Ferromagnetic/semiconductor hybrid structures for spin manipulations

In recent years, spintronics has attracted a great deal of interest for both fundamental physics and device engineering aspects. By utilizing electron (or hole) spins in electronic devices one can obtain another degree of freedom for information processing, perhaps quantum information processing also. The two-state system of a spin makes it a natural candidate for a quantum bit (qubit) in quantum computation. In this dissertation, efforts from sample growths and preparations to measurements and analysis are given. Ferromagnetic/semiconductor hybrid structures such as Fe/GaAs and (Ga,Mn)As/GaAs are grown using molecular beam epitaxy (MBE). One of the potential problems in Fe/GaAs structures is the interdiffusion between Fe and GaAs at the interface. We have shown that we can reduce this effect using extremely low temperature MBE and an ultra-thin Al interdiffusion barrier. The Fe/GaAs structures are used in investigating the ferromagnetic proximity polarization using the time-resolved Faraday rotation (TRFR) technique and we find that the polarization direction is opposite to that of MnAs/GaAs structures. Spin injections for both electron and hole from (Ga,Mn)As into self-assembled InAs quantum dots (QD) have been demonstrated using spin light emitting diodes (spin-LED). The circular polarizations observed from the spin-LEDs are ∼1% for both cases which correspond to those of quantum well spin-LEDs. One unique property observed in the QD spin-LED is that the injection efficiency or spin polarization in the QDs seems to be dependent on the size and/or material composition of the QDs. Finally, optically active Mn-doped self-assembled InAs QDs are also grown. The research performed in this dissertation could be important for future implementation of qubits using spins in QDs.