Synthesis and Raman Scattering Studies of Novel Semiconductor Nanostructures: Si, Ge and GaAs Twinning Superlattice Nanowires

Open Access
Adu, Kofi Wi
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
March 15, 2004
Committee Members:
  • Karen Eklund, Committee Chair
  • Vincent Henry Crespi, Committee Member
  • Paul Sokol, Committee Member
  • Albert Welford Castleman Jr., Committee Member
  • Phonon confinement
  • Quantum confinement
  • Twinning Superlattice
  • Nanowires
  • Raman Scattering
  • Fano Interference Scattering
  • Semiconductor nanostructures
  • Pulsed Laser Vaporization(PLV)
  • Synthesis
  • Oxidation
This thesis work focuses on synthesis of novel semiconductor nanostructures: nanowires, quantum dots and twinning superlattices by Pulsed Laser Vaporization (PLV) and Thermal Batch Process (TBP) and the studies of their optical properties. We have developed growth and synthesis routes to produce Ge and Si nanowires of diameters ranging from 3nm to 10nm that exhibit quantum confinement effects. The asymmetric Raman lineshape predicted for scattering from confined optical phonon by Richter, and the Fano interference lineshape in n-type Si and Ge are found to be similar. However, the physics of either phenomenon is distinctively different. We performed a series of Raman scattering experiments on Si and Ge nanowires to determine the true origin of the asymmetry. The experiments were conducted under two conditions: high wire density and low wire density, corresponding to poor thermal anchorage and good thermal anchorage on substrate. We identified three physical phenomena that contribute to the lineshape: Phonon confinement, photo-stimulated and thermally induced Fano scattering. We can distinguish each of these processes based on the diameter of the wire and the laser flux dependence of the scattering and the evolution of the lineshape at low laser power with nanowire diameter. Which effect dominates depends on the contact of the Raman sample to the substrate, the substrate thermal properties, and the diameter of the nanowires. We have proposed a coupled phenomenological model which takes the phonon confinement and Fano processes into account that best describe the observed asymmetry in the phonon lineshapes. We are the first to report true phonon confinement in Si and Ge nanowires that shows the evolution of the lineshape asymmetry with diameter (3 nm to 30 nm) first predicted by Richter. We have also investigated the effect of oxide layer, and strain induced by the oxide layer on the first order Raman scattering from Si nanowires. Our data reveal that: a) one can either introduce strain or suppress strain depending on the dry oxidation route; in situ suppresses strain where as ex situ induces strain and b) the asymmetric lineshape in the phonon mode is affected by the strain and the thickness of the oxide layer. Finally, we have used photoluminescence to demonstrate that twinning superlattice of GaAs nanowires of classical size d ³ 30nm can exhibit quantum confinement effects e.g., a red shift in the band gap (as predicted by theoretical investigations). In addition, Raman scattering spectrum from the GaAs twinning superlattice reveals a new strongly enhanced peak at 515 cm-1 that is not observed in the bulk. We attribute this peak to an undetermined twinning.