Ellipsometric characterization and density-functional theory analysis of anisotropic optical properties of single-crystal α-SnS

We report on the anisotropic optical properties of single-crystal tin monosulfide (SnS). The components ε_a, ε_b, and ε_c of the pseudodielectric-function tensor ⟨ε⟩ = ⟨ε₁⟩ + i⟨ε₂⟩ spectra are taken from 0.73 to 6.45 eV by spectroscopic ellipsometry. The measured ⟨ε⟩ spectra are in a good agreement with the results of the calculated dielectric response from hybrid density functional theory. The ⟨ε⟩ spectra show the direct band-gap onset and a total of eight above-band-gap optical structures that are associated with the interband-transition critical points (CPs). We obtain accurate CP energies by fitting analytic CP expressions to second-energy-derivatives of the ⟨ε⟩ data. Their probable electronic origins and implications for photovoltaic applications are discussed.