Spin-dependent transport properties of Fe3O4/MoS2/Fe3O4 junctions

Magnetite is a half-metal with a high Curie temperature of 858 K, making it a promising candidate for magnetic tunnel junctions (MTJs). Yet, initial efforts to exploit its half metallic nature in Fe3O4/ MgO/Fe3O4 MTJ structures have been far from promising. Finding suitable barrier layer materials, which keep the half metallic nature of Fe3O4 at the interface between Fe3O4 layers and barrier layer, is one of main challenges in this field. Two-dimensional (2D) materials may be good candidates for this purpose. Molybdenum disulfide (MoS2) is a transition metal dichalcogenide (TMD) semiconductor with distinctive electronic, optical, and catalytic properties. Here, we show based on the first principle calculations that Fe3O4 keeps a nearly fully spin polarized electron band at the interface between MoS2 and Fe3O4. We also present the first attempt to fabricate the Fe3O4/MoS2/Fe3O4 MTJs. A clear tunneling magnetoresistance (TMR) signal was observed below 200 K. Thus, our experimental and theoretical studies indicate that MoS2 can be a good barrier material for Fe3O4 based MTJs.Our calculations also indicate that junctions incorporating monolayer or bilayer MoS2 are metallic

Fabrication of sub-5 nm uniform zirconium oxide films on corrugated copper substrates by a scalable polymer brush assisted deposition method

We demonstrate a polymer brush assisted approach for the fabrication of continuous zirconium oxide (ZrO2) films over large areas with high uniformity (pin-hole free) on copper (Cu) substrates. This approach involves the use of a thiol-terminated polymethyl methacrylate brush (PMMA-SH) as the template layer for the selective infiltration of zirconium oxynitrate (ZrN2O7). The preparation of a highly uniform covalently grafted polymer monolayer on the Cu substrate is the critical factor in fabricating a metal oxide film of uniform thickness across the surface. Infiltration is reliant on the chemical interactions between the polymer functional group and the metal precursor. A following reductive H2 plasma treatment process results in ZrO2 film formation whilst the surface Cu2O passive oxide layer was reduced to a Cu/Cu2O interface. Fundamental analysis of the infiltration process and the resulting ZrO2 film was determined by XPS, and GA-FTIR. Results derived from these techniques confirm the inclusion of the ZrN2O7 into the polymer films. Cross-sectional transmission electron microscopy and energy dispersive X-ray mapping analysis corroborate the formation of ZrO2 layer at Cu substrate. We believe that this quick and facile methodology to prepare ZrO2 films is potentially scalable to other high-κ dielectric materials of high interest in microelectronic applications

Two dimensional semiconducting transition metal dichalcogenides via thermally assisted conversion - synthesis, characterisation and electronic properties

2D transitional metal dichalcogenides (TMDs) are of major interest to the research and electrical engineering community. A number of TMDs are semiconducting and have a wide range of bandgaps, they can exhibit n- or p- type behaviour, and the electronic structure changes with the number of layers. These interesting properties hold much promise for a host of electrical applications including low or high power field effect transistors, sensors and diodes. Moreover the unique optical properties and direct bandgap of monolayer TMDs make them attractive for optoelectronic applications such as Light Emitting Diodes (LEDs), photodiodes, couplers and photovoltaic cells. Many reports of 2D TMDs electrical properties involve devices created by micromechanical exfoliation. While providing pristine material, this method is laborious and is not suitable for anything larger than lab-scale demonstrations. In order to fulfil the potential of these materials, a synthesis route which is well controlled, scalable, and reproducible is required. A further prerequisite is the ability of the materials to be integrated with semiconductor industry process flows. Thermally assisted conversion (TAC), a variant of chemical vapour deposition (CVD), shows much promise for meeting these requirements. This thesis provides a detailed treatise on the development and subsequent characterisation of 2D TMDs produced from TAC. The TAC process produces thin films of polycrystalline TMDs. In order to synthesise and investigate these materials, a Low Pressure Chemical Vapour Deposition (LPCVD) system was designed and built in house. Firstly sulfide based TMDs are considered, particularly MoS2 and WS2. Due to well-regulated metal deposition conditions, TAC was shown to be suitable for producing continuous MoS2 thin films from bulk like thicknesses (≥ 20 nm) down to films with monolayer character. This is notable because monolayer MoS2 is a direct bandgap material, and thus optoelectronic applications come within the remit of this scalable synthesis method. Films were characterised by a suite of methods including scanning Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and electron microscopy. Electronic devices fabricated from MoS2 include Hall bars, Thin Film Transistors (TFTs) and chemiresistor gas sensors; these probed the electrical quality of the TAC materials. Unencapsulated devices exhibited room temperature Hall mobilities of 0.5-1 cm2V-1s-1. These values are too low for logic applications, however they compare well to literature values for MoS2films synthesised in an industrially relevant manner. Electrical Double Layer (EDL)-TFTs showed mobilities of 1.4 cm2V-1s-1 and this was increased to 10 cm2V-1s-1 when the synthesis temperature was increased to 1000 ?C. Chemiresistor gas sensors are a very suitable application for 2D TMDs due to their high surface area to volume ratio. Sensors were created featuring low power room temperature operation, fast response times and ultra-high sensitivity. MoS2sensors demonstrated an empirical limit of detection (LOD) of 300 ppb, and a theoretical LOD of 51 ppb for NH3. TAC growth was then extended to include the corresponding selenides. Another LPCVD system was designed and built, affording the opportunity to include automation and data-logging capabilities. Selenide thin film synthesis recipes for MoSe2 and WSe2 were developed and optimised. Temperature was shown to be a critical parameter in film growth. The films were characterised using the same techniques as were used for the sulfides. Electrical characterisation of the selenide family of TMDs showed generally better performance than the sulfides. EDL-TFTs made from MoSe2synthesised at 800 ?C showed a mobility of 4.3 cm2V-1s-1, this increased to 20.6 cm2V-1s-1 for samples grown at 1000 ?C. Chemiresistor sensors from selenides were also very sensitive. Devices utilising MoSe2 exhibited a LOD of 0.8 ppm for NO2and 2 ppm for NH3. WSe2 sensors displayed a LOD of 0.8 ppm to NH3. The TAC process is a viable method to produce 2D TMDs in an industrially relevant manner. It produces high quality thin films and it is potentially applicable for synthesising many different TMDs. The electronic quality of the material produced is such that logic applications are limited. However, there are a host of areas where TAC holds much promise, including sensing, catalysis and optoelectronics

Optimization and Control of Large Block Copolymer Self-Assembly via Precision Solvent Vapor Annealing

The self-assembly of ultra-high molecular weight (UHMW) block copolymers (BCPs) remains a complex and time-consuming endeavor owing to the high kinetic penalties associated with long polymer chain entanglement. In this work, we report a unique strategy of overcoming these kinetic barriers through precision solvent annealing of an UHMW polystyrene-block-poly(2-vinylpyridine) BCP system (Mw: ?800 kg/mol) by fast swelling to very high levels of solvent concentration (?s). Phase separation on timescales of ?10 min is demonstrated once a thickness-dependent threshold ?s value of ?0.80?0.86 is achieved, resulting in lamellar feature spacings of over 190 nm. The threshold ?s value was found to be greater for films with higher dry thickness (D0) values. Tunability of the domain morphology is achieved through controlled variation of both D0 and ?s, with the kinetically unstable hexagonal perforated lamellar (HPL) phase observed at ?s values of ?0.67 and D0 values of 59?110 nm. This HPL phase can be controllably induced into an order?order transition to a lamellar morphology upon further increase of ?s to 0.80 or above. As confirmed by grazing-incidence small-angle X-ray scattering, the lateral ordering of the lamellar domains is shown to improve with increasing ?s up to a maximum value at which the films transition to a disordered state. Thicker films are shown to possess a higher maximum ?s value before transitioning to a disordered state. The swelling rate is shown to moderately influence the lateral ordering of the phase-separated structures, while the amount of hold time at a particular value of ?s does not notably enhance the phase separation process. These large period self-assembled lamellar domains are then employed to facilitate pattern transfer using a liquid-phase infiltration method, followed by plasma etching, generating ordered, high aspect ratio Si nanowall structures with spacings of ?190 nm and heights of up to ?500 nm. This work underpins the feasibility of a room-temperature, solvent-based annealing approach for the reliable and scalable fabrication of sub-wavelength nanostructures via BCP lithography

High-Performance Sensors Based on Molybdenum Disulfide Thin Films

High-performance sensors based on molybdenum disulfide (MoS2) grown by sulfurization of sputtered molybdenum layers are presented. Using a simple integration scheme, it is found that the electrical conductivity of MoS2 films is highly sensitive to NH3 adsorption, consistent with n-type semiconducting behavior. A sensitivity of 300 ppb at room temperature is achieved, showing the high potential of 2D transition metal-dichalcogenides for sensing

Saturation of Two-Photon Absorption in Layered Transition Metal Dichalcogenides: Experiment and Theory

The saturation of two-photon absorption (TPA) in four types of layered transition metal dichalcogenides (TMDCs) (MoS₂, WS₂, MoSe₂, WSe₂) was systemically studied both experimentally and theoretically. It was demonstrated that the TPA coefficient is decreased when either the incident pulse intensity or the thickness of the TMDC nanofilms increases, while TPA saturation intensity has the opposite behavior, under the excitation of 1.2 eV photons with a pulse width of 350 fs. A three-level excitonic dynamics simulation indicates that the fast relaxation of the excitonic dark states, the exciton–exciton annihilation, and the depletion of electrons in the ground state contribute significantly to TPA saturation in TMDC nanofilms. Large third-order nonlinear optical responses make these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications

A New 2H-2H′/1T Cophase in Polycrystalline MoS₂ and MoSe₂ Thin Films

We report on 2H-2H′/1T phase conversion of MoS₂ and MoSe₂ polycrystalline films grown by thermally assisted conversion. The structural conversion of the transition metal dichalcogenides was successfully carried out by organolithium treatment on chip. As a result we obtained a new 2H-2H′/1T cophase system of the TMDs thin films which was verified by Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The conversion was successfully carried out on selected areas yielding a lateral heterostructure between the pristine 2H phase and the 2H′/1T cophase regions. Scanning electron microscopy and atomic force microscopy revealed changes in the surface morphology and work function of the cophase system in comparison to the pristine films, with a surprisingly sharp lateral interface region

Structural and electrical investigation of MoS2 thin films formed by thermal assisted conversion of Mo metal

Large-area synthesis is of great demand for the preparation of high-performance transition-metal-dichalcogenides (TMD) devices, however there are only limited reports to date of device operation on large-area TMDs. In this work we fabricate MoS2 devices based on Thermal Assisted Conversion (TAC) of metal layers, and characterize the thin-films with material analysis combined with electrical device parameter extraction. Specifically we report on temperature dependent parameter extraction for Ti/Au contacts to MoS2 thin-films to determine sheet resistance (Rsh), resistivity (ρ), and the activation energy (EA) of on-state current flow. For undoped MoS2, ρ was determined to be 191 Ω.cm at 25°C. The activation energy of the on-state current was found to be 0.18 eV, pointing to the presence of deep levels in MoS2

Fabrication of high-κ dielectric metal oxide films on topographically patterned substrates: polymer brush mediated depositions

Fabrication of ultrathin films of dielectric (with particular reference to materials with high dielectric constants) materials has significance in many advanced technological applications including hard protective coatings, sensors, and next generation logic devices. Current state-of-the-art in microelectronics for fabricating these thin films is a combination of atomic layer deposition and photolithography. As feature size decreases and aspect ratios increase, conformality of the films becomes paramount. Here, we show a polymer brush template assisted deposition of highly conformal, ultrathin (sub 5 nm) high-κ dielectric metal oxide films (hafnium oxide and zirconium oxide) on topographically patterned silicon nitride substrates. This technique, using hydroxyl terminated poly-4-vinyl pyridine (P4VP-OH) as the polymer brush, allows for conformal deposition with uniform thickness along the trenches and sidewalls of the substrate. Metal salts are infiltrated into the grafted monolayer polymer brush films via solution deposition. Tailoring specific polymer interfacial chemistries for ion infiltration combined with subsequent oxygen plasma treatment enabled the fabrication of high-quality sub 5 nm metal oxide films