Expanded Stability of Layered SnSe-PbSe Alloys and Evidence of Displacive Phase Transformation from Rocksalt in Heteroepitaxial Thin Films.
basic_science · Level V
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- Record sourced from PubMed, PMID 38717390.
- Also identified by DOI 10.1021/acsnano.4c04128.
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Abstract
Bulk PbSnSe has a two-phase region, or miscibility gap, as the crystal changes from a van der Waals-bonded orthorhombic 2D layered structure in SnSe-rich compositions to the related 3D-bonded rocksalt structure in PbSe-rich compositions. This structural transition drives a large contrast in the electrical, optical, and thermal properties. We realize low temperature direct growth of epitaxial PbSnSe thin films on GaAs via molecular beam epitaxy using an <i>in situ</i> PbSe surface treatment and show a significantly reduced two-phase region by stabilizing the <i>Pnma</i> layered structure out to Pb<sub>0.45</sub>Sn<sub>0.55</sub>Se, beyond the bulk limit around Pb<sub>0.25</sub>Sn<sub>0.75</sub>Se at low temperatures. Pushing further, we directly access metastable two-phase films of layered and rocksalt grains that are nearly identical in composition around Pb<sub>0.50</sub>Sn<sub>0.50</sub>Se and entirely circumvent the miscibility gap. We present microstructural and compositional evidence for an incomplete displacive transformation from a rocksalt to layered structure in these films, which we speculate occurs during the sample cooling to room temperature after synthesis. <i>In situ</i> temperature-cycling experiments on a Pb<sub>0.58</sub>Sn<sub>0.42</sub>Se rocksalt film reproduce characteristic attributes of a displacive transition and show a modulation in electronic properties. We find well-defined orientation relationships between the phases formed and reveal unconventional strain relief mechanisms involved in the crystal structure transformation using transmission electron microscopy. Overall, our work adds a scalable thin film integration route to harness the dramatic contrast in material properties in PbSnSe across a potentially ultrafast crystalline-crystalline structural transition.