A Raman and Rayleigh Scattering Study of Electrical and Phonon Properties of Semiconducting Nanowires

Open Access
Author:
Lu, Qiujie
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 10, 2010
Committee Members:
  • Nitin Samarth, Dissertation Advisor
  • Nitin Samarth, Committee Chair
  • Gerald Dennis Mahan, Committee Member
  • Vincent Henry Crespi, Committee Member
  • John V Badding, Committee Member
  • Jayanth R Banavar, Committee Member
Keywords:
  • Raman
  • Rayleigh
  • Antenna Effect
  • Hot Tip Effect
  • FDTD simulation
  • Semiconducting Nanowire
Abstract:
Semiconducting nanowires have the potential to become the next generation of building blocks for nanoelectronics, optoelectronics and sensors. Due to the their quasi-one-dimensional nature and high aspect ratio, semiconducting nanowires exhibit distinct phonon and electrical properties compared to the bulk. This thesis collects three original studies focusing on both computational and experimental results of light scattering from semiconducting nanowires. The first study focuses on Raman scattering results on Si1-xGex nanowires (0<x<1) grown by the vapor-liquid-solid (VLS) growth mechanism using a chemical vapor deposition (CVD). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the morphology growth axis and lattice constant of these materials. Typical wire diameters were in the range 80-130 nm. Based on Raman scattering studies of the bulk, three Raman bands are expected that can be identified as a perturbed Si-Si (~500 cm-1) mode, a Ge-Ge (~280 cm-1) mode and a new mode (~390 cm-1) assigned to Si-Ge or Ge-Si clusters. Peaks in this region are also observed in the case of our nanowires, although the frequencies are a few cm-1 lower than observed in the bulk. We also observe that the compositional (x) dependence of the Si-Ge band in nanowires is somewhat different than that in the bulk. The second research project studies the Rayleigh and Raman scattering from GaP semiconducting nanowires with different polarized incident excitations. GaP nanowires were grown using pulsed laser vaporization (PLV). The diameters of the nanowires range from 50 nm to 500 nm. Rayleigh and Raman spectra were obtained from single GaP nanowires suspended over TEM grid holes. Experiments show that the plots of the scattering intensity vs. the polarization of the incident laser depend on the diameters of the nanowires. Mie theory, the discrete dipole approximation (DDA) and the finite difference time domain (FDTD) methods were used to explain this dependence. The third study focuses on the internal electric field’s dependence on the position of the laser spot relative to the nanowire. About 60% of the Raman scattering spectra from the tips of GaP nanowires were greater than the scattering intensities from the center of the same nanowire. This enhancement factor (ratio of the TO mode intensity at a tip to the TO mode intensity at the center) is around 3~5 for GaP nanowires measured on Si substrates and about 1~2 for GaP nanowires suspended over TEM grid holes. The effect of polarization and energy of the incident excitation on the enhancement factor was studied experimentally with GaP nanowires over TEM grid holes. Further, the FDTD method was applied to compute the theoretical enhancement factor. We carried out a statistical exploration to understand the deviation of the enhancement factor from its computed value.