Muon probes of spin-polarized electrons in gallium arsenide

With the advance of spintronics materials, it becomes crucial to develop techniques that can directly probe conduction electron spin polarization (CESP) in many semiconductor systems. In this dissertation I demonstrate a proof-of-concept muon spin relaxation (muSR) experiment, in which spin-polarized muon beams are utilized to directly probe the CESP in semiconductors.;A positively charged muon implanted in semiconductor crystal forms a muonium by capturing a conduction band electron. Depending on the electron spin polarization either parallel or antiparallel to the muon spin, the formed muonium is either in a spin triplet (S = 1) or singlet (S = 0) state respectively. Since the singlet state is in a superposition of muon spin up (with electron spin down) and down (electron spin up), it quickly disappears from the muSR signal, while the triplet state remains the same. This unique nature of muonium is the principle of the experiment.;To prove the feasibility of the concept, it is ideal to demonstrate it in a system, in which electronic spin properties are already well known. The CESP of Si doped n-type GaAs has been well studied using the photoluminescence and time-resolved Kerr rotation (TRKR) techniques. Furthermore it is known that n-GaAs has a strong spin-orbit coupling and relatively long spin lifetime in low temperature. To induce the CESP in n-GaAs, optical spin orientation is utilized with circularly polarized laser light whose wavelength tuned near the bandgap. But since there is a strong absorption at the bandgap energy, it is necessary to use a wavelength below the bandgap so that it transmits through the sample and makes enough overlap of the photoexcited CESP and implanted muon. The optical spin orientation using below-bandgap excitation is confirmed here using the two-color TRKR experiment. Having established the below-bandgap optical spin orientation, the muSR experiment is performed under various experimental conditions and found a signal change, which confirms the above mentioned principle. This newly developed technique can be applied to various semiconductors regardless of their particular properties such as spin-orbit coupling or bandgap.