Paul McIntyre1,Michael Braun1,John Lentz1,Ishaa Bishnoi1,Andrew Meng2
Stanford University1,University of Pennsylvania2
Paul McIntyre1,Michael Braun1,John Lentz1,Ishaa Bishnoi1,Andrew Meng2
Stanford University1,University of Pennsylvania2
Germanium-tin (GeSn) is a promising silicon-compatible group IV semiconductor that can achieve a direct band gap and has been the subject of increasing research in recent years for both electronic and photonic applications. However, the compositions that are reported to produce a direct band gap in unstrained GeSn, ~10 at% tin, are metastable. The bulk solubility limit of Sn in solid solution with Ge is ~1 at%.<sup> 1</sup> Metastability of GeSn at high Sn contents limits the thermal processing window for growth and annealing of epitaxial layers.<sup>2, 3</sup> Detrimental Sn phase separation has been previously reported after annealing of strained GeSn epitaxial layers. We report X-ray photoelectron spectroscopy (XPS) and correlated electron microscopy studies of surface morphology after annealing of random core/shell Ge/Ge<sub>1-x</sub>Sn<sub>x</sub> nanowire assemblies both <i>ex-situ</i> in the XPS chamber and <i>in-situ</i> in the reduced pressure chemical vapor deposition growth reactor. We have previously demonstrated the elastic compliance of small diameter Ge core nanowires in the core/shell structure to enable largely strain-free epitaxial Ge<sub>1-x</sub>Sn<sub>x</sub> shells.<sup>4</sup> Utilizing the strain minimization of this geometry, we investigate the annealing characteristics of the GeSn surface and native oxide for Sn contents in the range of 2 at%–11 at%, unaffected by substrate-induced misfit strain. Samples that form a native oxide through post-growth air exposure show the presence of a Sn-rich oxide that exhibits a composition dependent thermal decomposition temperature measured via <i>ex-situ </i>annealing in the XPS chamber. The surface composition evolution of the as-grown GeSn surface was also examined via samples annealed <i>in-situ </i>without exposure to the atmosphere. We show a dramatic difference in the surface chemical and morphological stability between H<sub>2</sub> and rough vacuum annealing with H<sub>2</sub> annealed samples exhibiting surface Sn segregation at temperatures more than 100 °C lower than vacuum annealed samples.<br/><br/><sup>1</sup>M. Seifner, A. Dijkstra, J. Bernardi, A. Steiger-Thirsfield, M. Sistani, A. Lugstein, J. Haverkort, S. Barth, “Epitaxial Ge<sub>0.81</sub>Sn<sub>0.19</sub> Nanowires for Nanoscale Mid-Infrared Emitters,” <i>ACS Nano</i> <b>13</b>, 7 (2019), 8047-8054.<br/><sup>2</sup>S. Wu, L. Zhang, B. Son, Q. Chen, H. Zhou, C. Tan, “Insights into the Origins of Guided Microtrenches and Microholes/rings from Sn Segregation in Germanium-Tin Epilayers,” <i>J. Phys. Chem. C</i> <b>124 </b>(2020), 20035-20045.<br/><sup>3</sup>W. Wang, L. Li, Q. Zhou, J. Pan, Z. Zhang, E. Tok, Y. Yeo, “Tin surface segregation, desorption, and island formation during post-growth annealing of strained epitaxial Ge<sub>1-x</sub>Sn<sub>x</sub> layer on Ge(001) substrate,” <i>Applied Surface Science</i> <b>321</b> (2014), 240-244.