Structural and optical tunability by reaction time of selenization in Cu₂FeSnSe₄ thin films

Abstract Cu₂FeSnSe₄ (CFTSe) thin films were formed by selenization of sputtered metal layers on glass substrates. X-ray diffraction and Raman spectra show that all peaks are belong to the CFTSe. Moreover, the bond-length of Cu-Se (d_Cu-Se) decreases from 2.463 to 2.443 Å, and then increases to 2.450 Å as reaction time elevate to 20 min. The tuning of grain growth along (220/204)-orientation can be achieved by varying the reaction time, also, the intensity of (002) and (103) peaks will be restrained with the more formation of crystal surface along (220/204)-orientation. Based on the Keating's mode and combine with d_Cu-Se, it is well expounded the A₁ modes of CFTSe which exhibit a red-shift with the reaction time up to 15 min and then an opposite trend (i.e. blue-shift). Furthermore, the various hybridizations of Cu d and Se p orbital, which is connected with the d_Cu-Se, drive the redistribution of the band gap of CFTSe. The values of the band gap are 1.26, 1.25 1.14 and 1.23 eV for thin films selenized using 5, 10, 15 and 20 min, respectively. These results are helpful to understand the structure, crystal orientation control and band gap variations in polycrystalline Cu-Fe-Sn-Se(S) system materials deeply.