M.S. Thesis Defense: Hyun Soo Kim
Wednesday, November 13, 2013
4:00 p.m. 2328 AVW Building
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ANNOUNCEMENT: M.S. Thesis Defense
Name: Hyun Soo Kim
Location: A. V. Williams 2328
Date/Time: Wednesday, November 13. 2013 at 4pm to 7pm
Prof. Ian Appelbaum, Chair and Advisor
Prof. Isaak D. Mayergoyz
Prof. John Melngailis
Title: Hot Electron Injection into Uniaxially Strained Silicon
In semiconductor spintronics, silicon attracts great attention due to its long electron spin lifetime. Silicon is also one of the most commonly used semiconductor in microelectronics industry. The spin relaxation process of diamond crystal structure such as silicon is dominated by Elliot-Yafet mechanism. Yafet predicts the spin relaxation rate is related to temperature by the power law, ~T^(5/2), assuming that intravalley scattering is dominant. The conduction electron spin lifetime measured by electron spin resonance measurements and electronic measurements using ballistic hot electron method agrees well with Yafets theory. However, recent theory predicts a strong contribution of the intervalley scattering process such as f-process in silicon. The conduction band minimum is close the Brillouin zone edge X point, which causes strong spin mixing at the conduction band. A recent experiment of electric field-induced hot electron spin relaxation also shows the strong effect of f-process in silicon. In uniaxially strained silicon along crystal axis , it is predicted that the suppression of f-process leads to enhanced electron spin lifetime. By inducing a change in crystal structure via  uniaxial strain, the six fold degeneracy in silicon becomes a two fold degeneracy, which is known as valley splitting. According to recent theory, as the valley splitting increases, the intervalley scattering is reduced, which increases electron spin lifetime by a factor of four in 0.5% uniaxially strained silicon.
In this thesis, we demonstrate ballistic hot electron injection into silicon under various uniaxial strain. Spin polarized hot electron injection under strain is experimentally one of the most challenging parts of measuring conduction electron spin lifetime in silicon. Hot electron injection makes use of a tunnel junction, which is a thin oxide layer between two conducting materials. The two conducting materials are only tens of nanometer and a thin oxide layer is only a few angstroms so 0.5% strain on silicon along  axis can easily destroy thin films on the silicon substrate. In order to confirm the performance of tunnel junction, we use spin polarized hot electron injection method. The tunnel junctions consist of two kinds of ferromagnetic metals, normal metals and an oxide layer as tunnel barrier in order to measure spin valve effect in silicon. Using silicon as a collector with a Schottky barrier interface between the metal and silicon, ballistic hot spin polarized electron injection into silicon is demonstrated. We also observed a change of the coercive field and magnetoresistance due to modification of local states in ferromagnetic metals and surface states at the interface between metal and silicon due to strain.