Oxidation Mechanism of Si_1–<i>x</i>Ge_<i>x</i> Nanowires with Au Catalyst Tip as a Function of Ge Content

Si_1–<i>x</i>Ge_<i>x</i> nanowires (NWs) (0.22 ≤ <i>x</i> ≤ 0.78) were synthesized using a vapor–liquid–solid procedure with a Au catalyst. We measured the intrinsic physical, chemical, and electrical properties of the oxidized Si_1–<i>x</i>Ge_<i>x</i> NWs using several techniques, including transmission electron microscopy, X-ray photoemission spectroscopy, and optical pump-THz probe spectroscopy. We suggest two distinct oxidation mechanisms depending on the Ge content in the Si_1–<i>x</i>Ge_<i>x</i> NWs: (i) when the Ge content is around 0.22, a Au catalytic effect brings about oxidation in both the axial and lateral directions; and (ii) when the Ge content is greater than 0.22, the Au tip is detached from the NW body and does not act as a catalyst, which is a result of the high degree of Ge-atom participation in the oxidation process. Additionally, we measured the photoconductivity decay time distribution for the Si_1–<i>x</i>Ge_<i>x</i> NWs before and after oxidation process; the decay time is significantly shortened in oxidized Si_1–<i>x</i>Ge_<i>x</i> NWs (0.22 < <i>x</i>), whereas it is maintained for Si-rich Si_1–<i>x</i>Ge_<i>x</i> NWs (<i>x</i> ≈ 0.22) as compared to the as-grown Si_1–<i>x</i>Ge_<i>x</i> NWs. It indicates that the number of defect states is generated with the formation of defective Ge oxide at the oxide-shell-layer/Si_1–<i>x</i>Ge_<i>x</i>-core-NW interface