Low-temperature growth of crystalline Tin(II) monosulfide thin films by atomic layer deposition using a liquid divalent tin precursor

Abstract

In this study, better-quality stoichiometric SnS thin films were prepared by atomic layer deposition (ALD) using a liquid divalent Sn precursor, N, N'-di-t-butyl-2-methylpropane-1,2-diamido tin(II) [Sn(dmpa)], and H2S. A relatively high growth per ALD cycle (GPC) value of approximately 0.13 nm/cycle was achieved at 125 degrees C. Furthermore, crystalline SnS films could be grown from room temperature (25 degrees C) to a high temperature of 250 degrees C. Density functional theory (DFT) calculations were used to examine the surface reactions and self-limiting nature of the Sn precursor. Mixed phases of cubic (pi) and orthorhombic (o) SnS films were deposited at low temperatures (25-100 degrees C), whereas only the orthorhombic phase prevailed at high growth temperatures (>125 degrees C) based on the complementary results of X-ray diffractometry (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses. The optoelectronic properties of the SnS films were further evaluated by spectroscopic ellipsometry (SE) analysis. The results from the SE analysis supported the observed change from mixed pi-SnS and o-SnS to o-SnS with increasing deposition temperature and allowed the determination of the energy bandgap (similar to 1.1 eV) and a relatively broad semi-transparent window (up to 3000 nm). Overall, this new ALD process for obtaining a good quality SnS is applicable even at room temperature (25 degrees C), and we foresee that this process could be of considerable interest for emerging applications.