Polymorph-Selective Deposition of High Purity SnS Thin Films from a Single Source Precursor

Abstract

Metal chalcogenide thin films have a wide variety of applications and potential uses. Tin­(II) sulfide is one such material which presents a significant challenge with the need for high quality SnS, free of oxide materials (e.g., SnO2) and higher tin sulfides (e.g., Sn2S3 and SnS2). This problem is compounded further when the target material exhibits a number of polymorphic forms with different optoelectronic properties. Unlike conventional chemical vapor deposition (CVD) and atomic layer deposition (ALD), which rely heavily on having precursors that are volatile, stable, and reactive, the use of aerosol assisted CVD (AA-CVD) negates the need for volatile precursors. We report here, for the first time, the novel and structurally characterized single source precursor (1), (dimethylamido)­(N-phenyl-N′,N′-dimethylthiouriate)­tin­(II) dimer, and its application in the deposition, by AA-CVD, of phase-pure films of SnS. A mechanism for the oxidatively controlled formation of SnS from precursor 1 is also reported. Significantly, thermal control of the deposition process allows for the unprecedented selective and exclusive formation of either orthorhombic-SnS (α-SnS) or zinc blende-SnS (ZB-SnS) polymorphs. Thin films of α-SnS or ZB-SnS have been deposited onto Mo, fluorine doped tin oxide (FTO), Si, and glass substrates at the optimized deposition temperatures of 375 and 300 °C, respectively. The densely packed polycrystalline thin films have been characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis. These data confirmed the phase purity of the SnS formed. Optical analysis of the α-SnS and ZB-SnS films shows distinctly different optical properties with direct band gaps of 1.34 and 1.78 eV, respectively. Furthermore, photoelectrochemical and external quantum efficiency (EQE) measurements were undertaken to assess the optoelectronic properties of the deposited samples. We also report for the first time the ambipolar properties of the ZB-SnS phase.

Chemical and Lattice Stability of the Tin Sulfides

“…There are currently five reported or proposed phases, viz. the ground-state orthorhombic Pnma phase, 8 a high-temperature Cmcm phase, 9 and three cubic phases: rocksalt, 10 zincblende, 11−13 and the recently reported P 2 1 3 (“π-cubic”) phase with a 64-atom primitive cell. 14−17 The sesquisulfide Sn 2 S 3 has also been a source of confusion, as it is relatively easy to prepare and clearly distinguishable from the other tin sulfides 18 yet is frequently predicted to be unstable with respect to decomposition into SnS and SnS 2 in theoretical studies (e.g., as in the current Materials Project 19 entry, mp-1509 20 ).…”

Metastable cubic tin sulfide: A novel phonon-stable chiral semiconductor

“…8 A rocksalt-structured phase (Fm3m type) of SnS is accessible under external pressure or epitaxial strain; 9 however, the more recent reports concern thin-films stable under ambient conditions. [10][11][12] To date, this cubic phase has been synthesised using a variety of deposition techniques including aerosol-assisted chemical-vapour deposition (CVD), 13 chemical-bath deposition, 11 and successive ionic-layer adsorption and reaction. 12 The widespread assignment of a zincblende (F43m type) crystal structure to the cubic phase was challenged on the basis of first-principles density-functional theory (DFT) calculations, which demonstrated that this phase was both high in energy and not dynamically stable in finite-temperature molecular-dynamics simulations.…”

“…14 It was suggested that the observed cubic phase was, instead, related to the rocksalt structure. Nonetheless, assignment of a zincblende phase of SnS continued in the literature, 13 until, recently, a new cubic crystal structure was resolved, using electron and X-ray diffraction, corresponding to a 2×2×2 supercell expansion of the rocksalt structure with large internal distortions. 15 The phase has been named π-SnS (see Figure 1).…”

“…16 The assignment was further supported by X-ray diffraction data. 15 The cubic phase has reported optical bandgaps ranging from 1.6 to 1.8 eV, 11,13,17 and has been used to make a working solar cell. 18 Starting from the reported crystal structure parameters, we performed a local optimization of the structure (lattice vectors and internal positions) within density-functional theory (DFT) following a quasi-Newton minimization procedure.…”

“…No conversion is observed at room temperature, while at 375 • C thin films have been reported to show a phase transition from cubic to orthorhombic over a period of 5 h under N 2 . 13 As an additional test, we compare the total energy of π-SnS to the other phases of this system, which have previously been computed. 14 The new phase is 2.19 kJ/mol above the orthorhombic ground state and 70 kJ/mol below the zincblende phase.…”