The semiconductor industry is fast approaching the fundamental scaling limit of conventional complementary metal-oxide-semiconductor (CMOS) devices. This has spurred numerous exploratory researches in the last decade, on technologies that can sustain Moore's law and eventually replace CMOS based devices. One such magnonics based, beyond CMOS device is called the spin wave device. A spin wave device is a low power magnetic logic device that uses a propagating wave of precessional spins, called a spin wave, to perform computations. Traditionally, spin waves have been generated using energetically inefficient, inductively coupled antennas. In order to attain atto-joule energy per operation, which is substantially lower than that of the current generation of CMOS devices, we propose and demonstrate a novel method of spin wave generation using the strain mediated magneto electric effect. We have conducted extensive experimental and numerical studies to design, engineer and fabricate such nano scale spin wave devices. These devices could in effect outperform and eventually replace the CMOS based logic devices.

Spin waves are collective excitations in magnetically ordered materials. Early spin-wave devices find applications in microwave technologies. In more recent years, spin waves have been widely considered as an information carrier in the field of magnonics for energy-efficient applications. Spin waves in insulators are free from Ohmic losses but still suffer from magnetic damping. When it comes to thin films, even the ferrimagnetic insulator, yttrium iron garnet (YIG), with extremely low magnetic damping is far from ideal for practical device applications. The spin-wave damping, however, can be compensated with spin-orbit torque (SOT) sourcing from a high spin-orbit coupling (SOC) material in adjacent to the magnetic material. We demonstrate spin-wave amplification at the edge of spin-wave passband with a topological insulator (TI) as the SOC material. We also show that by using alternating Pt and Ta bars as the SOC materials to generate a SOT wave and matching it in phase with the spin waves, the spin waves can be efficiently amplified at a specific wavelength in the main spin-wave passband. On the other hand, the realization of low dissipative spin superfluid transport in a non-local spin transport configuration has been proposed theoretically, with some recent experimental signatures demonstrated. We show our non-local experimental results based on ferrimagnetic insulator YIG and antiferromagnetic insulator bismuth ferrite (BFO). Electrical control of spin superfluid transport in a magnetic Josephson junction is proposed and investigated numerically for practical spin superfluid device applications.

Spintronics, or spin electronics, utilizes the spin property of electrons alongside their charge to enhance data processing and storage capabilities. This field has transformed from early magnetic navigation tools to foundational technologies in modern storage solutions like Hard Disk Drives and Magnetic Random Access Memory (MRAM). Furthermore, the development of Magnetoelectric Spin-Orbit (MESO) devices extends the utility of spintronics into logic operations, leveraging high-efficiency processes that could drastically reduce power consumption in future computing systems. MRAM is recognized as a promising non-volatile storage solution for mobile devices and microcontrollers, possibly replacing traditional memory types like last-level cache due to its speed and density. However, conventional Spin-Transfer-Torque (STT) MRAM is hindered by slow and energy-intensive writing operations. To overcome these limitations, we propose using Voltage-Controlled Magnetic Anisotropy (VCMA) for writing. Our research, through a collaboration between foundry and lab, has successfully integrated VCMA-based MRAM with CMOS technology, demonstrating ultra-fast switching times of 0.7 ns with a 1.8V write voltage and achieving read times of 8.5 ns with endurance exceeding 1011 cycles, validating VCMA-MRAM as a viable non-volatile memory option. Further, our study advances VCMA-based True Random Number Generators (TRNGs), crucial for cryptography. We have developed TRNGs that operate without the traditionally required external in-plane magnetic field by applying a 1.7 V, 5 ns pulse. This method meets the National Institute of Standards and Technology (NIST) standards for randomness and introduces a novel mechanism that allows for faster relaxation times compared to superparamagnetic systems, paving the way for TRNGs that are fast, compact, energy-efficient, and highly reliable. Additionally, MESO devices face challenges with spin-to-charge conversion efficiency. Topological insulators (TIs), with their unique topology-protected surface states, present a promising solution to enhance this efficiency. Our study identified that a Ru layer insertion between the TI and the magnet can boost conversion efficiency by 60%, highlighting the significant role of interface engineering. Additionally, we proposed and validated a physical model for the conversion between spin and charge in topological insulators, which can guide the further enhancement of efficiency. This research establishes a solid foundation for advancing spin-to-charge conversion technologies, opening new possibilities for their integration into future electronic devices.

Quantum anomalous Hall effect (QAHE) is the zero-field version of the quantum Hall effect which shows dissipationless chiral edge electron transport and quantized Hall resistance without the help of external magnetic field. In this Dissertation, we study the growth of magnetic topological insulator (MTI) Cr-doped (BiSb)2Te3 thin film, which is the host of the QAH states, using molecular beam epitaxy. We report the first observation of QAHE in a milli-meter sized device with the thin film thickness beyond the two-dimension (2D) limit. In addition, by controlling the thin film thickness down to the 2D limit, we observe a metal-to-insulator transition behavior due to the hybridization effect introduced by the vertical quantum confinement.

We further investigate the temperature limiting factors of QAHE in the Cr-doped (BiSb)2Te3 system by studying the thickness and doping effects. We identify that for thin film thickness larger than 6 quintuple layers (QLs), the limiting factor is the overlap of Fermi level with the trivial bulk band. For thin film thickness equal to 6 QLs, weak ferromagnetism and pronounced superparamagnetism are the temperature limiting factors. However, modulation doping technique can significantly strengthen the ferromagnetism at this thickness. In addition, the metal-to-insulator transition behavior can also be tuned according to the doping profile of the thin film which can potentially affect the phase transition behavior in this material system.

To manipulate the QAH states, we investigate the heterogenous integration of the QAH insulator or TI with antiferromagnets. We first study the QAH insulator/Cr2O3 bilayer structure and realize the QAHE on a magnetically ordered system for the first time. We identify the positive exchange bias in this structure and discover the magnetic coupling between the two material systems using polarized neutron reflectometry (PNR). We then study the undoped TI/CrSe heterostructure and realize the antiferromagnet induced magnetic proximity effect in the TI layer confirmed by both PNR and X-ray magnetic dichroism techniques. We also discover the interface-dependent proximity behavior in the heterostructure that may help our understanding towards high temperature and functional QAH states.

Since the dawn of Big Data, the exponentially increasing demands for larger data volumes and higher information processing speeds have given the field of spintronics an astonishing momentum. In spintronics, the electron spins and their associated magnetic moments interplay with electronic charges, lattices, and even photons. These diverse interactions open endless possibilities for a new generation of fast, efficient, and non-volatile memory and logic devices to serve and fuel Big Data. Lying at the heart of innovating spintronic memory and logic devices is the search for advanced materials and mechanisms to control spin and magnetism.Following this line of research, this dissertation focuses on exploring two emerging material classes, namely, topological insulators and ferrimagnets, which hold great promise for efficient and fast magnetization manipulation. More specifically, topological insulators exhibit an extraordinary charge-spin conversion efficiency owing to their exotic surface states and can be employed to manipulate magnetic moments with minimal energy. Ferrimagnets, by contrast, are of technical interest for fast magnetization manipulation because their two non-equivalent and antiparallel aligned sublattices uniquely combine the antiferromagnet-like ultrafast dynamics with the ferromagnet-like readability/controllability for well-established techniques. However, these novel materials have been difficult to investigate using conventional magnetometers or magnetic resonance techniques. To address these challenges, an experimental platform integrating a magneto-optical Kerr effect magnetometer, a femtosecond optical pump-probe technique, and common magneto-transport measurements, was first established. Using this experimental platform, the charge-spin conversion efficiency was investigated and accurately quantified for a topological insulator-based magnetic bilayer, and a critical role of the topological surface states with spin-momentum locking was unveiled. With innovative material engineering, topological insulators were integrated with widely used metallic ferromagnet in a topological insulator/Mo/CoFeB/MgO structure. This topological insulator/Mo/CoFeB/MgO structure demonstrates high thermal stability, robust magnetic properties, and efficient magnetization switching driven by spin-orbit torques. The systematically calibrated efficiency confirms that, for a room temperature magnetic memory, topological insulators are at least one order of magnitude more efficient than conventional heavy metals. Moreover, the annealing effects were carefully studied in this structure, and desirable thermal compatibility with modern CMOS technology has also been achieved, empowering the development of advanced spintronic applications. To realize faster control of magnetic moments, the dynamical characteristics of a compensated ferrimagnetic GdFeCo film with a vertical compositional gradient were investigated through the laser-induced ultrafast spin dynamics. It is found that the vertical composition gradient significantly alters the ultrafast spin dynamics. Surprisingly, these distinct spin dynamics can be handily controlled by tuning the power of laser excitation, indicating the existence of more efficient energy pathways to control magnetization with high speed. These observations motivate ferrimagnets with a composition gradient as an ideal candidate for efficient and fast magnetization manipulation. Emboldened by the findings in this dissertation, topological insulators and ferrimagnets undoubtedly possess a vast potential in increasing the efficiency and speed of magnetization manipulation for advancing spintronic memory and logic devices.

Electrical detection of spin transport in a semiconductor (SC) channel is one of the key requirements to realize spintronics devices. Among various SC materials, the high-mobility two-dimensional electron gas (2DEG) confined in a modulation doped quantum well structure (MODQW) is of particular interest for device applications. This is because the high mobility promises for a long spin diffusion length of coherent transport as well as large spin signal for easy sensing. Meanwhile, the effective spin manipulation is achievable either by enhanced Rashba spin-orbit interaction from an asymmetric E-field structure, or by direct control of discrete density of states (DOS) within the quantum well structure. Despite of these merits, very few studies of direct electrical spin injection into 2DEG have been reported so far, mainly because of the difficulty in making reliable ferromagnetic (FM) contacts to the buried 2DEG channel. In literature, only a few reports in Si/SiO2 and III-V matrices are available up to now; however, electrical detection of spin transport in the high-mobility 2DEG in a Si/SiGe MODQW has not been reported. To make continuous progress of Si-based spintronics and to take full advantage of current CMOS technology, there is an urgent need to develop Si-based spintronics devices.

In this thesis we present two related projects: first is spin injection in Ge; second is spin injection in Si two-dimensional electron gas (2DEG) system. It is the knowledge built up from the Ge project helps us successfully demonstrate electrical spin injection in Si 2DEG in a Si/SiGe MODQW using FM Mn-germanosilicide (Mn(Si0.7Ge0.3)x) end contacts, which is a new approach to circumvent the difficulty of etching process adopted for the typical spin valve devices. The experiments show that the spin-polarized electrons could be laterally injected into one side of the 2DEG confined at the Si/SiGe interface, and subsequently detected from the other side by the magnetoresistance (MR) of a FM/2DEG/FM spin valve. Most important of all, symmetric resistance steps were clearly observed from a series of FM/2DEG/FM spin valve devices with different channel lengths (Lch = 1.5~3.5 μm), by which the spin diffusion length and spin lifetime are calculated to be 4.5 um and 16 ns at 1.9 K, respectively.

Over 50 years, Moore’s law has successfully predicted the progress of the silicon electronics industry. However, Moore’s law is approaching to the end recently, and new material and novel device type will be needed for next-generation devices. Two-dimensional (2D) layered material is one of the promising candidates due to its intrinsic desirable features, such as diverse electronic and magnetic properties, carrier mobility protection with decreasing body thickness, and flexibility for wearable applications. Furthermore, hetero-structure comprising of 2D crystals is of growing interest since various combinations are possible for multiple purposes as more and more van der Waals materials have been discovered. In a hetero-structure, electrons can propagate not only within 2D in-plane direction but also in the vertical out-of-plane direction. However, our understanding of the vertical carrier transport is greatly less than that on the lateral. Thus, using lateral electron energy band diagrams are still the main vehicle in 2D vertical hetero-structure device analysis, which may not be correct.

In this dissertation, silicon-based tunneling devices were fabricated and used to investigate electron transport properties when electrons go into or go across a 2D materials with the measurement of electron tunneling spectroscopy. We firstly examine the role of 2D sheet when electrons propagate perpendicularly across it. Here graphene is used as a platform since it is the earliest discovered one, and it has the most mature development including understanding and growth techniques. Graphene together with its neighboring van de Waals gaps serves as a tunnel barrier and barely has interaction with the vertically tunneling electrons. However, since graphene can still trap a fraction of carriers, we can take advantage of it and control the carrier flux via the adjustment of graphene electrical potential.

In addition to vertically propagating across a graphene sheet, electrons can go into graphene lateral band structure and transport within graphene. In chapter 3, we introduce a new model of the interfacial oscillation states at graphene-silicon hetero-junction, which is found and confirmed for the first time. Because of the presence of this discrete interfacial quantum state, Fano-Feshbach resonance is induced by its interaction with graphene’s continuum lateral energy diagram. This study provided a further elucidation of the interfacial effect in a low-dimension materials based system.

The capability of our silicon-based tunneling device along with electron tunneling spectroscopy is not limited to the graphene-silicon interface but also able to investigate electron in-plane transport behavior within 2D hetero-structure. Since large-size devices and their macroscopic characteristics would be needed for our everyday applications in the future, the strength of our tunneling device over the conventional scanning tunneling spectroscopy with a sharp tip is its scalable detecting area. Here, a study on graphene/hexagonal boron nitride hetero-stack prepared by chemical vapor deposition and large-area wet transfer techniques shows multiple secondary Dirac points and the preferred relative rotation angle of ~4� and ~7�. The theoretical calculation was also implemented to support our experimental observation. Raman spectroscopy and scanning tunneling microscope were carried out to confirm the Moiré pattern formation. This study provides a useful way to macroscopically conduct research on the electronic behavior of a van der Waals material, and our findings may be used when graphene/hexagonal boron nitride hetero-structure is pushed to practical applications. Undoubtedly, further careful study is needed for more detailed verification.

Topological insulator (TI) is a new class of quantum material with inverted band structure caused by strong spin-orbit coupling (SOC), resulting an insulating bulk and topologically protected surface states. By introducing magnetism to break the time-reversal symmetry, it can host emerging topological quantum phases such as quantum anomalous Hall (QAH) effect, axion insulator quantum phase, and can host chiral Majorana fermion when coupled to superconductors. The goal of this dissertation is to study these quantum phases by material growth and transport measurements.

First, we studied the growth condition by molecular beam epitaxy (MBE) and achieved high quality magnetically doped topological insulator (MTI) film growth that can host QAH effect. To understand the different origins of zero Hall conductance plateau in QAH insulators and axion insulators, we carried out the thickness dependent study on MTI sandwiched structures and observed a dimension crossover between 2D QAH and 3D axion insulators by minor loops measurements and scaling analysis. Then, we investigated the QAH insulator and antiferromagnetic heterostructures which offer an additional degree of freedom to manipulate the QAH state in MTI. By proximity coupling the QAH insulator to antiferromagnetic insulator Al-Cr2O3, we observed an exchange biased QAH effect, and the exchange bias can be effectively controlled by a field training process. In addition, chiral Majorana edge modes are reported to exist in QAH insulator and superconductors heterostructures. Aiming to understand the discrepancy of recent experimental reports, we fabricated QAH insulator with superconductor and normal metals hybrid devices, and we found the details of material characteristics, interface conditions play important roles on the experiment observations. Finally, two collaborative works closely related to the main scope of this dissertation are introduced. To probe the size limit and to understand the inter-channel scattering of QAH effect, we fabricated mesoscopic scale sub-micron size QAH insulator devices and carried out size dependent studies. Enabled by the successful growth of QAH film on insulating 2D van-der-Waals substrate MICA, we were able to fabricate dual-gate QAH device by exfoliation and transfer techniques. By applying a vertical electric field, we observe a topological phase transition from QAH to a trivial insulator state. This research will greatly improve the understanding of emerging topological quantum physics, and lead to the new revolution of quantum electronic devices.

Controlled heteroepitaxy and integration of arsenide based III-V compounds onto Si surfaces have been an important enabling technology for high efficiency solar cells and light emitters in satellite and optical interconnect applications. However, obtaining high crystal quality III-V compounds on Si, such as GaAs on Si is still challenging: (1) anti-phase domain (APD) boundary formation as the result of the polar GaAs growth on non-polar Si system, (2) a high density of threading dislocations generated by 4.1% lattice constant mismatch, and (3) the 62% thermal expansion coefficient mismatch leading to cracks during the cooling process.

The objective of this research is to obtain arsenide based III-V compounds monolithically integrated onto Si surfaces being APD-free with chemically abrupt GaAs/Si interfaces and possessing excellent optoelectronic properties. Patterned growth scheme by molecular beam epitaxy (MBE) is the approach I undertook to integrate GaAs based III-V compounds onto exactly oriented Si substrates. And the research consists of the following three stages to fulfill the objective.

(1) Precise positioning and low defect density selective area epitaxy for self-assembled/catalyst-free GaAs nanodisks on SiO2 masked exactly oriented Si(100) substrates:

Pure zincblende GaAs nanodisks with precise positioning and low defect density are demonstrated by selective area epitaxy. Defects in the epilayers are reduced by strain relaxation through facets formation and by a lateral overgrowth scheme atop the SiO2 mask.

(2) High-quality and defect-free GaAs thin film on SiO2 masked exactly oriented Si(111) substrates by a two-step growth technique:

Taking advantages of low energy for both Si(111) surface and GaAs/Si(111) interface, the two-step grown GaAs of total ∼175 nm atop patterned Si(111) substrates exhibits atomically smooth surface morphology, single crystallinity and a remarkably low defect density.

(3) Successful integration of InGaAs/GaAs double heterostrucure onto SiO2 masked exactly oriented Si(111) substrates with remarkably reduced thermal stress:

The atomically smooth and high crystalline quality InGaAs/GaAs DH is realized. The confined misfit dislocations at the nucleation layer and nearly threading dislocation-free buffer layer contribute to the atomically sharp GaAs/Si interface. The remarkable reduction in the thermally induced stress corroborates the effectiveness of the square shape pattern design. Optical properties and carrier dynamics are characterized by micro-photoluminescence (µ-PL) and time-resolved PL.

Two-dimensional material family is long time potential semiconductor candidate material system, for its ability to suppress short channel effect while scaling gate length. For a long time since the graphene monolayer exfoliation successfully in 2004, the researches in the field focused on charge-orientated devices such as thin film transistors, negative capacitance transistors, or applying its atomic thin thickness as quantum barrier for tunneling devices. Recently, because of more characterizations discovered in its special crystal structures, researches of spin and magnetism manipulations in 2-D material become prospering. Still, as a emerging stage of the field, little understanding of the mechanism inside is always the most significant challenge.

In this dissertation, the author focused on introducing magnetism into MoS2, one of transition metal dichalcogenides, through magnetic proximity effect (MPE). In chapter 2, the author demonstrated room temperature proximity-induced spin polarization in MoS2, with yttrium iron garnet (YIG) as magnetic proximity host, which was the firstly demonstrated in the world. In this chapter, the author also demonstrated the importance of shallow state in MoS2 for magnetism introduction through spin resolved photoluminescence (SR-PL). In 2-4, white light magnetic circular dichroism (MCD) was implemented to further characterize the inner mechanism of MPE. Through the MCD results, the mechanism between MoS2/YIG heterostructure was confirmed to be antiferromagnetically exchange coupling. The reference MCD of YIG and MoS2/Al2O3/YIG further verified the MPE induced MoS2 spin polarization. It was the first time people have better scope of the mechanism of MPE. In the end of this chapter, an illustration of band structure is given as explanation for the charge transfer mechanism and the resulting antiferromagnetically exchange coupled MoS2 and YIG’s magnetic moments. In chapter 3, the antiferromagnetic MPE was studied based on the foundation of chapter 2. An AFM perovskite, SmFeO3, was selected as MPE host in this experiment. The SR-PL results showed the significance of uncompensated spin contribution in SmFeO3/MoS2 heterointerface. The strong external field SR-PL also demonstrated the magnetic canting effect in AFM proximity effect is only crucial when uncompensated spin plane of host is presented. The results showed the proximity effect triggering in TMD is extremely crucial from the very interfacial atomic layer of host. In the last chapter, the author lists several potential future works in this field.