Ultra-low Energy Ion Implantation Research Articles

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.

Controlling electron and hole concentration in MoS2 through scalable plasma processes

Conventional high-energy ion implant processes lack implant depth precision and minimally damaging properties needed to dope atomically thin two-dimensional (2D) semiconductors by ion modification without undesirable side effects. To overcome this limitation, controllable, reproducible, and robust doping methods must be developed for atomically thin semiconductors to enable commercially viable wafer-scale 2D material-based logic, memory, and optical devices. Ultralow energy ion implantation and plasma exposure are among the most promising approaches to realize high carrier concentrations in 2D semiconductors. Here, we develop two different plasma processes using commercially available semiconductor processing tools to achieve controllable electron and hole doping in 2H-MoS2. Doping concentrations are calculated from the measured Fermi level shift within the MoS2 electronic bandgap using x-ray photoelectron spectroscopy. We achieve electron doping up to 1.5 × 1019 cm−3 using a remote argon/hydrogen (H2) plasma process, which controllably generates sulfur vacancies. Hole doping up to 4.2 × 1017 cm−3 is realized using an inductively coupled helium/SF6 plasma, which substitutes fluorine into the MoS2 lattice at sulfur sites. The high doping concentrations reported here highlight the potential of scalable plasma processes for MoS2, which is crucial for enabling complementary circuits based on 2D semiconductors.

Silicon nitride resistance switching MIS cells doped with silicon atoms

Stoichiometric SiNx layers (x = [N]/[Si] = 1.33) are doped with Si atoms by ultra-low energy ion implantation (ULE-II) and subsequently annealed at different temperatures in inert ambient conditions. Detailed material and memory cells characterization is performed to investigate the effect of Si dopants on the switching properties and performance of the fabricated resistive memory cells. In this context extensive dc current–voltage and impedance spectroscopy measurements are carried out systematically and the role of doping in dielectric properties of the nitride films is enlightened. The dc and ac conduction mechanisms are investigated in a comprehensive way. Room temperature retention characteristics of resistive states are also presented.

Periodic Arrays of Dopants in Silicon by Ultralow Energy Implantation of Phosphorus Ions through a Block Copolymer Thin Film.

In this work, block copolymer lithography and ultralow energy ion implantation are combined to obtain nanovolumes with high concentrations of phosphorus atoms periodically disposed over a macroscopic area in a p-type silicon substrate. The high dose of implanted dopants grants a local amorphization of the silicon substrate. In this condition, phosphorus is activated by solid phase epitaxial regrowth (SPER) of the implanted region with a relatively low temperature thermal treatment preventing diffusion of phosphorus atoms and preserving their spatial localization. Surface morphology of the sample (AFM, SEM), crystallinity of the silicon substrate (UV Raman), and position of the phosphorus atoms (STEM- EDX, ToF-SIMS) are monitored during the process. Electrostatic potential (KPFM) and the conductivity (C-AFM) maps of the sample surface upon dopant activation are compatible with simulated I-V characteristics, suggesting the presence of an array of not ideal but working p-n nanojunctions. The proposed approach paves the way for further investigations on the possibility to modulate the dopant distribution within a silicon substrate at the nanoscale by changing the characteristic dimension of the self-assembled BCP film.

Bond defects in graphene created by ultralow energy ion implantation

Ultralow energy (ULE) ion implantation is being increasingly applied to the modification of 2D materials, in particular, for substitutional doping and intercalation of graphene. Implantation-induced defects, whether desired or not, have a strong impact on the properties of graphene. Significant research has been devoted to vacancy-related defects however, disorder induced by ion irradiation in the ULE limit, that is, for energies below the vacancy-formation threshold, remains poorly understood. Here, we focus on that regime and report the formation of defects resulting from the breaking of C–C sp2 bonds and formation of C-substrate bonds. As a model system we used epitaxial graphene grown on Cu(111) and on Pt(111), subsequently implanted with He, Ne and Ar at energies between 15 eV and 40 eV. The bond defect density is found to increase with increasing energy and atomic number of the implanted element. These findings significantly advance our understanding of disorder induced in graphene by ULE ion implantation, while simultaneously revealing the potential for exploiting such bond defects for physical or chemical functionalization. In particular, these bond defects can be generated with a high degree of selectivity since they occur in the low-energy limit (at least down to 15 eV), significantly below the energies required to form stable vacancies.

(Invited, Digital Presentation) Single-Atom Quantum Magnetism in 2D Materials

With the advent of 2D materials, the playground to study spins in dilute and non-dilute phases has expanded. This is appealing for utilizing the additional degrees of freedom of electron systems such as spin and valley and, from the fundamental point of view, to better understand atomic scale magnetic phenomena in low dimensional materials. Dilute magnetism in 2D materials can lead to complex magnetic phenomena (e.g., Kondo effect, RKKY-interactions, quantum relaxation and coherence), with potential for applications in spintronics (e.g., spin FETs) and quantum technologies (e.g., single-atom quantum memories). We are investigating how to selectively incorporate substitutional magnetic atoms (3d transition metals and 4f rare earths) in 2D materials, using ultra low energy ion implantation, and we characterize their structural, electronic, and magnetic properties [1]. Ultra-low energy (ULE) ion implantation allows us to precisely tune the kinetic energy of the ions, providing control over the form of incorporation and concentration while preserving the structural and electronic properties of graphene. Our approach is based on a wide range of characterization techniques (structural and electronic), including scanning tunneling microscopy and spectroscopy (STM/STS), Raman spectroscopy, synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD), among others. These experimental studies are complemented by density functional theory (DFT) and molecular dynamics (MD) simulations. The new insights provided by our work establish a framework for the controlled incorporation of magnetic dopants in 2D materials, using ULE ion implantation.[1] P. C. Lin et al., ACS Nano 15(3), 5449-5458 (2021).

Thermal Annealing of Graphene Implanted with Mn at Ultralow Energies: From Disordered and Contaminated to Nearly Pristine Graphene

Ultralow-energy (ULE) ion implantation is increasingly being explored as a method to substitutionally dope graphene. However, complex implantation-related effects such as defect creation and surface contamination, and how they can be minimized by thermal annealing, remain poorly understood. Here, we address these open questions taking as the model case epitaxial graphene grown on Cu(111), which was subsequently ULE implanted with Mn at 40 eV and then studied as a function of annealing temperature under ultrahigh vacuum. While significant surface cleaning occurs at annealing temperatures as low as 200 °C, recovery from the implantation-induced disorder requires at least 525 °C. Upon high-temperature annealing, in the 600–700 °C range, the Mn atoms that were incorporated upon implantation as intercalated species (between graphene and the Cu surface) experience diffusion into the Cu layer, creating a subsurface alloy. Annealing at 700 °C restored implanted graphene to a nearly pristine state, with a well-ordered graphene lattice with substitutional Mn atoms and a well-defined Dirac cone. In addition to the insight into the complex physicochemical effects induced by thermal annealing, our results provide useful guidelines for future experimental studies on graphene that is modified (e.g., substitutionally doped) by using ULE ion implantation.

Computational Study of Low-Energy Pt-Ion Implantation into Graphene for Single-Atom Catalysis

Single-atom catalysts (SACs) represent the ultimate goal of nanocatalysis fields. However, complex synthesis processes and pyrolysis inactivation problems are the two main challenges that plague the development of SACs. In this work, we propose that the ultralow-energy ion-implantation (ULEII) method could be utilized to simply and efficiently synthesize stable SACs. Our simulation results of Pt-ion implantation into graphene indicate that the total doping efficiency, including direct displacement doping and indirect trap doping, can be effectively optimized by delicately adjusting the energy of incident ions. Further systematic molecular dynamics simulations and first-principles calculations demonstrate that irradiation-induced vacancy defects can effectively capture and anchor adsorbed metal atoms on the graphene surface. The stability and migration characteristics of various defects are also clearly elucidated. Theoretically, by selecting an optimal ion energy, the ULEII method can achieve a doping efficiency as high as 73.4% .

Introducing Ultra-Low Energy Ion Implantation of Radioactive Isotopes at ISOLDE, CERN for (Near-)Surface Characterization: The ASPIC and ASCII Vacuum Chambers

Solid-state physics research has long employed radioactive isotopes to investigate the crystallographic, electric and magnetic properties of nanostructures. Ion implantation (1–100 keV) is the method of choice for incorporating radioactive nuclei into the crystal structure. However, the enormous scientific interest in 2D materials, multiferroics and their interfaces of the last decades has lead to more stringent demands for isotope incorporation. Ultra-low energy (ULE) ion implantation (10–100 eV) provides the ability to precisely tune the depth of the implanted radioactive probes, even in the case of atomically thin 2D materials. To unlock this potential and expand the experimental capabilities of the ISOLDE collaboration in CERN, the apparatus for surface physics and interfaces at CERN (ASPIC), an experienced ultra-high vacuum chamber dedicated to surface characterization and modification, is refurbished and upgraded with a new component: the ASPIC’s ion implantation (ASCII) chamber, designed for ULE ion implantation of radioactive probes. This paper describes the scientific context, design and application of these vacuum chambers.

Bond Defects in Graphene Created by Ultralow Energy Ion Implantation

Bond Defects in Graphene Created by Ultralow Energy Ion Implantation

Laterally controlled ultra-low energy ion implantation using electrostatic masking

We describe the ongoing development of an ultra-low energy (ULE) ion implantation system for the implementation of a laterally controlled ion incorporation into 2D materials like graphene or transition metal dichalcogenides (TMDs). For this purpose, we use ion optics simulations to demonstrate how the generation of an electrostatic potential gradient laterally along the sample surface can be used to control the size of the implanted sample area. The simulations are complemented by experiments on the incorporation of 25eV Mn ions into thin tetrahedral amorphous carbon (ta-C) films on Si substrates. With the help of RBS measurements, it is shown how potential gradients of different strengths influence the amount of introduced Mn.

Sputter hot filament hollow cathode ion source and its application to ultra-low energy ion implantation in 2D materials

We describe the design of a new ion source that combines a hot filament hollow cathode source with a sputtering source. This combination makes it possible to provide ion beams of a wider range that previously could not be used due to respective high melting points and low vapor pressure of the needed elements. We demonstrate the sources viability for molybdenum, cobalt, niobium and the rare-earth element gadolinium. An implantation current of 20–900nA on the sample, depending on the element, could be achieved. Moreover, we experimentally prove that these ion beams are suitable for ultra-low energy implantation into 2D materials like graphene and transition metal dichalcogenides (TMDs). Furthermore, computational studies were conducted to investigate the ion trajectories within the source and ion implantation into 2D materials. RBS and PIXE measurements were performed to quantitative investigate the implanted samples.

(Invited) Atomic-Scale Functionalization and Characterization of 2D Materials

As 2D materials set their path into our next generation nanoelectronics, enriching their functionalities and complementing their limitations become more important. In particular, novel strategies for controllable incorporation of substitutional and intercalated atoms need to be developed. Here, we report on modifying 2D materials at the atomic-scale by ultra-low energy ion implantation to functionalize their properties and explore their limits. I will present our recent work on graphene on two topics: substitutional manganese (Mn) atoms for magnetic doping [1] and intercalation of noble gases to produce nanobubbles with small radii and high aspect ratios. On the one hand, we have characterized in detail the atomic structure of substitutional Mn and graphene’s Dirac-like band structure doped with up to 1% substitutional Mn. On the other hand, the structural inspection of our graphene nanobubbles has revealed a so far unexplored small-radius limit, associated with high aspect ratios as well as extreme pressure and strain. Our approach is based on a wide range of characterization techniques (structural and electronic), including scanning tunneling microscopy and spectroscopy (STM/STS), Raman spectroscopy, synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD), among others. These experimental studies are complemented by density functional theory (DFT) and molecular dynamics (MD) simulations.[1] P. C. Lin et al., ACS nano 15(3), 5449-5458 (2021). Figure 1

Differential nonlinear photocarrier radiometry for characterizing ultra-low energy boron implantation in silicon

The measuring of the depth profile and electrical activity of implantation impurity in the top nanometer range of silicon encounters various difficulties and limitations, though it is known to be critical in fabrication of silicon complementary metal–oxide–semiconductor (CMOS) devices. In the present work, SRIM program and photocarrier radiometry (PCR) are employed to monitor the boron implantation in industrial-grade silicon in an ultra-low implantation energy range from 0.5 keV to 5 keV. The differential PCR technique, which is improved by greatly shortening the measurement time through the simplification of reference sample, is used to investigate the effects of implantation energy on the frequency behavior of the PCR signal for ultra-shallow junction. The transport parameters and thickness of shallow junction, extracted via multi-parameter fitting the dependence of differential PCR signal on modulation frequency to the corresponding theoretical model, well explain the energy dependence of PCR signal and further quantitatively characterize the recovery degree of structure damage induced by ion implantation and the electrical activation degree of impurities. The monitoring of nm-level thickness and electronic properties exhibits high sensitivity and apparent monotonicity over the industrially relevant implantation energy range. The depth profiles of implantation boron in silicon with the typical electrical damage threshold (Y ED) of 5.3 × 1015 cm−3 are evaluated by the SRIM program, and the determined thickness values are consistent well with those extracted by the differential PCR. It is demonstrated that the SRIM and the PCR are both effective tools to characterize ultra-low energy ion implantation in silicon.

(Invited) Processing of 2D Materials Using Ultra-Low Energy Ion Implantation

Functionalization of 2D materials generally requires the modification of their physicochemical properties. Several approaches have been explored: the use of different substrates, creation of intrinsic defects (e.g. vacancies), adsorption and intercalation (of atoms, clusters or molecules), substitutional doping, among others. For incorporation of substitutional or intercalated elements, despite the numerous approaches that have been investigated, a major challenge remains: the limited control over the concentration and form of incorporation. An alternative approach is to incorporate the foreign species by ultra-low energy (ULE) ion implantation, precisely tuning the number of implanted ions and their kinetic energy. Here, I review our recent work on ULE ion implantation of 2D materials, with emphasis on substitutional doping and intercalation of graphene. Our approach is based on a wide range of characterization techniques (structural and electronic), including scanning tunneling microscopy and spectroscopy (STM/STS), synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD), transport measurements, among others. These experimental studies are complemented by density functional theory (DFT) and molecular dynamics (MD) simulations.

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.

(Invited) Processing of 2D Materials Using Ion Implantation

Functionalization of 2D materials generally requires the modification of their physicochemical properties. Several approaches have been explored: the use of different substrates, creation of intrinsic defects (e.g. vacancies), adsorption and intercalation (of atoms, clusters or molecules), substitutional doping, among others. For incorporation of substitutional or intercalated elements, despite the numerous approaches that have been investigated, a major challenge remains: the limited control over the concentration and form of incorporation. An alternative approach is to incorporate the foreign species by ultra-low energy (ULE) ion implantation, precisely tuning the number of implanted ions and their kinetic energy. Here, I review our recent work on ULE ion implantation of 2D materials, with emphasis on substitutional doping and intercalation of graphene. Our approach is based on a wide range of characterization techniques (structural and electronic), including scanning tunneling microscopy and spectroscopy (STM/STS), synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD), transport measurements, among others. These experimental studies are complemented by density functional theory (DFT) and molecular dynamics (MD) simulations.

Ion beam synthesis of indium-oxide nanocrystals for improvement of oxide resistive random-access memories

We report on the direct ion beam synthesis of a delta-layer of indium oxide nanocrystals (In2O3-NCs) in silica matrices by using ultra-low energy ion implantation. The formation of the indium oxide phase can be explained by (i) the affinity of indium with oxygen, (ii) the generation of a high excess of oxygen recoils generated by the implantation process in the region where the nanocrystals are formed and (iii) the proximity of the indium-based nanoparticles with the free surface and oxidation from the air. Taking advantage of the selective diffusivity of implanted indium in SiO2 with respect to Si3N4, In2O3-NCs have been inserted in the SiO2 switching oxide of micrometric planar oxide-based resistive random access memory (OxRAM) devices fabricated using the nanodamascene process. Preliminary electrical measurements show switch voltage from high to low resistance state. The devices with In2O3-NCs have been cycled 5 times with identical operating voltages and RESET current meanwhile no switch has been observed for non implanted devices. This first measurement of switching is very promising for the concept of In2O3-NCs based OxRAM memories.

The Effect of Low Energy Ion Implantation on MoS2

The use of ultra low energy ion implantation is investigated as a doping method for MoS2. 200 eV Cl and Ar implants at doses between 1 × 1013 and 1 × 1015 cm−2 were introduced into exfoliated MoS2 flakes. XPS results for Cl implants show a decrease in core peak binding energies for Mo3d and S2p with increasing dose, implying a p-doping effect. Implantation of MoS2 device channel regions is shown to reduce the channel's conductivity. However, isolated implantation of the contact region with low doses (1 × 1013 cm−2) of Cl and Ar are shown to improve output characteristics by linearizing the IDS-VDS curves and by increasing current through the device. Cl was shown to be more effective than Ar at increasing the current, implying there is a potential chemical effect as well as damage effect. For higher doses (≥ 1 × 1014 cm−2), the current through the device is reduced with increasing dose for both implant species. This report presents the as-implanted, unannealed results. Post-implantation anneals may be necessary to activate the dopants and fully realize the potential of this doping method.

Doping of Graphene by Low-Energy Ion Beam Implantation: Structural, Electronic, and Transport Properties.

We investigate the structural, electronic, and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultralow energy ion implantation, the influence of different ion species (boron, nitrogen, and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and can reduce or raise the position of the Fermi level, respectively, (12)C(+) carbon ions are used to study possible defect creation by the bombardment. For low-temperature transport, the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples, we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for (11)B(+)- and (12)C(+)-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, because they result in weak localization effects at low temperatures.