Low Energy Ion Sputtering Research Articles

Drift of two-dimensional vacancy islands on the Si(100) surface under electromigration conditions

The paper presents an experimental study of morphological changes of the Si(100) surface under electromigration conditions that was carried out using the in situ method involving ultrahigh vacuum reflection electron microscopy. The study aims to determine the temperature dependence of the effective electric charge of an adsorbed atom on the Si(100) surface. A system of concentric two-dimensional vacancy islands was formed on the surface of Si(100) samples by low-energy argon ion sputtering and subsequent high-temperature annealing. Quasi-equilibrium conditions were created on the sample surface by compensating the sublimating flow from the surface from an external silicon source. The video images of the drift of vacancy islands were recorded under the conditions of electromigration with the compensation of sublimation. Based on the processing and analysis of video images, the authors described the dependence of the velocity of motion of vacancy islands on the Si(100) surface for various temperatures and the direction of the electric current along and across the dimer rows with the (2 × 1) superstructure inside the island. It is shown that the drift rate of vacancy islands does not depend on their size under quasi-equilibrium conditions. A simplified one-dimensional theoretical model has been constructed. It includes one atomic step moving by a detachment of atoms from the step and their drift under the force of electromigration in the absence of desorption and deposition of atoms on the surface. Based on the proposed model, the effective electric charge is estimated, and the temperature dependence of the effective charge in the temperature range of 1010 to 1120 °C is obtained. The absolute value of the effective charge decreases linearly with increasing temperature. The sign of the effective charge is negative, and its average value is Z = –0.5 ± 0.3 elementary charges. The obtained results can be used for creating structures with a countable number of atomic steps and act as secondary measures of height with reference to the silicon crystal lattice.

Nonlinear effects in low-energy ion sputtering of solids

In this paper, we demonstrate that the sputtering of solids by ions of low (<30 keV) and even ultralow energies (<2 keV) is accompanied by nonlinear effects. These nonlinear effects are the result of heating a significant part of the cascades of collisions above the effective melting temperature of the material with the formation of local melts or “thermal spots” (a special case of the thermal spike regime). Nonlinear effects should primarily be observed in dense targets irradiated by heavy and cluster ions. In this paper, the question of the lifetime of such spots and their effect on the emission of secondary particles and the formation of specific surface topography is examined in detail. Another important assumption of the model is the existence of an intermediate ion sputtering regime, in that both linear cascades and thermal spots can exist simultaneously. A phenomenological model based on the effect of a rapid decrease in the cascade volume with a monotonic decrease in the energy of primary ions below 3 keV, which leads to the formation of thermal spots at ultralow energies (<2 keV), is also proposed. The suggested thermal spot model allows us to explain several experimental data that have not yet been explained in the framework of the traditional sputtering models. In particular, a simple explanation is given for the formation of pores in the surface layer during implantation and the formation of a relief on an ion-bombarded surface such as nanodots, pits (holes), and in the form of a foam-like surface. The small stationary concentration of cesium on the surface of the target sputtered by low-energy cesium ions is also explained.

Analysis of the Mo/Mo Nanocrystalline Optical Coating Obtained with the Aid of Simultaneous Deposition and Ion Sputtering

Structure, optical properties, and resistance to sputtering are studied for a reflecting Mo-coating that is fabricated using magnetron deposition with simultaneous low-energy ion sputtering at the deposition rate that is higher than the etching rate. A Mo-polycrystalline mirror is used as a substrate. It is shown that the coating exhibits textured nanocrystalline structure with a relatively low spread of crystallite sizes and high resistance to sputtering. It is also demonstrated that the spectral reflection coefficient of such a Mo-coating differs from the spectral reflection coefficient of polycrystalline and single-crystalline Mo and the difference results from the effect of the structure of coating on its optical properties. A theoretical model of the coating formation is proposed.

Stable Au-C bonds to the substrate for fullerene-based nanostructures.

We report on the formation of fullerene-derived nanostructures on Au(111) at room temperature and under UHV conditions. After low-energy ion sputtering of fullerene films deposited on Au(111), bright spots appear at the herringbone corner sites when measured using a scanning tunneling microscope. These features are stable at room temperature against diffusion on the surface. We carry out DFT calculations of fullerene molecules having one missing carbon atom to simulate the vacancies in the molecules resulting from the sputtering process. These modified fullerenes have an adsorption energy on the Au(111) surface that is 1.6 eV higher than that of C60 molecules. This increased binding energy arises from the saturation by the Au surface of the bonds around the molecular vacancy defect. We therefore interpret the observed features as adsorbed fullerene-derived molecules with C vacancies. This provides a pathway for the formation of fullerene-based nanostructures on Au at room temperature.

Kinetic Monte Carlo simulations of self organized nanostructures on Ta surface fabricated by low energy ion sputtering

Surfaces bombarded with low energy ion beams often display development of self assembled patterns and quasi-periodic structures. Kinetic Monte Carlo simulations have been performed to describe ion sputtered Tantalum surfaces. A weak nonlinearity in the relaxation process has been introduced and the results show that the positive Schwoebel barrier, produced by the nonlinear Hamiltonian, is necessary in describing ion bombarded Tantalum surfaces. Furthermore, their scaling exponents suggest presence of a class other than KPZ.

Morphology simulation of the surface subjected to low-energy ion sputtering

A new 2D method of simulating the morphology of a surface subjected to low-energy ion sputtering with regard to sputtered material redeposition is suggested. The object of simulation is the profile of microgrooves arising on the silicon surface exposed to slow argon ions from the dense plasma of an rf induction discharge. Numerical simulation data and experimental data are in good agreement.

Anisotropic plasmonic Cu nanoparticles in sol-gel oxide nanopillars studied by spectroscopic Mueller matrix ellipsometry

Broadened plasmon resonances of Cu nanoparticles in nanopatterned mixed oxide sol-gel nanopillars are shown to be readily detected by spectroscopic Mueller matrix ellipsometry. The plasmonic nanomaterials are obtained by low energy ion sputtering of a CuO sol-gel film. Both s- and p-polarized plasmon resonances are observed in the off-block-diagonal and the block-diagonal Mueller matrix elements as well as in the generalized ellipsometric parameters. The resonant features in all elements correlate with both maximum depolarization and a minimum in the reflected intensity. The spectral position and the polarization character of the plasmon resonances are discussed phenomenologically through effective medium theory.

Controlling the Structure and Size of Au Nanocrystals by Annealing and Ion Sputtering

In situ thermal annealing was used for the first time to observe directly that Au nanoparticles, which were originally fully embedded in the near-surface region of TiO(2), can be tailored into hemispheres exposed at the surface at elevated temperature. Precise control of the size of the Au hemispheres was achieved by subsequent low-energy ion sputtering. This method can be used to control the structure and size of a wide variety of nanoparticles in a matrix where surface structure and particle size are required to obtain specific material properties.

Polycrystalline Ni thin films on nanopatterned Si substrates: From highly conformal to nonconformal anisotropic growth

The growth of polycrystalline Ni thin films on nanorippled Si templates is investigated in situ by grazing incidence small angle x-ray scattering as well as ex situ by atomic force microscopy and cross-sectional transmission electron microscopy. The templates have been fabricated by low-energy ion sputtering which leads to the spontaneous formation of a periodic ripple pattern with about 35 nm periodicity and about 3 nm peak-to-peak height. Highly conformal growth of the Ni film is observed under normal incidence deposition with the film surface perfectly replicating the substrate morphology up to a film thickness of at least 120 nm. Grazing incidence deposition perpendicular to the ripple orientation leads to the formation of one-dimensional nanowires on one side of the ripples due to geometrical shadowing. At a film thickness of about 10 nm, a transition to anisotropic columnar growth with rapidly decreasing conformity is observed. In this regime, the nanowires act as growth seeds for the columns and further geometrical shadowing leads to a film consisting of rows of tilted columns.

Real-time in situ Mueller matrix ellipsometry of GaSb nanopillars: observation of anisotropic local alignment

The formation of GaSb nanopillars by low energy ion sputtering is studied in real-time by spectroscopic Mueller matrix ellipsometry, from the initial formation in the smooth substrate until nanopillars with a height of 200-300 nm are formed. As the nanopillar height increased above 100 nm, coupling between orthogonal polarization modes was observed. Ex situ angle resolved Mueller polarimetry measurements revealed a 180° azimuth rotation symmetry in the off-diagonal Mueller elements, which can be explained by a biaxial material with different dielectric functions εx and εy in a plane parallel to the substrate. This polarization coupling can be caused by a tendency for local direction dependent alignment of the pillars, and such a tendency is confirmed by scanning electron microscopy. Such observations have not been made for GaSb nanopillars shorter than 100 nm, which have optical properties that can be modeled as a uniaxial effective medium.

Formation of CoNx ultra-thin films during direct-current nitrogen ion sputtering in ultrahigh vacuum

This study reports the formation of ultra-thin cobalt nitride (CoNx) films on a Co/ZnO(002) crystal by low-energy ion sputtering of nitrogen in an ultrahigh vacuum system. The CoNx film formed during ion bombardment in which the nitrogen plasma (N+) results in both sputtering and implantation in the formation process of CoNx, especially for the Co adsorbed layers. Auger electron spectroscopy analysis shows that the composition ratio x as a function of sputtering time was highly related to the N+ ion energy that was varied from 0.5 to 2keV. The composition ratio x of CoNx films is inversely proportional to the ion energy. Low-energy ion sputtering is possible to fabricate ultra-thin CoNx films and to adjust their chemical compositions.

Effect of low energy ion irradiation on CdTe crystals: Luminescence enhancement

In this work we show that low energy ion sputtering is a very efficient technique as a cleaning process for CdTe substrates. We demonstrate, by using several techniques like grazing-angle x-ray diffraction, cathodoluminescence, microluminescence, and micro-Raman spectroscopy that the luminescent properties of CdTe substrates can be very much increased when CdTe surfaces are irradiated with low energy Argon ions. We postulate that this enhancement is mainly due to the removal of surface damage induced by the cutting and polishing processes. The formation of a low density of nonluminescent aggregates after the sputtering process has also been observed.

Molecular dynamics simulations of low-energy argon ion sputtering of copper clusters on polyethylene surfaces

The normal bombardment of the targets consisted of single 13-, 27- or 39-atom copper cluster on a surface of polyethylene by Ar ions with energies of 100, 200 and 400eV is examined using molecular dynamics simulation incorporating long-range many-body covalent bonding potential for hydrocarbons and a potential based on a embedded atom model for copper. Sputtering yield and its dependence on the energy of bombarding ion and size of the pre-deposited copper cluster are discussed.

Photoluminescence Study of Rapid Thermal Annealed GaAs/AlGaAs Quantum Dots Formed by Low Energy Ion Sputtering

Photoluminescence Study of Rapid Thermal Annealed GaAs/AlGaAs Quantum Dots Formed by Low Energy Ion Sputtering

Characterization of inclined GaSb nanopillars by Mueller matrix ellipsometry

Inclined GaSb nanopillars prepared by low energy ion sputtering with oblique ion beam incidence have been characterized by two different Mueller matrix ellipsometric tools. The optical properties of the nanopillars were found to be well described by a uniaxial anisotropic graded effective medium model. The pillar height and inclination angle were determined by fitting the parameters of the effective medium model to spectroscopic (1.44–2.88 eV) Mueller matrix measurements at multiple azimuth sample orientations. A set of different samples with various average pillar height and inclination angle was studied; results from the optical characterization correspond well with those from scanning electron microscopy analysis. For samples with nanopillars inclined by 45° or less, the height could be determined from a single Mueller matrix measurement at only one azimuth orientation, allowing real-time in situ observation of the formation. The nanopillars were also studied using a single wavelength angle resolved Mueller polarimeter, which also can be used to determine height and inclination of the pillars, in addition to validating the optical model over a wide range of incident and azimuth angles.

Modelling of low energy ion sputtering from oxide surfaces

The main aim of this work is to present a way to estimate the values of surface binding energy for oxides. This is done by fitting results from the binary collisions approximation code Tridyn with data from the reactive sputtering processing curves, as well as the elemental composition obtained from x-ray photoelectron spectroscopy (XPS). Oxide targets of Al, Ti, V, Nb and Ta are studied. The obtained surface binding energies are then used to predict the partial sputtering yields. Anomalously high sputtering yield is observed for the TiO2 target. This is attributed to the high sputtering yield of Ti lower oxides. Such an effect is not observed for the other studied metals. XPS measurement of the oxide targets confirms the formation of suboxides during ion bombardment as well as an oxygen deficient surface in the steady state. These effects are confirmed from the processing curves from the oxide targets showing an elevated sputtering rate in pure argon.

The morphology of amorphous SiO2 surfaces during low energy ion sputtering

The morphology of different amorphous or amorphizedSiO2 surfaces, including thermally grown films, fused silica, and single crystalline quartz, duringlow energy ion sputtering has been investigated by means of atomic force microscopy. Forall three materials, the formation of periodic ripple patterns oriented normal to the direction ofthe ion beam is observed at intermediate incident angles. At near-normal incidence, theSiO2 surfaces remain flat, whereas a rotation of the ripple patterns is observed at grazing incidence.At intermediate angles, the patterns on the different surfaces exhibit wavelength coarseningof different strengths, which can be attributed to different amounts of near-surface masstransport by the surface-confined ion-enhanced viscous flow. In the framework of the recenthydrodynamic model of ion erosion, the observed differences in ripple coarsening are consistentwith this interpretation and indicate that the surface energies of thermally grownSiO2 and amorphized quartz are lower and higher than that of fused silica, respectively.

Real-time in situ spectroscopic ellipsometry of GaSb nanostructures during sputtering

We demonstrate that real-time in situ spectroscopic ellipsometry can be used to measure the height evolution of nanostructures during low energy ion sputtering of GaSb. A graded anisotropic effective medium approximation is used to extract the height from the optical measurements. Two different growth regimes have been observed, first exponential then followed by a linear regime. The linear regime is not expected from the traditional sputtering theories. The in situ results correspond well to ex situ atomic force microscopy measurements.

Minimization of topological defects in ion-induced ripple patterns on silicon

The evolution of self-organized nanoscale ripple patterns induced by low-energy ion sputtering of silicon is investigated. The quality of the patterns is monitored by calculating a normalized density of topological defects from atomic force microscopy images. A strong dependence of the normalized defect density on the applied ion fluence is observed with a well-pronounced minimum at intermediate fluences. Simulations using the damped Kuramoto–Sivashinsky equation yield good agreement with the experiments and are further used to study the dynamics of single pattern defects.

Electromechanical properties of nichrome (80/20 wt-%) thin film fabricated by low energy ion sputtering

The electromechanical properties of nichrome (Ni–Cr 80/20 wt-%) used as a common material for application in thin film strain gauges have been studied. The composition and oxidation of Ni–Cr film fabricated by low energy ion beam sputtering has been studied by Auger electron spectroscopy. The microstructure of the film was determined by a TEM. The temperature coefficient of resistance has been determined by a Nanovolt/Microohm meter. The gauge factor has been determined by the cantilever method. Low stable temperature coefficient of resistance values of resistance stability and the desired sheet resistance have been obtained. It meets the main demanded properties for its application to strain sensitive film.