Spin polarized tunneling and spin injection in Fe-GaAs hybrid structures
Spin electronics, or spintronics, is a new branch of electronics whereby the spin degree of freedom in electronic devices is employed. For understanding the physics of spin injection in semiconductors, this thesis is aimed at contributing to fabricate ferromagnetic metal-semiconductor hybrid structures, typically Fe-GaAs hybrid structures, in which the spin-polarized transport phenomena are studied. In order to understand the spin transport at the Fe/GaAs interface, the spin-polarized tunneling is studied first in this work. The Fe/GaAs/Fe/Co magnetic tunneling junctions are fabricated and the TMR effect as well as I-V characteristics is measured at different temperatures. Interpretations of the experimental data by the theoretical model allow us to characterize the junction quality, which shows that apart from the conductivity mismatch problem, the oxidation of the semiconductor surface and the interdiffusion between Fe and GaAs are key issues in the fabrication of high quality ferromagnet-semiconductor hybrid structures. The spin-polarized tunneling through a sulphur-passivated GaAs barrier is studied to clarify the passivation effect. However, our experiments show no positive influence of sulphur passivation. The spin injection in the ferromagnetic metal-semiconductor hybrid structures is investigated in the second part of this work. Before performing the spin injection experiments, we try to measure the interface resistivity of Fe/GaAs Schottky barriers with different doping densities at low temperatures. Using the measured interface resistance as a guide for experimental design, the magnetic p-n junction diodes and Fe/GaAs/Fe structures are fabricated, and spin injection is investigated in these devices. In the magnetic p-n junction diode, a negative GMR-like effect is found under a large applied bias, when the relative magnetizations of the two magnetic electrodes are changed from parallel to antiparallel. The experimental finding agrees with the theoretical prediction very well. For spin injection in Fe/GaAs/Fe structures, the experiments are carefully performed by different surface treatments with different doping profiles of the GaAs. The small but clear magnetoresistance could only be found in the device with 50nm homogeneous heavily doped GaAs under a large bias, indicating a surface spin polarization of 2.6% in the Fe/GaAs/Fe structure.