Growth and properties of Cu2ZnSnS4 thin films prepared by multiple metallic layer stacks as a function of sulfurization time

In this paper, we report the two stage growth of Cu2ZnSnS4 (CZTS) thin films as a function of sulfurization time. First, magnetron sputtered metallic precursors were deposited sequentially (Zn/Cu/Sn/Cu) over rotating glass substrates held at 230 °C. Later, the sputtered precursors were heat treated at 500 °C in the ambiance of sulfur for various time durations in the range, 10–120 min. The sulfur treated samples were examined using various analytical tools to understand the role of sulfurization time on the CZTS growth and properties. From composition and structural analysis, Zn/Cu/Sn/Cu precursors sulfurized for shorter duration (10 and 20 min) revealed severe deficiency of sulfur that resulted in several metallic, bi-metallic and metal sulfide phases. With the increase of sulfurization time to 30 min, sulfur incorporation was enhanced and reached stoichiometric ratio (~50% S) for CZTS growth, however, samples were poorly crystalline in nature and consisted of prominent Cu2−xS phase as well. The Zn/Cu/Sn/Cu precursors sulfurized for 60 min exhibited prominent CZTS phase without Cu2−xS phase. Further, rise in sulfurization time to 120 min enabled drastic improvement in crystallinity of CZTS phase. Raman mapping over 60 µm × 60 µm for these films confirmed the homogeneous phase growth of CZTS. XPS study revealed the oxidation states of Cu1+, Zn2+, Sn4+ and S2− in CZTS films. The optimized films showed high absorption coefficient of 105 cm−1 with an optical band gap of 1.51 eV. These films showed leaf like grain morphology with high mobility and low resistivity of 18.2 cm2/V-s and 0.7 Ω-cm, respectively.