Understanding Stability in Ge-Sn Surfaces and Nanoparticles with First-Principles Calculations

Group IV semiconductors have demonstrated promise as candidates for next-generation low-temperature optical computing and communications technologies due to their promising optoelectronic properties. In particular, the bandgap of Ge can be tuned and made direct through the incorporation of Sn, motivating the study of Ge(Si)Sn nanomaterials and surfaces. However, the development of practical devices utilizing these elements is limited by the incorporation of Sn in high quality and uniform synthesizable materials. In this presentation, we will share our recent computational insights into fundamental properties governing the stability of Sn in Ge surfaces and nanoparticles.<br/><br/>Using density functional theory (DFT) calculations, we show that the stability of GeSn systems changes non-monotonically with Sn content. The incorporation of Sn depends strongly on morphology, considering both {111} and {100} crystal orientations; structure, considering extended surfaces and nanoparticle models of varying sizes; surface termination, considering a range of terminating species including H, halogens, and alkyl groups; and atomic-scale arrangements of Sn and Ge atoms. Our calculations predict improved Sn incorporation in {100}-terminated nanocrystals compared to {111}-terminated structures, enabled by the increased geometric freedom imparted to the larger Sn atoms at the more open {100} surfaces. The nature of the surface termination strongly influences atomic arrangements at the surface, with stronger-binding species (e.g., -Cl) exhibiting significantly different stabilization of Ge and Sn and thereby affecting the preferred segregation of components within surfaces and nanoparticles. Our results also suggest that the relative stability of nanoparticles depends on the nanoparticle size, though the nature of the size effects depend on surface termination (species and geometry). We finally show the effects of applied strain on the stability of Sn in surfaces, including in the presence of adsorbates. These fundamental insights suggest possible routes toward the synthesis of GeSn materials with tunable and high Sn content.