Local ordering in Ge/Ge-Sn semiconductor alloy core/shell nanowires revealed by extended x-ray absorption fine structure (EXAFS)

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 asymptotic to 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 asymptotic to 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 (asymptotic to 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.