UC Santa Barbara

This dissertation explores how modifications to and within a parent zincblende lattice, chemical composition and reduced dimensionality, can be used to engineer materials with functionality beyond that of conventional III-V semiconductors.

The first part explores the use of dimensionality to control the electronic properties. Here we examine the growth mechanisms and properties of semimetallic rare earth monopnictide (RE-V) nanostructures embedded coherently within a semiconducting III-V matrix. While previous work has focused primarily on the growth of embedded RE-V nanoparticles (e.g. ErAs or ErSb) for applications in thermoelectrics, here we show that by increasing the Er composition during simultaneous growth with GaSb, a wide range of new nanostructures form including nanorods, branched nanotrees, and lamellar nanosheets. These ErSb nanostructures form simultaneously with the GaSb matrix, and by combining molecular beam epitaxy (MBE) with in-situ scanning tunneling microscopy (STM), we image the growth surfaces one atomic layer at a time and show that the nanostructured composites form via a surface-mediate self assembly mechanisms that is controlled entirely at the growth front and is not a product of bulk segregation. Using angle resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS), we measure the momentum (k-) and spatially- resolved electronic structure of the embedded RE-V nanostructures and show that despite the predictions of simple quantum confinement models, they remain semimetallic down to their smallest dimensions. These nanostructured composites show great promise for applications in thermoelectrics, tunnel diodes, and fully epitaxial polarization filters.

The second part focuses on Half Heusler alloys, which are a ternary analogue to the zincblende III-Vs. The Full and Half Heusler alloys are an attractive family of multifunctional materials with tunable electronic and magnetic properties. These include both semiconducting and metallic behavior as well as magnetism, half metallic ferromagnetism, superconductivity, topological insulator behavior, and the shape memory effect. In this dissertation we demonstrate the MBE growth of NiTiSn and CoTiSb, two prototypical semiconducting Half Heusler compounds. The films are epitaxial, single crystalline, and show semiconducting-like transport properties with higher electron mobilities and lower electron densities than their bulk counterparts. We also show that both CoTiSb(001) and NiTiSn(001) exhibit a number of different surface reconstructions with phase behavior similar to that of III-V semiconductors, and we propose atomic models for these reconstructions. Using angle resolved photoemission spectroscopy (ARPES) we report the first direct measurement of the surface and bulk electronic bandstructure of semiconducting Half Heuslers and show that these compounds are semiconductors with a bulk bandgap, but with metallic surface states within the gap. These studies lay the groundwork for future studies on all-Heusler heterostructures.