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Polarization-dependent electrolyte electroreflectance (EER) measurements were carried out on the oriented Cu2ZnSiS4 and Cu2ZnSiSe4 single crystals at room temperature. Thin blade single crystals of Cu2ZnSiS4 and Cu2ZnSiSe4 were grown by chemical vapor transport technique using iodine as a transport agent. Laue pattern normal to the basal plane of the as-grown crystal revealed the formation of orthorhombic structure with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The polarized EER spectra in the vicinity of the direct band edge of Cu2ZnSiS4 displayed distinct structures associated with transitions from two upper-most valence bands to the conduction band minimum at Γ point. In the E⊥c configuration, the feature designated as EA ∼ 3.345 eV was detected and for Е‖c, only EB ∼ 3.432 eV appeared. For Cu2ZnSiSe4, three features denoted as EA, EB, and EC at around 2.348, 2.406 and 2.605 eV, respectively, were recorded for E⊥c polarization, whereas in the Е‖c, only EB and EC were the allowed transitions. Based on the experimental observations and a recent band-structure calculation by Chen et al. [Phys. Rev. B 82 (2010) 195203], plausible band structures near the direct band edge of Cu2ZnSiS4 and Cu2ZnSiSe4 have been proposed. |
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