Investigation of crystal structure and optical properties of Cd_1−xMn_xS epilayers

High quality Cd_1−xMn_xS epilayers were grown on GaAs (100) substrates via the hot-wall epitaxy method. The crystal structure of the grown Cd_1−xMn_xS epilayers was changed from cubic to hexagonal with Mn compositions higher than x = 0.05, as determined from x-ray diffraction patterns and scanning electron microscope images. In order to investigate the binding states and their chemical composition, Cd_1−xMn_xS epilayers have been characterized using x-ray photoelectron spectroscopy. The optical properties of the Cd_1−xMn_xS epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry (SE) at room temperature. The data obtained using SE were analyzed in order to find the critical points of the pseudodielectric function spectra, <ɛ(E)> = <ɛ₁(E)> + i<ɛ₂(E)>. In addition, the second derivative spectra, d²ɛ(E)/dE², of the pseudodielectric function of the Cd_1−xMn_xS epilayers were numerically calculated in order to determine the critical structures. Four structures, E₀′ (⁠$H3v→H3c$⁠), F₁ (⁠$M4v→M3c$⁠), and two E₂ $(K3v→K2c,K2v→K2c)$ structures from 6.0 eV to 8.0 eV, were observed with variation of the Mn composition at 300 K via ellipsometric measurements for the Cd_1−xMn_xS epilayers. The phase transition process from a cubic structure to a hexagonal structure was observed and investigated, for the first time, for Cd_1−xMn_xS epilayers via SE.