Investigation of precursor sequence and post-annealing effects on the properties of Cu2SnZnSe4 thin films deposited by the elemental thermal evaporation
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Cu2ZnSnSe4 (CZTSe) thin films were deposited onto soda lime glass substrates by thermal evaporation using the pure elemental sources. The influence of the sequential deposition order of Zn and Sn precursor layers for different evaporation cycles were investigated. In situ annealing at 400 degrees C under Se evaporation was applied to promote conversion of precursor layers to quaternary compound structure and additionally, subsequent post-annealing processes under nitrogen atmosphere at 450 degrees C was carried out to improve the crystalline behavior. The analyses concluded that the substrate temperature kept at 400 degrees C during selenization was not adequate to form homogenous CZTSe structure and as a result of post-annealing treatment, the polycrystalline quaternary CZTSe film structure was obtained. Structural analysis showed that in comparison with the initial Sn precursor layer, the growth process starting with Zn was found to be the preferable method to form better crystalline CZTSe structure. Furthermore, the stacking layer order and annealing processes showed the important effect on the stoichiometry and surface morphology of the films. The optical band gap energies were around 1.10 eV and also from Tauc plots, 1.40 eV was observed due to the admixture of secondary phases. The room temperature resistivity values and hole carrier densities were obtained around 10(-2) Omega cm and 1019 cm(-3), respectively. The mobility values of the samples were found in between 0.51 and 0.75 cm(2) V-1 s(-1).