Amit Sharma, Ph.D. (Advisor); Brent D. Foy, Ph.D. (Committee Member); Sarah F. Tebbens, Ph.D. (Committee Member)
Master of Science (MS)
Group IV elements based nanoelectronics devices (mainly Si and Ge based devices) have been developed and improved over a long period of time and are the most influencing materials of semiconductor electronics, but due to their indirect bandgap their use in optoelectronics is limited. Alternatively, new Group IV alloys comprised of Ge, Si, and Sn semiconductor materials have emerged as attractive options for various electronic and optoelectronic applications. The binary and ternary alloys provide strain and energy bandgap engineering by controlling element content, a route for realizing direct-transition semiconductors, improvement in interface and defect properties, and a reduction of the process temperature related to the crystal growth. However, there are many obstacles and challenges for the crystal growth of Ge-Sn alloy on the Silicon or Germanium substrate. One of the problems in Ge-Sn growth is Sn precipitation from Ge-Sn. Theoretical calculation predicts that Ge transitions from an indirect semiconductor to a direct semiconductor by incorporation of Sn on Ge matrix. For tensile strained Ge-Sn alloys, the transition is predicted at 6.3% Sn concentration. This is the main driving force for the growth of epitaxial Ge-Sn crystals on Si substrates. The epitaxial growth of Ge-Sn is very challenging because of huge lattice mismatch between Ge and Sn and, the strong surface segregation of Sn on Ge and extremely low equilibrium solubility of Sn on Ge. In the recent past, a lot of progress has been made for the development of epitaxial growth techniques. Besides other techniques like MBE for the deposition of Ge-Sn on the substrate of Si, chemical vapor deposition has been achieved. Similarly, pulsed laser-induced epitaxy is also another technique for the deposition. Besides the experimental efforts to study the Ge-Sn-Si elemental binary and ternary alloys, Molecular Dynamics (MD) modeling provides insight into atomic configurations and structural dynamics, which requires the accurate inter-atomic potential for Ge-Sn-Si binary and ternary system. Present work is an effort to generate Embedded Atom Method (EAM) potential for this system, which can then be used with the MD method to study epitaxial growth. The work presented here uses classical molecular dynamics approach and EAM potential fitting code to develop the EAM potential, which can be used to study the properties of Ge-Sn, Ge-Si, Si-Sn, and Ternary Ge-Sn-Si system. Density Functional Theory (DFT) calculations are performed for each binary pair - Ge-Sn, Ge-Si and Si-Sn using Vienna Ab initio Simulation Package, better known as VASP for a range of temperatures in the range of 1200K- 1500K. The interatomic potential fitting code, MEAMfit, is used to fit EAM potentials to energies and atomic forces generated from DFT calculations. The data to be fitted are directly read from “vasprun.xml” files from VASP. Three different methods were used to test the accuracy of developed potentials, namely, testing the fit for its predictability of DFT energies in the testing set; computing elastic properties, and crystal properties such as phonon band-structure with fitted potential and comparing those with direct DFT calculations.
Year Degree Awarded
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