Above-band-gap dielectric functions of ZnGeAs2: Ellipsometric measurements and quasiparticle self-consistent GW calculations

We report the above-band-gap dielectric-function spectra epsilon(a) = epsilon(a1) + i epsilon(a2) of single-crystal ZnGeAs2 grown epitaxially on (001)GaAs, and study it theoretically. After surface overlayers were removed chemically to minimize artifacts, pseudodielectric-function spectra = + i were acquired ellipsometrically from 1.5 to 6.0 eV with the sample at room temperature. The epsilon(a) spectra were then extracted by multilayer analysis. The procedure ensures that the result is a close approximation to the a-axis component of the dielectric-function tensor epsilon = epsilon(1) + i epsilon(2) of ZnGeAs2. The data exhibit numerous spectral features associated with critical points. The energies of these critical points are determined accurately by fitting standard line shapes to second energy derivatives of the data obtained by a combined method of spectral analysis. We compare our results to the predictions of quasiparticle self-consistent GW calculations. Good agreement is achieved for the major critical-point features, and their probable origins are identified.