Substrate Effects on Crystal Phase in Atomic Layer Deposition of Tin Monosulfide

Abstract

Obtaining single-phase tin monosulfide (SnS) films at low temperatures is challenging since cubic (π-SnS) and orthorhombic (α-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 α-SnS. Meanwhile, ozone treatment or preannealing of the same substrate leads to mainly π-SnS. Just a few ALD cycles of another oxide or sulfide can even more substantially alter the outcome. Substrates that favor α-SnS growth typically produce initially enhanced growth rates, while those promoting π-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; π-SnS preferentially evolves on substrates and π-SnS surfaces that are rich in highly reactive dangling bonds, while chemically inert substrates and α-SnS surfaces promote α-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.

Ultrathin Solar Cells Based on Atomic Layer Deposition of Cubic versus Orthorhombic Tin Monosulfide

“…As everything is common apart from the SnS phases here, the observable differences are limited in scope and could be further enhanced through optimization of (especially) the relatively unexplored π-SnS device structure. Apart from the bandgaps and band edge positions, the morphologies of the two SnS polymorphs differ substantially ,, and could play a role here. In brief, π-SnS films consist of densely packed and small (15–20 nm) grains.…”

“…Considering that SnS deposited on Ti grows predominantly in the cubic phase under our standard ALD conditions, just as it does on Al 2 O 3 , we compared back contacts of Ti with and without the two cycles of Al 2 O 3 . The results demonstrate rather similar performances with a slightly greater J sc for the sample with Al 2 O 3 and a slightly greater V oc for the sample without Al 2 O 3 .…”

“…In the present work, the back metal contact was modified by either two ALD cycles of Al 2 O 3 to promote the growth of π-SnS or by 30 ALD cycles of ZnS to promote α-SnS. These depositions were performed shortly before and at the same temperature (120 °C) as the SnS ALD, under conditions that were previously demonstrated to produce the desired phases . The Sn­(acac) 2 source bottle was maintained at 100 °C to provide sufficient vapor pressure of the liquid precursor and, hence, conditions for saturated SnS growth.…”

“…In the H 2 S half-cycle, the duration of steps 1 to 6 was 1, 1, 1, 2, 1, and 3 s while the H 2 S line flow was maintained at 120 SCCM. Typically, 1000 SnS ALD cycles were performed, resulting in 35 and 45 nm-thick SnS films of the cubic and orthorhombic phase, respectively, the thickness difference being due to disparate nucleation behavior of the two cases …”

“…A second objective is to investigate the solar cell behavior of the relatively unexplored cubic SnS polymorph and to compare it to the more commonly employed orthorhombic polymorph. To do this, we make use of the recent finding that the SnS phase resulting from ALD can be very well controlled by means of substrate surface modifications . We compare several back contact materials (Mo, Ti, and Al) and compositions of the Zn­(O,S) buffer layer in terms of their effects on the photovoltaic behavior.…”