Effect of stacking sequence on the structural ordering and phase formation in the sequentially deposited copper zinc tin sulfide thin films

In this work, we have synthesized the copper zinc tin sulfide (CZTS) thin films using a two-step process, that is, sequential evaporation and sulfurization. Copper (Cu), zinc sulfide (ZnS) and tin (Sn) were used as precursors and their stack sequence was varied to deeply investigate the impact of stack orders on the Cu/Zn disorder and phase. Each stack variation was subjected to sulfurization at 520°C, 550°C and 580°C. CZTS thin films with soda-lime glass (SLG)/ZnS/Sn/Cu stack order exhibited kesterite structure and all the samples exhibited tetragonal crystal structure with preferred CZTS orientations. Order parameter Q obtained from Raman analysis showed significant variation with the change in stack order, which describes the Cu/Zn disorder in the system, implying its substantial dependence on the stacking sequence. The stack order SLG/ZnS/Sn/Cu had near stoichiometric composition while other sequences had Zn in excess and were Cu deficient. Morphological analysis showed a visible bifurcation of surface modification and growth due to stack order variation. Atomic force microscopy studies suggested that stack order and sulfurization temperature can be used as a tuning factor to vary the surface roughness. An optical band gap of 1.54 eV was obtained for stack order SLG/ZnS/Sn/Cu subjected to sulfurization temperature at 580°C. Broad asymmetric peak shape was obtained from photoluminescence (PL) measurement and analysis showed the shift of PL maxima toward lower energy, suggesting the increase in antisite disorder.