This dissertation focuses on the growth and characterization of compounds, devices, and heterostructures with large lattice constant III-V semiconductors (SMs) with high spin orbit coupling. Spin orbit coupling originates from the interaction of the orbital angular momentum and the spin angular momentum of an electron and is a key component in spintronic devices and devices for studying Majorana zero mode physics. The ultraclean environment and precise growth controls of molecular beam epitaxy were used to grow a combination of III-V materials exhibiting high spin-orbit coupling, superconductor-semiconductor devices, and ferromagnet-semiconductor heterostructures.

Rashba spin-orbit interaction is responsible for splitting the degeneracy of spin-up and spin-down electrons. The broken degeneracy is useful for spin generation and detection or creating topological insulator-like devices by applying a magnetic field. Semiconductors exhibiting high spin-orbit coupling can be gated to control both the carrier density and the strength of the Rashba interaction. Here, we use InSb/GaAs (001) to demonstrate a simple and inexpensive heterostructure, which exhibits spin-orbit coupling.

InAs quantum wells (QWs) grown on InP (001) also exhibit large Rashba spin-orbit interaction, but have demonstrated mobilities of µ>1,000,000 cm^2/Vs. In this work, we demonstrate the effects of strain compensation on QW mobility by varying both the strain in the cladding layer and the thickness of the InAs QW. We observe a maximum mobility of µ=1,160,000 cm^2/Vs, which suggests that strain compensation in near surface QWs may improve mobility while enabling proximitized superconductivity.

Until recently, Al was the superconductor of choice for superconductor-semiconductor (SuperSemi) devices. Because of the epitaxial growth mode of Al on InAs and InSb nanowires, the Al/InAs and Al/InSb systems were primarily utilized for SuperSemi devices intended to study Majorana physics. Here we demonstrate that like Al, β-Sn exhibits a hard gap, large critical field, and 2e-charging, which are all requirements for Majorana devices.

Eliciting the Majorana zero mode in a SuperSemi structure currently requires the application of a large, global magnetic field. New proposals have suggested that the large global magnetic field can be replaced with either local micromagnets or ferromagnetic (FM) contacts. However, interfacial reactions between elemental FMs and III-V’s are well known to occur. By using an antimonide-based FM, MnSb, we demonstrate a FM that has tunable magnetic properties and is compatible with III-V heterostructures.

Spintronic devices, where information is carried by the quantum spin state

of the electron instead of purely its charge, have gained considerable interest

for their use in future computing technologies. For optimal performance, a pure

spin current, where all electrons have aligned spins, must b e generated and

transmitted across many interfaces and through many types of materials. While

conventional spin sources have historically been elemental ferromagnets, like Fe

or Co, these materials pro duce only partially spin polarized currents. To increase

the spin polarization of the current, materials like half-metallic ferromagnets,

where there is a gap in the minority spin density of states around the Fermi

level, or topological insulators, where the current transport is dominated by

spin-locked surface states, show promise. A class of materials called Heusler

compounds, with electronic structures that range from normal metals, to half metallic ferromagnets, semiconductors, sup erconductors and even top ological

insulators, interfaces well with existing device technologies, and through the

use of molecular beam epitaxy (MBE) high quality heterostructures and films

can b e grown. This dissertation examines the electronic structure of surfaces and

interfaces of both top ological insulator (PtLuSb– and PtLuBi–) and half-metallic

ferromagnet (Co2MnSi– and Co2FeSi–) III-V semiconductor heterostructures.

PtLuSb and PtLuBi growth by MBE was demonstrated on AlxIn1−xSb (001)

ternaries. PtLuSb (001) surfaces were observed to reconstruct with either (1x3)

or c(2x2) unit cells depending on Sb overpressure and substrate temperature.

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The electronic structure of these films was studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission sp ectroscopy. STS measurements as well as angle resolved photoemission spectropscopy (ARPES) suggest that PtLuSb has a zero-gap or semimetallic band structure. Additionally,

the observation of linearly dispersing surface states, with an approximate crossing point 240meV above the Fermi level, suggests that PtLuSb (001) films are

topologically non-trivial. PtLuBi films also display a Fermi level position approximately 500meV below the valence band maximum.

Co2MnSi and Co2FeSi were also grown by MBE on GaAs (001) for use as

spin injectors into GaAs lateral spin valve devices. By the growth of the quaternary alloy Co 2FexMn1−xSi and varying x, electron doping of the full Heusler

compound was demonstrated by observation of a crossover from a majority spin

polarization of Co 2MnSi to a minority spin polarization in Co2FeSi. Co2MnSi

films were studied as a function of the nucleation sequence, using either Co–

or MnSi– initiated films on c(4x4) GaAs. Studies using x-ray photoemission

spectroscopy (XPS), STM/STS, and transmission electron microscopy (TEM)

suggest that the bulk of the Co2MnSi films and the interfacial structure between

Co2MnSi and GaAs is not modified by the nucleation sequence, but a change

in spin transport characteristics suggests a modification of semiconductor band

structure at the Co2MnSi/GaAs interface due to diffusion of Mn leading to compensation of the Schottky barrier contact. Diffusion of Mn into the GaAs was

confirmed by secondary ion mass spectrometry (SIMS) measurements. The proposed mechanism for the modified spin transport characteristics for MnSi initiated films is that additional diffusion of Mn into the GaAs, widens the Schottky

barrier contact region. These studies suggest that the ideal initiation sequence

for Co2MnSi/GaAs (001) lateral spin valve devices is achieved by deposition of

Co first.

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.

The invention of integrated circuits (ICs) in the 1960s and the subsequent microelectronics revolution fundamentally altered every aspect of modern technology, ranging from communication to computation and healthcare. Through the last few decades, continued miniaturization of such ICs has led to rapid improvements of device functionalities as well as exponentially reducing cost, thus making these technologies more accessible to everyone. This aggressive scaling has however, pushed device features towards sub-100 nanometer regimes, challenging conventional fabrication techniques. Semiconductor nanostructures can circumvent these challenges and enable further scaling for advanced logic and memory devices. Additionally, such nanostructures exhibit electronic and optical properties, that are of tremendous interest for alternative computing architectures such as quantum computing and photonic circuits, or to build power efficient and ultrafast platforms through low-energy and spin-based devices. Fabricating such nanostructures is not trivial but can be achieved through both “top-down” and “bottom-up” approaches. Top-down approaches, which are commonly followed in the semiconductor industry typically start with a bulk material and fabricate nanostructures through various etching steps. Although this technique is scalable and has been extremely successful in building modern ICs, the damage induced from etching can prove detrimental to the performance of low-dimensional systems. Bottom-up techniques generally refer to growing the nanostructures in an additive method in pre-defined positions on a wafer. Bottom-up approaches have the inherent advantage of exhibiting less defects, due to the elimination of etching steps and can be combined with in-situ patterning to build novel hybrid devices. However, these techniques are simultaneously more challenging to integrate and scale up reliably. For optimal performance of such nanostructures, it is critical to have precise control over their chemical composition, geometries, and material qualities. In this thesis, we will explore both bottom-up and top-down approaches, to achieve such defect-free scalable nanostructures in the context of low-power electronics and quantum computing. For bottom-up approaches, we investigate two templated epitaxial growth techniques: confined epitaxial lateral overgrowth (CELO) to grow III-V lateral heterostructures and selective area growth (SAG) to grow in-plane III-V nanowires coupled to superconductors. We first discuss the inherent advantages of CELO to fabricate low-power tunneling devices. Next, we explore the use of growth conditions to reduce defects, engineer facets and improve the material qualities and morphologies in CELO nanostructures. Using this, we demonstrate high-quality lateral III-V quantum wells for heterojunction tunnel transistors. For in-plane selective area growths, we investigate the effect of fundamental parameters such as growth temperature, cooldown processes and nanowire orientation and geometries on the nucleation of highly lattice mismatched heterostructures (indium arsenide on indium phosphide). These nanowires can lead to the fabrication of scalable systems with enhanced electrical and optical properties. We also develop in-situ shadowing techniques to create patterned heterostructures of dissimilar materials with pristine disorder-free interfaces. Subsequently, we use this to demonstrate superconductor-semiconductor hybrid nanowire networks for probing low-temperature effects in topologically non-trivial systems. In the last part of this dissertation, we will shift our focus to top-down techniques for defining aluminum/silicon/aluminum trilayer nanostructures with extreme aspect ratios. Such nanostructures can reduce footprint and enable scaling up of Josephson-junction based superconducting qubit systems. In conclusion, using both bottom-up and top-down techniques we combine advanced fabrication and epitaxial growth to achieve defect-free III-V scalable nanostructures with disorder-free interfaces. In the future, these nanostructures will enable the scalable fabrication of next-generation efficient computing platforms.

Theoretically proposed by Ettore Majorana in 1937, Majorana fermions are a unique class of particles which are their own anti-particles. This concept is realized in Majorana Zero Modes (MZMs), which are quasi-particles bound to zero energy, with no measurable charge and mass. Arising out of topological states of matter, signatures of MZMs were first experimentally observed in 2012.

These quasi-particles are predicted to exhibit non-abelian braiding statistics, allowing them to “remember” whether they were moved clockwise or counterclockwise around each other, forming a braid in time and position. Being their own anti-particles, fusion or annihilation of a pair of MZMs is expected to lead to a different outcome based on how they were braided, making a pair of MZMs the simplest quantum bit or ‘qubit’, forming the basis of Topological Quantum Computation.

As MZMs are indistinguishable from each other and quantum information is encoded in the exchange of MZMs, it is protected from environmental perturbations (noise), referred to as topological protection. Computation based on these qubits is predicted to be fault tolerant and scalable.

This work focusses on the device heterostructure design, Molecular Beam Epitaxy (MBE) growth and low temperature electrical characterization of superconductor-semiconductor hybrid systems hosting MZMs. Low dimensional electron systems (2D quantum wells, 1D nanowires) in semiconductors with strong spin-orbit interaction (e.g. InAs, InAsxSb1-x and InSb), transparently coupled to a superconductor (e.g. Aluminum), have been investigated.

With an emphasis on improving electron mobility, the first demonstration of an InSb quantum well on InSb substrate, as part of this work, showed a record quantum mobility of 50,000 cm2/Vs. Top and bottom gate control of InSb quantum wells on GaSb substrates has also been demonstrated. MBE of InAs and InAsSb nanowires has also been studied, with demonstration of induced superconductivity in InAsSb nanowires. Recent work focused on MBE of near surface InAsSb quantum wells showing gate-controlled depletion and observation of quantized conductance through a Quantum Point Contact.

Lastly, in-vacuo growth of Aluminum partial shells on InSb nanowires, led to the first observation of quantized Majorana conductance, consistent with predictions. While further proof is necessary, these 1D and 2D hybrid systems, coupled with in-situ selective area evaporation of dielectrics, pave the way for realization of complex topological networks and subsequent demonstration of a fault tolerant topological qubit.

This dissertation explores the growth and the electronic properties of materials with strong spin orbit coupling. Thin films of materials with strong spin-orbit coupling are of interest for their potential applications in spintronics and topological quantum computing. The thin films are grown with molecular beam epitaxy, and their structural and electronic properties are investigated with scanning tunneling microscopy and low temperature electron transport.

The first part of this dissertation will investigate the engineering of the mobility and spin-orbit coupling of high mobility InGaAs quantum wells by digital alloying. The mobility of InGaAs quantum wells is significantly limited by alloy disorder scattering. The effect of digital alloying on the mobility of InGaAs quantum wells is investigated, and digital alloying is found to enable record high electron mobilities for InGaAs quantum wells. This record high mobility is due to a reduction in the alloy disorder scattering. In addition, we demonstrate tuning of the spin-orbit coupling in the InGaAs quantum wells by up to 138 meV\textnormal{\AA} by digital alloying. These results demonstrate the ability of digital alloying to engineer and enhance the properties of ternary quantum wells.

The Fermi-level pinning of various InSb surfaces is studied in the second part of this dissertation. The observed Fermi-level pinning is consistent with a charge neutrality point near the valance band maximum. Sb is found to shift the Fermi-level pinning position towards the conduction band minimum, with the (001) surface becoming under electron accumulation when sufficient Sb is on the surface. In contrast, the (111)B surface is found to be strongly pinned, with only minor changes to the Fermi-level pinning upon Sb deposition. The surface treatment of InSb is found to be important for the Fermi-level pinning position, which can be exploited to tailor the properties of InSb based devices.

The last part of this dissertation focuses on the formation of strain solitons in epitaxial bismuth thin films. Synthesizing ultrathin epitaxial bismuth thin films on nonmetallic substrates has remained a significant challenge. Here, the growth of high quality epitaxial bismuth thin films on an InSb (111)B substrate is demonstrated. In these thin films, the epitaxial strain induced in the bismuth from the substrate is relaxed by strain soliton formation. Strain solitons are topological defects in van der Waals materials that can host novel electronic properties, but have only been observed in exfoliated flakes or freestanding layers on mechanically strained bulk crystals. The first scalable approach towards the generation of strain solitons by epitaxial growth is demonstrated. Evidence of edge state localization within the strain solitons is also observed.

Quantum information science has made significant progress over the last several decades, but the eventual form a quantum computer will take has yet to be determined. Several physical systems have been shown to operate as quantum bits, or qubits, but each faces a central challenge: the qubit must be sufficiently isolated from its environment to maintain quantum coherence while simultaneously having sufficient coupling to the environment to allow quantum mechanical interactions for manipulation and measurement. An approach to achieve these conflicting requirements is to create qubits that are insensitive to small perturbing interactions within their environment by using topological properties of the physical system in which the qubits are formed. This dissertation presents studies on low-dimensional semiconductor heterostructures of InAs, GaSb and AlSb fabricated by molecular beam epitaxy with focus on relevant properties for their utilization in forming a topologically protected (TP) qubit.

The theoretical basis regarding the semiconductor characteristics suitable for realizing TP qubits stipulates the need for strong spin-orbit coupled semiconductors with high carrier mobility. A comparative study of InAs/AlSb heterostructures wherein structure parameters were systematically varied led to a greater understanding of the limits to mobility in InAs quantum wells. Magnetotransport measurements using a dual-gated device geometry and a comparison of experiment to models of carrier mobility as a function of carrier density were used to identify dominant scattering mechanisms in these heterostructures.

The development of dual-gated devices and high quality InAs channels with AlSb barriers led to a demonstration of the gate control of spin-orbit coupling in a high mobility InAs/AlSb quantum well in which the gate-tuned electron mobility exceeded 700,000 cm$^{2}$/V${\cdot}$s. Analysis of low temperature magnetoresistance oscillations indicated the zero field spin-splitting could be tuned via the Rashba effect while keeping the two-dimensional electron gas charge density constant.

Findings from the work on InAs quantum wells were applied to investigations on InAs/GaSb bilayers, a system predicted to be a two-dimensional topological insulator (TI). The temperature and magnetic field dependence of the resistance in dual-gated InAs/GaSb heterostructures gate-tuned to the predicted TI regime were found consistent with conduction through a disordered two-fluid system. The impact of disorder on the formation of topologically protected edge states and an insulating bulk was considered. Potential fluctuations in the band structure for realistic levels of disorder in state-of-the-art heterostructures were calculated using a gated heterostructure model. Potential fluctuations were estimated to be sufficiently large such that conduction in the predicted TI regime was likely dominated by tunneling between localized electron and hole charge fluctuations, corresponding to a symplectic metallic phase rather than a topological insulator. The implications are that future efforts must address defects and disorder in this system if the TI regime is to be achieved.

Organic semiconductors tend to have highly ordered, anisotropic morphologies. These structural anisotropies lead to optical anisotropies which we have shown directly affect the efficiency of photonic devices such as organic photovoltaics (OPVs). Measuring the direction-dependent optical properties of organic films would thus give unique insights into film structure and inform the design of organic optoelectronics. In this work we develop momentum-resolved spectroscopies to precisely measure the emitting and absorbing dipole strengths as well as complex refractive indices of uniaxial organic films. We then demonstrate how these techniques can be used to investigate film morphology by measuring the tilt angle and processing-dependent structural changes in the polymer system P(NDI2OD-T2). Finally, we show how molecular orientation impacts device properties by studying changes in surface plasmon dispersion and photoluminescence enhancement in films of the small molecule p-SIDT(FBTTh2)2 on gold.

Heusler compounds are an exciting class of intermetallics due to their diverse electrical and magnetic properties, including semiconducting, half metallic, and topologically insulating behaviors. With a crystal structure and lattice parameters similar to III-V compound semiconductors, the possibility of Heusler/III-V semiconductor as well as Heusler/Heusler heterostructures with unique properties is achievable. However, the integration of epitaxial Heusler compounds into functional devices still faces a number of challenges. The tunability of the electronic and magnetic properties of Heusler compounds must first be better understood. In addition, Heusler-based device demonstrations have been limited to magnetic Heuslers, leaving the semiconducting and topologically non-trivial variants relatively unexplored. Finally, most experimental studies of the semiconducting half-Heusler compounds have been limited to bulk polycrystalline samples, which cannot be easily used for device applications.

Here, the semiconducting half-Heusler, CoTiSb is grown by molecular beam epitaxy and used to explore the both the tunability of Heusler compound properties as well as the interface properties of Heusler alloys with III-V and other Heusler compounds. A semiconductor to metal transition is examined by substitutionally alloying Ni for Co. The evolution of the electronic structure in the alloy Co1-xNixTiSb is examined by electrical transport, Seebeck measurements, and angle-resolved photoemission spectroscopy. A gradual transition from semiconducting to metallic behavior is observed for films with x≥0.1. The effects of Ni alloying as well as the surface reconstruction on the valence band, conduction band, and Fermi level positions are examined.

Next, the introduction of ferromagnetism into CoTiSb is achieved by substitutionally alloying Fe on the Ti site, a predicted half-metallic compound for intermediate levels of alloying. The magnetic, electronic, and nano-structural properties of the epitaxial thin films are shown to depend strongly on Fe composition. In particular, a large dependence of the magnetic moment on the site which Fe atoms occupy is found. In addition, evidence of nano-level phase separation is observed and shows a clear compositional dependence.

Finally, the band alignments between CoTiSb and the III-V compounds InAlAs and InGaAs as well as another semiconducting half-Heusler, NiTiSn, are explored using a combination of X-ray photoemission spectroscopy, density functional theory (DFT), and electrical transport. Here good agreement is found for the valence band offsets between the three techniques. However, a discrepancy between the conduction band offsets predicted by DFT and those inferred from electrical transport is found. An effective reduction in the CoTiSb bandgap is used to explain this discrepancy.

This work ultimately advances the current understanding of the tunability of Heusler compounds and lays the foundation for future Heusler based heterostructure devices.

In the past several decades, nitride-based semiconductors have impacted everyday life in sectors such as energy efficient lighting and high-resolution display technology. While the (Al, In, Ga)N alloys are the most heavily utilized nitride-based semiconductors, hexagonal boron nitride (hBN) is essential to the two-dimensional (2D) material system, mainly due to its ability to passivate 2D materials better than conventional dielectric materials like SiOx.1 hBN plays an instrumental role in 2D material and device research, making the maturation of scalable, thin-film hBN growth techniques critical to the field.

This thesis presents plasma-enhanced growth techniques for next-generation nitride thin-films. Nucleation and growth processes were examined in-vacuo and ex-situ for a greater understanding of these synthesis techniques. A high-temperature 1450-1500°C, plasma-enhanced chemical beam epitaxy (PE-CBE) process was utilized to grow hBN on silicon carbide (SiC) substrates. Film morphology and epitaxial alignment were examined via in-situ reflection high-energy electron diffraction (RHEED), as well as ex-situ atomic force microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). Spectroscopic techniques, such as in-vacuo X-ray photoelectron spectroscopy (XPS) and ex-situ energy-dispersive X-ray spectroscopy (EDS), provided information on stoichiometry and interface bonding. Characterization of hBN nuclei showed that a 30° metastable rotational alignment between hBN and a graphinated SiC surface could be stabilized by tuning substrate morphology and growth parameters, providing a route to grow 2D heterostructures with incommensurate alignments without relying on manual rotation of the layers. These ''twisted” hBN/graphene structures could be utilized in future twistronic devices for their exotic physical properties.2

Low-temperature (<400°C) atomic layer epitaxy (ALE) was explored in collaboration with the U.S. Naval Research Lab as a means to produce novel heterostructures. These depositions utilized in-vacuo RHEED and XPS to greater understand the ALE mechanism and the required properties of the substrate's starting surface. To facilitate these depositions, low-temperature cleaning processes for bulk GaN substrates were developed and optimized for subsequent ALE regrowth. A unique, crystalline superconductor-semiconductor-superconductor Josephson junction (JJ) for future applications in quantum computation was chosen as a test structure for low-temperature ALE. The ALE AlN barriers were deposited on Al epitaxially grown at cryogenic temperatures on GaAs(001) substrates. Cross-sectional TEM analysis showed the low temperature ALE allowed for the Al to remain epitaxial, resulting in improved junction crystallinity. These crystalline JJs may provide enhanced tunneling performance over junctions with amorphous barrier dielectrics.

1 C.R. Dean, et al., Nat. Nanotechnol. 5, 722 (2010).

2 Y. Cao, et al., Nature. 556, 43-50 (2018).