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This thesis is dedicated to the electronic properties of few-layer twistronic graphene, and in particular, the electron transport in twisted bilayer graphene. I introduce the continuum-model Hamiltonian to describe twisted graphene heterostructures and give an overview of the band structures for twisted bilayer graphene, twisted double bilayer graphene and twisted monolayer-bilayer graphene. In this thesis, I show that the electronic properties in twisted bilayer graphene are different for varying twist angles. For relatively large angles of approximately 2 degrees, I model transverse magnetic focusing and theoretically explain the ballistic transport observed in the experiment. In addition, we study the effects of an applied displacement field to the system where we observe selective focusing from each minivalley. At relatively smaller angles below 1 degree with a large perpendicular electric field, we found that there are independent, perfect one-dimensional channels propagating in three different directions in the lattice. Using the continuum-model Hamiltonian, we demonstrate that an applied bias causes two well-defined energy windows on either side of zero energy that contain the one-dimensional channels. Lastly, the resistivity from umklapp electron-phonon interaction is analytically calculated for twisted bilayer graphene and twisted double bilayer graphene. This is a specific mechanism where an electron tunnels from one layer to another layer whilst transferring momentum to the superlattice. We find that there is a weak contribution to resistivity even at room temperature.

More than a decade after the successful isolation of the first monolayer two-dimensional (2D) layered materials in 2004, a major focus in contemporary material science research is to expand the 2D family with novel layered materials or vertical layered heterostructures (VLHs) presenting extraordinary properties. Especially, 2D layered semiconductors with various bandgaps are considered as promising candidates for versatile applications in nanoelectronics, optoelectronics, sensors, etc. This DPhil project aims to contribute to the field by developing fundamental knowledge in the synthesis of 2D layered semiconductors and their VLHs via vapour phase deposition, and also in subsequent characterisation of their material properties for optoelectronic applications. Simple direct sulfurization process was first developed for 2D semiconductor MoS2 which employs glass as a precursor-soluble substrate. This approach enables low cost and large-area uniformity as well as self-limiting crystal thickness and small nucleation density. Understanding of the diffusion-controlled dissolution and interface-limited precipitation processes facilitated morphological tuning of the MoS2 single crystals. Then, thickness- and stoichiometric-controlled growth of 2D GaS, a wide bandgap semiconductor, was achieved by chemical vapour deposition (CVD). A detailed investigation was performed into the stoichiometry- and thickness-dependent stability, optical and optoelectronic properties of GaS. Both layer-by-layer growth and stacking assembly are commonly used for fabrication of 2D VLHs. Based on the Frenkel-Kontorova model, a general framework was proposed for understanding the origin of small twist angles in high-mismatch epitaxially-grown VLHs. Experimentally, the framework was tested on CVD-grown epitaxial GaS/MoS2 and GaS/graphene by scanning transmission electron microscopy. Finally, the non-stoichiometric GaS0.87 was stacked with WS2 to investigate optoelectronic behaviour of the bilayer VLH. The band alignment and charge transfer behaviour of the isotype type-II heterojunction were probed by various spectroscopic characterisation techniques. In addition, both large linear dynamic range and large photoresponsivity across the visible light spectrum were observed for GaS0.87/WS2 photodetectors.

The initial aim of this thesis was to cool two-dimensional electrons down to temperatures below 10 mK. In order to cool a high mobility two-dimensional electron gas (2DEG) at a GaAs–AlGaAs heterojunction to milliKelvin temperatures, different processing techniques and recipes for low resistance Ohmic contacts based on alloys of Au, Ni, and Ge are fabricated on these semiconductor devices. Scanning electron microscope (SEM) and Energy Dispersive X-ray (EDX) images establish that the Ohmic contacts have the same inhomogeneous microstructure observed in the literature. The unexpected result from electrical measurements of the contact resistance RC, the four-terminal resistance along the top of a single contact Rtop, and the vertical resistance RV, is that there is superconductivity in the Ohmic contact below 0.9 K. Measurements for different Ohmic contacts investigated, show some have multiple superconducting transitions, whereas others have a single transition; there is no correlation discovered between the annealing recipe and the number of transitions. I-V measurements show the superconductor is turned completely normal with a DC current of 2.1 mA and in a magnetic field, the superconductor is turned completely normal with a magnetic field of 0.15 T. This thesis speculates on the superconductor, suggesting that the Ohmic contact is a granular superconductor comprised of multiple alloys of different phases, and reviews the possible compounds that may be superconducting below 0.9 K. There is a discussion on how this superconductivity affects electrical transport measurements in similar systems such as quantum conductance, four-terminal Hall measurements and electron cooling experiments in 2DEGs below 0.1 K.

This thesis highlights the challenges faced when cooling low dimensional electrons through the progress made towards achieving the coldest possible electron temperature and optimisation of cooling methods. AlGaAs/GaAs heterostructures are two dimensional systems and provide an interesting playground for understanding quantum mechanics. Many fundamental physical phenomena can be explored, including using common effects for thermometry, metrology studies as well as many body effects. At low temperatures, the electrons isolate from the lattice due to phonon freezing and the main source of cooling is through leads. The two–dimensional electron gas is connected to the cryostat by AuGeNi ohmic contacts in order to conduct as well as thermalise through leads to the coldest point of a dilution refrigerator. In order to achieve low electron temperature the contact resistance of the electrical contacts to the sample needs to be minimised. The sample itself must have high enough mobility and sufficient carrier density, while having low resistance to allow thermalisation throughout the system of study. The fabrication, use and characterisation of such devices upon investigating their compatibility with ultra-low-temperature measurements in the micro-Kelvin regime is described. Optimisation of ohmic contacts to these devices was achieved through a systematic study of the contact resistance and investigation of the contacting mechanism. The optimal annealing condition for layered AuGeNi contacts was found to be a rapid thermal anneal at 430?C for 80 seconds, giving a reproducibly low contact resistance of 1 ?mm. Measurement techniques at 4.2 K are compared, with direct electrical measurements made possible by a new test device giving insight into contact resistance. A study of our ohmic contact structure through systematic imaging studies (SEM/EDS) shows they are similar to those in literature. Measurements of standard two-dimensional electron gases showed a superconducting transition below 1 K. This is due to superconductivity in the ohmic contacts to the devices, in series with the measurement, and can be observed. AuGeNi ohmic superconductivity was investigated at lowtemperature on samples of different wafers, annealing conditions and geometries. Through investigation of the contacting mechanism, drawing from literature and the study on our ohmic contacts, this superconductivity is attributed to gold-gallium compounds forming in the contact. The experimental results and their implications are discussed as well as advances to achieving the coldest possible electron temperature. The implications on cooling lowdimensional electrons to less than 1 mK are considered.

This thesis studies the physics of multilayer heterostructures grown from topological insulators (TIs), primarily bismuth selenide and antimony telluride, and other topologically trivial materials. This is done by extending a standard low-energy 3D TI Hamiltonian and varying its associated material parameters across the simulation domain. New results arising from the position-dependent TI interface model are found. For the first time, this method is incorporated into a density-functional theory (DFT) solver in order to study the self-consistent charge density in multilayer TI heterostructures due to the interface states. The thesis is structured as follows. The introduction (Ch. 1) presents a pedagogical review of the theory of 3D TIs and low-energy Hamiltonians used to study them, as well as typical methods in solid state physics that are made use of throughout the thesis. Chapter 2 presents the position-dependent Hamiltonian, showing new evidence for topological features of bulk states including varying degrees of band mixing and inversion; also, interface state tunnelling is shown to be affected by atomic layer orbital overlap, and incomplete localisation of surface states is demonstrated for antimony telluride. Chapter 3 presents a new DFT model of TI heterostructure interfaces and shows how conduction through TI interface states can be controlled with an electric field. Chapter 4 covers the extension of the model in Ch. 1 to 2D cross-sections of TI wires and heterostructures, showing for the first time evidence of localisation of conduction almost entirely within the inner interfaces of a 2D heterostructure wire. Chapter 5 presents our work with magnetic fields, demonstrating evolution of interface and bulk states with changing magnetic field and Landau level, as well as presenting new evidence for more complex spin structures in bismuth selenide arising from Landé factor signs. Our conclusions are presented in Chapter 6.

This thesis consists of three studies which use atomic-resolution aberration-corrected TEM techniques to investigate the structure and composition of air-sensitive crystals. The first two studies investigate the oxidation of graphene-encapsulated 2D crystals, black phosphorus (BP) and NbSe2. Recent graphene-encapsulation techniques are performed in a glovebox environment, protecting thin (few-layered to monolayer) stacks of these crystals from instantaneously degrading from oxidation. Such protection allows for some of the first detailed structural and compositional studies of these crystals in pristine condition. BP is investigated specifically with a focus on electron beam damage, which can be utilised to sculpt stable features for applications in electronic devices. Intrinsic atomic defects in NbSe2 are identified with atomicresolution imaging, geometric phase analysis (GPA) and compared to density functional theory (DFT) calculations. Point defects such as vacancies and substitutional dopants are identified and play a crucial role in the overall electronic properties of the material. The final study investigates the nucleation and growth of aluminium oxide islands using atomicresolution in-situ environmental TEM (ETEM). Electron beam sputtering is used to remove initial self-healing oxide layers and oxygen is then introduced into the ETEM chamber allowing for the growth of metal oxides at room temperatures to be observed dynamically. Oxidation is studied with respect to oxygen pressure and crystallographic orientation of the aluminium. The study reveals atomic-scale insights into the oxidation mechanism which were previously elusive to bulk spectroscopy and surface science techniques.

This thesis reports the successful rational design of three highly active photocatalytic semiconductor nanocrystal (SNC) systems by exploiting morphology effects and the electronic properties of type II semiconductor heterojunctions. Novel architectures of colloidal SNCs are produced with the aim of suppressing exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. Rod-shaped niobium pentoxide Nb2O5 nanocrystals (NCs) are shown to exhibit significantly enhanced activity (10-fold increase in rate constant) relative to spherical-shaped NCs of the same material. The increase is attributed to Nb5+ Lewis acid site rich (001) surfaces, present in higher proportions in the rod morphology, which bind organic substrates from solution resulting in direct interaction with photogenerated charges on the surface of the NC. Building on the insights into morphology-activity dependence, type II semiconductor heterojunctions are exploited for their ability to increase exciton lifetimes and spatially separate charges. Two novel II-VI heterostructured semiconductor nanocrystals (HSNCs) systems are investigated: a series of CdX/ZnO (X = S, Se, Te) HSNCs and ZnS/ZnO HSNCs capped with two different surface ligands. In the first case, substantial photocatalytic activity improvement is observed for HSNCs (relative to pure ZnO analogues) according to the following trend: CdTe/ZnO > CdS/ZnO > CdSe/ZnO. The observed trend is explained in terms of heterojunction structure and fundamental chalcogenide chemistry. In the second case, both ZnS/ZnO HSNCs exhibit activity enhancement over analogous pure ZnO, but the degree of enhancement is found to be a function of surface ligand chemistry. Photocatalytic activity testing of all the materials investigated in this work is performed via the photodecomposition of methylene blue dye in aerated aqueous conditions under UVA (350 nm) irradiation. The synthetic techniques employed for the synthesis of colloidal SNCs investigated in this thesis range from chemical precipitation and solvothermal techniques to several different organometallic approaches. A wide variety of analytical techniques are employed for the chemical, structural and optical characterisation of SNC photocatalysts including: XRD, XPS, TEM, UV-vis absorption, PL spectroscopy and FTIR. Atom Probe Tomography (APT) is employed for the first time in the structural characterisation of II-VI heterojunctions in colloidal HSNCs. Overall, this thesis provides a useful contribution to the growing body of knowledge pertaining to the enhancement of photocatalytic SNCs for useful applications including: solar energy conversion to chemical fuels, the photodecomposition of pollutants and light-driven synthetic chemistry.

Solar-driven water splitting offers a promising avenue for renewable hydrogen (H2) production, due to their simplicity and potential for decentralized applications. Ternary tungstates have emerged as notable photoactive materials for these solar-driven applications, thanks to their narrow band gaps that facilitate efficient sunlight utilization. However, they often suffer from poor oxygen evolution reaction (OER) activity on their surfaces and a relatively fast recombination rate of photogenerated electron-hole pairs, limiting their solar-to-hydrogen conversion efficiencies. The work presented in this thesis employs computational approaches to address these issues. The study begins by evaluating the OER performance of four ternary tungstates: CuWO4, NiWO4, FeWO4, and SnWO4, based on calculations of overpotentials. For the three transition metal tungstates, the second step is identified as the potential-determining step (PDS), while SnWO4 exhibits the third step as the PDS. The influence of dopants on OER performance is also investigated. Among all the compounds studied, Fe is anticipated to be the most effective dopant for SnWO4, while adding Sn as a dopant in FeWO4 yields a significant improvement in the OER performance. Delving deeper into the OER performance of FeWO4 and Fe2WO6, this work explores two different reaction pathways, the adsorbate evolution mechanism (AEM) and the lattice oxygen mechanism (LOM), and examines a broader range of dopants at higher concentrations. The results reveal that FeWO4 primarily favors the AEM pathway whereas Fe2WO6 exhibits a stronger affinity for the LOM. Dopants with high-energy occupied states, such as Ti, Sn, and Sb, tend to favor AEM, while those resistant to easy oxidation, such as Ni or Zn, lean towards LOM. Finally, two distinct heterostructures, WO3/SnWO4 and WO3/ FeWO4, are investigated, each with various interfacial planes.WO3(001)/SnWO4(010) and WO3(001)/ FeWO4 (100) are identified as having optimized structures with the least distortion and the lowest formation energies. Band alignment shows that all of them exhibit type-II heterostructures in their band alignment, which enhances charge separation for improved performance. In summary, this thesis demonstrates potential approaches for enhancing the solar water splitting performance of ternary tungstates by improving their OER performance through doping and increasing charge separation through the construction of heterostructures.

Plastic strains during investment casting of single crystal Ni-based superalloys arise from differences in thermal contraction between the metal and ceramic mould-and-core. If deformation is above a critical limit, subsequent solution heat treatment of the alloy causes recrystallisation. Crack nucleation and propagation is preferred at the recrystallisation grain boundaries, and this significantly reduces the creep and fatigue properties of the alloy. The as-cast microstructure (Chapter 3) is characteristic of high temperature deformation (~1050�(), where dislocations primarily form loops and networks at the y/y' interface. This validates the process modelling performed by our collaborators. The dislocation density was found to be higher in the interdendritic areas and the dendrite cores were virtually dislocation-free, indicating that deformation occurs at temperatures close to they' solvus, which is 1250-1310�C for CMSX 4, where dislocations accumulate in the interdendritic areas that are the first to precipitate they'. Critical strains for recrystallisation were determined as a function of temperature (Chapter 3). Recrystallisation was found to be sensitive to the surface finish, where alloys with the cast surface were more prone to recrystallisation, tolerating plastic strains of 1-2% at temperatures ~1000�C. On examining the cast surface of CMSX 4, two sources of nucleation for recrystallisation were identified (Chapter 4). Firstly, micro-grains of y', 2-30 ?m deep, forming high angle boundaries with the bulk single crystal were found within the surface eutectic; these grow larger during the heat treatment and maintain high angle misorientations with the matrix. Secondly, in regions where surface eutectic is absent, the metal adheres to the mould and forms intense local deformation, 5- 20 ?m deep, during subsequent detachment. During the heat treatment local surface recrystallisation occurs, where small grains develop in orientations similar to the deformed matrix and subsequently twin to form high angle boundaries. Experimental trials show that in the presence of deformation in the bulk the nuclei in the casting surface can cause recrystallisation. By removing the cast surface with etching, recrystallisation can be completely mitigated . Recrystallisation studies on alloys varying systematically in composition (Chapter 5) show that high . Co alloys (with up to 8 wt%) are more prone to recrystallisation. Co lowers they' solvus temperature and the stacking fault energy of y. Ru, Mo and W appear to have no direct effect on recrystallisation. The nucleating grains for recrystallisation form in orientations similar to the deformed matrix, and subsequently twin to form high angle boundaries and proliferate within the deformed microstructure. They' phase and topologically close packed phases hinder grain boundary migration.

The quantum confined Stark effect (QCSE) and ultrafast absorption dynamics near the bandedge have been investigated in p-i-n waveguides comprising quantum confined heterostructures grown on GaAs substrates, for emission at 1.3um. The materials are; isolated InAs/InGaAs dot-in-a-well (DWELL) quantum dots (QD), bilayer InAs quantum dots and GaInNAs multiple quantum wells (MQW). The focus was to investigate these dynamics in a planar waveguide geometry, for the purpose of large scale integration in optical systems. Initial measurements of the QCSE using photocurrent measurements showed a small shift for isolated QDs whilst a significant shift of 40nm (at 1340nm) was demonstrated for bilayer dots, comparable to that of GaInNAs MWQ (30nm at 1300nm). These are comparable to InP based quaternary multiple quantum wells used in modulator devices. With the use of a broadband continuum source the isolated quantum dots exhibit both a small QCSE (15nm at 1280nm) and minimal broadening which is desirable for saturable absorbers used in monolithic modelocked semiconductor lasers (MMSL). A robust experimental set-up was developed for characterising waveguide modulators whilst the electroabsorption and electro-refraction was calculated (dn=1.5x10⠻³) using the Kramers-Kronig dispersion relation. Pump probe measurements were performed at room temperature using 250fs pulses from an optical parametric oscillator (OPO) on the three waveguide samples. For the isolated QDs ultrafast absorption recovery was recorded from 62ps (0V) to 700fs (-10V and the shortest times shown to be due to tunneling. Additionally we have shown good agreement of the temperature dependence of these dots and the pulse width durations from a modelocked semiconductor laser using the same material. Bilayer QDs are shown to exhibit ultrafast absorption recovery from 119ps (0V) to 5ps (-10V) offering potential for applications as modelocking elements. The GaInNAs multiple quantum wells show absorption recovery of 55ps (0V), however under applied reverse bias they exhibit long lived field screening transients. These results are explained qualitatively by the spatial separation of electrons and holes at heterobarrier interfaces.

Strained SiGe heterostructures possess transport properties superior to Si. Their integration in the channel of MOSFETs can lead to an enhanced speed performance and is therefore an alternative to the scaling of the channel length. This thesis reports on the fabrication and characterization of p-MOSFETs with a strained SiGe channel. As both design and material properties determine the performance of MOSFETs, the mobility in the SiGe heterostructure must be seen in context with the MOSFET design. SiGe heterostructures fabricated by molecular beam (MBE) and solid phase (SPE) epitaxial growth have been successfully integrated in the channel of p-MOSFETs. A manufacturable SiGe CMOS process based on SPE has been demonstrated. A slightly enhanced oxidation rate and an increased interface state density have been observed for Si oxidized after Ge⁺ implantation. The low performance of the SiGe p-MOSFETs is explained by the scattering and-trapping of carriers at the degraded interface. This suggests that changing the annealing conditions could improve the SiGe p-MOSFETs. Devices have been fabricated on a locally grown MBE layer. A considerable mobility enhancement over Si references has been shown for devices with a SiGe layer grown by MBE - more than or nearly a factor of two for devices with an LTO or thermal gate oxide, respectively. The devices fabricated on pillars display considerably better transport properties than the devices not fabricated on pillars. Altering the standard cleaning procedures has enabled us to achieve the required process control of one nanometer. Otherwise, the carriers would not be confined in the buried SiGe channel and would not profit from its enhanced transport properties. Low source and drain resistances have been obtained for SiGe p-MOSFETs subject to a maximum processing temperature of 800°C.

This work studied sputter deposited conventional spin valves (SV) and related structures. In SV layered structures, two ferromagnetic layers are separated by a non-magnetic spacer. Under an external magnetic field, the relative orientation of the magnetization changes in the ferromagnets, exhibiting the giant magnetoresistive effect. The controlled switching of ferromagnets in convention SV is facilitated by the exchange bias (EB) effect, which is achieved by depositing an antiferromagnetic layer next to one of the ferromagnetic layers in a magnetic field. Two highly related investigations were performed in this work. In the first part the exchange bias effect in the Ni80Fe20/Fe50Mn50/Co trilayer structure was studied. Samples were deposited in a low field condition that permitted EB to be established in NiFe/FeMn but not in FeMn/Co bilayer structures. Temperature-dependent magnetic measurements were performed on the trilayer sample, as well as the corresponding NiFe/FeMn and FeMn/Co bilayer samples. Recent literature on similar system showed that an AF spiral could be formed in the trilayer, which was probed by relative EB directions of the NiFe and Co layers. In this work, no exchange bias was found to propagate from the NiFe/FeMn system into the FeMn/Co system, showing that the AF spiral was induced by the specific magnetic treatment and was not the cause of EB effect. Besides, exchange bias field and coercivity of the samples indicated the influence of the EB system in the presence of an adjacent EB system. Explanations of the effect were made with some existing EB models. In the second part of the work, conventional SV of target structure Nb/NiFe/Cu/Co/FeMn/Nb was studied in a 'built-up samples' strategy. A batch of these built-up samples, which corresponded to the different stages of the deposition of the target top conventional SV structure, were prepared by terminating the sputtering process after a certain number of layers were deposited. These samples were thoroughly characterized by structural, magnetic and electrical measurements. In terms of structural characterization by x-ray techniques, more reliable information concerning the morphology and microstructure of the layers was obtained by probing the built-up samples, instead of relying solely on the information of the full SV structure. For the electric and magnetic measurements, a number of unexpected observations were made in the built-up samples, although the final performance of the full SV structure was of comparable quality to the literature. These results showed the ability of the 'built-up samples' strategy in critical characterization and optimization of magnetic multilayered structures.

This thesis presents work done on electrical transport through dilute magnetic semiconductor (DMS) single barriers in both zero and non-zero magnetic fields. The fields are applied either perpendicular or parallel to the DMS layers. The main samples under investigation consist of 100Å and 200Å CdTe/Cd₀.₈Mn₀.₂Te/CdTe single barrier heterostructures. In addition electrical characterisation of the non-magnetic layers is performed. Current through the barrier is measured as a function of voltage, magnetic field and temperature. A theoretical model is derived in order to calculate the current as a function of barrier height, barrier width, emitting layer carrier concentration, applied bias and temperature. These variables are then treated as fitting parameters and comparisons are made between the calculated and the experimental currents. The barriers are shown to produce non-Ohmic transport. The roles of quantum mechanical tunnelling and thermal activation across the barrier are investigated and shown to be highly mixed. An unexpectedly high degree of tunnelling is found to occur at high temperatures, within the region previously assumed to be dominated by thermal activation. Moreover the barrier height is found to be lower and the width greater than expected. These observations suggest that a high level of Mn diffusion occurs, possibly due to In dopant related effects. This suggestion is validated by the high emitting layer carrier concentration suggested by the fitting. At low temperatures and voltages the thicker barrier sample is shown to contain a parasitic leak path which short-circuits the barrier. This leak may exist in both samples but only becomes dominant where the barriers are sufficiently opaque to the incident carriers. Changes in a magnetic field are expected to be due to sp-d exchange induced giant Zeeman splitting in the barrier and either normal spin splitting or sp-d exchange effects in the emitter regions. The application of a magnetic field is shown to produce either an enhancement or suppression of the barrier dominated current, dependent on the field orientation and the level of transmission. Current enhancement is seen when the transmission through the barrier is low, i.e. at low fields and voltages, whereas suppression is seen in the high transmission barrier current and in the leak dominated conduction. Comparisons of the experimental and calculated results suggest that the low voltage current through the barrier is dominated by emission from three dimensional states in the CdTe layers.

The use of SiGe/Si heterostructures in the fabrication of electronic devices results in an improvement of the device performances with respect to bulk silicon. Ion implantation has been proposed as one of the possible technologies to produce these structures and, thus, the aim of this work is to develop an ion beam technology to fabricate strained SiGe heterostructures. The formation of extended defects in SiGe alloy layers formed by high dose Ge+ ion implantation followed by Solid Phase Epitaxial Growth (SPEG) has been investigated by transmission electron microscopy. Rutherford backscattering spectroscopy has also been used to determine the chemical composition and the crystalline quality of the synthesised structures. In addition, X-ray diffraction has been used to evaluate the strain level in selected samples. Two different structures have been studied in this project. The first consisted of "all-implanted" layers, where the Ge+ implants were followed in some cases by additional implants of Si+ and/or C+ ions, prior to SPEG, to investigate methods to inhibit defect formation. The second was achieved by capping the ion beam synthesised SiGe alloy layer by the deposition of a thin film of silicon, in order to realise structures compatible with device dimensions. Single crystal device worthy SiGe alloy layers have been achieved by implantation of Ge+ ions at energies ranging from 70 keV to 400 keV, where the only extended defects observed are EOR defects at a depth correspondent to the a/c interface formed during the Ge+ implant. In some cases, "hairpin" dislocations have also been observed in the vicinity of the EOR defects and extending up to the surface. Both types of defects are annihilated after post-amorphisation with 500 keV Si+ and replaced with dislocation loops at a depth of about 1 fj,m. For each Ge+ implantation energy a critical value of the peak germanium concentration exists above which the structures relax through the formation of stacking faults or "hairpin" dislocations nucleated in the vicinity of the peak of the germanium concentration depth profile and extending up to the surface. A critical value of the elastic energy stored in the structures (~300 mJ/m2) has been determined above which ion beam synthesised SiGe alloys relax, independently of the implantation energy. This empirical approach has been found to successfully account for the results obtained in this work as well as in many other studies reported in the literature. "Hairpin" dislocations formed under different experimental conditions have been investigated by plan view TEM and have been found to have the same crystallographic orientation () and Burgers vector (b= a ). Their formation has been explained within a "strain relaxation model". For a regrowth temperature of 700° C, all samples investigated by XRD have been found to be almost fully strained, including samples containing relaxation-induced defects, indicating that, under these conditions, the energy transferred to the defects is very low. C+ co-implantation has been successfully used to reduce both relaxation-induced defects and EOR dislocation loops. It is noted that a mixed technology entailing both layer deposition and ion implantation to produce the Si/SiGe/Si device structures requires extra process steps to control surface contaminations, pre cleaning and/or native oxide formation, resulting in increased fabrication costs. In this work an " all-implanted" route to the synthesis of Si/SiGe/Si device structures is therefore described, which exploits all of the advantages given by ion implantation.

The conduction band of InAs lies lower in energy than the GaSb valence band. In order to preserve continuity of the Fermi level across the interface, charge transfer takes place resulting in a confined quasi two dimensional electron gas (2DEG) in the In As and a confined quasi two dimensional hole gas (2DHG) in the GaSb. This is the first detailed study into vertical transport in an n-InAs/p-GaSb single heterojunction (SHET). Application of a forward bias (InAs negative with respect to GaSb) increases the 2DEG and 2DHG concentrations and, therefore, their confinement energies. Eventually a critical bias is reached where the electron confinement energy moves above the hole confinement energy (the theoretical voltage induced semimetallsemiconductor transition Vc). Any subsequent increase in voltage is expected to result in a current decrease, and a region of negative differential resistance (NDR) should occur. The SHET can be grown with two distinct interface types, 'InSb-like' and 'GaAslike'. This study shows for the first time that the SHET vertical transport characteristic is very dependent upon this interface monolayer. For example, the temperature dependence of the I/V trace in a SHET with a 'GaAs-like' interface is found to be weak, with similar current peak to valley ratios (PVR) at 300 and 77K. The 'InSblike' SHET, however has a PVR that is very close to 1 at 300K, rising above 2 at 77K. Hydrostatic pressure is used to alter reversibly the InAs conduction/GaSb valence band overlap Δ. Vertical transport measurements taken at pressure confirm that Δ reduces at the same rate for both interface types and that it is larger for the 'InSb-like' interface. Experimental I/V traces at various pressures are compared with the corresponding results from self-consistent band profile calculations. The subsequent discoveries are that NDR occurs after Vc for both interfaces, and that each interface supports a different conduction mechanism - with the 'GaAs-like' interface exhibiting NDR when the band overlap is calculated to be ~ -100 meV. Magnetic fields have been applied both perpendicular and parallel to the SHET interface. The perpendicular field results provide additional evidence that the conduction process must be different at both interfaces and that NDR occurs after Vc. Parallel field I/V traces reveal an entirely different response for the two interface types.

Although the ultra-wide bandgap (UWBG) semiconductor aluminum nitride (AlN) has been routinely used in optoelectronics such as deep ultraviolet (DUV) light emitting diodes (LEDs) and lasers, its adoption in RF and power electronics remains largely unexplored. This dissertation demonstrates the significant potential of AlN platform for the next generation electronics to merge logic, memory and RF communication, the three pillars of electronic information systems. Built upon structurally-pure single crystal AlN substrates with high thermal conductivity, the high and best-balanced performances of p-channel and n-channel devices presented in this dissertation challenge the common misconception that high performance complementary logic is impossible with nitride semiconductors. Together with the radio frequency (RF) filters that have already been demonstrated on the same platform thanks to the high piezoelectricity of AlN, the results in this dissertation are expected to enable a fully-integrated monolithic RF signal processing solution on AlN. In this dissertation, utilizing polarization engineering, conductive channels are generated on the electrically-insulating AlN by adding a thin layer of GaN or AlGaN on top. Taking advantage of the capability to maintain sharp heterointerfaces by molecular beam epitaxy (MBE), combined with the state-of-the-art fabrication process, devices with record performances were achieved. First, the observation and properties of polarization-induced 2d hole gases (2DHGs) in GaN/AlN heterostructures on metal-polar single crystal AlN substrates are presented. The reduced dislocation densities on single crystal AlN substrates compared to foreign substrates such as SiC and sapphire improve hole mobility, and a record high hole mobility of ∼ 280 cm2/V·s is measured at 10 K. By leveraging the highly-conductive 2DHG in conjunction with a highly-scaled 3D-gating architecture, the fastest (fT/fMAX = 25/45 GHz) p-channel FinHFETs that deliver record-high on-currents(ION =1.3A/mm at room temperature) and >0.5 W/mm at 6 GHz RF output power are achieved. This is the first time RF output power has been obtained in nitride pFETs, marking the entrance of nitride transistor technology into the new frontier of RF CMOS. Next, by flipping the polarity of the structure, 2d electron gases (2DEGs) can be induced in N-polar GaN/AlN heterostructures. Unlike metal-polar AlN, the homoepitaxy of N-polar AlN is challenging, primarily due to the high reactivity of N-polar AlN surface. A new atomic surface cleaning technique— Al-assisted surface cleaning is developed and enables MBE homoepitaxy of electronic- and optical-grade N-polar AlN. 2DEGs are successfully observed in N-polar GaN/AlGaN heterostructures with sheet resistances of ∼300 Ω/□. These are among the lowest sheet resistances reported in III-nitride 2DEGs. The first N-polar HEMTs on AlN are enabled by these 2DEGs, showing a high on-current of 2.6 A/mm, a high speed (fT/fMAX = 68/100 GHz) as well as >3 W/mm RF output power at 6 GHz. While there is large room for future improvement, these exciting performances already demonstrated in the first generation devices mark important milestones towards highly reliable RF electronics with excellent thermal management based on N-polar AlN HEMTs. Finally, as an important first step towards direct integration of magnetic memory on the same nitride semiconductor platform, the MBE growth of ferrimagnetic Mn4N hosting desirable properties for spintronic applications is explored on GaN to form a all-nitride ferrimagnet/semiconductor heterostructure. Through exploration of nucleation and growth conditions, the MBE growth condition for c-axis aligned Mn4N on GaN with smooth surface morphologies is uncovered. Instead of direct nucleation of Mn4N on GaN substrates, a homoepitaxial GaN buffer layer is found to be helpful for improving the quality and surface morphology of the Mn4N epilayer. The ferrimagnetism evidenced by the clear anomalous Hall hysteresis loops at room temperature, along with the smooth surface morphology of Mn4N on GaN, lay the groundwork for bringing magnetic memory onto the AlN platform. As an aside, because of the rich magnetic phases within the Mn-N system and the large potential of antiferromagnetic materials for future spintronics, the magnetic properties of antiferromagnetic MnN are studied using optical second harmonic generation (SHG). The point group symmetry of MBE grown antiferromagnetic MnN films is identified as 2/m1’ and a loose upper bound on the domain sizes of 0.65 μm is placed. These results demonstrate the effectiveness of SHG for detecting the Neel order in metallic antiferromagnets and are expected to contribute to the recent efforts in using antiferromagnets for spintronic applications. With the results presented in this dissertation, the vision of fully-integrated electronics capable of logic, memory and RF communication functionalities on the single crystal AlN platform is very close to being realized, and will hopefully be greater than the sum of its parts.

Recently, two-dimensional (2D) heterostructures have attracted extensive attention in nanomaterials science. They have been successfully fabricated and applied to nanotechnology in many fields, such as nanoelectronics, solar cells, sensors, energy stores, quantum information, etc. The most common heterostructures are 2D-lateral heterostructure (LH) and 2D-vertical heterostructure (VH) where each of them exhibits unique features depending on the direction of assembly, i.e., along in-plane or out-of-plane direction. Beyond the van der Waals-VH which possess of van der Waals (vdW) interaction, there are other types of heterostructures made of 2D polar materials that possess different types of chemical bonding nature, e.g., chemical bonds with less (e.g., SiC monolayer) or more (e.g., GeC and SiGe monolayers) charge transfer between atoms, forming covalent bonds with a certain ionicity. The goal of this work focused on shedding light on the physical aspects of 2D LH and VH, constructed by such polar materials (e.g., ������, GeC, and SiGe monolayers). This work is a theoretical study by employing Density Functional Theory to unravel the unique physical properties of such heterostructures. Because an artificial strain will be induced by the lattice mismatch in building heterostructures, the effect of strain on the electronic properties of, and monolayers was first investigated. It was found that these monolayers can tolerate strain up to 8%, and such strain can induce modifications on the physical properties. Interestingly, it was found that ������ and monolayers undergo a direct-indirect band gap transition; while, monolayer undergoes a metal-semimetal transition, which made them attractive candidates for building heterostructures. Second, a systematic study on the aspect of 2D polar-LH of and has been conducted. It was found that the synergistic effect of the lattice mismatch induced strain, the chemical bonding nature at the interface, and quantum confinement can lead to several interesting phenomena. For instance, their electronic properties can be modulated by tuning the domain size, the chemical bonding nature, and the designing of interface. Accordingly, a lateral spontaneous p-n junction triggered by the in-plane charge transfer was detected which implies the promising applications such as visible light photocatalyst. Third, the roles of the stacking species arrangement and the interlayer interactions (including vdW and electrostatic forces) on stabilizing the structure and modulating electronic properties of 2D polar-VH of were deeply studied. It was found that, in addition to the redistribution of the in-plane net-charge transfer, a net charge redistribution also occurs between layers and leads to a polarization in the interfacial region that induces a built-in electric field and helps to reduce the recombination of photogenerated electron-hole pairs.

Graphene, a monolayer of carbon atoms, has gained prominence to augment existing chip-scale photonic and optoelectronic applications, especially for sensing in optical radiation, owing to its distinctive electrical properties and bandgap as well as its atomically thin profile. As a building block of photodetection, graphene has been co-integrated with mature silicon technology to realize the on-chip, high-performance photo-detecting platforms with broad spectral response from the deep-ultraviolet (UV) to the mid-infrared (MIR) regime. The recent state-of-the-art graphene-based photodetectors utilizing the combination of colloidal quantum dots (QDs) and graphene have been intensively studied, where QDs function as the absorber and the role of graphene is as a fast carrier recirculating channel. With such a configuration, an ultrahigh sensitivity can be achieved on account of the photogating mechanism; however, the response time is slow and limited to the millisecond-to-second range. To achieve balance between high sensitivity and fast response time, we have demonstrated a new photodetector that is based on graphene/two-dimensional heterostructures. The homogeneous thickness and the large contact of the heterostructure give rise to fast carrier transporting between the thin absorber layer and the graphene, leading to a fast response time. This thesis carefully investigates the optimization of fabrication as well as optoelectronic characterization of photodetectors based on graphene/semiconductor heterostructures field-effect transistors (GFETs). GFETs with different architectures were demonstrated and systematically studied under optical illumination ranging from deep-UV to MIR at varying optical powers. Noise behaviors have been studied under different device parameters such as device structure, area and gate-bias. Results show that the flicker noise of graphene-based devices can be explained by the McWhorter model in which the fluctuation of carrier numbers is the dominant process of noise in low frequencies; thus, it can be scaled down by reducing the number of introduced charged carriers with optimized fabrication. Besides, the impact of absorber on top of graphene and the bottom substrate has been comprehensively explored through various experimental techniques including current-voltage (IV), photo-response dynamics, and noise characterization measurements. With our configuration, the high sensitivity and fast response time of photodetectors can be obtained at the same time. In addition to this, the study of the bottom substrate with different doping levels suggests a concept of dual-photogating effect which is induced by the top absorbent material and the photoionization of the doped silicon substrate. In summary, this thesis showcases novel device architecture and fabrication procedures of GFETs photodetectors, optimizes device structure, quantifies the performance and evaluates the effect of various absorbent materials and substrate. It provides insight into the improvement of possible routes to achieve a broadband photo-detecting system with higher sensitivity, faster response time and lower noise level.

In quantum materials, quantum mechanical effects survive over a wide range of temperatures and length scales, which leads to fascinating phenomena, such as charge density waves, spin liquids, and magnetism. These remarkable properties originate from a delicate balance among many interacting degrees of freedom. Even small perturbations such as temperature and doping can induce many diverse phases and colossal changes in the material's functionalities. It remains a mystery how to detect these novel phenomena without complicated fabrication and extreme conditions. This thesis contributed to this field by studying collective modes in quantum materials via inelastic light scattering. After fabricating the devices in a glovebox, we directly transfer them to the measurement platform under a high vacuum. Using Raman spectroscopy, a new quasi-particles--axial Higgs mode in charge density wave systems was discovered and further symmetry breaking accompanied with it suggests the charge density wave is unconventional. In addition to symmetry analysis breakthroughs, this thesis also provided direct evidence of the fractional nature as well as the energy and temperature limits of Kitaev interactions in quantum spin liquid candidate systems, which is the building block for topological quantum computers. Not limited to 2D Kitaev materials, the non-Fluery-Loudon single magnon scattering process was detected in a 3D Kitaev system. Other than using Raman to probe the fundamental nature, we also employed it to reveal, for the first time, a clean way to realize modulation doping in 2D materials, where the acceptor carrier density has reached 10$^{14}$ cm$^{-2}$. This method can be applied to dope magnetic materials or twisted heterostructures to find new phases.

Band-gap engineering is central to the design of heterojunction devices. It is a powerful technique to overcome the constrain of natural material properties and realize novel functionalities of materials. Monolayer transitional metal dichalcogenides (TMDs) provides unprecedented opportunities for band-gap engineering with their atomically-thin thickness, direct bandgap in the near-infrared to the visible region, large excitonic effect, and strong spin-orbit coupling.For heterojunctions involving monolayer TMDs, the electronic properties of the atomically thin films are widely tunable by external perturbations. For instance, the carrier concentration of monolayer MoS2 can vary significantly depending on the amount of charge transfer between the MoS2 and the substrate. This makes substrates with a range of charge neutrality levels—as is the case for complex oxide substrates—a powerful addition to electrostatic gating or chemical doping to control the doping of overlying MoS2 layers. In this thesis, I demonstrate this approach (charge transfer doping) by growing monolayer MoS2 on perovskite (SrTiO3 and LaAlO3), spinel (MgAl2O4), and SiO2 substrates with multi-inch uniformity. The as-grown MoS2 films on these substrates exhibit a controlled, reproducible, and uniform carrier concentration ranging from (1-4) ×1013 cm-2, depending on the oxide substrate employed. The observed carrier concentrations are further confirmed by our density-functional theory calculations based on ab initio mismatched interface theory (MINT). This approach is relevant to large-scale heterostructures involving monolayer-thick materials in which it is desired to precisely control carrier concentrations for applications.

Advances in crystal handling techniques specialized to van der Waals (vdW) materials - crystals with only weak vdW bonds between strongly bonded crystal planes - have brought about a new generation of devices with emergent properties in atomically thin layers and interfaces, which do not appear in their bulk counterpart. Many vdW materials, such as the cuprate high temperature superconductor Bi2Sr2CaCu2O8+x (BSCCO), spontaneously react with air and thermally decompose even at room temperature, rendering the powerful arsenal of vdW crystal handling techniques inapplicable. In this Dissertation, we develop a versatile suite of novel vdW fabrication techniques so that we can handle chemically sensitive vdW crystals like any other. We first demonstrate that the vdW heterointerface between graphene and hexagonal boron nitride can be electrochemically intercalated with lithium in a controlled way, like the interface between graphene crystals. Next, we use a scanning nano X-ray diffraction probe to visualize the crystal structure of an exfoliated BSCCO crystal 2 unit cells thick with 100 nm spatial resolution. We find that while the exfoliated and bulk crystals have the same incommensurate lattice modulations (ILM), the ILM spatial distribution is different in the exfoliated crystal, where it is correlated with the local strain. Next, we create BSCCO Hall bar devices down to 2 unit cells, where we observe Hall sign reversal both above and below the superconducting transition temperature. As the samples become thinner, the region over which the Hall resistance reverses sign enlarges due to enhanced superconducting fluctuations in the atomically thin samples as well as a decrease of carrier mobility. We obtain quantitative agreement between theory and experiment, which establishes excess charges at the core of superconducting vortices as the origin of Hall sign reversal below Tc. Finally, we engineer twist Josephson junctions between BSCCO crystals, with quality approaching that of intrinsic Josephson junctions in single crystals. Our angle-dependent critical current clearly reflects the expected d-wave symmetry of the order parameter. At 45 degree twist angle, we observe fractional Shapiro steps which indicate the presence of a second harmonic in the Josephson junction current-phase relation, originating from co-tunneling of Cooper pairs at the twist junction. Such a process is expected to support an emergent interfacial topological superconducting phase, persisting to the superconducting transition temperature of the cuprate crystal.

Spin currents and spin defects are exciting topics of research because of their applications in spintronics and quantum information. A central challenge for studying spin-dependent phenomena is creating materials with the characteristics that are ideally suited for the generation and injection of highly spin polarized carriers. Likewise, many transport phenomena rely on electron-defect interactions, where a commonly occurring challenge is in the tunability and reproducibility of the interacting defects. Heterostructures of stacked 2D materials can potentially provide the flexibility to engineer the charge and spin transport properties of a device, and can even provide defect centers that can be exploited for the device operation. In this thesis I will discuss two different types of stacked 2D material devices, both utilizing graphene as the base 2D material. In the first device, graphene is stacked on top of an assembled layer of superparamagnetic iron oxide nanoparticles (SPIONs). The interaction of the spin carriers in graphene with the assembled SPIONs gives rise to the observation of an Anomalous Hall Effect in the electron-doped regime. Demonstrating a magnetic proximity effect that can be integrated into a more complex 2D heterostructure systems.The second device studied in this thesis is a tunnel-junction with a hexagonal boron nitride (h-BN) tunnel barrier, with graphene acting as the tunneling electrode reservoirs. Tunneling transport spectroscopy shows possible spin interaction with defect states in the h-BN tunnel barrier. This device geometry demonstrates the promise of stacked 2D material systems to create new functional devices.

In this work, we present a comprehensive computational and theoretical study of the structural phase space of multiferroic Bi-Fe-O and the influence of the structure on functional properties. Complex oxides are known for their many different functionalities, including ferroelectricity, ferromagnetism, or piezoelectricity, which are relevant for technological applications. Multiferroicity, in which at least two different ferroic orders (ferromagnetism, ferroelectricity, and ferroelasticity) coexist and couple in a single phase is of particular interest and can enable, for example, control of magnetic properties using an applied electric field. BiFeO3 (BFO) is one of the most studied multiferroic materials because of the coexistence of magnetic and polar orders at room temperature promising for applications. The substantial intrinsic functionalities of complex oxides can be further enhanced by growing them as thin films in superlattices or heterostructures. In most cases identified to date, this is a result of the change in the material’s lattice constants caused by epitaxial strain imposed by the substrate. This change in lattice constants can modify the functionalities, for example causing a phase change to a higher polar state in BFO. In heterostructures of polar and non-polar materials, the polar discontinuity at the interface results in an accumulation of surface charges, which in turn create an electric field, known as the depolarising field. This de- polarising field can cause the formation of domains in the polar material and even change the orientation of the polarisation, in order to reduce the energy penalty of the interfacial charge. We start our study by considering the case in which a phase transition to a non-polar phase with low relative energy is preferred over polar domains formation and present a new phase with a larger unit cell size and surprising properties matching recent experimental results. We then explore the phase space of BFO using density functional theory (DFT) calculations and reveal several low-energy phases with large unit cells which could be stabilised by a polar dis- continuity at the interface. Finally, we consider a heterostructure in which the polar discontinuity is partially reduced by the presence of differently charged ionic layers and show that this causes an isosymmetric phase transition from two phases with the same symmetry but different polar states in highly-strained BFO. Next, we study a new candidate multiferroic material, bismuth hexaferrite, which exhibits net magnetic and possible ferroelectric moments at room temperature, experimentally demonstrated. We show that the net magnetic moment is likely related to the presence of Fe vacancies and compute the energy barrier between opposite orientations of the polarisation in order to evaluate the likelihood for it to be ferroelectric. This result paves the way for the exploration of new materials in the Bi-Fe-O compositional phase space with technologically relevant properties. Finally, we conclude our work by proposing an accelerated method combining DFT, irreducible phonon modes, and machine learning to explore the potential energy surface of BFO and we predict several low-energy structures, suggesting that the phase space of BFO is still far from being completely known. This thesis highlights the richness of the phase space of Bi-Fe-O and the importance of the boundary conditions provided by the heterostructure, in particular, the electrostatics at the interface in determining the properties of functional oxides. Finally, the methods that we developed provide a new accelerated approach to structure discovery. They are broadly applicable and can certainly lead to discoveries in other materials.

As conventional semiconductors reach their materials limits for modern high power switching applications, we must look towards new materials systems. Ultrawide bandgap semiconductors provide opportunities for future efficient high voltage switches due to their ability to withstand high electric fields. In particular, β-Ga2O3 shows promise due to its high critical electric field (6-8 MV/cm), availability of high-quality melt grown bulk substrates, and donor and deep acceptor doping possibilities. This work focuses on growth and doping of β-Ga2O3 and its alloys via plasma-assisted molecular beam epitaxy (PAMBE). Conventional PAMBE shows promise for (010) β-Ga2O3 growth, however other crystallographic orientations have lower growth rates and poor film quality due to significant suboxide desorption during growth. An indium catalyzed growth mechanism using an additional indium flux during PAMBE growth of β-Ga2O3 is demonstrated, allowing for significantly improved growth rates across various crystallographic orientations. This metal oxide catalyzed epitaxy (MOCATAXY) allows for improved film quality, demonstrated by minimal extended defects and smoother surface morphologies, particularly for (001) β-Ga2O3. The supplied In flux during MOCATAXY growth acts as a catalyst, allowing for growth at high growth temperatures and Ga fluxes for which growth would not occur for conventional PAMBE. This In limits suboxide desorption during growth and does not incorporate into the film for sufficiently Ga rich growth conditions. Donor doping is necessary for achieving a variety of device designs, with its use in contacts, channels, modulation doping, and drift regions. Donor doping with Ge, Sn, and Si is demonstrated for β-Ga2O3 grown on various orientations. While Ge doping can be used for a range of concentrations for conventional PAMBE, at higher growth temperatures and Ga fluxes its incorporation decreases, limiting its use for MOCATAXY. Sn shows the ability to achieve high doping concentrations in conventional PAMBE, however surface segregation during growth and a delay in incorporation into the film is observed for lower concentrations. Sn doping during MOCATAXY growth, however, allows for sharp, controllable doping profiles for a variety of Sn concentrations across various orientations. Furthermore, Sn doping of (010) β-Ga2O3 via MOCATAXY demonstrates the highest electron mobility for continuously doped β-Ga2O3 grown via MBE. Sn doping of (001) β-Ga2O3 via MOCATAXY shows significantly higher electron mobility than conventional PAMBE. Si doping is also investigated, showing degradation of (010) β-Ga2O3 film quality, however promising electron mobility and high doping concentrations were achievable for (001) oriented growth via MOCATAXY. Deep acceptor doping allows for realization of potential barriers in β-Ga2O3, as well semi-insulating regions of the device, such as current blocking layers for vertical structures and an intentionally compensated film-substrate interface for lateral devices. Mg is investigated as an intentional dopant in conventional PAMBE growth of (010) β-Ga2O3. While sharp doping profiles and a range of doping concentrations are achievable, annealing at high temperatures (≥ 925 °C) allows for diffusion of Mg, limiting its application to lower growth temperature epitaxial techniques and processing steps. A mechanism of Mg diffusion via the mobile Mg interstitial species is proposed, involving interactions of point defects in the film during annealing. Additionally, Fe incorporation into β-Ga2O3 films grown on Fe doped substrates is shown to be the result of surface segregation, rather than diffusion. This incorporation can be limited using a low temperature Fe trapping buffer layer prior to growth of critical regions of the film structure. Finally, growth of heterostructures with β-(AlxGa1-x)2O3 via MOCATAXY is investigated. Maximum Al contents for (010) β-(AlxGa1-x)2O3 of 22% are achieved with high quality, coherently strained films. (001) β-(AlxGa1-x)2O3 films with Al contents up to 15% are also grown with smooth surface morphology and no evidence of extended defects or relaxation. A relationship between out of plane lattice parameter is derived using the fundamental stiffness tensor and stress and strain expressions for (001) β-(AlxGa1-x)2O3 coherently strained to the β-Ga2O3. Confirmation of the Al content in the films confirms the validity of the derived relationship. This demonstration of high quality (001) β-(AlxGa1-x)2O3 shows promise for future heterostructure based devices in this orientation.

In this work, we explored one material from the broad family of 2D semiconductors, namely WSe2 to serve as an enabler for advanced, low-power, high-performance nanoelectronics and optoelectronic devices. A 2D WSe2 based field-effect-transistor (FET) was designed and fabricated using electron-beam lithography, that revealed an ultra-high mobility of ~ 625 cm2/V-s, with tunable charge transport behavior in the WSe2 channel, making it a promising candidate for high speed Si-based complimentary-metal-oxide-semiconductor (CMOS) technology. Furthermore, optoelectronic properties in 2D WSe2 based photodetectors and 2D WSe2/2D MoS2 based p-n junction diodes were also analyzed, where the photoresponsivity R and external quantum efficiency were exceptional. The monolayer WSe2 based photodetector, fabricated with Al metal contacts, showed a high R ~502 AW-1 under white light illumination. The EQE was also found to vary from 2.74×101 % - 4.02×103 % within the 400 nm -1100 nm spectral range of the tunable laser source. The interfacial metal-2D WSe2 junction characteristics, which promotes the use of such devices for end-use optoelectronics and quantum scale systems, were also studied and the interfacial stated density Dit in Al/2D WSe2 junction was computed to be the lowest reported to date ~ 3.45×1012 cm-2 eV-1. We also examined the large exciton binding energy present in WSe2 through temperature-dependent Raman and photoluminescence spectroscopy, where localized exciton states perpetuated at 78 K that are gaining increasing attention for single photon emitters for quantum information processing. The exciton and phonon dynamics in 2D WSe2 were further analyzed to unveil other multi-body states besides localized excitons, such as trions whose population densities also evolved with temperature. The phonon lifetime, which is another interesting aspect of phonon dynamics, is calculated in 2D layered WSe2 using Raman spectroscopy for the first time and the influence of external stimuli such as temperature and laser power on the phonon behavior was also studied. Furthermore, we investigated the thermal properties in 2D WSe2 in a suspended architecture platform, and the thermal conductivity in suspended WSe2 was found to be ~ 1940 W/mK which was enhanced by ~ 4X when compared with substrate supported regions. We also studied the use of halide-assisted low-pressure chemical vapor deposition (CVD) with NaCl to help to reduce the growth temperature to ∼750 °C, which is lower than the typical temperatures needed with conventional CVD for realizing 1L WSe2. The synthesis of monolayer WSe2 with high crystalline and optical quality using a halide assisted CVD method was successfully demonstrated where the role of substrate was deemed to play an important role to control the optical quality of the as-grown 2D WSe2. For example, the crystalline, optical and optoelectronics quality in CVD-grown monolayer WSe2 found to improve when sapphire was used as the substrate. Our work provides fundamental insights into the electronic, optoelectronic and quantum properties of WSe2 to pave the way for high-performance electronic, optoelectronic, and quantum-optoelectronic devices using scalable synthesis routes.

Researches on graphene and other two-dimensional (2D) layered materials remain exponential growth, driven by the fundamental interests and potential applications. Isolated atomic layers are intrinsically the building blocks that can be reassembled into vertically stacked heterostructures. Those so-called van der Waals heterostructures exhibit intriguing, unique properties that cannot be found in their single-layer counterparts. Recent deterministic placement methods have further opened up new possibilities to fabricate even more complex heterostructures with high performance. Here, this report provides insights into the recent progress of 2D heterostructures with an emphasis on their fabrication-structure-performance datacubes. First, we introduce a detailed description of state-of-the-art deterministic assembly and fabrication methods. We then compare different approaches, summarize their advantages and limitations, alongside the recommendations on choosing suitable techniques. Next, we will discuss the supreme electrical properties of heterostructures and the electron transfer mechanisms that make them outstanding. Then, we present some typical examples of state-of-the-art high-performance electronic applications. Finally, the perspectives and challenges will be addressed for future developments of 2D heterostructures.

Modern material growth techniques allow for nano-engineering highly complex three dimensionally nanostructured materials. These nano-engineered materials possess highly anisotropic physical properties that are significantly different from that of their bulk counterparts. The magnetization properties of nano-engineered materials can be modified through a close range interaction known as magnetic exchange. These materials are referred to as magnetic exchange-coupled materials. Exchange-coupled magnetic materials are composite magnetic materials where the magnetization of one material is influenced by the magnetization state of the neighboring materials. The author describes the creation of a representative sample set of exchange-coupled nanoengineered magnetic materials. These materials are created by glancing angle deposition (GLAD), which is a physical vapor deposition process. By means of a self shadowing, highly ordered nanostructures are created which are columnar in shape and form a quasi-thin film. The nanocolumnar films are heterostructured by subsequent material depositions of cobalt and permalloy (Ni80Fe20). Atomic layer deposition (ALD) is utilized to introduce ultra thin dielectric gates between cobalt and permalloy in order to modify the exchange-coupling. ALD is further used to passivate the overall structures with conformal Al2O3 coatings to prevent oxidation. The structural properties of the nanoengineered materials are determined by scanning electron microscopy and generalized ellipsometry. The magnetic properties of the nanoengineered materials are determined at room temperature by vibrating sample magnetometry and by vector magneto-optical generalized ellipsometry (VMOGE). Vector magneto-optical generalized ellipsometry is a setup that combines generalized ellipsometry with a programmable vector magnet. The eight pole vector magnet is capable of producing magnetizing fields of arbitrary amplitude within the given physical limits and arbitrary spatial orientation to perform hysteresis loop measurements on quasi thin film samples. Mueller matrix generalized ellipsometry data are gathered from the sample with and without the magnetizing field present. Through a differencing process, the magnetically induced changes to the data are isolated. The data is analyzed with a layered optical model to determine the three dimensional magnetically induced changes to the dielectric tensor. These changes to the dielectric tensor of the magnetized material are proportional to the magnetization of the material. To quantify the magnetization properties of the materials the author develops hysteresis based models. The hysteresis models are compared to the characterization techniques that were performed on the representative sample set. The comparison of data for the representative sample set demonstrate the tailoring of magnetic properties, in particular the energy product, based on the geometry and material selection. The studies performed on the representative sample set by specialized means have found that the highly anisotropic magnetic properties are able to tuned by material selection and geometry. In particular, the use of GLAD in coordination with ALD allows for the control of the magnetic reversal by tuning the exchange-coupling between material subsections in the representative sample set. Advisor: Mathias Schubert

The modern world's dependency on electronics provides a constant need to discover new materials and devices. A promising technique to fabricate a new device is to create a heterostructure; a device consisting of two bulk crystals joined at an interface. These materials often support a low dimensional electron gas confined to the interface, which exhibits properties different than both the parent materials. These materials have led to the creation of MOSfets, the discovery of the quantum Hall effect, and in recent years the discovery of Majorana edge modes in nanowires. In this thesis, we study several different heterostructures. We begin with one of the most famous heterostructures, AlGaAs/GaAs. Modern AlGaAs/GaAs heterostructures support a high mobility two-dimensional electron gas (2DEG) in a quantum well. The 2DEG is provided by two remote donor $\delta$-layers placed on both sides of the well. Each $\delta$-layer is located in the midplane of a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. The excess electrons screen both the remote donors and background impurities, and are responsible for the observed high mobility. Still, we find that the mobility is substantially lower than theoretical estimates, which may be due to significant disorder in the donor layers, most likely roughness of the interfaces or spreading of the donors out of the midplane of the layer. Thus one should take care to make sure that the donor layers are as ideal as possible. We next move on to oxide heterostructures involving SrTiO$_3$ (STO). More specifically, we study the electron gas in accumulation layers of these heterostructures characterized by a density profile $n(x)$, where $x$ is the distance from the STO surface. SrTiO$_3$ at liquid helium temperatures has the highest dielectric constant which strongly enhances the role of nonlinear dielectric effects. It was recently shown that the nonlinear dielectric response results in an electron density profile $n(x)$ that slowly decays as $1/x^{12/7}$. We show that such a long tail of $n(x)$ causes the magnetization and the specific heat of the accumulation layer to diverge at large $x$. We explore the truncation of the tail by the finite sample width $W$, the transition from the nonlinear to linear dielectric response with dielectric constant $\kappa$, and the use of a back gate with a negative voltage $-\abs{V}$. We find that as a result both the magnetization and specific heat are anomalously large and obey nontrivial power law dependences on $W$, $\kappa$, or $\abs{V}$. In the linear dielectric regime under a strong magnetic field, the large dielectric constant of STO makes it easy to reach a quasi-one-dimensional state known as the extreme quantum limit (EQL) in which all electrons occupy the lowest Landau level. We present a theory of the EQL phase in STO accumulation layers. We find a phase diagram of the electron gas in the plane of the magnetic field strength and the electron surface concentration for different orientations of the magnetic field. In addition to the quasi-classical metallic phase (M), there is a metallic EQL phase, as well as an insulating Wigner crystal state (WC). Remarkably, the insulating Wigner crystal phase depends on the orientation of the magnetic field. We show that these effects can be measured through quantum capacitance measurements of the STO accumulation layer. The third material we study is semiconducting quantum wires. Though it is not a heterostructure, it supports a low dimensional electron gas which is often tuned with an external gate, making it similar to many of the devices we have studied. We have theoretically investigated the influence of interface roughness scattering on the low temperature mobility of electrons in quantum wires when electrons fill one or many subbands. We find the Drude conductance of the wire as a function of the linear concentration $\eta$ has a sharp peak. The height of this peak grows as a large power of the wire radius $R$, so that at large $R$ the conductance $G_{max}$ exceeds $e^2/h$ and a window of concentrations with delocalized states (which we call the metallic window) opens around the peak. Thus, we predict an insulator-metal-insulator transition with increasing concentration for large enough $R$. Furthermore, we show that the metallic domain can be sub-divided into three smaller domains: 1) single-subband ballistic conductor, 2) many-subband ballistic conductor 3) diffusive metal, and use our results to estimate the conductance in these domains. Finally we estimate the critical value of $R_c(\mathcal{L})$ at which the metallic window opens for a given length $\mathcal{L}$. We conclude the thesis with a discussion of a newer class of materials known as transition metal dichalcogenides (TMDs). We study a capacitor made of three monolayers TMD separated by hexagonal boron nitride (hBN). We assume that the structure is symmetric with respect to the central layer plane. The symmetry includes the contacts: if the central layer is contacted by the negative electrode, both external layers are contacted by the positive one. As a result a strong enough voltage $V$ induces electron-hole dipoles (indirect excitons) pointing towards one of the external layers. Antiparallel dipoles attract each other at large distances. Thus, the dipoles alternate in the central plane forming a 2D antiferroelectric with negative binding energy per dipole. The charging of a three-layer device is a first order transition, and we show that if $V_1$ is the critical voltage required to create a single electron-hole pair and charge this capacitor by $e$, the macroscopic charge $Q_c = eSn_c$ ($S$ is the device area) enters the three-layer capacitor at a smaller critical voltage $V_{c} < V_{1}$. In other words, the differential capacitance $C(V)$ is infinite at $V = V_{c}$. We also show that in a contact-less three-layer device, where the chemically different central layer has lower conduction and valence bands, optical excitation creates indirect excitons which attract each other, and therefore form antiferroelectric exciton droplets. Thus, the indirect exciton luminescence is red shifted compared to a two-layer device.

Perovskite oxide (ABO3 type compounds) is an important class of materials exhibiting a wide range of functionalities. However, in comparison to conventional semiconductors such as silicon, they possess orders of magnitude lower room-temperature electron mobilities. For example, in doped SrTiO3, the best reported room-temperature value of electron mobility has remained below 10 cm2 V−1 s−1 for over five decades. The realization of a perovskite oxide semiconductor with high room-temperature mobility would constitute a significant advancement, enabling novel physical and perhaps even a plethora of new and more realistic device concepts. Very recently a key step in this direction was taken via the growth of bulk doped BaSnO3, where room-temperature mobilities as high as 320 cm2 V−1 s−1 was reported. Thin films of BaSnO3 show much lower room-temperature mobility values ranging between 1-180 cm2 V−1 s−1 and highly dependent on the growth method, choice of substrate, and dopants. Although these findings have been encouraging for fundamental studies and potential applications in room-temperature oxide electronics, there still remains many open fundamental questions and challenges including the role of defects on the properties of BaSnO3 and the scattering mechanisms that limit the mobility in thin films from reaching values close to bulk mobility. These questions will be addressed in this thesis by studying thin films of BaSnO3 grown by molecular beam epitaxy. One of the challenges with the growth of BaSnO3 is the high electronegativity (low oxidation potential) of tin suggesting that stronger oxidizing conditions such as ozone or high-pressure oxygen plasma are required to achieve full oxidation of Sn. Such extreme oxidation conditions in an ultra-high vacuum molecular beam epitaxy system may lead to undesirable consequences such as oxidation of elemental sources leading to flux-instabilities, filament oxidation, and potential damage to vacuum pumps. As the first step in this direction, a new radical-based hybrid MBE approach for tin-based compounds is developed. For BaSnO3 growth, Ba is supplied through effusion from a cell, Sn using a chemical precursor (hexamethylditin – (CH3)6Sn2), and oxygen using a radio frequency plasma source. The unique aspect of our approach is that hexamethylditin forms highly reactive Sn• radicals, which facilitate the growth of phase-pure, stoichiometric films even in weak oxidizing environment such as molecular oxygen. Using this approach, synthesis of phase-pure and epitaxial BaSnO3 with scalable growth rates and layer-by- layer control over thicknesses is reported. Reflection high-energy electron diffraction is used to describe the strain relaxation behavior of BaSnO3. Various characterization techniques are employed for establishing the stoichiometric growth condition such as X-ray diffraction for lattice parameter measurements, Rutherford backscattering spec- trometry for quantification of cation (Sn:Ba) ratio, atomic force microscopy for imaging the surface morphology, electronic transport for measuring the carrier concentrations, resistivity, and electron mobility in lanthanum-doped BaSnO3 films, and time-domain thermoreflectance for determining the thermal conductivity. With the combination of these techniques, existence of a self-regulating “growth-window” is demonstrated. Through controlled La-doping in BaSnO3 films, a highest room-temperature electron mobility of 120 cm2 V−1 s−1 is achieved on a -5.12 % lattice-mismatched SrTiO3 substrate. The optimal doping range for the highest mobility is found to be 5.0 × 1019 cm−3 to 5.0 × 1020 cm−3. Mobility decreases at higher or lower doping concentrations. Temperature-dependent measurements of mobility provide insights into the scattering mechanisms limiting the mobility at different doping concentrations and temperatures. While dislocation scattering is found to be dominant at low doping regime, ionized impurity scattering plays a major role at high doping levels. At intermediate doping concentrations, both scattering mechanisms control the transport behavior. Phonon scattering accounts for the decreasing trend in mobility with increasing temperature. Building upon these findings which revealed mobility-limiting mechanisms in uniformly doped BaSnO3, the final step involves the development of modulation doping approach n BaSnO3-based heterostructures. The basic idea behind modulation doping is to sep- arate electrons from their ionized donors. Favorable band offsets in BaSnO3–SrTiO3 and BaSnO3–SrSnO3 systems are established. Taking BaSnO3–SrSnO3 as the model heterostructure, electron transfer from La-doped SrSnO3 to BaSnO3 is demonstrated, resulting in dramatic changes in the transport behavior. Results are encouraging and clearly suggest that electrons in BaSnO3 can be separated from ionized dopants. The transport, however, is still limited by dislocations and defects at the interface which should be the focus of future studies.

Multiferroic oxide heterostructures exhibit superior functional properties as a result of the interplay between the lattice, charge, orbital and electronic degrees of freedom. In particular these interactions can lead to emergent functionalities such as ferromagnetism and ferroelectricity at the interfaces. This offers excellent prospects for advanced devices based on the manipulation of the spin and charge degrees of freedom. However, the understanding of the underlying physics at the interface of these complex structures is still incomplete. This thesis investigates, (i) the effect of structural, electronic and compositional dependencies on the ferroelectric domain switching properties of bismuth ferrite (BiFeO3) multiferroic thin-films and, (ii) the emergent magnetism at the interface of perovskite manganite superlattices comprised of lanthanum strontium manganese oxide (LaMnO3) and calcium manganese oxide (CaMnO3) layers. Pulsed laser deposition (PLD) equipped with in-situ reflection high energy electron diffraction (RHEED) technique is employed for the deposition of the samples to ensure atomically precise layer by layer growth. The deposited structures are characterized using an array of characterization techniques such as atomic force microscopy (AFM), X-ray diffraction techniques, piezoresponse force microscopy (PFM) and physical property measurement systems (PPMS) to investigate the effect of internal and external influences upon the strong electron correlations in multiferroics with exciting potential for future spintronic applications. The thesis is made of three key studies; the first part focuses on the fabrication of BiFeO3 (BFO) thin-films of varying thickness on a SrTiO3 (STO) substrate of two different orientations, namely (100) and (110) with and without a bottom electrode. Through unique epitaxial constraints of a (110)-oriented STO substrate and an intermediate layer of SrRuO3 (SRO), single domain BFO thin-films are realized. Further, the samples revealed that the crystallographic orientation and electronic boundary conditions play a vital role in determining the structural and electronic properties in nanoscale BFO thin-films. Presence of the bottom electrode and the thickness of the film played a vital role in imposing considerable strain in BFO thin-film. It is shown that in the absence of the SRO layer when the thickness is increased from 50 to 100 and 150 nm, BFO films relaxes into a two-domain state. A significant reduction in the out-of-plane lattice parameter has also been observed with the increase in the thickness of the film. The results demonstrate that oxide heteroepitaxy of BFO can be carefully tuned to achieve various ferroic ground states. As the second stage, thin BFO films (~12 nm) have been successfully realised on LaAlO3 (100) substrates with LaSrMnO3 (LSMO) buffer layer in between. AFM topography scans show the absence of the characteristic stripe-like morphology. It is demonstrated that the surface roughness increases with the increase in bottom electrode layer thickness. XRD and RSM reveal the evolution of the phase with the electrode thickness. BFO films with 5nm and 10nm LSMO Buffer layers revealed a typical rhombohedral like R’ phase whereas with the increase LSMO layer thickness to 20nm; the phase evolved to tilted S’ like a phase. The PFM studies revealed that the domain size increases with the increase in thickness of the bottom electrode and sample exhibited both in-plane and out of plane domain signals. As the final stage of this study and the groundwork for future possibilities of coupling perovskite manganites and multiferroic BFO, LMO/CMO superlattices are fabricated on STO (100) substrate. The structures are then characterized using AFM, XRD and PPMS techniques. It is observed that varying the periodicity of the SL layers; we can vary the presence of manganese (Mn) ions with different valence states (Mn2+ and Mn3+) and thus introduce an antiferromagnetic state that co-exists with a robust ferromagnetic state. In summary, the study shows that it is possible to engineer novel functional performances via the interfacial engineering of mechanical and electrical boundary conditions in complex metal oxide heterostructures.

Van der Waals solids are a class of materials made up of loosely-bound, molecularly-thin layers. These layers can be readily detached from one another due to their weak interlayer van der Waals bonds, giving the materials their name. In 2004, the isolation of graphene, a single-atom-thick layer of carbon, from graphite, a bulk van der Waals material, prompted a surge of research interest in the nearly two-dimensional layers making up these materials. Their mechanical strength, stability in ambient conditions, flexibility, and near-transparency are intriguing for the production of flexible electronics. Meanwhile, their ultimate thinness opens an avenue for exploring extreme physics, offering the possibility of new kinds of devices, and making these materials intriguing to researchers in the short term. Graphene, the most famous, is a conductor. However, some van der Waals materials are semiconductors, which are necessary to produce electronic switches and light-emitting diodes (LEDs), while other van der Waals materials are insulators. There is considerable potential in combining these different van der Waals materials to produce so-called heterostructures, which can re-create existing functionality, such as light emission, or produce new devices with unique capabilities.The difficulty of manipulating atomically-thin semiconductors has prevented them from achieving commercial application. Existing processes for manipulating van der Waals semiconductors do not offer control over the shape or size of the single-layer features produced.By interrogating the role of stamp mechanics in the success of producing single layers, this thesis develops a method that enables the creation of van der Waals heterostructure arrays. After an introduction to the current state of the art in scalable manufacture of van der Waals monolayers and its tradeoffs in Chapter 1, Chapter 2 demonstrates a new process for creating patterned arrays of semiconducting monolayers from a bulk source. In order to measure the effectiveness of the method, this work introduces yield metrics relevant to nanomanufacturing of 2D material patterns. To further understand the role of the stamp in the process of exfoliation, Chapter 3 presents a finite element model bridging macroscale and nanoscale physics to understand the interaction between van der Waals layers and an elastic stamp. The results from this model carry implications for nanomanufacturing process design.Chapter 4 then concludes with discussion of the usefulness and limitations of the methods developed in this work, and considers future directions that will enable high-yield mass production of van der Waals heterostructures.

Transition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.

Two-dimensional layered crystals are a promising class of materials for post-silicon electronics. Due to their atomic thinness, flexibility, and versatile electrical properties (i.e. conductors, semiconductors, and insulators), we can envision future ultra-small, flexible computers completely comprised of various two-dimensional materials. For this application, lateral heterostructures of two-dimensional materials play a major role in the realization of wholly two- dimensional devices, as they are the fundamental elements in a circuit, such as p-n junctions and metal-semiconductor contacts. This dissertation will employ transmission electron microscopy and related techniques to address how different two-dimensional materials merge to form lateral heterostructures, specifically between two distinct two-dimensional semiconductors (analogous to p-n junctions) and two-dimensional conductor- semiconductor heterostructures (analogous to metal-semiconductor contacts). Within the heterostructures between two semiconductors, Chapter 2 and 3 will discuss atomically sharp interfaces and gradual interfaces in lateral heterostructures, respectively. Chapter 4 will describe the conductor-semiconductor interconnects between two-dimensional materials with dissimilar lattice structures. Our results demonstrate how the strain is relaxed in epitaxial lateral heterostructures, as well as how the heterostructure between crystallographically distinct two-dimensional materials forms. These findings can unravel how to use or engineer distortions in two-dimensional lateral heterojunctions, predict the mechanical strength and devices performance, and inform the mechanism of chemical synthesis at the interface between atomically thin films.

This thesis is devoted to improving the spectroscopic performance of the gallium antimonide (GaSb)/aluminum arsenide antimonide (AlAsSb) energy-sensitive detector. The heterostructure device is realized by integrating the GaSb absorber and the AlAsSb digital-alloy, combined with a field-control layer to optimize the electric field profile. This work attempts to enhance the energy resolving ability of the detector by lowering its surface leakage current, which is the dominant dark current source and limits the signal to noise ratio (SNR) of the detection system. To achieve the proposed goal, a layer of aluminum oxide (Al2O3) is deposited on the detector surface by ALD (Atomic Layer Deposition), which suppresses the formation of surface leakage path by interface chemistry between Al2O3 and the native oxides (e.g. Ga2O3 and Sb2O3). Well-improved dark current levels are measured in the Al2O3-passivated device, and the spectroscopic performance also indicates reduced electronic noise.

Since the discovery of graphene, two-dimensional (2D) van der Waals materials research has flourished, with new crystals and novel phenomena discovered at a rapid pace. In 2014, black phosphorus (BP) was rediscovered as a member of 2D materials, displaying high quality electronic transport with in-plane anisotropy, though the first generation of layered devices suffered from poor stability and relatively low mobility. In this thesis, we overcome these challenges by employing van der Waals assembly techniques to create BP devices sandwiched between thin hexagonal boron nitride layers. These devices are air-stable, for more than two weeks, and boast mobility up to 500 cm2V-1s-1 at room temperature and 4,000 cm2V-1s-1 at low temperature. Using two-point and four-point geometries with global and local gates, we observe Shubnikov de Haas oscillations that yield effective mass of holes ~0.25 me to 0.31 me, where me is the mass of the electron. From weak localization measurements, we obtain phase relaxation length of ~30 nm to 100 nm. Moreover, we determine mobility bottleneck and transport mechanism from temperature dependence measurements: when the device is highly hole doped, the mobility µ exhibits power law dependence on temperature T, µ ~ T0.6, suggesting that it is limited by charged impurity scattering; close to the band edge, conduction is dominated by variable range hopping, with estimated localization length ~ 1.7 nm – 30 nm. The studies presented here contributed to our understanding of electronic properties of atomically thin BP, and will help to guide future efforts in fabrication, exploring, and engineering devices based on 2D semiconductors.

Scaling limits of conventional complementary metal oxide semiconductor (CMOS) technology has motivated the research of numerous beyond CMOS device concepts. One such device is the interlayer tunnel FET (ITFET). This device is demonstrated using the two-dimensional (2D) materials bilayer graphene and hexagonal boron nitride (hBN). Stacking these materials together, we fabricate a double bilayer graphene and hBN heterostructure where the two graphene layers function as the top and bottom electrodes and the hBN as the tunnel barrier of the ITFET. Significant negative differential resistance (NDR) in the interlayer current-voltage characteristic is demonstrated at room temperature. Electrostatic analysis reveals that the multiple NDR peaks are due to energetic band alignment between the two sub-bands of the top and bottom bilayer graphene at the K-point of the Brillouin zone. Temperature dependent and parallel magnetic field measurements are conducted to further confirm that the conduction mechanism is momentum and energy conserving resonant tunneling. In addition, we demonstrate how the NDR can be used for implementing a one-transistor static random access memory element. Improvements in the transfer method which allowed rotationally aligned top and bottom electrode layers, made possible extensive experimental studies of 2D heterostructure based ITFETs. Utilizing such technique, we conducted experiments involving thicker multilayer graphene. We analyze how the graphene thickness and stacking order can influence the resonance condition and how this affects the overall device characteristic. The effects of interlayer hBN scaling such as increased tunneling current and peak position shifting is also briefly dealt with. Current-voltage simulations based on Bardeen transfer Hamiltonian approach were conducted for these devices, and it is shown that the peak positions predicted by theory match well with those obtained through measurements.

In this dissertation, a wide variety of optical spectroscopies including photoluminescence (PL), photoluminescence excitation (PLE), photoluminescence imaging, time-resolved photoluminescence (TRPL), as well as photocurrent (PC) spectroscopy have been used to explore the optical and electronic structures of Zn-doped GaAs twinning superlattices (TSL) and GaAs/AlGaAs quantum well tube (QWT) nanowires.Low Temperature PL spectra of a single Zn-doped GaAs TSL nanowire reveal a broad band low energy emission centerd at 1.47 eV in addition to the free exciton emission of zincblende GaAs. We do not observe any changes in the relative ratio of these emissions as a function of excitation power, suggesting that the low energy emission is not related to defects or carbon impurities, this is further confirmed by PC measurement. A simple theoretical model with holes confined in the wurtzite segment is developed in order to understand the origin of the emissions.In GaAs/AlGaAs QWT nanowires, low temperature PL spectra of 8 and 4 nm QWTs show that the exciton confinement energy have been shifted up by 57 meV and 185 meV with respect to the GaAs core. A first excited state transition has been observed in PLE spectra of only the 8 nm QWT. A simple theoretical modeling using a cylindrical quantum well shows the calculations are in good agreement with both direct and indirect transitions observed in PL and PLE measurements. A more detailed model including the hexagonal symmetric facets shows that the electron and holes ground states are strongly localized to the corners, while the excited states are confined to the long facets of the QWT.In order to gain more information about quantum confined energy in these radial heterostructures, we then studied a series of QWTs with thicknesses ranging from 1.5 to 8 nm. High resolution spatially-resolved PL images reveal several ultranarrow emission lines (localized states) distributed at different spatial positions along the long axis of 2 and 4 nm QWTs, but no such localized states are observed in the 6 and 8 nm QWT nanowires. This indicates that narrow QWTs are more sensitive to well width and alloy fluctuations.In the 2 nm QWT, high resolution PL spectra reveal many ultranarrow emission lines on the high energy side at low temperatures. These localized states quench rapidly compared to the low energy emissions as a function of temperature. The overall emission linewidth also decreases as a function of temperature. Time-dependent PL spectra show that the localized states are stable at low temperatures but become metastable at higher temperature. We developed a rate equation model where localized states are decoupled with the delocalized states at low temperatures, but become strongly coupled once the excitons have enough thermal energy to surmount the energy barrier at higher temperature. Parameters extracted from this model show a thermal activation energy of 10.9 meV for the localized states. The model provides a view of the evolution of the complex energy landscape in these QWTs as a function of temperature from 10 to 120 K.

Graphene, as a famous Van der Waals material, has attracted intensive attention from research group and industry all over the world after 2004, while hexagonal boron nitride (h-BN), as an excellent two-dimensional (2D) dielectric layer, has been studied intensively mainly for its compatibility with graphene and other 2D materials. To realize the technological potential of 2D system, it is essential to synthesize large-area, high-quality 2D thin films through a scalable and controllable method in order to investigate novel phenomenon in fundamental physics and promising device applications. In this thesis, the growth of graphene, h-BN and their vertical and lateral heterostructures by molecular beam epitaxy (MBE) is mainly discussed. In addition, the growth mechanism, fundamental physics and possible applications are also studied.In-situ epitaxial growth of graphene/h-BN heterostructures on cobalt (Co) film substrate was achieved by using plasma-assisted MBE in Chapter 2. We demonstrated a solution for direct fabricating graphene/h-BN vertical stacking structures. Various characterizations, such as Raman spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), were carried out to confirm and evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1º. Chapter 3 studied the growth of graphene/h-BN heterostructures on Co foil substrate by plasma-assisted MBE. It is found that the coverage of h-BN layers on the epitaxial thin graphite layer is growth-time dependent. Large-area, uniform-quality h-BN film was successfully deposited on thin graphite layer. Based on the as-grown h-BN (5-6 nm)/G (26-27 nm) heterostructure, without using any transferring process, we fabricated capacitor devices with Co(foil)/G/h-BN/Co(contact) configuration to evaluate the dielectric properties of h-BN film. The measured breakdown electric field showed a high value of 2.5-3.2 MV/cm. Both I-V and C-V characteristics indicated that the epitaxial h-BN film is of good insulating nature.Following with the lateral growth of graphene on in situ epitaxial h-BN flakes by plasma-assisted MBE is discussed in Chapter 4. Single-crystal h-BN domains were grown on Co film substrates at a substrate temperature of 850~900 oC using plasma-assisted MBE. Three-point star shape h-BN domains were observed by SEM, and confirmed by Raman and XPS. The h-BN on Co template was used for in situ growth of multilayer graphene, leading to an h-BN/graphene heterostructure. Carbon atoms preferentially nucleate on Co substrate and edges of h-BN and then grow laterally to form continuous graphene. Further introduction of carbon atoms results in layer-by-layer growth of graphene on graphene and lateral growth of graphene on h-BN until it may cover entire h-BN flakes.The final part (Chapter 5) is related to the growth of large-area and multi-layer hexagonal boron nitride film on polished Co foils by plasma-assisted MBE. The coverage of h-BN layers can be readily controlled by growth time under appropriate growth conditions. A large-area, multi-layer h-BN film is confirmed by various characterizations. Dielectric property of as-grown h-BN film is evaluated by characterization of Co(foil)/h-BN/Co(contact) capacitor devices. Breakdown electric field is in the range of 3.0-3.3 MV/cm, which indicates that the epitaxial h-BN film has good insulating characteristics. In addition, the effect of substrate morphology on h-BN growth is discussed regarding different domain density, lateral size, and thickness of the h-BN films grown on unpolished and polished Co foils.

Transistion metal dichalcogenides (TMDs) are an emerging class of materials for thin film transistors (TFTs). The inherent transparent properties, functionality and large lateral dimensions offer material selection for sensor technologies and transparent flexible displays. Low temperature processing of large crystallographic ordered films is needed to prevent thermal degradation of modern flexible electronic substrates. In this study, polycrystalline TMD growth mechanisms and, for the first time, a scalable processing technique used at room temperature will be demonstrated for application of semiconducting TMDs directly onto substrates by laser processing. A means of controlling the material structure by photon-phonon excitation provides a low temperature approach to controlling structural changes in materials; in particular, these studies are conducted on thin-film molybdenum disulfide and tungsten disulfide. A significant variance in both structure and electronic properties upon laser processing are observed in all samples. The direct deposition of molybdenum disulfide and tungsten disulfide on cross-linked polymers followed by localized laser annealing results in few atomic layer hexagonal sheets confirmed by Raman analysis, x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The strong enhanced A1g and E2g bands in Raman and bonding characteristic in XPS suggests the layered S-Mo-S and S-W-S bonding. Conductive atomic force microscopy (C-AFM) on laser exposed samples showed a change in topography and electrical properties from insulating material to semiconducting after laser annealed. Laser annealing of amorphous films on polymer substrates is a simple and scalable approach to patterning complex patterns of semiconducting MoS2 film in an insulating matrix or generating large areas of semiconducting material for fabrication of 2D based flexible/stretchable electronics.

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.

The increasing demand for fossil fuels and the need to reduce greenhouse gases require clean energy sources and more efficient utilization of energy. Thermoelectric materials provide a means toward achieving these goals since they convert heat, including waste heat, directly into an electric potential difference. Metal-semiconductor heterostructures can work as Schottky barriers in thermoelectric materials to increase thermoelectric efficiency.In this project, nickel silicide phases were introduced into silicon nanowires (SiNWs) to build up the Schottky barrier. SiNW arrays were fabricated using a metal-assisted chemical process, creating SiNWs about 200 nm in diameter and 30ìm in length. Different methods were adopted for nickel deposition: electroless nickel deposition, electro nickel deposition, E-beam deposition, and thermal evaporation. The samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that depositing nickel on SiNWs in an aqueous solution without electricity is a simple way to deposit nickel particles, and the morphology of nickel particles depends on the concentration of the deposition bath. However, an aqueous solution will cause oxidation of the SiNWs and hinder the formation of nickel silicide. To solve this problem, depositing nickel on SiNWs in organic solutions inside an oxygen-free glove box is a way to prevent oxidation, and nickel can diffuse into silicon substrates easily via annealing when there no oxidation layer on the surface of SiNWs. The dominant phase formed in these samples is NiSi2 after being annealed at 650°C for one hour in a tube furnace.

Silicon Germanium thin films are the most stable thermoelectric materials at high temperatures. Nonetheless, low efficiency and limited knowledge of such structures are still a challenge to research. Understanding the various mechanisms taking place in the matter and their relationships is the next step to boost research and to enhance the TE efficiencies, widening the range of applications. In this thesis, we develop a mathematical approach based on solid state theory to calculate the figure of merit from an electronic band structure. Added to DFT, this near ab initio method rapidly assesses virtual structures as possible TE materials. In this report, the method is applied to bulk silicon germanium with good agreement with experimental results. Throughout the development of this method, we also conclude that TE semiconductors band-gaps are related to the range of temperature where the material show higher values of ZT. We also show that doping with donors and acceptors, which is a common enhancement, need optimization for high temperatures applications due to its contribution to the thermal conductivity. Moreover the carriers' mobility is a prevalent parameter but its calculation remains complex. So, we implement deformation potential theory with DFT to calculate the electron-phonon interactions in silicon germanium alloys. We find no change of interactions with the alloy composition. With enough computing power, these methods are applicable to low dimensions structures. In addition to our theoretical study, we report a sputter deposition method of silicon germanium alloy thin films with controlled composition and thickness, grown on a sputtered layer of silicon dioxide. XRD study shows the appearance of a crystal phase beginning at deposition temperatures of 650 C . Reflectivity and TEM provide a consistent measurement of deposition thickness in the range of 20 to 100 nm with average interfaces roughness around 2 nm.