UC Santa Barbara

As Moore's law approaches its end, the search for other computing architectures becomes more important. One of these alternatives is spintronics, which relies on the generation of spin-polarized electrons in thin film heterostructures. In recent years, topological materials have been found as a promising method of producing solid state spin-polarized electrons. In addition, III-V photocathodes can be used to produce spin-polarized electrons in vacuum. This dissertation presents growth optimization of different materials intended for spin-polarized electron production. After this, we sought a better understanding of the properties and electronic structure of these materials through a variety of in situ and ex situ characterization techniques.

Co₂FeSn is a Heusler alloy proposed to have giant anomalous Hall and Nernst conductivities at room temperature. A variety of methods before, during, and after growth were explored to improve the crystalline ordering of Co₂FeSn thin films; it was found that higher disorder actually led to higher anomalous transport coefficients at room temperature. Further development of this material system could lead to high performance room temperature spin-transfer and spin-orbit torque devices.

α-Sn thin films grown by molecular beam epitaxy on InSb substrates have been found to be relatively easily tuned through multiple topologically non-trivial phases, each with different applications. This material has already demonstrated exceptional potential for low power current-induced spin-orbit torque based devices at room temperature---likely owing to the presence of spin-polarized surface states. Using spin- and angle-resolved photoemission spectroscopy, we clarified the number and the nature of spin-polarized surface states in this system. The confinement-induced three-dimensional topological insulator phase was clearly benchmarked on the topological phase diagram of α-Sn. Hybridization with the substrate changes the film thickness-induced boundaries in the topological phase diagram. In addition, the substrate appears to contribute to a bulk inversion symmetry breaking in α-Sn films which further modifies the α-Sn topological phase diagram.

Alloying isostructural, isovalent Ge into α-Sn allows for the application of tensile strain to these films, opening up an as-yet unexplored section of this topological phase diagram. High concentrations of Ge (>5 %) were alloyed into ultrathin α-Sn films, leading to the discovery of unexpected spin-polarized surface states and an unexpected topological phase transition. These studies provide an avenue for deterministic control of the topological phase of the α-SnGe system. Control of the bulk band gap and the dispersion of spin-polarized surface states is essential for optimizing the performance of devices which use this material.

Finally, we developed strained superlattice InAlGaAs/AlGaAs spin-polarized photocathodes grown by molecular beam epitaxy. This materials system presents a less expensive alternative to the usual GaAs/GaAsP superlattice system; InAlGaAs/AlGaAs superlattices are both easier to grow and more compatible with epitaxial wafer foundries. We successfully demonstrated acceptable spin polarization and quantum efficiency from InAlGaAs/AlGaAs photocathodes and further improved performance by incorporating a digital alloy scheme. These photocathodes may be used in a range of materials characterization techniques which make use of incident electrons and as spin-polarized electron sources in high energy physics and nuclear physics experiments.