Andrew Meng1 Michael Braun1 Colleen Fenrich1 David Diercks2 Brian Gorman2 Marie-Ingrid Richard3 James Harris1 Paul McIntyre1

1, Stanford University, Stanford, California, United States
2, Colorado School of Mines, Golden, Colorado, United States
3, European Synchrotron Radiation Facility, Grenoble, , France

Germanium–tin is a promising alloy system for silicon-compatible optoelectronic elements due to the potential for achieving a direct band gap for efficient light emission1 and the possibility of low processing temperatures. Lasers, on-chip photonic interconnects, and emitters in the telecommunications wavelengths are examples of potential applications for this material. In particular, core-shell nanowires offer the benefit of decoupling the device from compressive misfit strains imposed by lattice matching to silicon substrates, which inhibits formation of a direct semiconductor gap. In addition, the core-shell structure presents an avenue for optical property engineering through strain manipulation and carrier confinement. Core-shell Ge/GeSn nanowires synthesized using VLS exhibit greatly enhanced light emission properties compared to bare Ge NWs. 2-3 We hypothesize that core-shell strain plays an important role in the enhancement in emission. In this study, we present core-shell cross-sectional 4D scanning transmission electron microscopy strain mapping in tandem with energy dispersive x-ray spectroscopy elemental mapping to quantify and visualize strains in core-shell Ge/GeSn nanowires. Atom probe tomography is used to confirm the elemental composition of the core-shell structures, which is consistent with collected STEM EDS data. Furthermore, synchrotron microdiffraction is used to obtain detailed strain information from small groups of as-grown nanowires. The strain measured in this way from as-grown nanowires differs significantly from results inferred from TEM cross-section analysis due to elastic strain relaxation in thin TEM samples. By correctly measuring and understanding the interplay of strain and composition in core-shell Ge/GeSn nanowires, improved materials properties for optoelectronic applications can be obtained.

1. Wirths, S.; Geiger, R.; von den Driesch, N.; Mussler, G.; Stoica, T.; Mantl, S.; Ikonic, Z.; Luysberg, M.; Chiussi, S.; Hartmann, J. M.; Sigg, H.; Faist, J.; Buca, D.; Grützmacher, D., Lasing in direct-bandgap GeSn alloy grown on Si. Nat Photon 2015, 9 (2), 88-92.
2. Meng, A. C.; Fenrich, C. S.; Braun, M. R.; McVittie, J. P.; Marshall, A. F.; Harris, J. S.; McIntyre, P. C., Core-Shell Germanium/Germanium–Tin Nanowires Exhibiting Room-Temperature Direct- and Indirect-Gap Photoluminescence. Nano Lett. 2016, 16 (12), 7521-7529.
3. Assali, S.; Dijkstra, A.; Li, A.; Koelling, S.; Verheijen, M. A.; Gagliano, L.; von den Driesch, N.; Buca, D.; Koenraad, P. M.; Haverkort, J. E. M.; Bakkers, E. P. A. M., Growth and Optical Properties of Direct Band Gap Ge/Ge0.87Sn0.13 Core/Shell Nanowire Arrays. Nano Lett. 2017, 17 (3), 1538-1544.