Substrate Effects On Crystal Phase In Atomic Layer Deposition Of Tin Monosulfide

Obtaining single-phase tin monosulfide (SnS) films at low temperatures is challenging since cubic (pi-SnS) and orthorhombic (alpha-SnS) polymorphs have similar energies of formation and grow under similar conditions. Here, we show that in atomic layer deposition (ALD) of polycrystalline SnS using tin(II) acetylacetonate and H2S precursors, the substrate surface greatly influences the SnS phase evolution. For example, a silicon (100) substrate, with a highly hydroxylated surface, favors the growth of alpha-SnS. Meanwhile, ozone treatment or preannealing of the same substrate leads to mainly pi-SnS. Just a few ALD cycles of another oxide or sulfide can even more substantially alter the outcome. Substrates that favor alpha-SnS growth typically produce initially enhanced growth rates, while those promoting pi-SnS are partially surface-poisoned by the acetylacetonate precursor ligands. Growth of either polymorph is self-sustained after its initiation, and the sustaining factor appears to be the surface-ligand interaction; pi-SnS preferentially evolves on substrates and pi-SnS surfaces that are rich in highly reactive dangling bonds, while chemically inert substrates and alpha-SnS surfaces promote alpha-SnS. While lattice matching is less central, the role of ligand bonding in SnS ALD also helps explain the previously reported phase dependence on growth temperature and H2S precursor dose and shows promise for area-selective ALD of SnS.