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First Principles Studies of Mechanical, Physical, and Electronic Properties of a-Si - also, Diffusion of a Self-interstitial Atom in an Ultra-thin fcc Film Via Lattice Statics

Tabatabaei, Maryam | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44812 (09)
  4. University: Sharif University
  5. Department: Civil Engineering
  6. Advisor(s): Mohammadi Shodja, Hossein
  7. Abstract:
  8. By employing first principles density functional theory-based (DFT) molecular dynamics (MD), the influences of dangling and floating bonds as well as distorted tetrahedral bonds are studied on the mechanical, physical, and electronic properties of amorphous Si (a-Si). For further examination of the effects of these geometrical defects, two distinct amorphous samples, namely as-quenched and annealed are generated and examined. To verify the validity of the representative cells, the obtained radial distribution function, pair correlation function, and cohesive energy are compared with those corresponding results presented in the literature. Moreover, the surface energy is calculated at final rupture and is compared with the available theoretical and experimental results and those calculated form Tyson's empirical relation and universal binding energy relations (UBERs). The calculated elastic constants using the symmetry-general scheme satisfy well the isotropic relation with a small discrepancy of about 0.17% at T=0K and that of about 3% at T=300K. The ideal tensile strength is calculated as 8.08GPa under uniaxial tension and 10.28GPa under uniaxial extension. The chemistry of the embryonic crack reveals that the preferred site of crack initiation is threefold-coordinated Si atom. The calculated pseudogap at 0K is about 1.4eV for uniaxial tensile strains below 0.07, and sharply decreases by applying strain beyond 0.07. The phenomenon of diffusion is considered in an ultra-thin Cu film. Determination of the interstitial sites and saddle points corresponding to the diffusion of an interstitial atom in thin film is of particular interest. To address this problem a lattice statics method which incorporates Rafii-Tabar and Sutton interatomic potential function, suitable for binary alloys is developed. For the sake of demonstration, the values of the barrier height energy pertinent to a diffusing self-interstitial atom in the bulk material are computed using both the first principles DFT and the developed technique, indicating reasonable correspondence
  9. Keywords:
  10. Amorphous Silicon ; First Principles Molecular Dynamics Calculations ; Gap Tuning ; Theoretical Tensile Strength ; Crack Initiation Chemistry

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