are deposited in a metastable state.…”