Deposition of the tin sulfide thin films using ALD and a vacuum annealing process for tuning the phase transition

Abstract

Tin sulfide (SnSx) thin films deposited by atomic layer deposition (ALD) under various deposition conditions have unique properties. Such films have two phases, tin monosulfide (SnS) and tin disulfide (SnS2), that depend on the composition ratio of the thin film, and these materials can be applied to a wide range of technologies, such as solar cells, optical sensors, and transistors. In this study, we deposit amorphous tin sulfide thin films at 100 °C using ALD and perform post annealing at various temperatures to control the phase transition from SnS2 to SnS. The post annealing temperature-dependent phase transition was analyzed through Raman spectroscopy, grazing-incidence X-ray diffraction (GI-XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV–vis) spectroscopy, and ultraviolet photoelectron spectroscopy (UPS). Thermodynamic calculations were performed to determine that the initial temperature of the phase transition correspond to the post annealing temperature of 350 °C. The phases and crystal structures of the films annealed at different temperatures were investigated by Raman and GI-XRD. The chemical bonding of the films was analyzed through XPS, revealing that the binding energy of Sn4+ states shifted to the lower binding energy of Sn2+ states with increasing annealing temperatures. The electronic band structures of the films were calculated by UV–vis and UPS. The band gaps of the 300 °C, 350 °C, 400 °C annealed films were 2.26 eV, 2.05 eV, 1.63 eV, respectively. The tin sulfide thin films had n-type characteristics when annealed at 300 °C, which changed to p-type characteristics when annealed at 350 °C and 400 °C. Through this study, we can investigate phase transition of tin sulfides and correlate fabrication conditions to both n-type and p-type characteristics, which can be applied to synthesize the desired films for application in various devices.