Indium tert-butylthiolates as single source precursors for indium sulfide thin films: Is molecular design enough?

Abstract

The dimeric indium thiolates [R 2In(μ-S tBu)] 2 R  tBu ( 1), nBu ( 2), Me ( 3), and [( tBuS)MeIn(μ-S t Bu)] 2 ( 4) have been synthesized and used as single source precursors for the metal-organic chemical vapor deposition (MOCVD) of In/InS and InS thin films. In the case of the atmospheric pressure film grown from either 1 or 2, deposition at temperatures between 290 and 350°C results in the formation of indium rich films (In: S ∼ 2) consisting of indium metal and orthorhombic InS, while at 400°C a single phase; the tetragonal high pressure phase of InS, is the only product deposited. Use of compound 3 as the precursor results in amorphous indium rich films being deposited at 300°C. While films grown from 3 at 400°C have a In: S ratio of 1, they consist of an indium rich phase and In 2S 3. The dependence of the film composition i.e., indium rich versus stoichiometric InS and structure (orthorhombic versus tetrag onal InS) with the deposition temperature and molecular precursor is discussed with respect to the decomposition pathways available to the precursor molecules ( 1– 3). Based on these results compound 4 was proposed to be a suitable precursor for the low temperature deposition of stoichiometric InS, indeed its solid state pyrolysis does yield InS. However, although low pressure MOCVD using 4 yields amorphous films of stoichiometry InS, upon annealing β-In 2S 3 is formed as the crystalline phase. The efficacy of molecular design of solid state materials is discussed. The indium thiolates were characterized by 1H and 13C NMR spectroscopy and mass spectrometry. Analysis of the deposited films has been obtained by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), with associated energy dispersive X-ray analysis (EDX).