Dielectric functions of Cu₂ZnSnSe₄ and Cu₂SnSe₃ semiconductors

The dielectric functions of co-evaporated Cu₂ZnSnSe₄ (CZTSe) and Cu₂SnSe₃ (CTSe) polycrystalline layers are determined accurately from self-consistent spectroscopic ellipsometry analyses. To minimize the effects of the compositional modulation and light scattering induced by rough surfaces, quite thin CZTSe and CTSe layers (<50 nm) having the single-phase stoichiometric compositions are characterized. The dielectric functions of CZTSe and CTSe show rather similar spectral features with almost identical critical point energies for the transition peaks at 2.4 and 3.9 eV. The CTSe dielectric function, however, indicates strong free carrier absorption, expressed by the Drude model, due to high p-type conductivity in the layer. We find that CZTSe and CTSe show quite large absorption coefficients exceeding 10⁵cm⁻¹ at 2.0 eV with band gap values of 0.91 ± 0.02 eV and 0.68 ± 0.05 eV, respectively. To characterize the optical transition in CZTSe in more detail, the dielectric response of each interband transition is calculated by applying density functional theory. The calculation result reveals that the strong visible light absorption in CZTSe is induced by the high joint density of states at the P point in the Brillouin zone. The optical constants of CZTSe and CTSe deduced in this study are further parameterized in an energy range up to 6.0 eV by expressing the transition peaks using the Tauc-Lorentz model. From the above results, we discuss the fundamental optical properties of (Cu,Se)-based compound semiconductors.