Dielectric Function Spectra and Critical-Point Energies of Cu2ZnSnSe4 from 0.5 to 9.0 eV

We present dielectric function ε = ε ₁ iε ₂ spectra and critical-point energies of Cu ₂ZnSnSe ₄ determined by spectroscopic ellipsometry from 0.5 to 9.0 eV. We reduce artifacts from surface overlayers to the maximum extent possible by performing chemical-mechanical polishing and wet-chemical etching of the surface of a Cu ₂ZnSnSe ₄ thin film. Ellipsometric data are analyzed by the multilayer model and the ε spectra are extracted. The data exhibit numerous spectral features associated with critical points, whose energies are obtained by fitting standard lineshapes to second energy derivatives of the data. The experimental results are in good agreement with the spectra calculated within the GW quasi-particle approximation, and possible origins of the pronounced critical-point structures are identified.