Growth of Ge-Sn Digital Alloys toward Group IV Topological Materials
Poster for NAMBE
Bradicich, Adelaide Maria [Sandia National Laborat↗
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Composition dependence of atomic order in strain-relaxed, metastable GeSn alloys
Extended x-ray absorption fine structure (EXAFS) measurements of single-crystal Ge/GeSn radial heterostructure nanowires are used to examine the effects of composition on both short-range order (SRO) and longer-range disorder in GeSn alloys. GeSn has prompted significant interest because it can achieve a direct band gap for sufficient Sn concentrations beyond the equilibrium solid solubility limit in an all-group IV system. Short-range order in this material is particularly interesting as it has been predicted to affect the band gap independent of average composition or strain effects. By independently controlling the Sn composition and GeSn thickness during chemical vapor deposition of misfitting GeSn shells around ultrathin, elastically compliant, Ge core nanowires, the elastic misfit strain in the GeSn is minimized for Sn compositions over the studied range ≈Ge 0.96 Sn 0.04 to Ge 0.88 Sn 0.12 . The degree of SRO was found to decrease with increasing Sn composition. Additionally, damping of the EXAFS signal was observed as the Sn content increased, particularly for increasingly distant atomic shells about the absorbing atom, even for scattering paths not involving Sn atoms. This result is quantified as an increase in the mean-squared relative displacement parameters of the shells. These measurements reveal the accommodation of local strain due to the presence of the highly size-mismatched Sn atoms in the Ge diamond cubic lattice (≈14%), which may have effects on the band structure of the material in addition to the influence of short-range atomic order. Comparison among the nanowire samples allows for calculation of the topological rigidity parameter, a ∗∗ , for the first-neighbor bond lengths. Furthermore, these exhibit chemically distinct values for Ge-Ge, Ge-Sn, and Sn-Sn, and they are consistent with the value a ∗∗ = 0.75 ± 0.07 confirming the general applicability of the model to alloys with both large amounts of natural misfit strain and the potential for short-range order.
Crystal structure Semiconductors Transmission elec↗
Materials Data on SnGe3 by Materials Project
Sn1Ge3 is beta Cu3Ti-like structured and crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Sn is bonded to twelve Ge atoms to form SnGe12 cuboctahedra that share corners with four equivalent SnGe12 cuboctahedra, corners with eight equivalent GeSn4Ge8 cuboctahedra, edges with eight equivalent SnGe12 cuboctahedra, edges with sixteen equivalent GeSn4Ge8 cuboctahedra, faces with four equivalent SnGe12 cuboctahedra, and faces with fourteen GeSn4Ge8 cuboctahedra. There are four shorter (3.09 Å) and eight longer (3.12 Å) Sn–Ge bond lengths. There are two inequivalent Ge sites. In the first Ge site, Ge is bonded to four equivalent Sn and eight Ge atoms to form distorted GeSn4Ge8 cuboctahedra that share corners with twelve equivalent GeSn4Ge8 cuboctahedra, edges with eight equivalent SnGe12 cuboctahedra, edges with sixteen GeSn4Ge8 cuboctahedra, faces with four equivalent SnGe12 cuboctahedra, and faces with fourteen GeSn4Ge8 cuboctahedra. There are four shorter (3.09 Å) and four longer (3.12 Å) Ge–Ge bond lengths. In the second Ge site, Ge is bonded to four equivalent Sn and eight equivalent Ge atoms to form distorted GeSn4Ge8 cuboctahedra that share corners with four equivalent GeSn4Ge8 cuboctahedra, corners with eight equivalent SnGe12 cuboctahedra, edges with twenty-four GeSn4Ge8 cuboctahedra, faces with six equivalent SnGe12 cuboctahedra, and faces with twelve GeSn4Ge8 cuboctahedra.
36 MATERIALS SCIENCE↗
Materials Data on SnGe by Materials Project
GeSn is graphite-derived structured and crystallizes in the trigonal P3m1 space group. The structure is two-dimensional and consists of one GeSn sheet oriented in the (0, 0, 1) direction. Sn is bonded in a trigonal non-coplanar geometry to three equivalent Ge atoms. All Sn–Ge bond lengths are 2.63 Å. Ge is bonded in a trigonal non-coplanar geometry to three equivalent Sn atoms.
36 MATERIALS SCIENCE↗