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Core-shell Ge/GeSn nanowires provide a route to dislocation-free single crystal germanium-tin alloys with desirable light emission properties because the Ge core acts as an elastically compliant substrate during misfitting GeSn...

Short-range atomic order in semiconductor alloys is a relatively unexplored topic that may promote design of new materials with unexpected properties. Here, local atomic ordering is investigated in Ge–Sn alloys, a group-IV system that is attractive for its enhanced optoelectronic properties achievable via a direct gap for Sn concentrations exceeding ≈10 at. %. The substantial misfit strain imposed on Ge–Sn thin films during growth on bulk Si or Ge substrates can induce defect formation; however, misfit strain can be accommodated by growing Ge–Sn alloy films on Ge nanowires, which effectively act as elastically compliant substrates. In this work, Ge core/Ge1−xSnx ( x ≈  0.1) shell nanowires were characterized with extended x-ray absorption fine structure (EXAFS) to elucidate their local atomic environment. Simultaneous fitting of high-quality EXAFS data collected at both the Ge K-edge and the Sn K-edge reveals a large (≈ 40%) deficiency of Sn in the first coordination shell around a Sn atom relative to a random alloy, thereby providing the first direct experimental evidence of significant short-range order in this semiconductor alloy system. Comparison of path length data from the EXAFS measurements with density functional theory simulations provides alloy atomic structures consistent with this conclusion.

As a direct bandgap Group IV alloy, metastable Ge 1−x Sn x (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 Ge 1−x Sn x 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 oxidefree 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.