Local electric field enhancement at the heterojunction of Si/SiGe axially heterostructured nanowires under laser illumination

Abstract

We present a phenomenon concerning the electric eld enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The electric eld is sensed by micro Raman spectroscopy, which permits to reveal the enhancement of the Raman signal arising from the heterojunction region; the Raman signal per unit volume increases at least 10 times with respect to the homogeneous Si, and SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a 3-dimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by nite element methods. A local enhancement of the electric eld at the heterojunction was deduced; however, the magnitude of the electromagnetic eld enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this e ect promises a way to improve the photon harvesting using axially heterostructured semiconductor NWs.