Oxide Decomposition and Sn Surface Segregation on Core/Shell Ge/GeSn Nanowires

As a direct bandgap Group IV alloy, metastable Ge1–xSnx (x > ∼0.1) is an extremely interesting optical and electronic material. Germanium core/germanium-tin coaxial heterostructures offer an opportunity to study Sn surface segregation from Ge1–xSnx alloys in the technologically interesting composition range that exceeds the maximum solid solubility of tin in diamond cubic structure germanium. We investigate the annealing characteristics of the germanium-tin surface and native oxide for tin contents in the range of 2 to 12 at% for initial conditions ranging from intentional air exposure to surface oxide-free nanowires. For air-exposed samples, we show the presence of a tin-rich oxide that exhibits a composition dependent temperature for thermal decomposition during postdeposition annealing in the XPS chamber. Across the range of Sn compositions investigated, the decomposition temperatures of tin oxide and germanium oxide were found to be the same, indicating a single-phase oxide in which both components decompose simultaneously. Utilizing nominally air-free transfer of freshly synthesized and rapid thermally annealed Ge/GeSn nanowires, we investigated the effects of hydrogen and vacuum (∼50 mTorr) annealing and show the inhibition of Sn segregation to the GeSn shell surface when a surface oxide forms. Formation of a surface oxide during an anneal inhibits further Sn surface segregation and, compared to hydrogen anneals, permits an approximately 175 °C increase in the annealing temperature window before changes occur in the nanowire surface morphology, thus promoting thermal stability needed for many device fabrication processes.