Magnetism, semiconductors and spins: Hybrid ferromagnetic semiconductor and semiconductor spintronic systems

The physics of semiconducting and magnetic materials is of considerable interest for both fundamental condensed matter science and technological applications. Over the past few decades, rapid advances in the fabrication of hetero- and nano-structured materials have resulted in an accelerated focus on the effects of restricted dimensionality in both semiconducting and magnetic materials. However, such studies have largely followed parallel but separate tracks. In this dissertation, we report on experiments in “semiconductor spintronics” that bridge these disparate areas, with the generic goal of exploring spin-dependent phenomena in semiconductors. We focus on three distinct classes of condensed matter systems (paramagnetic semiconductor heterostructures, hybrid ferromagnet/semiconductor heterostructures, and optically-pumped conventional semiconductor heterostructures) whose defining commonality is the importance of spin.; The first experiments describe a variation on the traditional two dimensional gas (2DEG) in a modulation doped semiconductor quantum well. Unlike a conventional 2DEG, we study a system in which the electron gas is strongly coupled to magnetic moments. We develop growth methods that improved the carrier mobility in these “magnetic” 2DEGs, enabling the fabrication of 2DEG superlattices in which the magnetically altered Landau level energy spectrum leads to novel magnetotransport characteristics.; The next system considered is a hybrid ferromagnet/semiconductor heterostructure in which MnAs (a metallic ferromagnet) is integrated with both a II-VI semiconductor (ZnSe) and a III-V semiconductor (GaAs). We report the first detailed studies of carrier transport in epitaxial MnAs, demonstrating the presence of two types of carriers (electrons and holes) in this material. This information is of importance to future experiments that will use MnAs as a spin injector in semiconductor spin devices. The strain present when MnAs is grown epitaxially on these zinc blende templates also results in novel magnetic and structural properties, including an intrinsic exchange biasing effect originating in the presence of a strain-stabilized secondary phase.; Finally, we discuss studies of conventional (non-magnetic) semiconductor heterostructures (ZnSe/GaAs) in which spin effects are made dominant by using circular polarized light to pump spin polarization into the conduction band Fermi sea. Femtosecond time resolved spectroscopy demonstrates the coherent transfer of spin information across a semiconductor heterointerface (from GaAs into heteroepitaxial ZnSe). In the absence of an applied electric field, a short burst of spin polarized electrons crosses the heterointerface with an efficiency of 10–15 percent. The application of an external electric field enhances this spin transfer efficiency by as much as 500%, as well as creating a new persistent mode of coherent spin transfer. In addition, the fabrication of p-GaAs/n-ZnSe bipolar heterojunctions show that the internal electric field of a p-n junction can also significantly enhance the relative spin transfer efficiency and move coherently precessing electrons from an environment of rapid dephasing (p-GaAs) to one where decoherence is minimized (n-ZnSe). The ability to independently tune the coherent spin transfer process using either electric or magnetic fields creates new opportunities in multifunctional semiconductor spintronics.