Optical properties of Cl-doped ZnSe epilayers grown on GaAs substrates

We have determined the optical properties of a series of Cl-doped ZnSe epilayers grown on GaAs substrates using ellipsometry and prism coupling. Initially, the carrier concentrations were determined using Hall measurements for samples between 6.30×10¹⁶ cm⁻³ and 9.50×10¹⁸ cm⁻³. Using a variable angle spectroscopic ellipsometer in the energy range between 0.7 eV and 6.5 eV, we then obtained experimental spectra for each of the samples. By incorporating a three-layer model to simulate the experimental data, we determined the complex dielectric functions for these Cl-doped ZnSe epilayers. In order to facilitate this modeling procedure, we have complemented the ellipsometric results with prism coupling experiments that measured the film thickness and the index of refraction (at discrete wavelengths) very precisely. For the fundamental band gap, we observe a blue shift with respect to the doping concentration, which can be explained by the Burstein-Moss effect. In addition, we have determined the critical point parameters related to these specimens by fitting the second derivatives of both the real and the imaginary parts of the dielectric functions. Similar to several doped III-V semiconductors reported thus far, we find that in Cl-doped ZnSe epilayers, both E₁ and E₁ + Δ ₁ red shift, as well as a broadening with respect to the doping concentration.