Morphological, structural, and electrical properties of PbS thin films deposited on HfO₂, SiO₂, and Al₂O₃ for TFTs applications

PbS thin films were deposited by the chemical bath deposition (CBD) technique at 43 °C on HfO₂, SiO₂, and Al₂O₃ dielectric films, which were previously deposited by atomic layer deposition (ALD) on <100> silicon wafers. The effect of the dielectric surface on the final morphology and structure of the PbS films was studied to find the most suitable dielectric/PbS system for thin film transistors (TFTs) applications. The PbS films showed a polycrystalline structure related to the galena cubic phase. The surface morphology of the PbS films was analyzed by scanning electron microscopy (SEM), which evidenced a gradual coverage of the surface of the dielectric with the deposition time. The shape of the PbS grains evidenced a strong dependence with the dielectric substrate employed. The PbS films deposited on HfO₂ achieved a well-defined and compact microstructure in comparison with the other dielectric substrates. The electrical properties of the PbS films were studied by Hall effect measurements. A p-type conductivity with Hall mobility up to 0.44 cm²V⁻¹cm⁻¹ and electrical resistivity values between 10²-10⁵ Ωcm were obtained. Finally, two TFTs were fabricated by using standard photolithography processes, where the PbS film was used as semiconductor active layer and HfO₂ (30 nm) as dielectric gate layer, respectively. The electrical response of the PbS-based TFTs evidenced a p-type behavior with a mobility in the saturation regime (μ_sat) in the range of 0.006–0.011 cm²/Vcm and I_ON/I_OFF current ratio in the order of 10². The TFTs operated in depletion mode with a low positive threshold voltage (V_TH) between 0.5 and 2 V.