Templated Liquid-Phase Growth of Textured Tin Sulfide Films

Tin sulfide (SnS) is a layered semiconductor with a band gap of 1.1 eV that is of interest for thermoelectrics and solar cells. It features non-toxic and Earth-abundant elements, excellent stability, good charge carrier mobility, and strong light absorption. However, the highest reported efficiency for SnS solar cells remains below 5%. This poor performance is likely due to defects introduced during film growth, including sulfur vacancies that are understood to be recombination-active lifetime killers. The layered crystal structure also poses challenges of making thin films that are compact and smooth, especially for high film deposition rate. Continued innovation in the growth and processing of SnS thin films is needed to overcome these material limitations and propel technology development.<br/><br/>Here, we extend the method of templated liquid-phase (TLP) growth to make SnS thin films with strong texturing. TLP growth involves depositing a metal thin film, capping it with a thin dielectric layer, and then annealing the film above its melting point in a reactive atmosphere to form a compound. To make SnS, we deposit a Sn metal film and anneal in H₂S gas. This process has previously been demonstrated to make polycrystalline III-V films with large grain size (<i>e.g</i>., InP), but has not previously been reported for chalcogenides. TLP growth of chalcogenide thin films represents a new opportunity in defect control. The high overpressure of H₂S used during metal-co-compound conversion (here, carried out at atmospheric pressure) suggests that films may form with a much lower concentration of chalcogen vacancies than film made by vacuum processing.<br/><br/>We find that TLP growth of SnS on a-plane Al₂O₃ produces single-phase films with a preferred out-of-plane orientation (<i>i.e</i>., texturing). A thin (50 nm) Sn film converts to a rough SnS film with 100-300 nm islands on top of the film. A thicker (200 nm) film converts to a rougher film with approximately 1 μm islands. SnS films grown from uncapped Sn metal have a fine, needle-like morphology, suggestive of high nucleation density and rapid crystal growth. We demonstrate that control of Sn metal de-wetting and the sulfurization rate are important to producing smooth, uniform SnS films. We report on how growth parameters control grain size, electrical conductivity, photoluminescence, and excited-state lifetime. As time allows, we may also present growth of (Sn,Pb)S and (Sn,Pb)Se alloys, also by TLP, of interest for infrared optoelectronics.