Optical characterization of nano-structured Cu2ZnSnS4 thin films deposited by GLAD technique

摘要

In this paper, Cu2ZnSnS4 (CZTS) thin films were elaborated at room temperature by thermal evaporation method using Glancing Angle Deposition (GLAD) technique at different incident angles gamma = 00 degrees, 20 degrees, 40 degrees, 60 degrees, 75 degrees and 85 degrees. XRD, Raman scattering analysis, (SEM) and UV-Visible-NIR spectroscopy were used to characterize the crystalline structure, morphology and optical properties of CZTS samples. The results have showed that the ellipsometric analysis leaded to an optical anisotropy due to the structural anisotropy for CZTS samples deposited at gamma = 85 degrees. All Cu2ZnSnS4 samples exhibited a high absorption coefficient (alpha > 10(4) cm(-1)) and a direct optical transition varied between 1.48 eV and 2.05 eV for CZTS thin films deposited at gamma = 00 degrees and 85 degrees, respectively. The value of the Urbach energy increased with incident angle, indeed, its value increased from 58 meV (gamma = 00 degrees) to 604 meV (gamma = 75 degrees) and decrease to be 368 meV for gamma = 85 degrees. This result is correlated with the Raman analysis. From transmittance data of CZTS thin films deposited at gamma = 00 degrees, 20 degrees and 40 degrees Swanepoel's method was used, to estimate the refractive index n. It allows us, using the Wemple-DiDomenico and Spitzer-Fan models, to calculate other optical parameters such as the oscillator energy E-0, dispersion energy E-d, zero frequency refractive index n(0), high frequency dielectric constant epsilon(infinity) and the electric susceptibility chi(e). On the other hand, to have an idea about the evolution of the nonlinear optical character, the nonlinear susceptibility chi((3)) and the nonlinear refractive index n(2) of CZTS thin films deposited at gamma = 00 degrees, 20 degrees and 40 degrees were investigated. Ellipsometric measurements of CZTS thin films has leaded to an optical anisotropy for gamma = 85 degrees. In addition, the generalized ellipsometry in Jones formalism have proved this property, which can be related to the nano-columnar slanted structure as revealed by (SEM) analysis.