Tuning Spin- and Valley-Degeneracies in Multicomponent Quantum Well Transport.

摘要

The theme of this thesis is manipulation of spin and valley degeneracies in two-dimensional electron systems (2DES) by locally or globally controlling the energy gaps between the two spin states or multiple valley states. Degeneracies in 2DES can be controlled internally or externally with magnetic, strain, and electrostatic fields. With magneto-transport measurements we can probe these spin and valley energy gaps. Spin degeneracies in quantum wells (QW) can be controlled with magnetic field by changing the tilt angle of the field with respect to the sample. Valley degeneracies can be controlled principally by growing QWs of a certain orientation and width. Furthermore, the valley energies can be controlled externally by applying strain or electrostatic gated devices.;We first consider transport signatures of controlled spin degeneracies. Magnetic fields can be used to control spin degeneracies and spin gaps by tuning the tilt angle of the field with respect to the sample plane. These spin dependencies can be observed at different tilt angles by conducting measurements of the longitudinal and Hall resistance. In particular, transport measurements in a Si/SiGe spin-split valley degenerate 2DES demonstrates anomalous rise of the transverse Hall resistance at certain quantized plateaus. With systematic tilted field data we map this anomaly to the longitudinal resistance, and also to directional derivatives of the longitudinal resistance. We also develop a theoretical model for estimating the spin-degenerate and spin-split density of states which we fit using the data on longitudinal resistance. We input the exactly calculated spin gaps at every tilt angle in the edge state model of quantum Hall effect, and we are able to provide a microscopic justification to the experimentally observed anomalous features by introducing a constant energy density of disordered states in our model.;We next consider transport signatures of controlled valley degeneracies. Valley degeneracies can be tuned with structural parameters such as growth orientation, QW width and additional confinement such as gating along specific directions. We detail a theory to calculate the ground energy of each valley in a multi-valley system, considering the influence of growth orientation, quantum confinement and miscut angles on valley degeneracies. We first study AlAs QWs grown along the high mobility (001) facet, which has two degenerate valleys. Since valley mass is anisotropic along different directions we can perform transport experiments with orientation sensitivity on specific sample geometries which permit us to distinguish between the valleys. The mass anisotropy also gives rise to anisotropy in valley resistance, and we measure the valley anisotropy ratio at various half-filling factors. The measurement of this resistance anisotropy ratio at half-filled Landau levels is the first evidence of valley ordering of Landau levels.;We also study AlAs QW grown along the lower mobility and lesser studied (111) facet with three degenerate valleys. Though they have not yet been experimentally demonstrated, on-axis AlAs (111) valleys would exhibit an SU(3)-like symmetry which is of interest due to the novelty of valley texture excitations which might arise. A small miscut angle to the principle growth axis allows us to grow defect-free (111) AlAs QWs but also breaks the degeneracy. We optimize the growth with AFM, TEM and XRD morphology characterization of the GaAs, AlGaAs and AlAs layers individually and in combination on (111) GaAs substrate. We show with numerical simulations that careful selection of miscut angle with respect to the valley orientation can exactly determine the valley degeneracy breaking in this SU(3)-like system. Furthermore, the valley degeneracies that we observe with transport characterization match with the numerical simulations. By choosing current to flow along the three valley orientations, and measuring the longitudinal resistance we can also infer the energetic ordering of the valleys.;Inspired by spintronic devices which manipulate the spin to achieve device functionality, we can propose devices which manipulate the valley or pseudo-spin state to identify new valleytronic devices. We introduce the basic design and concept behind the valley filter, devices whose function is to locally break the degeneracies in (001) AlAs QWs. We then show conceptual circuit elements such as valley diodes which block current as a measure of the fidelity of the valley filters. These circuit elements allow us to conceptualize a valleytronic non-volatile memory transistor device. We also detail prototype device fabrication steps for valley filter devices with deposition of external electrostatic and strain gate structures. Valleytronic devices might offer performance advantages over spin devices by substituting valley-ferromagnetism for spin-magnetism for a long lived memory state. (Abstract shortened by UMI.).