Fabrication of single-phase SnS film by H₂ annealing of amorphous SnS_x prepared by atomic layer deposition

This study evaluated a simple and novel route to fabricate single-phase SnS thin films, consisting of the growth of smooth amorphous SnS₂ films by atomic layer deposition at very low temperature using tetrakis(dimethylamino)tin {TDMASn, [(CH₃)₂N]₄Sn} and hydrogen sulfide followed by H₂ annealing at controlled higher temperatures. The properties of the SnS films fabricated by subjecting the amorphous as-grown SnS₂ films deposited at 100 ℃ to post-H₂ annealing at 360 ℃ were superior to those of the as-grown SnS films deposited at 200 ℃ in terms of their phase purity, optical band gap, adhesion, and surface roughness. Raman spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and atomic force microscopy consistently showed that single-phase, stoichiometric crystalline (orthorhombic structure) SnS films (Sn/S ratio: ~1), without any incorporated sulfur-rich tin sulfides, could be fabricated with a very low surface roughness [root mean square (RMS) roughness: ~7 nm] using the proposed scheme. On the other hand, the as-grown SnS film at 200 ℃ still contained a small amount of SnS₂ phase and its RMS surface roughness was as high as 16 nm. The optical band gap of the as-grown SnS film was 1.85 eV, whereas that of the annealed SnS film was ~1.2 eV, confirming the latter's improved phase purity. The SnS film fabricated by H₂ annealing was a p-type semiconductor with a carrier concentration of ~2.3 × 10¹⁶/cm³ and a hole mobility of ~15 cm²/V s. The present scheme to prepare high-quality SnS films might be useful for fabricating photovoltaic or solar conversion devices.