The creation and transport of spin -polarized carriers in semiconductor heterostructures.

摘要

In this dissertation we present a theory of the creation and transport of spin-polarized carriers in semiconductor heterostructures. We first present the theory of spin-dependent reflection of semiconductor electrons off the semiconductor-ferromagnet interface. A simplified one-dimensional effective-mass approximation model of the interface is used to elucidate the key physical features in the quantum-mechanical scattering off the spin-dependent ferromagnet band-structure. Spin-dependence under reflection is calculated for both electrical and optical excitation of carriers at or above the Fermi level in the semiconductor, and shows that high spin-polarizations are possible through proper tailoring of the interface properties through material selection or doping. The creation of spin-polarization through reflection is then calculated for the ferromagnet-semiconductor junction under an applied bias. A full three-dimensional calculation shows that efficient spin creation is possible through all-electrical means. We then study systems in which the semiconductor electrons are confined at the interface, specifically in the naturally formed surface layer and in the gate-bias induced inversion layer in the silicon field-effect transistor system. Due to the carriers being confined at the interface, the wavefunction penetrates into the ferromangetic gate and creates the spin-dependent properties. The transport of the spin-polarization through the two-dimensional electron gas is calculated including the effects of carrier leakage into the gate, drift due to an in-plane bias, and diffusion due to a gradient in the two-dimensional electron gas density. Finally, we introduce a device proposal, in which the single ferromagnet is replaced by two adjacent ferromagnets. The channel conductivity can be controlled by the relative orientation of the two ferromagnets' magnetizations (parallel vs. antiparallel), a device which could perform the role of magnetic non-volatile memory on the semiconductor chip.