The effect of structural and chemical bonding changes on the optical properties of Si/Si1−xCx core/shell nanowires

Si/Si1−xCx 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 Si1−xCx 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/Si1−xCx CS NWs as a function of annealing temperature. The Si1−xCx 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 Si1−xCx shell with increasing annealing temperature. Furthermore, the THz pulse emitted from the Si/Si1−xCx 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.