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The high current-carrying capacity of graphene is essential for its use as an interconnect in electronic and spintronic circuits. At the same time, knowing the breakdown limits and mechanism under high fields can enable new device design strategies. In this work, we push the current carrying capacity of the scalable form of chemical vapor deposited (CVD) graphene employing a high-thermal conducting single crystalline diamond substrate. Our experiments on CVD graphene reveal extremely high current densities > 10⁹ A/cm² in graphene on the diamond with both ohmic (low-resistive) and tunneling tunnel (high-resistive) contacts. Measurements on ferromagnetic (TiO_x/Co) and metallic (Ti/Au) contacts demonstrate current densities of ∼1.16×10⁹ A/cm² and ∼1.7×10⁹ A/cm², respectively. The tunnel (high-resistive) contacts exhibit a shunting of graphene under high currents via the bottom graphitized diamond, resulting in dielectric breakdown and via alternative conducting paths. Electrical measurements show a distinct threshold for conducting paths of graphitized diamond, in tune accordance with Middleton-Wingreen's theory. Our results of high current densities achieved in CVD graphene, with distinct dependence on ohmic and tunneling, contact resistance, and the observed breakdown mechanism, provide new insights for enabling high-current all carbon circuits.

Achieving enhanced and stable electrical quality of scalable graphene is crucial for practical graphene device applications. Accordingly, encapsulation has emerged as an approach for improving electrical transport in graphene. In this study, we demonstrate high-current treatment of graphene passivated by AlOx nanofilms as a new means to enhance the electrical quality of graphene for its scalable utilization. Our experiments and electrical measurements on large-scale chemical vapor-deposited (CVD) graphene devices reveal that high-current treatment causes persistent and irreversible de-trapping density in both bare graphene and graphene covered by AlOx. Strikingly, despite possible interfacial defects in graphene covered with AlOx, the high-current treatment enhances its carrier mobility by up to 200% in contrast to bare graphene samples, where mobility decreases. Spatially resolved Raman spectroscopy mapping confirms that surface passivation by AlOx, followed by the current treatment, reduces the number of sp3 defects in graphene. These results suggest that for current treated-passivated graphene (CTPG), the high-current treatment considerably reduces charged impurity and trapped charge densities, thereby reducing Coulomb scattering while mitigating any electromigration of carbon atoms. Our study unveils CTPG as an innovative system for practical utilization in graphene nanoelectronic and spintronic integrated circuits.

Graphene, the atomically thin material of carbon atoms, first isolated experimentally in 2004, exhibits remarkable properties and holds potential for applications in quantum, electrical, and spin-based devices. The chemical vapor deposition (CVD) method enables graphene production on a large scale, merging its exceptional characteristics with scalability and high-quality implementation. Despite the extraordinary promise of CVD graphene with structural imperfections, the main challenge for graphene electronics and spintronics lies in achieving reliability at the device and circuit levels with scalable materials and interfaces. To address these, it is essential to understand the intricacies, endurance, and performance issues in graphene devices. In this thesis, to understand graphene interfaces in devices, we first explored a critical aspect of graphene's interaction with metal oxides, particularly titanium oxide (TiO_x) and aluminum oxide (AlO_x), and their implications for graphene-based nanoelectronic and spintronic devices. Investigating the electrical characteristics of graphene, both with and without oxides, uncovers the distinct behaviors of TiO_x and AlO_x when interfaced with graphene, highlighting the charge transfer-induced p-type doping and the formation of sp³ defects, traps, and impurities, especially at the AlO_x/graphene interface. These findings bring new insights for graphene spintronic devices while opening possibilities for novel functionalities such as hybrid resistive switching devices. Advancing further towards van der Waals heterostructures in these studies, we could also observe the impact of monolayer MoS₂ on graphene’s properties. Next, we explored how CVD graphene devices withstand high current stress to elucidate device durability and resilience. We examine the impact of extreme electric currents on channel structures and resistive tunnel barrier interfaces, focusing on their feasibility for high-capacity electronic and spintronic applications. Here, despite the polycrystalline nature of CVD graphene, we could observe the highest current density of 5.2×10⁸ Acm^-2 in graphene on Si/SiO₂ substrates, elevating it further to 1.7×10⁹ Acm^-2 on diamond substrates, remarkably exceeding previous reports. Performing systematic cyclic electrical measurements, with a gradual increase in the applied high current, we could determine the limits of the reversible regime for safe device operation of both channels and contacts. This knowledge of high current limits and oxide interfaces with graphene leads to an innovative current-treated passive graphene (CTPG) system, where we passivated graphene with metal oxide and applied high current to enhance quality. This method addresses the challenge of interfacial defects and remarkably improves carrier mobility, thereby reducing Coulomb scattering while mitigating electromigration issues. The CTPG presents a scalable platform for stable nanoelectronic and spintronic circuits. The experiments and systems studied in this thesis open possibilities for the exploration of temperature-dependent charge and spin transport measurements via new heterostructures and interfaces with different material combinations.

Two-dimensional (2D) van der Waals heterostructures combine the distinct properties of individual 2D materials, resulting in metamaterials, ideal for emergent electronic, optoelectronic, and spintronic phenomena. A significant challenge in harnessing these properties for future hybrid circuits is their large-scale realization and integration into graphene interconnects. In this work, we demonstrate the direct growth of molybdenum disulfide (MoS₂) crystals on patterned graphene channels. By enhancing control over vapor transport through a confined space chemical vapor deposition growth technique, we achieve the preferential deposition of monolayer MoS₂ crystals on monolayer graphene. Atomic resolution scanning transmission electron microscopy reveals the high structural integrity of the heterostructures. Through in-depth spectroscopic characterization, we unveil charge transfer in Graphene/MoS₂, with MoS₂ introducing p-type doping to graphene, as confirmed by our electrical measurements. Photoconductivity characterization shows that photoactive regions can be locally created in graphene channels covered by MoS₂ layers. Time-resolved ultrafast transient absorption (TA) spectroscopy reveals accelerated charge decay kinetics in Graphene/MoS₂ heterostructures compared to standalone MoS₂ and upconversion for below band gap excitation conditions. Our proof-of-concept results pave the way for the direct growth of van der Waals heterostructure circuits with significant implications for ultrafast photoactive nanoelectronics and optospintronic applications.

Understanding the stability and current-carrying capacity of graphene spintronic devices is key to their applications in graphene channel-based spin current sensors, spin-torque oscillators, and potential spin-integrated circuits. However, despite the demonstrated high current densities in exfoliated graphene, the current-carrying capacity of large-scale chemical vapor deposited (CVD) graphene is not established. Particularly, the grainy nature of chemical vapor deposited graphene and the presence of a tunnel barrier in CVD graphene spin devices pose questions about the stability of high current electrical spin injection. In this work, we observe that despite structural imperfections, CVD graphene sustains remarkably highest currents of 5.2 × 108 A/cm2, up to two orders higher than previously reported values in multilayer CVD graphene, with the capacity primarily dependent upon the sheet resistance of graphene. Furthermore, we notice a reversible regime, up to which CVD graphene can be operated without degradation with operating currents as high as 108 A/cm2, significantly high and durable over long time of operation with spin valve signals observed up to such high current densities. At the same time, the tunnel barrier resistance can be modified by the application of high currents. Our results demonstrate the robustness of large-scale CVD graphene and bring fresh insights for engineering and harnessing pure spin currents for innovative device applications. 

Graphene, the first experimentally isolated atomically thin crystal has displayed numerous superlative properties for quantum and spin-based electronics, as evidenced by research results of more than a decade. The scalable form of graphene, produced by the chemical vapor deposition (CVD) method has been increasingly attracting scientific and technological interest, as outstanding properties are combined with large scalability and high quality. The high-performance devices based on large-scale polycrystalline graphene growth capabilities with efficient charge and spin transport make it prospective for practical implementation into future spintronic and quantum integrated circuits. While CVD graphene presents unlimited prospects for exploring spin currents, there exist challenges along the way in terms of scalability of efficient performance, and reliability. Deformations, wrinkles, and structural (electronic) modifications caused at the interfaces with contacts remain key concerns for device performance. In particular, oxide-based interfaces with graphene are central to both graphenes electronic and spintronic devices. For high-performance scalable devices, it is of crucial significance to understand the details of these interfaces and how devices of CVD graphene with polycrystallinity respond to high current limits. In this thesis, we discuss a systematic study of the effect of e-beam evaporated ultra-thin titanium oxide (TiO_x) and aluminum oxide (AlO_x) on graphene; which are conventionally used as tunnel barriers in spintronic and nanoelectronics devices. Characteristic topographic features of both metal oxides on the graphene surface were revealed by atomic force microscopy. To estimate the impact of these oxides on graphene, electrical measurements were performed on graphene spin devices with and without metal oxides on the same devices. These measurements show significant p-type doping for both metal oxides, with sustained sheet conductance (σ₀) and mobility (μ) values. Strikingly, Raman spectroscopy and X-ray photoelectron spectroscopy show the emergence of significant sp3 carbon for AlO_x on graphene, in sharp contrast to TiOx. Our results and observations, together with theoretical calculations provide new insights into how sp3 carbon for AlO_x can lead to new memristive mechanisms and explicate enhanced spin relaxation into graphene with AlO_x devices, which was widely attributed to the presence of interface pinholes. Here we also investigate how CVD graphene-based devices respond to high current stress to understand their stability and robustness. Despite the grainy and wrinkled structure, we observed the highest till-date current density of 5.2 × 10⁸ A/cm², remarkably higher than previously reported values for multilayer graphene and graphene nanoribbons. The recorded reversible regime (~10⁸ A/cm²) for device operation allows reliable spin transport measurements with an observable spin signal up at such high current density. Furthermore, our investigation also encompasses cyclical current-voltage electrical measurement, to unveil the stability of graphene/ultra-thin oxide interfaces in graphene devices. Overall, these results present significance for CVD graphene device engineering for nanoelectronics and spintronics.

Neuromorphic computing mimics the brain's architecture to create energy-efficient devices. Reconfigurable synapses are crucial for neuromorphic computing, which can be achieved through memory-resistive (memristive) switching. Graphene-based memristors have shown nonvolatile multibit resistive switching with desirable endurance. Through first-principles calculations, we study the structural and electronic properties of graphene in contact with an ultra-thin alumina overlayer and demonstrate how one can use charge doping to exert direct control over its interfacial covalency, reversibly switching between states of conductivity and resistivity in the graphene layer. We further show that this proposed mechanism can be stabilized through the p-type doping of graphene, e.g., by naturally occurring defects, the passivation of dangling bonds or defect engineering.

Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiO_x, the AlO_x/graphene interface shows the presence of appreciable sp³ defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp³ bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp³ hybridization at the AlO_x/graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlO_x barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.

Despite structural and processing-induced imperfections, wafer-scale chemical vapor deposited (CVD) graphene today is commercially available and has emerged as a versatile form that can be readily transferred to desired substrates for various nanoelectronic and spintronic applications. In particular, over the past decade, significant advancements in CVD graphene synthesis methods and experiments realizing high-quality charge and spin transport have been achieved. These include growth of large-grain graphene, new processing methods, high-quality electrical transport with high-carrier mobility, micron-scale ballistic transport, observations of quantum and fractional quantum Hall effect, as well as the spintronic performance of extremely long spin communication over tens of micrometers at room temperature with robust spin diffusion lengths and spin lifetimes. In this short review, we discuss the progress in recent years in the synthesis of high-quality, large-scale CVD graphene and improvement of the electrical and spin transport performance, particularly towards achieving ballistic and long-distance spin transport that show exceptional promise for next-generation graphene electronic and spintronic applications.

Emergent technologies are required in the field of nanoelectronics for improved contacts and interconnects at nano and micro-scale. In this work, we report a highly-efficient nanolithography process for the growth of cobalt nanostructures requiring an ultra-low charge dose (15 mu C cm(-2), unprecedented in single-step charge-based nanopatterning). This resist-free process consists in the condensation of a similar to 28 nm-thick Co-2(CO)(8) layer on a substrate held at -100 degrees C, its irradiation with a Ga+ focused ion beam, and substrate heating up to room temperature. The resulting cobalt-based deposits exhibit sub-100 nm lateral resolution, display metallic behaviour (room-temperature resistivity of 200 mu omega cm), present ferromagnetic properties (magnetization at room temperature of 400 emu cm(-3)) and can be grown in large areas. To put these results in perspective, similar properties can be achieved by room-temperature focused ion beam induced deposition and the same precursor only if a 2 x 10(3) times higher charge dose is used. We demonstrate the application of such an ultra-fast growth process to directly create electrical contacts onto graphene ribbons, opening the route for a broad application of this technology to any 2D material. In addition, the application of these cryo-deposits for hard masking is demonstrated, confirming its structural functionality.