Influence of growth kinetics on Sn incorporation in direct band gap Ge_1-xSn_x nanowires

Ge_1-xSn_x alloys with substantial incorporation of Sn show promise as direct bandgap group IV semiconductors. This article reports the influence of growth kinetics on Sn inclusion in Ge_1-xSn_x alloy nanowires through manipulation of the growth constraints, i.e. temperature, precursor type and catalyst. Ge_1-xSn_x nanowire growth kinetics were manipulated in a vapour-liquid-solid (VLS) growth process by varying the growth temperature between 425 and 470 °C, using Au and Ag alloys as growth catalysts and different tin precursors such as allyltributytin, tertaethyltin and tetraallyltin. The profound impact of growth kinetics on the incorporation of Sn; from 7 to 9 at%; in Ge_1-xSn_x nanowires was clearly apparent, with the fastest growing nanowires (of comparable diameter) containing a higher amount of Sn. A kinetically dependent “solute trapping” process was assigned as the primary inclusion mechanism for Sn incorporation in the Ge_1-xSn_x nanowires. The participation of a kinetic dependent, continuous Sn incorporation process in the single-step VLS nanowire growth resulted in improved ordering of the Ge_1-xSn_x alloy lattice; as opposed to a randomly ordered alloy. The amount of Sn inclusion and the Sn impurity ordering in Ge_1-xSn_x nanowires has a profound effect on the quality of the light emission and on the directness of the band gap as confirmed by temperature dependent photoluminescence study and electron energy loss spectroscopy.