Low-energy Argon Ion Bombardment Research Articles

Atomic Scale Etching of Diamond: Insights from Molecular Dynamics Simulations

Abstract Diamond is a promising material for multiple applications in quantum information processing and sensing as well as applications in microelectronics. However, diamond devices can be limited by surface defects that compromise charge stability and spin coherence, among others. Improved strategies in plasma etching of diamond could play an important role in minimizing or eliminating these defects. In this work, we explore plasma-assisted atomic scale etching of diamond using argon ions (Ar$^{+}$), hydrogen ions (H$^{+}$) and hydrogen atoms (H). We employ classical molecular dynamics (MD) simulations and test several interatomic potentials based on the Reactive Empirical Bond Order (REBO) form with comparisons to a variety of published experimental results. We performed MD simulations of low-energy hydrogen ($\le$50 eV) and argon ($\le$200 eV) ion bombardment of diamond surfaces. Ar$^{+}$ bombardment can be used to locally smooth initially rough diamond surfaces via the formation of an amorphous C layer, the thickness of which increases with argon ion energy. Subsequent exposure with hydrogen ions (or fast neutrals) will selectively etch this amorphous C layer, leaving the underlying diamond layer mostly intact if the H energy is maintained below about 10 eV. The simulations suggest that combining Ar$^+$ smoothing with selective, near threshold energy H removal of amorphous C can be an effective strategy for diamond surface engineering, leading to more reliable and sensitive diamond color center devices.

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.

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.

Study of the compositional, structural and stress evolutions induced by low-energy argon ions on chemically strengthened glass

The chemically strengthened glasses bombarded by low-energy (100 eV) argon ions at room temperature are investigated in order to understand the effect of ion-bombardment on the distribution of alkali metal ions within the glass network. The composition distribution, microstructure evolutions and physical properties of the chemically strengthened glasses before and after bombardment were analyzed by means of X-ray Photoelectron Spectroscopy, electron probe micro analysis, Raman spectroscopy and surface stress meter, respectively. Changes of atomic composition and bonding states of Na, K, Si and O were observed with increasing bombardment time. Out-diffused and migrated alkali ions in the exchanged region during bombardment, the peak of potassium ions concentration in the exchanged region decreases and moves into the interior of glasses to different degrees. And two sides of the peak value exhibit significantly different concentration distribution. Moreover, decrease of the scattering intensity of Raman spectral bands connected with silicon-oxygen tetrahedron containing non-bridging oxygens (Q2 and Q3 species) accompanies by the formation of high-coordinate Si species with increasing bombardment time. The values of compressive stress and depth of layer in the chemically strengthened glasses have a trend of decreasing markedly during bombardment. All results have shown that variation of composition distribution, Raman spectral components and surface compressive stress in the exchanged region is indicative of alterations of the silicate network induced by low-energy (100 eV) argon ion-bombardment, which provide importance data for the industrial application of the chemically strengthened glasses.

Polyelemental Nanolithography via Plasma Ion Bombardment: From Fabrication to Superior H2 Sensing Application

The development of complex nanostructures containing a homo- and heteromixture of two or more metals is a considerable challenge in nanotechnology. However, previous approaches are considerably limited to the number of combinations of metals depending on the compatibility of elements, and to the complex shape control of the nanostructure. In this study, a significant step is taken toward resolving these limitations via the utilization of a low-energy argon-ion bombardment. The multilayer films are etched and re-sputtered on the sidewall of the pre-pattern, which is a secondary sputtering phenomenon. In contrast to the precursor mixing method, most metallic combinations can be fabricated. The degree of mixing is tuned by the control of the sequence and thickness of multilayers. In addition, the feature shape and dimensions are controlled by changing the pre-pattern or by controlling the ion-beam angle. Using this method, the shortest response time (2 s to 1% H2 ) in comparison with those of Pd-based H2 sensors reported previously and a limit of detection below 1 parts per million (ppm) for Pd/Au and Pd/Pt bimetallic line arrays are achieved. This study is expected to realize a family of polyelements that can be used in various applications.

Effect of Low-Energy Ion-Plasma Treatment on Residual Stresses in Thin Chromium Films

The results of studying the effect of low-energy argon ion bombardment (~30 eV) on residual mechanical stresses in a thin chromium film are presented. The change in the mean value and stress gradient as a function of the ion bombardment duration was determined by the change in the bend of test micromechanical bridges and cantilevers. A method is proposed for calculating the depth of the stress modification in a film using these structures. It has been established that the long-term ion-plasma treatment at room temperature affects stresses at a depth of more than 100 nm.

Molecular dynamics simulation of bipartite bimetallic clusters under low-energy argon ion bombardment

The evolution of bipartite bimetallic atomic clusters within 5 ps under bombardment with monoenergetic argon ions at the initial energy ranging from 1 eV to 1.4 keV has been simulated by the classical molecular dynamics method with a target obtained from Ni‒Al and Cu‒Au clusters consisting of 78 and 390 atoms, equally divided between the corresponding monometallic parts, the simulated pairs of which have different heats of intermixing. The changes in the potential energy and temperature, the sputtering yields, and the intensity of the ion-stimulated movement of atoms at the interface of the monometallic parts of clusters of both sizes have been determined as functions of the energy of the bombardment.

直接接合技術を用いた<i>p</i>-ZnTe/<i>n</i>-ZnOヘテロ接合界面の作製と電気特性評価

We fabricated a p-ZnTe/n-ZnO heterojunction structure by a direct bonding technology. The surfaces of the p-ZnTe and n-ZnO substrates were activated by low-energy argon-ion bombardment under high vacuum, with keeping their roughness. Continuously, they brought into contact under controlled pressure at room temperature, and annealed in argon atmosphere as post process. Atomic-scale bonding was confirmed by transmission electron microscopy and scanning energy-dispersive X-ray spectroscopy. It was also confirmed that the electric rectifying characteristics depend on the bonding press time and the post anneal temperature.

Molecular dynamics simulation on selective etching of α -quartz and amorphous quartz substrate using low-energy argon ion bombardment model in dry etching process

We investigated the physical sputtering of low-energy argon bombardment onto α-quartz and amorphous quartz substrate using dynamics simulations. We reported the effects of etching selectivity, the effect of surface temperature, Ts, and the effect of incident energy, Ei on sputtering yield. The second generation charge-optimized many body (COMB10) potential was utilized to model the interatomic potential of quartz substrates. Simulations were conducted at incident energies of Ei=50, 100, and 150 eV and substrate temperatures of Ts=300, 500, and 800 K. α-quartz shows higher sputtering yield compared to amorphous quartz at any given incident energy, Ei and substrate temperature, Ts. α-quartz has also produced more stoichiometric yield compared to amorphous quartz.

Evolution of isolated copper clusters under low-energy argon ion bombardment

The molecular dynamics simulation of the evolution of clusters of copper in a vacuum under low-energy argon ion bombardment has been performed. The clusters consisting of 26, 78, and 390 atoms have been studied at a bombarding particle energy ranging from 1 to 800 eV. The potential of the interaction between copper atoms has been simulated using the embedded atom method, whereas the interaction of copper atoms with argon atoms has been described using the Ziegler-Biersack-Littmark potential. As a result of the simulation, time dependences of the temperature and potential energy and the numerical values of the sputtering coefficients of free clusters under the ion bombardment have been obtained.

Stress evolution in Si during low-energy ion bombardment

Abstract

Ion induced compositional changes and nanodroplet formation on GaN surface

Abstract Low energy argon ion bombardment of different fluence onto a GaN film grown on Al 2 O 3 (0 0 0 1) surface induces interesting compositional and morphological modifications. While atomic force microscopy (AFM) studies reveal the formation of a circular flat islands, Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) core-level and valence-band spectra confirm that these nanodroplets consist of pure Ga. Detailed XPS studies suggest breaking of Ga N bond, that releases Ga to diffuse on the surface and nucleate at the threading dislocation sites, defined by the interfacial surface free energy. The formation of droplet pattern formation is discussed in the light of the existing theories on ion induced surface modification.

Transition from ripples to faceted structures under low-energy argon ion bombardment of silicon: understanding the role of shadowing and sputtering

In this study, we have investigated temporal evolution of silicon surface topography under 500-eV argon ion bombardment for two angles of incidence, namely 70° and 72.5°. For both angles, parallel-mode ripples are observed at low fluences (up to 2 × 1017 ions cm-2) which undergo a transition to faceted structures at a higher fluence of 5 × 1017 ions cm-2. Facet coarsening takes place at further higher fluences. This transition from ripples to faceted structures is attributed to the shadowing effect due to a height difference between peaks and valleys of the ripples. The observed facet coarsening is attributed to a mechanism based on reflection of primary ions from the facets. In addition, the role of sputtering is investigated (for both the angles) by computing the fractional change in sputtering yield and the evolution of surface roughness.PACS81.05.Cy, 81.16.Rf, 61.80.Jh, 87.64.Dz

Raman scattering characterization of a carbon coating after low-energy argon ion bombardment

The characteristic of Raman scattering spectra of a carbon coating, which was modified by radio frequency argon plasma, has been investigated. The argon ion bombardment causes changes in the microstructure and amount of stress in the coating. Raman scattering spectra are discussed in terms of intensity, bandwidth and wavenumber. The evolvement of Raman spectra shows the following behavior with increasing bombardment time: the intensity changes of the disordered D band, amorphous D ″ band and graphite G band could be separated into several stages; low-energy argon ion bombardment over a short period can reduce the number of defects in the carbon coating, while a larger bombardment period can increase the number of defects; the widths of the D and G bands both increase, while that of the D ″ band decreases; the wavenumbers of all the three bands fluctuate according to the changes in electronic configuration and amount of stress in the carbon coating.

Characterization of oxygen and argon ion flux interaction with PET surfaces by in-situ XPS and ex-situ FTIR

The effects of low (2.5, 0.2 keV) energy reactive oxygen ion bombardment and argon ion bombardment on poly(ethylene terephthalate) thin film (PET) surface chemical composition were studied. PET films have a high potential as a material for biomedical and electrical industries. The source of ions was an ECR Ion Gun with settable acceleration voltages. PET films were sputtered by ion bombardment for variable process time and the modified films were investigated by in-situ X-ray Photoelectron Spectroscopy (XPS) and ex-situ Fourier transform infrared spectroscopy (FTIR). The significant changes in the chemical composition of surface layers were quantitatively studied by XPS. The ion bombardment scissions the chains in PET film surface layers. Selective sputtering of oxygen atoms from PET surface was observed when argon ion flux used. The 0.2 keV and 2.5 keV argon ion decreased O/C ratio from 0.37 to 0.25, 0.04 respectively. This phenomenon is responsible for the creation of carbon-rich up 96 at.% surface layer and the oxygen in ester bonds is detached first. The oxygen 2.5 keV ion bombardment had similar effect as argon ion bombardment; the ratio O/C was decreased. The ester bond was broken first. But oxygen 0.2 keV ion flux irradiation created an oxygen rich surface; the O/C ratio was in increased from 0.37 to 0.46. The changes in surface conductivity were investigated by shifts in C1s binding energy. Good agreement with atomic concentration of carbon in C–C bonds on the films surface was found. The FTIR analyses identified changes in chemical composition but with no obvious correlation to surface changes. Photons from the ion source irradiating the PET film during ion bombardment probably caused the observed changes in FTIR spectra.

Direct observation of defects in hexagonal boron nitride by near-edge X-ray absorption fine structure and X-ray photoemission spectroscopy

Formation of defects in hexagonal boron nitride (h-BN) under low-energy argon or nitrogen ion bombardment has been studied by near-edge X-ray absorption fine structure (NEXAFS) around boron and nitrogen K-edges and X-ray photoemission spectroscopy (XPS) from B1s and N1s core levels. Breaking of B–N bonds and formation of nitrogen vacancies have been identified in the B K-edge NEXAFS and B1s XPS measurements, followed by the formation of molecular nitrogen, N 2, at interstitial positions for both argon and nitrogen bombardments. The formation of N 2 produces a sharp resonance in the low-resolution NEXAFS spectra around the N K-edge, showing characteristic vibrational fine structure in high-resolution measurements.

New method for fabricating ZnO nanowires deposited onto CdTe substrates

In this work we present a novel thermal oxidation method for the fabrication of ZnO nanowires on CdTe substrates. The results obtained on the modification of their structural and optical properties by using low energy (<5 kV) argon ion bombardment are also analysed. Photoluminescence analysis reveals a decrease in the luminescent properties and a shift in the luminescent peak towards lower energies for the sample with nanowires when radiated with ions. However, columnar structures still remain after irradiation. A higher Cd content in the radiated surfaces has also been observed, indicating Cd diffusion enhancement due to the ion irradiation.

Multiparameter investigation of the sputtering behavior of Ag∕Cu Alloys for low energy argon ion bombardment

The sputtering behavior of Ag∕Cu alloys due to low energy (100–1500eV) Ar+ bombardment as a function of component composition, sample temperature, ion current density, ion dose, and ion energy has been completed. The results indicate that the surface topography development is a strong function of temperature and depends to a lesser extent on ion energy, ion dose, and ion current density. The surface at low temperature coupled with low ion dose and/or low ion current density is flat and faceted with widely dispersed large cones. As the temperature is raised, the rate of Ag surface diffusion increases faster than that of Cu and Ag begins to coat the Cu grains thereby, decreasing the subsequent erosion of the Cu. Selective sputtering of the higher yield Ag and continuous Ag diffusion toward the surface results in a decrease in the Ag concentration in the near surface region as measured by energy dispersive spectroscopy. Auger electron spectroscopy (AES) depth profiles indicate significant diffusion of both Cu and Ag when the electron beam was used in spot mode. Angular distributions of sputtered material indicate that Ag is ejected more preferentially at large angles to the surface normal. The total sputter yield is a function of temperature, ion energy, and dose. These effects are related to the growth of surface topography and the subsequent recapture of ejected material on to the sides of surface features. A method of increasing the thermal conductivity between the sample and holder using graphite colloid was found to affect the surface topography growth and AES profiles. The results of these experiments have implications for depth profiling of multicomponent metallic alloys.

Molecular dynamics simulations of GaAs sputtering under low-energy argon ion bombardment

Results from molecular dynamics (MD) simulations of low-energy (50–200eV) Ar+ ion bombardment on (110) GaAs surfaces are reported. A new analytical bond-order potential, originally developed for molecular beam epitaxy studies, is used and tested in the context of etching to investigate the nature and effects of physical sputtering on GaAs compounds. It is found that a thermal desorption model, which accounts for long time scale phenomena between MD simulated impacts, is necessary to achieve steady state sputtering. An initial rapid etch of both atomic species is observed up to 4×1016ions∕cm2 fluence with preferential sputtering of Ga atoms. At high fluences, simulations show the formation of an As-rich layer on the top surface, a subsurface enrichment of Ga, and a return to stoichiometry deeper in the solid. More than 97% of sputtered or desorbed species appear to be Ga or As atoms; sputtering of GaAs molecules is negligible. All these observations are in agreement with published experimental results. Finally, a significant fraction of the atoms leave the surface with more than 10% of the incident ion energy, which could alter passivation layers on sidewalls during etching.

Relaxation of process induced surface stress in amorphous silicon carbide thin films using low energy ion bombardment

The surface stress in amorphous silicon carbide (a-SiC) thin films has been modified after processing. We show that low energy argon ion bombardment in an inductively coupled plasma reactor causes relaxation of the induced surface stress. Cantilever beams of variable length (25–150μm) have been fabricated and their deflection profiles have been compared before and after ion bombardment using white light interferometry. Our experiments show that the ion bombardment relaxes the stress gradient of the cantilever beams. Both the duration of plasma exposure as well as the bias voltage contribute to relaxing the stress gradient in the a-SiC thin film.