nBu2Sn(SnBu)2 and nBu3SnEnBu (E = S or Se) – effective single source precursors for the CVD of SnS and SnSe thermoelectric thin films

Abstract

The use of single source precursors offers a convenient option for the chemical vapour deposition of thin film semiconductor materials with good stoichiometric control and precursor efficiency. Here we show that reaction of ⁿBu₃SnCl with NaSⁿBu or LiSeⁿBu, or ⁿBu₂SnCl₂ with 2 mol equiv. of NaSⁿBu, gives the molecular alkyltin chalcogenolate precursors, ⁿBu₃SnEⁿBu (E = S (1), Se (3)) and ⁿBu₂Sn(SⁿBu)₂ (2), respectively, in good yield as colourless (S) or yellow/orange (Se) oils. These were characterised by ¹H, ¹³C{¹H}, ⁷⁷Se{¹H} and ¹¹⁹Sn{¹H} NMR spectroscopy, microanalysis and thermogravimetric analysis. Low pressure CVD experiments using these precursors showed that (1) gave S-deficient SnS thin films, whereas using (2) and implementing short deposition times and low precursor loadings, gave stoichiometric SnS films. Stoichiometric SnSe films were also obtained using (3) and confirmed by grazing incidence XRD analysis, which revealed the films adopt the orthorhombic Pnma structure. SEM and EDX analysis, together with Raman spectroscopic data, were also used to identify the films deposited and to correlate with the deposition conditions employed. Variable temperature Seebeck and Hall effect characterisation confirm that the stoichiometric SnS and SnSe films are semiconducting and highly resistive, giving large positive Seebeck coefficients, with the overall power factor ranging from 0.017 at 300 K to 0.049 μW cm⁻¹ K⁻² at 450 K for SnS and increasing from 0.06 at 300 K to 0.4 μW cm⁻¹ K⁻² at 425 K for SnSe.