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., SnO 2 ) and higher tin sulfides (e.g., Sn 2 S 3 and SnS 2 ). 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.