Raman analysis of silicon and germanium nanowires
Résumé
Silicon and germanium nanowires are synthesized by the VLS (Vapor
Liquid Solid) growth process in a LPCVD reactor (Low Pressure Chemical Vapor
Deposition), using gold catalysts and silane (SiH4) and germane (GeH4)
respectively as gas precursors. The gold catalyst droplets can be formed by
either by dewetting of gold continuous layers or deposition of a colloidal solution
on the substrate. The second solution was used in the present work to get a
control on the nanowire diameter during growth, which is not possible when
starting with a continuous gold layer. These nanowires are characterized by
Raman spectroscopy to study their structure and electronic properties. In fact,
the Raman spectrum is sensitive to many factors that can cause its modification
(effect of inhomogeneous distribution of the diameters, inhomogeneous heating
effect and laser-induced phonon confinement effect). These effects can lead to
the shifting and broadening of Raman spectra compared to a Si or Ge
monocrystalline reference. On the Raman spectra from our individual nanowires,
we observe in the case of tapered nanowires, oscillations on the phonon line
intensities which we attribute to resonances related the local nanowire diameter.
Such effects have been predicted from theoretical simulations. [1] 1. Cao, L., B.
Nabet, and J.E. Spanier, Enhanced Raman scattering from individual
semiconductor nanocones and nanowires. Physical review letters, 2006. 96(15):
p. 157402.