Dielectric functions of Cu2ZnSnSe4 and Cu2SnSe3 semiconductors

Abstract

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 105 cm−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.

Narrow-bandgap Cu2Sn1−xGexSe3 thin film solar cells

“…this study is smaller than that of as-grown CTSe (~0.84 eV) in our previous study [12,15]. Recently, it was also reported that the bandgap of CTSe thin films decreases from 0.68 to 0.49 eV as the growth temperature increases from 370 to 455°C [17,18]. The discrepancy in the optical bandgap between the as-grown and annealed samples may be ascribed to the band tail, which has been correlated to the poor crystallinity of films [19].…”

Optical absorption, Raman spectra, and electrical properties of Mn-doped Cu2 SnSe3 semiconductor compound

“…On the other hand, in an attempt to explain the wide range of E G values for this compound reported, it has been suggested that values of E G higher than about 0.60 eV could be explained as due to the Burstein–Moss shift effect observed for degenerated semiconductors. In such a case, the band‐gap energy due to this effect is given by the expression where p deg is the critical hole concentration for which the semiconductor becomes degenerate, E Gnd the band‐gap energy for the non‐degenerate semiconductor and (ℏ 2 /8 m h *)(3 p /π) 2/3 the Burstein–Moss shift term.…”

“…Thus, although it has been established that stoichiometric Cu 2 SnSe 3 crystallizes in a monoclinic structure at low and in a cubic structure at elevated temperatures; much of the bulk materials , films , and nanocrystals crystallize in a cubic or wurtzite structures at room temperature. On the other hand, while first‐principles calculations and ellipsometry studies determine for Cu 2 SnSe 3 a band‐gap energy at room temperature from 0 to 0.45 and 0.49 to 0.68 eV, respectively, a considerable larger value of E G ranging from 0.5 to 1.7 eV was found from absorption spectra .…”

“…On the other hand, Cu 2 SnSe 3 , especially when it is doped with foreign elements like Mn, In, Ga, and Zn could be used as a high efficient thermoelectric material. For these reasons, bulk crystals , films , and nanocrystals of Cu 2 SnSe 3 have received considerable attention recently. Hence, a comprehensive understanding of its growth, optical, and electrical properties is essential at present since discrepancies still exist in the literature about them.…”

Influence of copper composition and reaction temperature on the properties of CZTSe thin films

“…The interaction between the Cu-3d orbitals and Se-4p orbitals pushes the valance band position to higher energy levels which gives rise to the reduction in the effective E g Ref. [29]. Therefore, decreasing Cu content in the CZTSe thin films leads to weaker interaction of CueSe, and thus higher band gap value.…”