(698d) First Principles Approach to Understanding Stability in Ge-Sn Nanomaterials

Semiconductor materials comprising Group IV elements (Si, Ge, and/or Sn) have demonstrated promising optoelectronic properties as candidates for next-generation low-temperature optical computing and communications technologies. However, the development of devices utilizing these elements has been limited by challenges in efficiently incorporating Sn, which is required to tune the band structure for direct transitions, in high quality and uniform materials. In this presentation, we will share our recent computational insights into fundamental trends governing the stability of Sn in Ge surfaces and nanoparticles.

Our density functional theory (DFT) calculations show that the stability of GeSn systems changes non-monotonically with Sn content. The preferred incorporation of Sn depends strongly on the surface geometry, considering terminations with {111} and {100} facets; 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 the local arrangement of Sn and Ge atoms. Our calculations predict improved Sn incorporation in {100}-terminated nanocrystals compared to {111}-terminated structures of similar sizes, due to the increased geometric freedom imparted to Sn 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. Our results also suggest that the relative stability of nanoparticles depends on the nanoparticle size, though these size effects are themselves dependent on surface termination (species and geometry). We finally show that applied strain can affect the relative incorporation of Sn in surfaces. Together, these fundamental insights suggest potential strategies for synthesizing GeSn surfaces and nanoparticles with tunable Sn content.