Achieving High Substitutional Incorporation in Mn-Doped Graphene.

Despite its broad potential applications, substitution of carbon by transition metal atoms in graphene has so far been explored only to a limited extent. We report the realization of substitutional Mn doping of graphene to a record high atomic concentration of 0.5%, which was achieved using ultralow-energy ion implantation. By correlating the experimental data with the results of ab initio Born-Oppenheimer molecular dynamics calculations, we infer that direct substitution is the dominant mechanism of impurity incorporation. Thermal annealing in ultrahigh vacuum provides efficient removal of surface contaminants and additional implantation-induced disorder, resulting in Mn-doped graphene that, aside from the substitutional Mn impurities, is essentially as clean and defect-free as the as-grown layer. We further show that the Dirac character of graphene is preserved upon substitutional Mn doping, even in this high concentration regime, making this system ideal for studying the interaction between Dirac conduction electrons and localized magnetic moments. More generally, these results show that ultralow energy ion implantation can be used for controlled functionalization of graphene with substitutional transition-metal atoms, of relevance for a wide range of applications, from magnetism and spintronics to single-atom catalysis.

Benchmarking First-Principles Reaction Equilibrium Composition Prediction.

The availability of thermochemical properties allows for the prediction of the equilibrium compositions of chemical reactions. The accurate prediction of these can be crucial for the design of new chemical synthesis routes. However, for new processes, these data are generally not completely available. A solution is the use of thermochemistry calculated from first-principles methods such as Density Functional Theory (DFT). Before this can be used reliably, it needs to be systematically benchmarked. Although various studies have examined the accuracy of DFT from an energetic point of view, few studies have considered its accuracy in predicting the temperature-dependent equilibrium composition. In this work, we collected 117 molecules for which experimental thermochemical data were available. From these, we constructed 2648 reactions. These experimentally constructed reactions were then benchmarked against DFT for 6 exchange-correlation functionals and 3 quality of basis sets. We show that, in reactions that do not show temperature dependence in the equilibrium composition below 1000 K, over 90% are predicted correctly. Temperature-dependent equilibrium compositions typically demonstrate correct qualitative behavior. Lastly, we show that the errors are equally caused by errors in the vibrational spectrum and the DFT electronic ground state energy.

Improving Accuracy and Transferability of Machine Learning Chemical Activation Energies by Adding Electronic Structure Information.

Predicting chemical activation energies is one of the longstanding and important challenges in computational chemistry. Recent advances have shown that machine learning can be used to create tools to predict them. Such tools can significantly decrease the computational cost for these predictions compared to traditional methods, which require an optimal path search along a high-dimensional potential energy surface. To enable this new route, we need both large and accurate datasets and a compact yet complete description of the reactions. Although data for chemical reactions is becoming increasingly available, the key step of encoding the reaction as an efficient descriptor remains a big challenge. In this paper, we demonstrate that including electronic energy levels in the description of the reaction significantly improves the prediction accuracy and transferability. Feature importance analysis further demonstrates that electronic energy levels have a higher importance than some structural information and typically require less space in the reaction encoding vector. In general, we observe that the results of the feature importance analysis relate well to the domain knowledge of fundamental chemical principles. This work can help to build better chemical reaction encodings for machine learning and thus improve the predictions of machine learning models for reaction activation energies. These models could ultimately be used to recognize reaction limiting steps in large reaction systems, allowing to account for bottlenecks at the design stage.

Area-Selective Deposition of AlOx and Al-Silicate for Fully Self-Aligned Via Integration.

The rush for better-performing electronics, and manufacturing processes that heavily rely on "top-down" patterning techniques, is making the integration of "self-aligned" fabrication methods, such as area-selective deposition (ASD), a critical objective for continued device scaling. The fully self-aligned via (FSAV) scheme is broadly proposed as a "killer application" to determine whether ASD can shift from an R&D process to high-volume manufacturing. Nevertheless, the lack of a suitable low-κ deposition process has prevented the realization of FSAV by dielectric-on-dielectric ASD. This is primarily due to the high temperature and/or strong oxidizers employed during low-κ dielectric deposition and their unsuitability in the presence of organic masks, such as self-assembled monolayers (SAMs), used to prevent material nucleation during ASD. In this work, AlOx and Al-silicate atomic layer deposition (ALD) processes are studied to provide suitable materials for ASD-enabled FSAV. Dimethylaluminum isopropoxide and H2O are utilized to deposit the metal oxide, whereas Al-silicate is grown by adding 2,2-dimethoxy-1,6-diaza-2-silacyclooctane (DMDAcO) pulses to the AlOx ALD cycle. The selectivity of such processes is demonstrated on 50 nm Cu/SiO2 structures, using octadecanethiol-derived SAMs to inhibit material nucleation on the metal lines. Scanning and transmission electron microscopies are employed to assess the quality of the ASD processes and investigate the mechanisms behind defect generation on a nongrowth surface. X-ray photoelectron spectroscopy measurements show the high purity of the AlOx film, whereas DMDAcO-ligand incorporation into the Al-silicate matrix is observed. Planar capacitor structures are used to assess the electrical properties of both ASD films, revealing that the silicate film exhibits a relatively low κ-value (5.3 ± 0.2), with a high acceleration field factor (32.4 ± 1.4) and a dielectric breakdown voltage of 6.0 ± 0.3 V at 100 °C.

Au3-Decorated graphene as a sensing platform for O2 adsorption and desorption kinetics.

The adsorption and desorption kinetics of molecules is of significant fundamental and applied interest. In this paper, we present a new method to quantify the energy barriers for the adsorption and desorption of gas molecules on few-atom clusters, by exploiting reaction induced changes of the doping level of a graphene substrate. The method is illustrated for oxygen adsorption on Au3 clusters. The gold clusters were deposited on a graphene field effect transistor and exposed to O2. From the change in graphene's electronic properties during adsorption, the energy barrier for the adsorption of O2 on Au3 is estimated to be 0.45 eV. Electric current pulses increase the temperature of the graphene strip in a controlled way and provide the required thermal energy for oxygen desorption. The oxygen binding energy on Au3/graphene is found to be 1.03 eV and the activation entropy is 1.4 meV K-1. The experimental values are compared and interpreted on the basis of density functional theory calculations of the adsorption barrier, the binding energy and the activation entropy. The large value of the activation entropy is explained by the hindering effect that the adsorbed O2 has on the fluxional motion of the Au3 cluster.

Breakdown of Universal Scaling for Nanometer-Sized Bubbles in Graphene.

We report the formation of nanobubbles on graphene with a radius of the order of 1 nm, using ultralow energy implantation of noble gas ions (He, Ne, Ar) into graphene grown on a Pt(111) surface. We show that the universal scaling of the aspect ratio, which has previously been established for larger bubbles, breaks down when the bubble radius approaches 1 nm, resulting in much larger aspect ratios. Moreover, we observe that the bubble stability and aspect ratio depend on the substrate onto which the graphene is grown (bubbles are stable for Pt but not for Cu) and trapped element. We interpret these dependencies in terms of the atomic compressibility of the noble gas as well as of the adhesion energies between graphene, the substrate, and trapped atoms.

A chemisorbed interfacial layer for seeding atomic layer deposition on graphite.

The integration of graphene, and more broadly two-dimensional materials, into devices and hybrid materials often requires the deposition of thin films on their usually inert surface. As a result, strategies for the introduction of surface reactive sites have been developed but currently pose a dilemma between robustness and preservation of the graphene properties. A method is reported here for covalently modifying graphitic surfaces, introducing functional groups that act as reactive sites for the growth of high quality dielectric layers. Aryl diazonium species containing tri-methoxy groups are covalently bonded (grafted) to highly oriented pyrolytic graphite (HOPG) and graphene, acting as seeding species for atomic layer deposition (ALD) of Al2O3, a high-κ dielectric material. A smooth and uniform dielectric film growth is confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurements. Raman spectroscopy showed that the aryl groups gradually detach from the graphitic surface during the Al2O3 ALD process at 150 °C, with the surface reverting back to the original sp2-hybridized state and without damaging the dielectric layer. Thus, the grafted aryl groups can act as a sacrificial seeding layer after healing the defects of the graphitic surface with annealing treatment.

Cyclic Plasma Halogenation of Amorphous Carbon for Defect-Free Area-Selective Atomic Layer Deposition of Titanium Oxide.

As critical dimensions in integrated circuits continue to shrink, the lithography-based alignment of adjacent patterned layers becomes more challenging. Area-selective atomic layer deposition (ALD) allows circumventing the alignment issue by exploiting the chemical contrast of the exposed surfaces. In this work, we investigate the selective deposition of TiO2 by plasma halogenation of amorphous carbon (a-C:H) acting as a growth-inhibiting layer. On a-C:H, a CF4 or Cl2 plasma forms a thin halogenated layer that suppresses the growth of TiO2, while nucleation remains unaffected on plasma-treated SiO2. The same halogenating plasmas preferentially etch TiO2 nuclei over films and thus enable the restoration of the halogenated surface of amorphous carbon. By embedding the intermediate plasma treatments in the ALD TiO2 sequence, an 8 nm TiO2 layer could be deposited with a selectivity of 0.998. The application of the cyclic process on a 60 nm half-pitch line pattern resulted in the defect-free deposition of TiO2 at the bottom of the trenches. Cyclic fluorination demonstrated better growth inhibition compared to chlorination due to more efficient defect removal and retention of the favorable surface composition during plasma exposure. While exploring the TiO2 nucleation defects at the limit of detection for conventional elemental analysis techniques (<1 × 1014 at/cm2), we additionally highlight the value of imaging techniques such as atomic force microscopy for understanding defect formation mechanisms and accurately assessing growth selectivity.

Multicomponent Covalent Chemical Patterning of Graphene.

The chemical patterning of graphene is being pursued tenaciously due to exciting possibilities in electronics, catalysis, sensing, and photonics. Despite the intense efforts, spatially controlled, multifunctional covalent patterning of graphene has not been achieved. The lack of control originates from the inherently poor reactivity of the basal plane of graphene, which necessitates the use of harsh chemistries. Here, we demonstrate spatially resolved multicomponent covalent chemical patterning of single layer graphene using a facile and efficient method. Three different functional groups could be covalently attached to the basal plane in dense, well-defined patterns using a combination of lithography and a self-limiting variant of diazonium chemistry requiring no need for graphene activation. The layer thickness of the covalent films could be controlled down to 1 nm. This work provides a solid foundation for the fabrication of chemically patterned multifunctional graphene interfaces for device applications.

Electron-induced fragmentation mechanisms in organic monomers and their implications for photoresist optimization for EUV lithography.

Secondary electrons generated during the Extreme Ultraviolet Lithography (EUVL) process are predominantly responsible for inducing important patterning chemistry in photoresist films. Therefore, it is crucial to understand the electron-induced fragmentation mechanisms involved in EUV-resist systems to improve their patterning performance. To facilitate this understanding, mechanistic studies were carried out on simple organic EUV-resist monomers, methyl isobutyrate (MIB) and methacrylic acid (MAA), both in the condensed and gas phases. Electron-stimulated desorption (ESD) studies on MIB in the condensed phase showed desorption peaks at around 2 and 9 eV electron energies. The gas-phase study on MIB showed that the monomer followed the dissociative ionization (DI) fragmentation pathway, under single collision conditions, which opened up at electron energies above about 11 eV. No signs of dissociative electron attachment (DEA) were detected for MIB in the gas phase under single collision conditions. However, DEA was an active process in MAA in the gas phase under single collision conditions at around 2 eV, showing that slight modifications of the molecular structures of photoresists may serve to sensitize them to certain electron-induced processes.

Doping Graphene with Substitutional Mn.

We report the incorporation of substitutional Mn atoms in high-quality, epitaxial graphene on Cu(111), using ultralow-energy ion implantation. We characterize in detail the atomic structure of substitutional Mn in a single carbon vacancy and quantify its concentration. In particular, we are able to determine the position of substitutional Mn atoms with respect to the Moiré superstructure (i.e., local graphene-Cu stacking symmetry) and to the carbon sublattice; in the out-of-plane direction, substitutional Mn atoms are found to be slightly displaced toward the Cu surface, that is, effectively underneath the graphene layer. Regarding electronic properties, we show that graphene doped with substitutional Mn to a concentration of the order of 0.04%, with negligible structural disorder (other than the Mn substitution), retains the Dirac-like band structure of pristine graphene on Cu(111), making it an ideal system in which to study the interplay between local magnetic moments and Dirac electrons. Our work also establishes that ultralow-energy ion implantation is suited for substitutional magnetic doping of graphene. Given the flexibility, reproducibility, and scalability inherent to ion implantation, our work creates numerous opportunities for research on magnetic functionalization of graphene and other two-dimensional materials.

Understanding ambipolar transport in MoS2 field effect transistors: the substrate is the key.

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS2 interface, but also from the MoS2-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO2 substrate. Furthermore, we analyse MoS2 FETs of different channel thickness on three different substrates, SiO2, hBN and Al2O3, by correlating the p-branch I ON/I OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al2O3 which enables us to observe ambipolar transport in a bilayer MoS2 FET. These results highlight that MoS2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS2 FETs is strongly correlated to its dielectric environment and processing conditions.

Dielectric Response Spectroscopy as Means to Investigate Interfacial Effects for Ultra-Thin Film Polymer-Based High NA EUV Lithography.

Extreme ultra-violet lithography (EUVL) is the leading-edge technology to produce advanced nanoelectronics. The further development of EUVL is heavily based on implementing the so-called high numerical aperture (NA) EUVL, which will enable even smaller pitches up to 8 nm half pitch (HP). In anticipation of this high NA technology, it is crucial to assess the readiness of the current resist materials for the high NA regime to comply with the demanding requirements of resolution, line-edge roughness, and sensitivity (RLS). The achievable tighter pitches require lower film thicknesses for both resist and underlying transfer layers. A concern that is tied to the thinning down is the potential change in resist properties and behavior due to the interaction with the underlayer. To increase the fundamental understanding of ultra-thin films for high NA EUVL, a method to investigate the interplay of reduced film thickness and different patterning-relevant underlayers is developed by looking at the glass transition temperature (Tg) of polymer-based resists. To minimize the ambiguity of the results due to resist additives (i.e., photoacid generator (PAG) and quencher), it was opted to move forward with polymer-only samples, the main component of the resist, at this stage of the investigation. By using dielectric response spectroscopy, the results obtained show that changing the protection group of the polymer, as well as altering the polymer film thickness impacts the dynamics of the polymer mobility, which can be assessed through the Tg of the system. Unexpectedly, changing the underlayer did not result in a clear change in the polymer mobility at the tested film thicknesses.

Photo-induced electrodeposition of metallic nanostructures on graphene.

Graphene, a single atomic layer of sp2 hybridized carbon, is a promising material for future devices due to its excellent optical and electrical properties. Nevertheless, for practical applications, it is essential to deposit patterned metals on graphene in the micro and nano-meter scale in order to inject electrodes or modify the 2D film electrical properties. However, conventional methods for depositing patterned metals such as lift-off or etching leave behind contamination. This contamination has been demonstrated to deteriorate the interesting properties of graphene such as its carrier mobility. Therefore, to fully exploit the unique properties of graphene, the controlled and nano-patterned deposition of metals on graphene films without the use of a sacrificial resist is of significant importance for graphene film functionalization and contact deposition. In this work, we demonstrate a practical and low-cost optical technique of direct deposition of metal nano-patterned structures without the need for a sacrificial lift-off resist. The technique relies on the laser induced reduction of metal ions on a graphene film. We demonstrate that this deposition is optically driven, and the resolution is limited only by the diffraction limit of the light source being used. Patterned metal features as small as 270 nm in diameter are deposited using light with a wavelength of 532 nm and a numerical aperture of 1.25. Deposition of different metals such as Au, Ag, Pd, Pb and Pt is shown. Additionally, change in the Fermi level of the graphene film through the nano-patterned metal is demonstrated through the electrical characterization of four probe field effect transistors.

On the van der Waals Epitaxy of Homo-/Heterostructures of Transition Metal Dichalcogenides.

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.

Area-Selective ALD of Ru on Nanometer-Scale Cu Lines through Dimerization of Amino-Functionalized Alkoxy Silane Passivation Films.

The selective deposition of materials on predefined areas on a substrate is of crucial importance for various applications, such as energy harvesting, microelectronic device fabrication, and catalysis. A representative example of area-confined deposition is the selective deposition of a metal film as the interconnect material in multilevel metallization schemes for CMOS technology. This allows the formation of multilevel structures with standard lithographical techniques while minimizing pattern misalignment and overlay and improving the uniformity of the structures across the wafer. In this work, area-selective deposition of Ru by atomic layer deposition (ALD) is investigated using alkoxy siloxane dielectric passivation layers. In this work, a comparison of several silane organic SAM precursors in terms of Ru ALD ASD performance is reported. The importance of the surface areal concentration of the passivation molecules is demonstrated. According to the in situ X-ray photoelectron spectroscopy film characterization, the ALD blocking layers derived from a (3-trimethoxysilylpropyl) diethylenetriamine (DETA) precursor have the ability to polymerize under ALD-compatible temperatures, such as 250 °C, which leads to a significant inhibition of Ru growth up to 400 ALD cycles. At the same time, the DETA layer can be selectively removed from the oxidized Cu surface by rinsing in acetic acid, which allows selective deposition of ca. 14 nm of Ru on Cu with no Ru detected on the DETA-coated surface by RBS. The approach is successfully tested on 50 nm half-pitch patterned SiO2/Cu lines.

Material-Selective Doping of 2D TMDC through AlxOy Encapsulation.

For the integration of two-dimensional (2D) transition metal dichalcogenides (TMDC) with high-performance electronic systems, one of the greatest challenges is the realization of doping and comprehension of its mechanisms. Low-temperature atomic layer deposition of aluminum oxide is found to n-dope MoS2 and ReS2 but not WS2. Based on electrical, optical, and chemical analyses, we propose and validate a hypothesis to explain the doping mechanism. Doping is ascribed to donor states in the band gap of AlxOy, which donate electrons or not, based on the alignment of the electronic bands of the 2D TMDC. Through systematic experimental characterization, incorporation of impurities (e.g., carbon) is identified as the likely cause of such states. By modulating the carbon concentration in the capping oxide, doping can be controlled. Through systematic and comprehensive experimental analysis, this study correlates, for the first time, 2D TMDC doping to the carbon incorporation on dielectric encapsulation layers. We highlight the possibility to engineer dopant layers to control the material selectivity and doping concentration in 2D TMDC.

Peculiar alignment and strain of 2D WSe2 grown by van der Waals epitaxy on reconstructed sapphire surfaces.

The increasing scientific and industry interest in 2D MX2 materials within the field of nanotechnology has made the single crystalline integration of large area van der Waals (vdW) layers on commercial substrates an important topic. The c-plane oriented (3D crystal) sapphire surface is believed to be an interesting substrate candidate for this challenging 2D/3D integration. Despite the many attempts that have been made, the yet incomplete understanding of vdW epitaxy still results in synthetic material that shows a crystallinity far too low compared to natural crystals that can be exfoliated onto commercial substrates. Thanks to its atomic control and in situ analysis possibilities, molecular beam epitaxy (MBE) offers a potential solution and an appropriate method to enable a more in-depth understanding of this peculiar 2D/3D hetero-epitaxy. Here, we report on how various sapphire surface reconstructions, that are obtained by thermal annealing of the as-received substrates, influence the vdW epitaxy of the MBE-grown WSe2 monolayers (MLs). The surface chemistry and the interatomic arrangement of the reconstructed sapphire surfaces are shown to control the preferential in-plane epitaxial alignment of the stoichiometric WSe2 crystals. In addition, it is demonstrated that the reconstructions also affect the in-plane lattice parameter and thus the in-plane strain of the 2D vdW-bonded MLs. Hence, the results obtained in this work shine more light on the peculiar concept of vdW epitaxy, especially relevant for 2D materials integration on large-scale 3D crystal commercial substrates.

Vapor-deposited zeolitic imidazolate frameworks as gap-filling ultra-low-k dielectrics.

The performance of modern chips is strongly related to the multi-layer interconnect structure that interfaces the semiconductor layer with the outside world. The resulting demand to continuously reduce the k-value of the dielectric in these interconnects creates multiple integration challenges and encourages the search for novel materials. Here we report a strategy for the integration of metal-organic frameworks (MOFs) as gap-filling low-k dielectrics in advanced on-chip interconnects. The method relies on the selective conversion of purpose-grown or native metal-oxide films on the metal interconnect lines into MOFs by exposure to organic linker vapor. The proposed strategy is validated for thin films of the zeolitic imidazolate frameworks ZIF-8 and ZIF-67, formed in 2-methylimidazole vapor from ALD ZnO and native CoOx, respectively. Both materials show a Young's modulus and dielectric constant comparable to state-of-the-art porous organosilica dielectrics. Moreover, the fast nucleation and volume expansion accompanying the oxide-to-MOF conversion enable uniform growth and gap-filling of narrow trenches, as demonstrated for 45 nm half-pitch fork-fork capacitors.