SIZE EFFECTS IN ALLOY AND HETEROSTRUCTURED Si1-xGex NANOWIRES

Open Access
Author:
Zhang, Xi
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 10, 2009
Committee Members:
  • Sarah Elizabeth Dickey, Dissertation Advisor
  • Elizabeth C Dickey, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Suzanne E Mohney, Committee Member
  • Mark William Horn, Committee Member
Keywords:
  • nanowires
  • SiGe
  • size effect diffusion
Abstract:
This dissertation focuses on studies of the size-dependent properties of Si1-xGex alloy and heterostructured nanowires grown by the vapor-liquid-solid (VLS) mechanism. Chemical vapor deposition (CVD) and pulsed laser vaporization (PLV) methods are used to synthesize alloy and heterostructured Si1-xGex nanowires. Transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS) are used to investigate structural and chemical properties of these nanowires. The first experiments investigate the size-dependent composition in Si1-xGex nanowires grown via Au-catalyzed CVD on Si (111) substrates or in nanoporous alumina (AAO) membranes. The Ge concentration in Si1-xGex nanowires is studied as a function of nanowire diameter by using quantitative EDS. The Ge concentration in Si1-xGex nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below ~50 nm. The size-dependent nature of Ge concentration in Si1-xGex nanowires is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth. The next set of experiments focus on the solid-sate diffusion of Ge in Si nanowires grown by PLV. Experiments are developed to study the effects of nanowire diameter and post-synthesis annealing temperature on the diffusion coefficient of Ge in Si nanowires. The activation energy calculated from the Arrhenius fitting suggests that a surface diffusion mechanism dominates in nanowires. The third part is a theoretical analysis of solid-state diffusion of Ge in Si nanowires by taking into account the contributions from both surface and bulk diffusions. Computer simulation is employed to model the Ge concentration in Si nanowires by solving the differential equation of diffusion in a cylindrical coordinate system and an analytical solution is obtained. The validity of the solutions is demonstrated by comparing theoretical results with experimental measurements. Based on this model, the diameter dependence of the diffusion coefficient of Ge in Si nanowire is also discussed. Finally, thermal oxidation of Si1-xGex nanowires is investigated. Early stage experimental results are presented and future work based on our findings is also recommended.