Expanded Stability and Evidence of a Displacive Phase Transformation in SnSe-PbSe Heteroepitaxial Thin Films

Phase change materials often transition between amorphous and crystalline states. There are few systems which transition between distinct crystal structures at accessible temperatures, and even fewer which maintain semiconducting characteristics, particularly while retaining semiconducting characteristics rather than shifting to and from an insulating state The PbSe-SnSe chalcogenide materials system is a unique example of both, where a unique interplay occurs between a Sn-rich 2D-bonded layered orthorhombic structure (<i>Pnma</i>) and a Pb-rich 3D-bonded rocksalt structure (<i>Fm3</i><i>m</i>). These phases have indirect and direct band gaps respectively, and require only a fraction of a unit cell displacement to transform between them [1]. This huge change in bonding results in a large contrast in the electrical, optical, and thermal properties, which can be driven by temperature change and intense light-fields [1,2]. The unusual combination of large property contrast between phases while retaining close proximity in structure has the potential for important phase-change devices, if the bulk two-phase region can be avoided.<br/>Recent developments in PbSnSe have demonstrated a direct transition between the two crystal structures, achieved through high-temperature synthesis (&gt;600 °C) and rapid quenching to stabilize metastable phases [1,3]. In contrast, we have achieved direct low-temperature synthesis (165–300 °C) of epitaxial PbSnSe thin films on GaAs using molecular beam epitaxy (MBE) with an <i>in situ</i> PbSe surface treatment. This approach has significantly reduced the two-phase region, extending the stability of the layered Pnma structure up to Pb_0.45Sn_0.55Se —beyond the bulk limit near Pb_0.25Sn_0.75at low temperatures—entirely circumventing the two-phase region [3]. Upon cooling, a single-phase cubic film undergoes a displacive phase transformation to the layered phase, accompanied by notable changes in electronic properties. Cryogenic Hall measurements suggest that this phase transition could facilitate highly doped layered phase PbSnSe films. Moreover, we find that this displacive phase transformation is reversible, with the transformed layered film reverting back to cubic when heated. Additionally, the temperature of this phase transition can be tuned, potentially allowing commercial thermoelectric coolers to leverage these phase-change capabilities.<br/>In addition to expanding the stability of single phase PbSnSe alloys, we access metastable two-phase films of layered and rocksalt grains at compositions of Pb_0.50Sn_0.50Se. And so, we directly probe the microstructure of the 3D to 2D phase transformation by investigating grain boundaries in these two-phase films. Detailed microstructural analysis of these grain boundaries reveals distinct interfaces with specific orientation relationships and strain-relief mechanisms. Importantly, this 3D to 2D transformation occurs laterally with minimal strain. This, in addition to the reversibility of the phase transformation, underscores the potential of the PbSe-SnSe materials system for cyclically operated phase-change devices. The thin film growth method on GaAs not only opens avenues to manipulate the energetic landscape using alloy compositions but also facilitates integration with existing photonic schemes for next-generation phase-change devices.<br/><br/>[1] T. Katase et al. ... T. Kamiya, <i>Science advances</i>, <b><i>7</i></b>(12), (2021).<br/>[2] Y. Nishimura et al. ... T. Kamiya, <i>Adv. Electron. Mater</i>, <b><i>8</i></b>(9), (2022).<br/>[3] H. Krebs et al. ... D. Kallen, <i>Z. Für Anorg. Allg. Chem</i>, <i>312</i> (5–6), 307–313, (1961).