Search

Topological insulators (TI) have been demonstrated as a unique electronic phase of matter, possessing topological surface states (TSS) with promising applications in spin-based logic and memory, heterostructures in 2D electronics and exotic physical phenomena such as Majorana quantum computing, axion electrodynamics and topological magnetoelectronics. Since the early stages of discovery, the field of applied research in TIs has evolved. However, demonstration of scalable applications remain challenging due to practical hurdles such as rapid prototyping of new TI compounds, and efficient probing of TSS for device applications. This research work endeavors to take a two-pronged approach: to address the challenges of reliable material growth and to explore TI transport physics. While indirect spectroscopy methods have indisputably shown the presence of TSS, transport in TI devices remains challenging, in part due to parasitic conduction channels. As an alternative to staple binaries, ternary and quaternary compounds (Bi₁₋[subscript y]Sb[subscript y])₂(Te₁₋[subscript x]Se[subscript x])₃ are being explored to reduce unintentional bulk-doping and gain better access to the Dirac point. The sulfur-based ternary Bi₂Te₂S has received little attention, even as its potential as a promising TI is theoretically predicted. We demonstrate first-time van der Waals epitaxial (vdWE) growth of crystalline Bi₂Te₂₋[subscript x]S₁₊[subscript x] (BTS) nanosheets on SiO₂, hBN and mica. We also perform detailed magnetotransport measurements on BTS devices, establishing BTS as a candidate TI with readily accessible TSS and providing a sound picture of multiple transport channels in TI devices. A versatile process for large-area custom-feature TI growth and fabrication is also demonstrated using BTS as the candidate TI, achieved through selective-area modification of surface free-energy on mica. TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag edges highly oriented with the substrate lattice. Unusual nonlinear thickness dependence on lateral dimensions and denuded zones are observed, explained by semi-empirical two-species surface migration modeling with robust estimates of growth parameters. TSS contribute up to 60% of device conductance at room-temperature, indicating excellent electronic quality. The process is constructed from highly adaptable microfabrication technology, and in conjunction with multi-species modeling, it can be customized for TI and other vdW materials device fabrication processes ranging from rapid prototyping to scalable manufacturing.

Topological insulators (TI) have been demonstrated as a unique electronic phase of matter, possessing topological surface states (TSS) with promising applications in spin-based logic and memory, heterostructures in 2D electronics and exotic physical phenomena such as Majorana quantum computing, axion electrodynamics and topological magnetoelectronics. Since the early stages of discovery, the field of applied research in TIs has evolved. However, demonstration of scalable applications remain challenging due to practical hurdles such as rapid prototyping of new TI compounds, and efficient probing of TSS for device applications. This research work endeavors to take a two-pronged approach: to address the challenges of reliable material growth and to explore TI transport physics. While indirect spectroscopy methods have indisputably shown the presence of TSS, transport in TI devices remains challenging, in part due to parasitic conduction channels. As an alternative to staple binaries, ternary and quaternary compounds (Bi₁₋[subscript y]Sb[subscript y])₂(Te₁₋[subscript x]Se[subscript x])₃ are being explored to reduce unintentional bulk-doping and gain better access to the Dirac point. The sulfur-based ternary Bi₂Te₂S has received little attention, even as its potential as a promising TI is theoretically predicted. We demonstrate first-time van der Waals epitaxial (vdWE) growth of crystalline Bi₂Te₂₋[subscript x]S₁₊[subscript x] (BTS) nanosheets on SiO₂, hBN and mica. We also perform detailed magnetotransport measurements on BTS devices, establishing BTS as a candidate TI with readily accessible TSS and providing a sound picture of multiple transport channels in TI devices. A versatile process for large-area custom-feature TI growth and fabrication is also demonstrated using BTS as the candidate TI, achieved through selective-area modification of surface free-energy on mica. TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag edges highly oriented with the substrate lattice. Unusual nonlinear thickness dependence on lateral dimensions and denuded zones are observed, explained by semi-empirical two-species surface migration modeling with robust estimates of growth parameters. TSS contribute up to 60% of device conductance at room-temperature, indicating excellent electronic quality. The process is constructed from highly adaptable microfabrication technology, and in conjunction with multi-species modeling, it can be customized for TI and other vdW materials device fabrication processes ranging from rapid prototyping to scalable manufacturing.

The recently discovered time-reversal-invariant topological insulator (TI) has led to the flourishing of unique physics along with promises for innovative electronic and spintronic applications. However, the as-grown TI materials are not truly insulating but with a non-trivial bulk carrier density, which makes difficulties to the transport methods. In our work, we study the fundamental transport properties of TI and its heterostructure, in which various approaches are utilized to better reveal the surface state properties. In particular, in Chapter 2, in-situ Al surface passivation of Bi2Se3 inside MBE is investigated to inhibit the degradation process, reduce carrier density and reveal the pristine topological surface states. In contrast, we show the degradation of surface states for the unpassivated control samples, in which the 2D carrier density is increased by 39.2% due to ambient n-doping, the Shubnikov-de Hass oscillations are completely absent, and a deviation from WAL weak antilocalization is observed. In Chapter 3, through optimizing the material composition to achieve bulk insulating state, we present the ambipolar effect in 4-9 quintuple layers (Bi0.57Sb0.43)2Te3 thin films. We also demonstrate the evidence of a hybridized surface gap opening in (Bi0.57Sb0.43)2Te3 sample with thickness below six quintuple layers through transport and scanning tunneling spectroscopy measurements. By effective tuning the Fermi level via gate-voltage control, we unveil a striking competition between weak antilocalization and weak localization at low magnetic fields in nonmagnetic ultrathin films. In Chapter 4, we study the magnetic properties of Bi2Se3 surface states in the proximity of a high Tc ferrimagnetic insulator YIG. Proximity-induced magnetoresistance loops are observed by transport measurements with out-of-plane and in-plane magnetic fields applied. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, which is an important step toward room-temperature application of TI-based spintronic devices. The engineering of a TI and FMI heterostructure will open up numerous opportunities to study high temperature TI-based spintronic devices, in which the TI is controlled by breaking the TRS using a FMI with perpendicular magnetization component. A YIG film with out-of-plane anisotropy at > 300 K could potentially manipulate the magnetic properties of a TI may even above room temperature.

Extensive research efforts have been devoted to the study and development of III-V compound semiconductor nanowires (NWs) and nanopillars (NPs) because of their unique physical properties and ability to form high quality, highly lattice-mismatched axial and radial heterostructures. These advantages lead to precise nano-bandgap engineering to achieve new device functionalities. One unique and powerful approach to realize these NPs is by catalyst-free, selective-area epitaxy (SAE) via metal-organic chemical vapor deposition, in which the NP location and diameter can be precisely controlled lithographically. Early demonstrations of electronic and optoelectronic devices based on these NPs, however, are often inferior compared to their planar counterparts due to a few factors: (1) interface/surface states, (2) inaccurate doping calibration, and (3) increased carrier scattering and trapping from stacking fault formation in the NPs. In this study, the detailed growth mechanisms of different III-As, III-Sb and III-P NPs and their heterostructures are investigated. These NPs are then fabricated into single-NP field-effect transistors (FETs) to probe their electrical properties. It is shown that these devices are highly diameter-dependent, mainly because of the effects of surface states. By growing a high band-gap shell around the NP cores to passivate the surface, the device performance can be significantly improved. Further fabrication and characterization of vertical surround-gate FETs using a high-mobility InAs/InP NP channel is also discussed. Aside from the radial NP heterostructures, different approaches to achieve purely axial heterostructures in InAs/In(As)P materials are also presented with excellent interface quality. Both single barrier and double barrier structures are realized and fabricated into devices that show carrier transport characteristics over a barrier and even resonant tunneling behavior. Antimonide-based NPs are also studied for their immense application in high-speed electronics and mid-IR optoelectronics. Different growth regimes are probed to achieve InSb NPs and InAsSb NPs.

Functional crystalline oxides with perovskite structure have a wide range of electrical properties such as ferroelectric, ferromagnetic, and superconductive, as well as unique properties that make them suited for a wide variety of applications including electro-optics, high-k dielectrics, and catalysis. Therefore, in order to realize the potential of perovskite oxides it is desirable to integrate them with semiconductors. Due to the high surface energy of oxides compared to that of semiconductors and the low number of oxides that are thermodynamically stable against SiO₂ formation, it has been extremely difficult to integrate epitaxial oxides with Si directly. However, in 1998, McKee and co-workers finally succeeded in depositing SrTiO₃ on Si directly using a Sr template via molecular beam epitaxy. This breakthrough opened the possibility of integrating the perovskite oxides with Si to realize potential device applications. In this dissertation, alkaline earth metal (Sr and Ba) templates on semiconductors, which enable epitaxial growth of complex oxides on semiconductors, are investigated using molecular beam epitaxy (MBE) for growth and in-situ X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) for the electronic structure analysis. An epitaxial layer of SrTiO₃ on Si using such alkaline earth templates is used as a pseudo-substrate for the integration of perovskite oxides on Si. Through the use of post-deposition annealing as a function of oxygen pressure and annealing time, the strain relaxation behavior of epitaxial SrTiO₃ films grown on Si is also investigated to determine how the SiO₂ interlayer thickness affects the SrTiO₃ lattice constant. This ability to control strain relaxation can be used as a way to manipulate the properties of other perovskite oxides grown on SrTiO₃/Si. Additionally, SrTiO₃ can be made conductive by doping with La. Conductive SrTiO₃ can be used as a thermoelectric, a transparent conductive layer, and a quantum metal layer in a quantum metal field-effect transistor (QMFET). The structural, electrical, and optical properties of strained conductive La-doped SrTiO₃ are studied in order to understand the relation between elastic strain and electrical properties for electronic device applications. Oxide quantum well systems based on LaAlO₃/SrTiO₃ are also investigated using spectroscopic ellipsometry to understand how the quantum well layer structure affects the electronic structure. Such quantum well systems are good candidates for the monolithic integration of functional perovskites on semiconductors. Oxides quantum wells can be used in various device applications such as in quantum well cascade lasers, laser diodes and high performance transistors. As part of the growth optimization for high quality complex oxide heterostructures, the surface preparation of SrTiO₃ substrates using several different methods was also extensively studied using angle-resolved photoemission spectroscopy (ARPES). We found that acid-free water-based surface preparation is actually more effective at removing SrOx̳ crystallites and leaving the surface TiO₂-terminated compared to the more commonly used acid-based methods.

Functional crystalline oxides with perovskite structure have a wide range of electrical properties such as ferroelectric, ferromagnetic, and superconductive, as well as unique properties that make them suited for a wide variety of applications including electro-optics, high-k dielectrics, and catalysis. Therefore, in order to realize the potential of perovskite oxides it is desirable to integrate them with semiconductors. Due to the high surface energy of oxides compared to that of semiconductors and the low number of oxides that are thermodynamically stable against SiO₂ formation, it has been extremely difficult to integrate epitaxial oxides with Si directly. However, in 1998, McKee and co-workers finally succeeded in depositing SrTiO₃ on Si directly using a Sr template via molecular beam epitaxy. This breakthrough opened the possibility of integrating the perovskite oxides with Si to realize potential device applications. In this dissertation, alkaline earth metal (Sr and Ba) templates on semiconductors, which enable epitaxial growth of complex oxides on semiconductors, are investigated using molecular beam epitaxy (MBE) for growth and in-situ X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) for the electronic structure analysis. An epitaxial layer of SrTiO₃ on Si using such alkaline earth templates is used as a pseudo-substrate for the integration of perovskite oxides on Si. Through the use of post-deposition annealing as a function of oxygen pressure and annealing time, the strain relaxation behavior of epitaxial SrTiO₃ films grown on Si is also investigated to determine how the SiO₂ interlayer thickness affects the SrTiO₃ lattice constant. This ability to control strain relaxation can be used as a way to manipulate the properties of other perovskite oxides grown on SrTiO₃/Si. Additionally, SrTiO₃ can be made conductive by doping with La. Conductive SrTiO₃ can be used as a thermoelectric, a transparent conductive layer, and a quantum metal layer in a quantum metal field-effect transistor (QMFET). The structural, electrical, and optical properties of strained conductive La-doped SrTiO₃ are studied in order to understand the relation between elastic strain and electrical properties for electronic device applications. Oxide quantum well systems based on LaAlO₃/SrTiO₃ are also investigated using spectroscopic ellipsometry to understand how the quantum well layer structure affects the electronic structure. Such quantum well systems are good candidates for the monolithic integration of functional perovskites on semiconductors. Oxides quantum wells can be used in various device applications such as in quantum well cascade lasers, laser diodes and high performance transistors. As part of the growth optimization for high quality complex oxide heterostructures, the surface preparation of SrTiO₃ substrates using several different methods was also extensively studied using angle-resolved photoemission spectroscopy (ARPES). We found that acid-free water-based surface preparation is actually more effective at removing SrOx̳ crystallites and leaving the surface TiO₂-terminated compared to the more commonly used acid-based methods.

Ultrafast angle-resolved two-photon photoemission (TPPE) is used to probe the electronic structure of highly ordered monolayers of organic materials deposited on the Ag(111) and Ag(100) surfaces. Studies focus on how the electronic states intrinsic to the silver surface are perturbed by the presence of the organic adlayer. These studies show that the strength of the interaction between the surface states and the organic adlayer depends on the nature of the surface state being investigated. For states localized at or near the organic adlayer, substantial changes in the surface state band structure were discovered, whereas for states residing farther in the vacuum the surface state band structure was perturbed surprisingly little. The results presented here give detailed insight into the interaction between the electronic states of organic adlayers and the silver surface.

In this dissertation, we present contributions to the nanoscale engineering of electronic and geometrical structure of dielectric-metal interfaces. Such structure is designed to support the interaction of light and matter for useful scientific and technological applications. Among our interests are electromagnetic subwavelength localization, propagation and storage; photonic manipulation, modulation, synchronization and its use in the confinement of atomic systems. The latter is directed towards a new generation of scalable devices for quantum information processing (QIP) and surface science studies.

Surface states place a limitation on the high-frequency behavior of Gallium Nitride devices by causing RF dispersion. They are also a source of undesirable 1⁄f noise. This thesis specifically aims to explore techniques to address known experimental observations of dispersion phenomena of unknown origin in the 1-30 GHz frequency range. A new method to investigate these at high frequencies is proposed. It involves the measurement of S-parameters between the drain and source of a GaN nin structure in the GHz frequency range. The basis of the proposed technique is the assumption that the behavior of surface states and other dispersion phenomena can be isolated by subtracting the behavior of the device from the measured Y-parameters.