This thesis focuses on the growth and characterization of compounds, devices, and structures for applications in Quantum computing. Quantum computing leverages the principles of quantum mechanics to process information, promising exponential speedup for certain problems. Here, we are interested in two branches - Superconducting and Topological Quantum Computing. Superconducting quantum computing uses circuits cooled to near absolute zero to create and manipulate qubits with high fidelity. Topological quantum computing, on the other hand, exploits the properties of quasiparticles called anyons to form qubits that are inherently resistant to errors, offering the potential for more stable and scalable quantum systems.

Motivated by recent advancements highlighting Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have chosen Tantalum as the superconductor. Here, we have explored the possibility of growth of Ta/Ta2O5/Ta Josephson Junctions for transmon qubits. The growth of such junctions was done at low temperatures using the LT-MBE.

Topological insulators can be used in topological quantum computing by exploiting their edge states, which are protected by time-reversal symmetry and are predicted to be robust against local perturbations. This protection reduces the error rates in quantum computations, enhancing the stability and coherence of qubits. Here, we demonstrate the growth of α-Sn on InSb(001) in ultrathin limit using tensile strain which is a potential topological insulator. The films were characterized electrically in a cryogenic setup using a Hall bar geometry. Fabrication of the hall bars using low-temperature processing techniques has also been demonstrated which ensures there is no phase transition from α to β-Sn.

This dissertation explores epitaxial growth and modifications to the electronic band structure of topological semimetal materials through heteroepitaxy, biaxial strain, and reduced dimensionality. High-quality thin films are grown via molecular beam epitaxy (MBE) and studied using a combination of angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and low-temperature magnetotransport.

Recent predictions of topological phases and observations of extremely large magnetoresistance in the class of rare-earth monopnictides, and specifically GdSb, have opened up a new research front aimed at studying the interplay between magnetoresistance, topology, and magnetic ordering. The first part of the dissertation focuses on magnetoresistance and band topology evolution in lattice-matched and biaxially strained GdSb (001) thin films. Lattice-matched GdSb films show a mobility and carrier concentration imbalance, deviating from the commonly assumed compensated charge carrier densities seen in bulk rare-earth monopnictides. Next, we established a clear connection between biaxial strain in GdSb films and the affected band dispersions based on their orbital composition. As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases.

The second part of this dissertation reports the first ARPES investigation conclusively assigning the topological character of bismuth as a trivial ℤ2 state by studying ultrathin Bi (111) films grown on InSb (111)B. Bismuth films hold promise for potential applications in spintronic devices and topological one-dimensional edge transport. Yet synthesizing high-quality, wafer-scale ultrathin bismuth films on non-metallic substrates remains challenging. We achieved large-area Bi (111) films with a single epitaxial domain orientation and mapped the dispersion of surface states and quantum well states. Strong film-substrate interactions were found to promote epitaxial stabilization of the (111) orientation and lead to inversion symmetry breaking. Our results demonstrate that interfacial bonds prevent the semimetal-to-semiconductor transition predicted for freestanding bismuth layers, highlighting the importance of controlled functionalization and surface passivation in two-dimensional materials.

Finally, the growth parameters of biaxially strained LuPtBi films on InSb were explored. The half-Heusler compound LuPtBi belongs to a unique group of superconductors with a topologically nontrivial band structure, and very low carrier densities (<10^20 cm^-3). A suitable growth window could not be identified for (111) oriented LuPtBi. Room-temperature nucleation of LuPtBi on LuPtSb/InSb (001) allowed higher incorporation of Bi in LuPtBi and produced smooth, coherently strained films.

Surfaces and interfaces are responsible for many of the properties observed in electronic devices. However, characterizing the properties and effects of these interfaces remains one of the most challenging problems in materials science. This is due to both the limited number of characterization techniques that are sensitive to small interfacial regions, as well as the problems associated with surface contamination when exposing a clean surface to atmosphere. In this thesis these problems are overcome through the use of in-situ growth and characterization. Taking advantage of the layer by layer interfacial control achievable with molecular beam epitaxy (MBE) highly precise surfaces and interfaces are created. In-situ surface characterization via x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) is then used to characterize the grown structures. Three different materials are examined, Al superconducting coplanar waveguides (SCW), metal to insulator transitions in rare earth nickelates, and self-assembled ErSb nanostructures. Collectively, these material systems demonstrate the importance of contaminants at interfaces, interfacial control of material properties, and the effect of surfaces on material growth.

Two level states are thought to be one of the primary loss mechanisms for qubits and resonators fabricated from SCW. In-situ XPS is used to analyze the physical changes that occur at the surface during cleaning procedures used to improve qubit coherence times. Possible sources of TLS are identified, and include the presence of hydrogen and carbon on the initial sapphire substrate. Requirements for creating a similar TLS free surface when transitioning to SCW fabrication using silicon substrates is also discussed.

Materials in the RNiO3 (R = a trivalent rare earth ion) family exhibit a metal to insulator transition(MIT) as a function of temperature and film thickness. Strain is shown to clearly suppress or enhance the MIT observed in PrNiO3 thin films. A separate MIT transition is observed as a function of LaNiO3 film thickness and characterized by in-situ STM and XPS. A clear gap is observed in the density of states for thin LaNiO3 films providing insight into the nature of the MIT and suggesting a source other than disorder. LaAlO3/LaNiO3 superlattices are characterized and show that the MIT and transport properties of LaNiO3 may be tunable via superlattice structure.

ErxGa1-xSb is a model system for examining self-assembled nanostructures producing nanoparticles, out-of-plane nanowires, in-plane nanowires and nanosheets. The morphology of the growth surface is characterized via in-situ STM. The growth mechanism responsible for the transition from out-of-plane to in-plane nanowire growth is identified as the formation of large macrosteps with step heights of ~7nm flowing across the surface and a surface energy model is developed to predict this behavior. A novel nanoparticle consisting of extremely thin in-plane nanowires was identified for low temperature, high Sb pressure growths and may prove beneficial to a number of technological applications.

Commercially viable spintronic devices require magnetic contacts with high electrical conductivity, high spin polarization, low Gilbert damping, and perpendicular magnetic anisotropy. The contact must also be amenable to thin film growth techniques to allow device scalability. Until now, this combination of properties had yet to be obtained in a single material. The exquisite control over crystal growth conditions and elemental composition imparted by molecular beam epitaxy can be leveraged to tune magnetic and electronic material properties closer to the ideal set desired by device researchers.

Ferromagnetic metals composed of elements with low atomic weight are commonly used for spintronics, but the industry standard CoFeB does not possess high spin polarization, and its perpendicular magnetic anisotropy depends on film thickness, limiting its versatility. On the other hand, Heusler compounds are a class of over 1000 ternary intermetallic materials with highly variable magnetic and electronic properties. The Heusler compound Co2MnSi is well known as a half-metal with 100% spin polarization at the Fermi level, making it an ideal source of spin-polarized current. However, Co2MnSi does not possess perpendicular magnetic anisotropy.

In this work, the magnetic anisotropy of Heusler compounds is engineered by breaking their cubic crystal symmetry. This can be accomplished by growing tetragonal crystal structures with the unique axis aligned out-of-plane, or by engineering superlattices composed of alternating layers of dissimilar Heusler compounds. In both cases, the resulting perpendicular magnetic anisotropy does not depend on film thickness, making the materials attractive for a broad range of spintronic device applications. Additionally, the Heusler compound superlattices studied here are composed of Co2MnAl and Fe2MnAl, which combine their electronic structures to produce 95% spin polarization as measured by spin-resolved photoemission spectroscopy. This combines two important magnetic properties never before seen in a single material system. The growth, structural, electronic and magnetic properties of the engineered films will be presented.

Finally, Co2TiGe is explored as a candidate of an exotic class of topological materials known as Weyl semimetals. These systems possess a unique band structure that arises due to broken time-reversal symmetry resulting from the internal magnetization. Electrons with energy and momentum near so-called Weyl points have zero effective mass and a discrete chiral charge, making them analogous to the elusive Weyl fermion. The signatures of Weyl semimetallicity in Co2TiGe are probed using magnetotransport and synchrotron-based angle-resolved photoemission spectroscopy.

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.