Electromagnetic field enhancement effects in group IV semiconductor nanowires. A Raman spectroscopy approach

Abstract

Semiconductor nanowires (NWs) are the building blocks of future nanoelectronic devices. Furthermore, their large refractive index and reduced dimension make them suitable for nanophotonics. The study of the interaction between nanowires and visible light reveals resonances that promise light absorption/scattering engineering for photonic applications. Micro-Raman spectroscopy has been used as a characterization tool for semiconductor nanowires. The light/nanowire interaction can be experimentally assessed through the micro-Raman spectra of individual nanowires. As compared to both metallic and dielectric nanowires, semiconductor nanowires add additional tools for photon engineering. In particular, one can grow heterostructured nanowires, both axial and radial, and also one could modulate the doping level and the surface condition among other factors than can affect the light/NW interaction. We present herein a study of the optical response of group IV semiconductor nanowires to visible photons. The study is experimentally carried out through micro-Raman spectroscopy of different group IV nanowires, both homogeneous and axially heterostructured (SiGe/Si). The results are analyzed in terms of the electromagnetic modelling of the light/nanowire interaction using finite element methods. The presence of axial heterostructures is shown to produce electromagnetic resonances promising new photon engineering capabilities of semiconductor nanowires.

About the Interaction Between a Laser Beam and Group IV Nanowires: A Study of the Electromagnetic Field Enhancement in Homogeneous and Heterostructured Nanowires

“…In ref. one can see the spectrum at the HJ of a SiGe/Si axially heterostructured NW with higher concentration of Ge (60%), where one can clearly appreciate the HJ contribution.…”

“…This model is solved by FEMs using COMSOL Multiphysics. The Maxwell equations are solved for the NW and its surrounding space when excited by a focused laser beam in similar conditions to the micro‐Raman experiments . The system formed by the air/NW/substrate was limited by cartesian perfectly matched layers (PMLs), which absorb the outgoing radiation eliminating secondary reflections, Figure .…”

“…The presence of the axial heterojunction breaks the longitudinal symmetry of the NW. Therefore, a 3D model must be applied, instead of the 2D model used for NWs without axial discontinuities, either homogeneous NWs or core/shell heterostructured NWs . The 3D model accounts for the presence of the axial HJ; and also takes account of the finite length of the NW.…”

“…The Raman spectra were acquired by scanning the laser beam along the NW axis in steps of 100 nm, and under TM light polarization with respect to the NW axis, that is, the electric field parallel to the NW axis. This orientation is selected because the NWs behave as electric dipoles face to the incident laser beam, and the Raman intensity as a function of the angle formed by the NW axis and the light polarization axis reveals an electric dipole behavior . The maximum Raman signal is therefore obtained for TM polarization with respect to the NW axis.…”

“…In our Raman experiments we used an Al substrate, which permits the optimization of the Raman signal, but also acts as an efficient heat‐sink avoiding the NW heating. The laser power was optimized to prevent any heating on the NWs . The Raman measurements were carried out under laser power densities around 5 × 10 8 W m −2 , for which laser heating was negligible.…”