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Optical Injection of Spin Polarized Electrons in Silicon

Abstract

Silicon is a popular semiconductor in spintronics. Advancements in research into

this material has only happened through contacts. There hasn’t been any research done

by means of optical injection due to its indirect bandgap. In this dissertation, I

demonstrate the optical injection of spin polarized electrons into silicon using a muon

spin relaxation technique that utilizes spin-polarized muons to probe for conduction

electron spin polarization.

When an antimuon, a positive muon, is implanted into a semiconductor, it

captures a conduction electron and forms a muonium atom. The spin of the electron can

either be parallel or antiparallel to the muon spin. If the spin is parallel, the spins are

fixed. But, if the spins are antiparallel, then they are in a superposition of both

configurations; muon spin-up with electron spin-down plus muon spin-down plusv

electron spin-up. This spin flipping is the basis of how the muons can probe for the

conduction electron spin polarization.

The experiment was completed on both n-type and intrinsic silicon to prove the

existence in two different types. The wavelength was scanned over an interval slightly

above the bandgap since there is virtually no absorption at the bandgap. Since silicon is

an indirect bandgap, the photons alone can’t be absorbed into the bottom of the bandgap

due to the momentum shift, so phonons are required to either be absorbed or emitted to

preserve momentum conservation. Due to the low temperature of the experiment,

phonon emission is the only practical path for absorption. Several parameters, including

wavelength, applied magnetic field, and laser power, were varied to find the and analyze

signal. The experiment was carried out over the course of several years, and trips, to the

ISIS pulsed muon source with a successful detection of the conduction electron spin

polarization in both the n-type and intrinsic silicon samples

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