Excitonic effects at the temperature-dependent <i>E</i>1 and <i>E</i>1 + Δ1 critical points of Ge

Abstract

We investigated excitonic effects in the complex dielectric function of Ge near the E1 and E1+Δ1 critical points as a function of temperature. By employing Tanguy’s theory for two-dimensional excitons [Tanguy, Solid State Commun. 98, 65 (1996)], we fitted the second derivative of the dielectric function to a temperature series of spectroscopic ellipsometry measurements ranging from 4 to 800 K [Emminger et al., J. Vac. Sci. Technol. B 38, 012202 (2020)]. We analyzed the temperature dependence of the effective masses, matrix elements, and exciton binding energies to develop a model for the dielectric function that requires no fitting parameters, apart from energy and broadening. Our calculations not only show a remarkable agreement between theory and experiment, but also provide a model for the absorption by two-dimensional excitons that can be adapted to other applications and materials.