Fabrication of high crystalline SnS and SnS₂ thin films, and their switching device characteristics

Representative tin sulfide compounds, tin monosulfide (SnS) and tin disulfide (SnS₂) are strong candidates for future nanoelectronic devices, based on non-toxicity, low cost, unique structures and optoelectronic properties. However, it is insufficient for synthesizing of tin sulfide thin films using vapor phase deposition method which is capable of fabricating reproducible device and securing high quality films, and their device characteristics. In this study, we obtained highly crystalline SnS thin films by atomic layer deposition and obtained highly crystalline SnS₂ thin films by phase transition of the SnS thin films. The SnS thin film was transformed into SnS₂ thin film by annealing at 450 °C for 1 h in H₂S atmosphere. This phase transition was confirmed by x-ray diffractometer and x-ray photoelectron spectroscopy, and we studied the cause of the phase transition. We then compared the film characteristics of these two tin sulfide thin films and their switching device characteristics. SnS and SnS₂ thin films had optical bandgaps of 1.35 and 2.70 eV, and absorption coefficients of about 10⁵ and 10⁴ cm^-1 in the visible region, respectively. In addition, SnS and SnS₂ thin films exhibited p-type and n-type semiconductor characteristics. In the images of high resolution-transmission electron microscopy, SnS and SnS₂ directly showed a highly crystalline orthorhombic and hexagonal layered structure. The field effect transistors of SnS and SnS₂ thin films exhibited on-off drain current ratios of 8.8 and 2.1 × 10³ and mobilities of 0.21 and 0.014 cm² V^-1 s^-1, respectively. This difference in switching device characteristics mainly depends on the carrier concentration because it contributes to off-state conductance and mobility. The major carrier concentrations of the SnS and SnS₂ thin films were 6.0 × 10¹⁶ and 8.7 × 10¹³ cm^-3, respectively, in this experiment.