Tuning the Phase and Optical Properties of Ultrathin SnS_x Films

Abstract

Novel materials suitable for optoelectronics are of great interest due to limited and diminishing energy resources and the movement toward a green earth. We report a simple film growth method to tune the S composition, x from 1 to 2 in semiconductor ultrathin SnS x films on quartz substrates, that is, single phase SnS, single phase SnS2, and mixed phases of both SnS and SnS2 by varying the sulfurization temperature from 150 to 500 °C. Due to the ultrathin nature of the SnS x films, their structural and optical properties are characterized and cross-checked by multiple surface-sensitive techniques. The grazing incidence X-ray diffraction (GIXRD) shows that the single phase SnS forms at 150 °C, single phase SnS2 forms at 350 °C and higher, and mixed phases of SnS and SnS2 form at temperature between. GIXRD shows structures of SnS film and SnS2 film are orthorhombic and 2H hexagonal, respectively. To complement the GIXRD, the reflection high energy electron diffraction pattern analysis shows that both pure phases are polycrystalline on the surface. Raman spectra support existence of pure phase SnS, pure phase SnS2, and mixed phases of SnS and SnS2. X-ray photoelectron spectroscopy reveals that the near surface stoichiometry of both single phase SnS and single phase SnS2 are close to Sn/S ratios of 1:1 and 1:2, respectively. UV–vis spectroscopy shows the optical absorption coefficient of SnS film is higher than 105 cm–1 above the optical bandgap of 1.38 ± 0.02 eV, an ideal optical absorber. A two-terminal device made of SnS film grown on SiO2 substrates shows good photoresponse. The SnS2 has an optical bandgap of 2.21 ± 0.02 eV. A photoluminescence (PL) peak of SnS2 film is observed at ∼542 nm. Time-resolved PL of the single phase ultrathin SnS2 film indicates a carrier lifetime of 1.62 ns, longer than sub nanosecond lifetime from multilayer SnS2. Our comprehensive results show that ultrathin SnS and SnS2 films have the required properties for potential photodetectors and solar cell applications but consume much less material as compared with current thin film devices.

Tin sulphide prepared by sulphurisation process using metallic tin film precursor obtained from dc magnetron sputtering method

“…The decay time and density of trap states of the films sulphurised at 300 and 350 o C were tabulated in Table 1. Our first decay time value in SnS phase is comparable to the previous reports [4,5] 3.0x10 -6 3.5x10 -6 4.0x10 -6 4.5x10 -6 5.0x10 -6 5.5x10 -6 6.0x10 -6 6.5x10 -6 7.0x10 …”

“…The E g increased from 1.70 to 2.45 eV with the increase of sulphurisation temperature from 300 to 500 o C, respectively. The band gap (Eg) values of 1.38 eV for SnS phase and of 2.21 eV for SnS2 phase were recently reported [4]. The difference in Eg values was mainly due to the difference in the deposition techniques employed and process parameters maintained during the growth of the films.…”

Effect of source temperature on the electrical transport, microstructural and optical properties of tin monosulfide thin films prepared by close spaced sublimation method

“…For all three typical films, only one Raman peak located around 220 cm -1 was presented in Figure 4. This Raman peak was attributed to LO phonon mode of SnS phase [6]. Regarding to the peak intensity, it suggested that the films prepared at source temperature of 600 o C revealed the best crystal quality due to the strongest peak intensity.…”

Fabrication of SnS nanowalls via pulsed plasma-enhanced chemical vapor deposition using a metal–organic single-source precursor

“…One commonly known technique is the deposition of a thin tin layer with subsequent sulfurization at high-vacuum conditions, as reported by Yang and coworkers for the fabrication of SnS x films. 13 Miles et al directly evaporated pure SnS powder and obtained a smooth tin-sulfide layer with a thickness of a few micrometers on glass substrates. 14 Depending on the substrate temperature, additional phases of SnS 2 or Sn 2 S 3 appeared.…”