Atomic Layer Deposition of Tin Monosulfide Using Vapor from Liquid Bis(N,N′-diisopropylformamidinato)tin(II) and H2S

The oxide and sulfide of divalent tin show considerable promise for sustainable thin-film optoelectronics, as transparent conducting and light absorbing p-type layers, respectively. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) provide attractive routes to these layers. The literature on volatile tin(II) compounds used as CVD or ALD precursors shows that new compounds can provide different growth rates, film morphologies, preferred crystallographic orientations, and other material properties. We report here the synthesis and characterization of a new liquid tin(II) precursor, bis(N,N'-diisopropylformamidinato)tin(II) (1), which is effective in ALD of SnS in combination with H2S between 65 and 180 degrees C. Like other highly reactive tin(II) precursors, the growth per cycle linearly decreases from 0.82 angstrom/cycle at 65 degrees C to 0.4 angstrom/cycle at 180 degrees C. This is obviously different from the case of previously reported SnS ALD using bis(2,4-pentanedionato)tin(II), Sn(acac)(2), and H2S; films grow at 0.22-0.24 angstrom/cycle almost independent of the substrate temperature (125-225 degrees C, J. Phys. Chem. C 2010, 114, 17597). Quartz crystal microbalance (QCM) experiments for SnS ALD using 1 at 80, 120, and 160 degrees C were carried out to study the linear decrease of the growth per cycle with increasing substrate temperature. On the basis of these QCM studies, although the mechanism of chemisorption-loss of one ligand or two-can be manipulated by changing the exposure of 1, the purging time, or the temperature, only the temperature changes the growth per cycle. We therefore attribute the decreasing growth per cycle with increasing temperature to a decreasing surface thiol density. Photovoltaic devices prepared from 1-derived SnS have a performance similar to those of the best devices prepared from other precursors, and the device yield and replicability of J-V properties are substantially increased by using 1.