The effect of structural and chemical bonding changes on the optical properties of Si/Si_1-xC_x core/shell nanowires

Si/Si_1-xC_x core/shell nanowires (CS NWs) were synthesized. First, a Si NW was grown via a Vapor-Liquid-Solid (VLS) procedure using Au as a catalyst. Next, a Si_1-xC_x shell was deposited by a chemical vapor deposition (CVD) method after the removal of the Au tip at the top of the Si NW. We investigated the physical, chemical, and optical properties of the Si/Si_1-xC_x CS NWs as a function of annealing temperature. The Si_1-xC_x shell was initially deposited on the Si core with small clusters of an amorphous state, which were remarkably transformed into larger clusters by recrystallization after annealing under vacuum. To relieve the strain induced by the huge difference between the atomic sizes of Si and C, substitutionally incorporated C atoms can combine with another C atom at the third-nearest-neighbor distance in the Si _1-xC_x shell with increasing annealing temperature. Furthermore, the THz pulse emitted from the Si/Si_1-xC_x CS NWs was observed and analyzed. In the case of annealing treatment at 600 °C, the THz pulse intensity was substantially increased, which is not ascribed to Drude absorption but to mid-IR absorption. Moreover, based on the simulation results, we suggest that the existence of substitutional C atoms and control of the shell thickness is a viable method to enhance the THz pulse amplitude.