Low Energy Ion Bombardment Research Articles

Amorphous Carbon Nitride Films: Surface and Subsurface Composition and Bonding.

To obtain quantitative information about the composition and bonding of atoms located at and beyond the analyzed solid surface nondestructively, we applied angle-resolved X-ray photoelectron spectroscopy aided by the maximum entropy method to air-exposed amorphous carbon nitride films deposited by pulsed laser deposition of diamond-like carbon modified by low-energy nitrogen ion bombardment during film growth. We demonstrate that the composition, chemical bonding, and mass density vary significantly from the top surface to a shallow subsurface region. The analyzed samples, in a shallow surface region of ∼1 nm, are composed of oxygen, nitrogen, hydrogen, and mostly carbon in sp2 hybridization. In a deeper region, the C sp3 content increases substantially going to a maximum, whereas the nitrogen percentage decreases to a minimum, then increases, and tends to saturate. Special attention has been paid to in-depth distributions of carbon atoms in trigonal and tetragonal arrangements because they specify numerous physical and chemical properties of carbon-based materials. These results indicate that the interaction of DLC:N surfaces with surroundings can be influenced, barring oxygen and nitrogen, by sp2-bonded carbon atoms located near the surface of the samples. The obtained results can be useful for developing a deeper understanding of the interaction between DLC:N layer surfaces and their surroundings and particularly with living tissue.

Investigation of Ripple Formation on Surface of Silicon by Low-Energy Gallium Ion Bombardment.

Regular wave patterns were created by a 2 kV gallium ion on Si(111) monocrystals at incidence angles between 60° and 80° with respect to the surface normal. The characteristic wavelength and surface roughness of the structured surfaces were determined to be between 35-75 nm and 0.5-2.5 nm. The local slope distribution of the created periodic structures was also studied. These topography results were compared with the predictions of the Bradley-Harper model. The amorphised surface layers were investigated by a spectroscopic ellipsometer. According to the results, the amorphised thicknesses were changed in the range of 8 nm to 4 nm as a function of ion incidence angles. The reflectance of the structured surfaces was simulated using ellipsometric results and measured with a reflectometer. Based on the spectra, a controlled modification of reflectance within 45% and 50% can be achieved on Si(111) at 460 nm wavelength. According to the measured results, the characteristic sizes (periodicity and amplitude) and optical property of silicon can be fine-tuned by low-energy focused ion irradiation at the given interval of incidence angles.

Unveiling the nature of atomic defects in graphene on a metal surface.

Low-energy argon ion bombardment of graphene on Ir(111) induces atomic-scale defects at the surface. Using a scanning tunneling microscope, the two smallest defects appear as a depression without discernible interior structure suggesting the presence of vacancy sites in the graphene lattice. With an atomic force microscope, however, only one kind can be identified as a vacancy defect with four missing carbon atoms, while the other kind reveals an intact graphene sheet. Spatially resolved spectroscopy of the differential conductance and the measurement of total-force variations as a function of the lateral and vertical probe-defect distance corroborate the different character of the defects. The tendency of the vacancy defect to form a chemical bond with the microscope probe is reflected by the strongest attraction at the vacancy center as well as by hysteresis effects in force traces recorded for tip approach to and retraction from the Pauli repulsion range of vertical distances.

A density functional tight-binding based strategy for modeling ion bombardment and its application to Ar bombardment of silicon nitride

In many modern applications, it is important to understand mechanisms of non-equilibrium chemistry and physics that are driven by low energy ion bombardment of solid surfaces. However, the study of these processes has been challenging as it demands a relatively unique balance between chemical fidelity and computational cost. To this end, we have proposed and constructed a new, high-throughput simulation pipeline based on density functional tight binding simulations. Additionally, we have extended the parameter set pbc-0-3 with the addition of Ar, thereby enabling the simulation of Ar bombardment. This pipeline was then applied to study the structural and compositional evolution of silicon nitride (SiN) under Ar bombardment. We identified a possible rate limiting step of bombardment-driven sputtering of SiN and suggested underlying mechanisms of Si and N removal. Damage from the bombardment, including generation of surface defects and Ar implantation, are further discussed. These findings and the newly developed simulation framework will serve as a useful foundation for further research in processes driven by ion bombardment.

Differential sputtering yield measurement of ion grid materials by novel arch array system

In this experimental study the differential sputtering yield and total sputtering yield of ion thruster grid materials (Mo and C/C composite) under low-energy ion bombardment has been investigated. A novel combinable aluminum arch array is developed to determine the spatial distributions of sputtered atoms in different azimuthal angles. The quartz crystal microbalance (QCM) method is used to validate the typical distribution profile of sputtered atoms at the 180° azimuthal angle. The distribution characteristics of sputtered atoms as well as the specific regions of maximum differential sputtering yield under different incident energies, incident angles and ion species (Xe, Kr, Ar) have been discussed. The sputtering resistance performance of C/C composite materials have been compared with Mo metal by measuring the total sputtering yield. The spatial distributions of C/C differential sputtering yield are found highly anisotropic even under normal incidence, a multi-region discontinuous distribution characteristic of maximum differential sputtering yield have been observed in C/C composite material. The experimental results about the spatial distribution of sputtered atoms are of great importance for establishing the sputter-redeposition models of grid apertures to accurately predict the evolution of grid morphology and lifetime of ion thruster.

Spectromicroscopic study of the transformation with low energy ions of a hematite thin film into a magnetite/hematite epitaxial bilayer

The search of new properties in novel oxide heterostructures requires the exploration of new fabrication methods and the study, at the microscopic level, of the processes involved during the synthesis. We present a synchrotron-based spectromicroscopic investigation of a magnetite/hematite bilayer on Pt(111) grown in a two-step process by thermal evaporation and Low Energy Ion Bombardment (LEIB). The characterization includes the study of structural, electronic, chemical, and magnetic properties using X-ray Absorption Spectroscopy (XAS), Low Energy Electron Microscopy (LEEM), Photoemission Electron Microscopy (PEEM), or X-ray Magnetic Circular Dichroism (XMCD). The aim is to obtain microscopic information of the thin film before, during, and after the ion bombardment. Ion bombardment gradually transforms the topmost layers of the hematite thin film into a defective sub-oxide, where magnetite nuclei grow and coalesce with increasing ion doses. Two rotational domains of magnetite coexist, which are typically a few tens of nanometres large and do not grow significantly with temperature annealings. The incoherent growth of the magnetite nuclei favours the formation of stable twin boundaries (TBs) and antiphase boundaries (APBs). Dichroic spectra show the characteristics of the ferrimagnetic (FiM) order of magnetite, and the spatial distribution of magnetic domains shows no apparent correlation with the structural image, displaying smooth domains separated by diffuse frontiers. These findings illustrate the importance of a spectromicroscopic characterization of novel oxide heterostructures for potential future applications.

Effect of Low-Energy Ion Bombardment on the Texture and Microstructure of Platinum Films

Effect of Low-Energy Ion Bombardment on the Texture and Microstructure of Platinum Films

Influence of Low-Energy Ion Bombardment on the Texture and Microstructure of Pt Films

Influence of Low-Energy Ion Bombardment on the Texture and Microstructure of Pt Films

Surface morphology and carbon structure effects on sputtering: Bridging scales between molecular dynamics simulations and experiments

Experiments report large scatter in the sputtering yield of carbon materials under the bombardment of low-energy noble gas ions. Here, we conduct scale-bridging molecular dynamics (MD) simulations on the xenon bombardment of carbon substrates across ion energies of 75 eV to 2 keV, and at ion incidence angles of 0°–75°, to resolve uncertainties in the sputtering data. Results show rapid amorphization of the carbon subsurface with ion bombardment, but the structural characteristics (sp/sp2/sp3 bond proportion, atomic density) eventually plateau once steady-state sputtering is achieved. In addition, we obtain virtually indistinguishable “steady-state” amorphous structures across the range of ion energies and ion incidence angles, as well as for several different carbon structures, which suggests that the steady-state sputtering yield data is independent of the initial carbon structure and prior sputtering history. By accounting for changes in the local incidence angle, surface shielding, and redeposition of the sputterants associated with surface morphology effects, we demonstrate that the sputtering predictions from MD are in perfect agreement with prior experiments across the range of ion energies and incidence angles. Using a Bayesian approach based on the MD data, we calibrate the parameters of a semi-empirical sputtering model for different surface morphologies.

Effect of low-energy ion bombardment on structural and optical properties of gold functionalized hBN nanosheets

This work reports the effect of low-energy ion (30 KeV Si−1) bombardment on the structural and optical properties of gold-functionalized hexagonal boron nitride (Au-hBN) nanosheets. FE-SEM and XRD results indicate the successful synthesis of highly crystalline Au-hBN nanosheets. The grain size modulation and strain variation in Au-hBN nanosheets with increasing ion fluence clearly indicate defect-induced structural modifications. The optical properties of Au-hBN are investigated by Raman and UV–vis absorption spectroscopy. A systematic drop in the band gap and gradual rise in the Urbach energy of Au-hBN nanosheets with increasing fluence is observed. In addition, the FTIR results reveal the growth of cBN, eBN and wBN phases after irradiation. The theoretical simulations are done to study the defect formation and estimate the damage yields. The experimental and computational results establish the efficacy of the low-energy ion bombardment approach in systematically altering the structural and optical behavior of Au-hBN nanosheets.

Study on Ion Bombardment Semiconductor Shaped Self-Assembled Nanostructures

Through Surface nanopatterning induced by ion beam bombardment has become an effective nanostructure technology, in a short period, large-area patterning can be induced on the surface of various materials through ion bombardment technology, and island-like, porous, and corrugated morphology can be formed on the surface of the material, effectively improving its surface morphology. In this review, we provide an updated description of the progress made when ions bombard semiconductor surfaces, focusing on the influence of different parameters on the evolution of surface morphology. Secondly, the physical model of the evolution of surface nanostructures and the application of nanostructures generated by low-energy ion bombardment are summarized.

Formation of Q-carbon with wafer scale integration

We describe the formation of highly uniform Quenched-carbon (Q-carbon) layers by plasma-enhanced chemical vapor deposition (PECVD) followed by low-energy Ar+ ion bombardment to achieve wafer-scale integration of Q-carbon films. After PECVD, 9 nm and 20 nm thick silicon-doped diamond-like carbon (Si-DLC) films showed complete conversion into Q-carbon using 250eV Ar+ ions via negative biasing. However, this conversion was only partial for 30 nm thick films. Detailed EELS, XPS, Raman, and EDS studies were carried out to confirm the formation of Q-carbon by this method. We discuss the mechanism of Q-carbon formation as a result of low-energy ion bombardment during PECVD of thin films. These ions during negative biasing are energetic enough to create Frenkel defects, which support the conversion of the three-fold coordinated sp2 carbon units in as-deposited carbon into sp3 bonded five-atom tetrahedron units in Q-carbon. This process enhances the atomic number density and fraction of sp3 bonded carbon. These diamond tetrahedra are randomly packed and provide easy nucleation sites for diamond. If the underlying substrate can provide an epitaxial template for diamond growth via domain matching epitaxy, then wafer-scale growth of diamond epitaxial films can be achieved for wafer-scale integration and next-generation novel device manufacturing from diamond-related materials.

Novel Titanium Nanospike Structure Using Low-Energy Helium Ion Bombardment for the Transgingival Part of a Dental Implant.

Aim(s): The aim of the study was to fabricate a nanospike surface on a titanium alloy surface using a newly established method of low-energy helium ion bombardment. Various methods to achieve nanospike formation on titanium have been introduced recently, and their antibacterial properties have been mainly investigated with respect to Escherichia coli and Staphylococcus aureus. Oral pathogens such as Porphyromonas gingivalis play an important role in the development of peri-implantitis. For that reason, the antibacterial properties of the novel, nanostructured titanium surface against P. gingivalis were assessed, and a possible effect on the viability of gingival fibroblasts was evaluated. Materials and Methods: Helium sputtering was employed for developing titanium surfaces with nanospikes of 500 nm (ND) in height; commercially available smooth-machined (MD) and sandblasted and acid-etched titanium disks (SLA) were used as controls. Surface structure characterization was performed through scanning electron microscopy (SEM) and atomic force microscopy (AFM). Following incubation with P. gingivalis, antibacterial properties were determined via conventional culturing and SEM. Additionally, the viability of human gingival fibroblasts (HGFs) was tested through MTT assay, and cell morphology was assessed through SEM. Results: SEM images confirmed the successful establishment of a nanospike surface with required heights, albeit with heterogeneity. AFM images of the 500 nm nanospike surface revealed that the roughness is dominated by large-scale hills and valleys. For frame sizes of 5 × 5 μm and smaller, the average roughness is dominated by the height of the titanium spikes. ND successfully induces dysmorphisms within P. gingivalis cultures following the incubation period, while conventional culturing reveals a 17% and 20% reduction for ND compared to MD and SLA, respectively. Moreover, the nanospike surfaces did not affect the viability of human growth fibroblasts despite their sharp surface. Conclusion(s): This study successfully developed a novel titanium-nanospike-based structuration technique for titanium surfaces. In addition, the nanospikes did not hinder gingival fibroblast viability. Enhanced antimicrobial effects for such a novel nanospike-based resurfacing technique can be achieved through further optimizations for nanospike spacing and height parameters.

Investigation of Defect InP(001) Surface by Low Energy Ion Scаttering Spectroscopy

In this paper presents the computer simulation results on the investigations of the ion scattering processe on the defect InP(001)<110>,<ī10> surface under low-energy grazing ion bombardment have been presented. The peculiarities trajectories of the scattered ions from surface defect, atomic chain and semichannel have been investigated by computer simulation. It was found some trajectories nearby surface atomic chain which have loop shape and a line form. At grazing ion incidence, from a correlation of the experimental and calculated energy distributions of the scattered particles, one may determine a spatial extension of the missing atom on the monocrystal surface damaged by the ion bombardment.

Formation of (100) texture in thin Ti films under low-energy ion bombardment

10-40 nm Ti films with mixed crystalline texture (100)+(001) are exposed to ion bombardment in inductively coupled Ar plasma by applying the bias -30 V to the films. It is found that such a treatment leads to the formation of (100) texture in films. This result is explained by the generation of the compressive stress in films as a result of ion bombardment. The thinner the film the less time is required to form the (100) texture. Keywords: crystalline texture, ion bombardment, titanium, thin films.

Copper thin films deposited using different ion acceleration strategies in HiPIMS

The growth of Cu thin films by low-energy ion-bombardment using bipolar and conventional HiPIMS pulse configurations to the target in combination with different biasing methods of the substrate were investigated. For bipolar HiPIMS with a substrate at floating potential, XRD measurements indicate minimal ion acceleration and change in the crystal growth when increasing the substrate holder potential to the same level as the applied positive voltage. In contrast, using bipolar HiPIMS with a substrate at ground potential results in a similar ion current profile as in conventional HiPIMS with a synchronized pulsed bias with the same delay and timing as the positive pulse. Furthermore, the trend in crystal growth is the same such that a significant increase in the (200) intensity is observed within an ion acceleration window, 125–175 V. Using conventional HiPIMS with a continuous DC bias also results in Cu films exhibiting significant (200) peaks, but the ion acceleration window is shifted to 175–225 V. The observed differences in the film growth could be explained not only by the energy of the ions but also by the type of ions (working gas vs metal ions) that are accelerated during either the positive pulse or substrate biasing.

Mechanism for anisotropic ejection of atoms from fcc (100) metal surface by low-energy argon ion bombardment: Molecular dynamics simulation

In order to thoroughly clarify the formation mechanism of “Wehner spots” during the low-energy sputtering of fcc (100) surface, molecular dynamics (MD) method was employed to investigate the sputtering of Cu (100) surface by 500 eV argon ion bombardment. Simulation program monitored the emission trajectories and velocities of sputtered atoms as well as the forces acting on them. Simulation results revealed that the preferential ejection of sputtered atoms along close-packed directions, which caused the formation of “Wehner spots”, resulted mainly from the regular interaction between sputtered atoms and their neighbor atoms. The formation mechanism of “Wehner spots” is that sputtered atom is first horizontally focused and then undergoes a ski-jump takeoff along close-packed direction. The generality of this mechanism was further verified by supplementary molecular dynamics simulations. Furthermore, this investigation demonstrated that local environment surrounding sputtered atom rather than simple collision processes transferring initial recoil energy to sputtered atom dominated the anisotropic sputtering of fcc (100) surface in low energy regime.

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases. While preferential n-type doping was observed in graphene domains in nitrogen-containing plasmas, preferential p-type doping was observed at grain boundaries in boron-containing plasmas.

A linked-scale coupled model of mass erosion and redistribution in plasma-exposed micro-foam surfaces

Surface evolution due to exposure to harsh environments is of importance in many scientific and technological applications. In plasma-exposed materials, the surface receives charged particles from the plasma, leading to a series of processes that drive the system far from equilibrium and may lead to the severe deterioration of the surface properties. Although surface morphological changes are driven by atomic collisions taking place over picoseconds and nanometers, these processes result in mass loss and redistribution of matter over much larger length and time scales. This necessitates a multi-scale approach capable of capturing the range of processes linking primary atomic collision events with engineering-level surface geometry changes. In this paper, we develop a computational model to simulate the morphological evolution and effective erosion rate of micro-architected tungsten foams during low-energy plasma ion bombardment. The model acts on several length scales, with the energy and angular dependence of the sputtering yield of flat tungsten surfaces determined using the SRIM code based on the binary collision approximation. This information is introduced into a low-fluence, short-term Monte Carlo raytracing model, and further into a high-fluence, long-term particle transport model. In the latter, material particles representing billions of atoms are described in a digitized 3-D representation of the foam structure from X-ray tomography data, and are sputtered off and redeposited using the atomistic information. We show that the redeposition of sputtered atoms leads to partial self-healing in the bottom layers with a sharp reduction in the sputtering coefficient of low density foams, roughly 25% of the solid W value. This is in qualitative agreement with recent experiments on low porosity W structures. At high fluence, the foam structure degrades considerably as there are fewer ligaments available to recapture sputtered atoms and, consequently, the sputtering rate increases again.

Defect-mediated sputtering process of boron nitride during high incident angle low-energy ion bombardment

Further development of hexagonal boron nitride (hBN) towards electronic devices requires the application of precise analytical techniques. High incident angle (¿ 65°) secondary ion mass spectrometry has been recently developed, and allows to reach atomic depth. However, the procedure has been optimized experimentally, and thus computer simulations are needed to validate and comprehend the experiment. It is revealed that a sample without any defects cannot be sputtered in such conditions — all ions are reflected from the surface. Only defects, particularly vacancies, can act as erosion centers. After prolong bombardment (dose in the range of 1017 ions cm-2), the number of defects and their sizes are sufficiently large that rapid removal of a top-most hBN layer can be observed. Computer simulations and additional experiments reveal that the sputtering process is defect-mediated and anisotropic — significantly more prominent along the incident direction.