Ion Irradiation Intensity Research Articles

Phase composition of polycrystalline HfNx (0.45 ≤ x ≤ 1.60) and effects of low-energy ion irradiation on microstructure, texture, and physical properties

We have investigated the phase composition of HfNx as a function of x and the effects of low-energy ion irradiation on the microstructure and physical properties of polycrystalline layers grown on SiO2 at 350 °C by ultrahigh vacuum reactive dc magnetron sputtering of Hf in mixed N2/Ar discharges. X-ray diffraction and Rutherford backscattering spectrometry results show that the phases obtained in polycrystalline HfNx layers with increasing x are hcp-structure α-Hf:N (x ≲ 0.6); multiphase mixtures consisting of α-Hf, NaCl-structure δ-HfN, rhombohedral ɛ-Hf3N2, and/or ζ-Hf4N3 (0.6 ≲ x ≲ 0.9); δ-HfN single phase (0.9 ≲ x ≲ 1.3); and mixtures of δ-HfN and higher nitrides (x ≳ 1.3). HfNx layers with 0.9 ≲ x ≲ 1.2 grown under mild ion irradiation (incident ion energy Ei ≃ 7 eV and ion-to-Hf flux ratios Ji/JHf = 1−3) are underdense with mixed orientation, low in-plane stress, and rough surface morphology due to limited adatom mobilities resulting in kinetic roughening and atomic shadowing during film growth. However, the use of intense ion irradiation (Ei = 25 eV and Ji/JHf = 4−20) results in HfNx layers, which are fully dense with strongly 111-oriented texture, compressive in-plane stress, and smooth surfaces due to ion irradiation enhanced adatom surface mobilities. In addition, the latter films have lower resistivity and higher hardness. For stoichiometric δ-HfN layers, ρ decreases from 69.7 to 35.2 μΩ cm and H increases from 22.1 to 27.4 GPa, with increasing ion-irradiation intensity. However, for HfNx layers with 1.2 ≲ x ≲ 1.6, the correspondingly higher steady state atomic N surface coverages during deposition alter growth kinetics in favor of 001 texture with a fully dense structure and compressive in-plane stress.

Investigation on friction force and surface modification of MoS2 flakes under Ga+ ion irradiation

The influence of Ga ion irradiation intensity on the surface of both chemical vapor deposition (CVD) and mechanical exfoliate MoS2 flakes were studied. By Raman spectroscopy, it was observed that the irradiation-induced defects and strain had an impact on the crystal structure of the MoS2. The ion-induced tensile strain initially caused lattice stretched of the MoS2 flakes to increase the work function. However, when the strain exceeded a few percentages (2%), the work function decreased. Friction force microscopy in contact mode was then used to determine the friction force and surface changes in the samples treated with different irradiation time. The result indicated that the friction force decreases as the exposure time increases. Also, the average mean value of irradiated and non-irradiated areas of the samples showed variation in the surface morphology. Furthermore, the bond between the atomic force microscope (AFM) tip and the sample increased, suggesting that the irradiation had caused a change in the MoS2 flakes.

Surface modification of multilayer graphene using Ga ion irradiation

The effect of Ga ion irradiation intensity on the surface of multilayer graphene was examined. Using Raman spectroscopy, we determined that the irradiation caused defects in the crystal structure of graphene. The density of defects increased with the increase in dwell times. Furthermore, the strain induced by the irradiation changed the crystallite size and the distance between defects. These defects had the effect of doping the multilayer graphene and increasing its work function. The increase in work function was determined using contact potential difference measurements. The surface morphology of the multilayer graphene changed following irradiation as determined by atomic force microscopy. Additionally, the adhesion between the atomic force microscopy tip and sample increased further indicating that the irradiation had caused surface modification, important for devices that incorporate graphene.

Ion beam based composition and texture control of titanium nitride

Titanium nitride was formed by electron beam evaporation of titanium in an atmosphere of backfilled nitrogen. The growing film was simultaneously irradiated with argon ions. From of a single-aperture ion source, a Gaussian-shaped ion beam was extracted. Each position on the sample was correlated with a particular ion beam current density. With this method, on a single substrate a large variation in ion irradiation intensity could be obtained. Since the titanium evaporation rate was uniform over this area, the parameter varied over the sample surface was only the ion-to-atom arrival ratio. It turns out that with a constant Ti condensation rate and nitrogen impact rate, the Ti:N ratio in the film is a strong function of the argon ion impact rate. Ion-beam induced nitrogen sorption saturates at a certain ion irradiation intensity. While under all conditions the TiN (mono)phase is obtained, it shows different preferred crystal orientations in dependence on ion bombardment intensity. At low level, the film grows in (111)-orientation, at high levels in (100)-orientation. In a small transition zone, (110)-orientation prevails. Thus, the orientation can be selected by the ion beam intensity, which is relevant for application.

Chromium nitride films formed by ion beam assisted deposition at low nitrogen ion energies in comparison to high energies

In ion beam assisted deposition (IBAD), apart from the relative ion irradiation intensity, which is the arrival ratio of impinging ions to deposited atoms (I/A-ratio), the kinetic ion energy E k plays a dominant role. The energy deposited into the growing film by the ions, given by the I/A-ratio and E k, influences the features of the film, such as the phase composition and microstructure, including crystallographic orientation. The influence of the kinetic nitrogen ion energy on chromium nitride films was investigated. Chromium was evaporated and condensed onto silicon wafers at ambient temperature. The growing film was concurrently bombarded with nitrogen ions at 1 kV acceleration voltage. The films were deposited with different chromium deposition rates. They were analyzed for their phase composition by X-ray diffraction, and for the elemental depth profile with secondary ion mass spectrometry. The results are compared with findings from earlier experiments with a high ion energy. It turns out that different ion energies yield different phase compositions of Cr/Cr 2N/CrN, different preferential crystal orientations and different nitrogen distributions in depth.

Ion beam assisted deposition of nitrogen-containing chromium films: A comparison of argon vs nitrogen ions

Chromium films were deposited onto different substrates by two ion beam assisted deposition techniques, one being based on nitrogen ion implantation, the other one being based on nitrogen gas sorption under rare gas ion irradiation. In both cases, a residual gas atmosphere of nitrogen was used. In the first case, nitrogen was both implanted and absorbed from the atmosphere, in the second case, it was only absorbed under the influence of rare gas ion irradiation. In dependence on ion irradiation intensity, the films consist of phase mixtures of nitrogen-containing chromium, di-chromium nitride and chromium nitride. Nitrogen content of the films is a function of both ionic species and ion irradiation intensity. In case of nitrogen irradiation, always more nitrogen is incorporated than for the argon ion case. Under moderate ion irradiation, the surface morphology of the films is almost the same for both, under intense irradiation, argon ion bombardment leads to larger grains.

Ion beam assisted deposition of nitrogen-containing chromium films: A comparison of argon vs nitrogen ions?

Chromium films were deposited onto different substrates by two ion beam assisted deposition techniques, one being based on nitrogen ion implantation, the other one being based on nitrogen gas sorption under rare gas ion irradiation. In both cases, a residual gas atmosphere of nitrogen was used. In the first case, nitrogen was both implanted and absorbed from the atmosphere, in the second case, it was only absorbed under the influence of rare gas ion irradiation. In dependence on ion irradiation intensity, the films consist of phase mixtures of nitrogen-containing chromium, di-chromium nitride and chromium nitride. Nitrogen content of the films is a function of both ionic species and ion irradiation intensity. In case of nitrogen irradiation, always more nitrogen is incorporated than for the argon ion case. Under moderate ion irradiation, the surface morphology of the films is almost the same for both, under intense irradiation, argon ion bombardment leads to larger grains.

Corrosion performance of thin amorphous carbon films on aluminum formed by ion beam-based coating techniques

Thin amorphous carbon films were deposited onto aluminum samples at ambient temperature by ion beam-assisted evaporation and by ion beam sputter deposition. Their corrosion protection effect in sodium chloride solution where aluminum suffers from pitting corrosion was tested by means of electrochemical polarization measurements. The results showed that in case of ion beam-assisted deposition at a given ion irradiation intensity, the corrosion protection effect is strongly influenced by the deposition rate. There exists an optimal rate corresponding to a certain ratio of deposited atoms to impinging ions. In case of ion beam sputter deposition, it turned out that lower energies and lower currents of the sputtering ions were more favourable for corrosion protection. A comparison of the results of both techniques shows that under the present experimental conditions with sputter deposited films, a better corrosion protection performance could be achieved than with those formed by evaporation under ion assist.

Behavior of Hydrogen in Al, Mg and MgAl Plasma Saturated Films

The behaviors of hydrogen in Al, Mg and MgAl thin films on stainless steel substrate were investigated in this work. The hydrogen ions extracted from plasma were used to load hydrogen into the film material. Glow discharge optical emission spectroscopy (GDOES) was applied to obtain the hydrogen depth profiles in Al films versus hydriding parameters. The MgH2, AlH3 and Mg(AlH4)2 hydrides were identified in plasma hydrided films using X-ray diffraction (XRD). It is shown that efficient supply transport of hydrogen from the surface into the bulk for Mg films takes place at temperature above 100 °C and ion irradiation intensity above 1 mAcm-2.

Studies on PAN-based carbon fibers irradiated by Ar + ion beams

In this work, the effects of Ar + ion beam irradiation on carbon fibers were studied using tensile and surface analytical techniques. The single-fiber pull-out test was executed in order to characterize the fiber/epoxy matrix interfacial adhesion. The Ar + ion beam was irradiated using an ion-assisted reaction (IAR) method in reactive gas conditions under an oxygen environment with 1×10 16 ions/cm 2 Ar + ion dose (ID), 6 sccm blown gas flow rate, and different ion beam energy intensities. From the experimental results, both the interfacial shear strength (IFSS) and fracture toughness ( G i ) were found to increase with increasing Ar + ion irradiation intensity. This was probably due to the fact that Ar + ion beam irradiation on carbon fibers was effective in altering their surface physical chemistry and structural morphology, resulting in improved interfacial adhesion in the fiber/epoxy matrix. The reliability of single-fiber pull-out test data could be improved by statistical analysis using the Weibull distribution, which served to predict the variation of the mechanical interfacial properties in a composite system.

Tantalum nitride films formed by ion beam assisted deposition: analysis of the structure in dependence on the ion irradiation intensity

Tantalum nitride (TaN) films are formed by evaporating Ta metal under simultaneous nitrogen ion irradiation at 10 kV. Films have been formed on Si substrates with different Ta evaporation rates, in order to study the structural properties and dependence on the nitrogen content and radiation damage. A systematic study of films prepared under different Ta evaporation rates shows that for high rates substoichiometric nitride and Ta metal inclusions in the TaN films are formed. Lowering the evaporation rate results in the formation of the cubic TaN phase. A small amount of the stable hexagonal TaN phase, however, is found for all parameters used in this study. The lower parts of the films are heavily radiation damaged so that a fine crystalline, continuous layer can be observed on the Si substrate. The Si substrate is amorphous in an approximately 50-nm thick region due to nitrogen ion bombardment at the beginning of the treatment. The upper part of the TaN film is composed of elongated grains forming a continuous film, which is more dense the higher the irradiation intensity is. In summary, it is shown that Ta evaporation under nitrogen ion irradiation (ion beam assisted deposition; IBAD) results in the formation of a mainly cubic TaN film, the microstructure and phase composition of which depends on the bombardment intensity.

Thin film oxides as probe for homogeneity measurements of 3-dimensional objects treated by plasma immersion ion implantation

Results on an optical method are presented for investigation of the lateral homogeneity of treatment of 3-dimensional objects by plasma immersion ion implantation (PIII). It is based on interference colours of thin oxide films of transition metals such as titanium, niobium or tantalum. The thickness of the oxides and hence their colour changes when they are sputter etched by the plasma immersion ion bombardment. With the example of cube-shaped samples which are treated by argon PIII, it is demonstrated how this method works. Cube-shaped samples with slots with both inner and outer surfaces are a particularly interesting and illustrative example both for homogeneity effects of PIII and its measurement. It turns out that large differences in ion irradiation intensity between the outer parts and the inner parts of such samples develop. For the outer sides, a gradient in irradiation intensity is found. In contrast, the inner walls are hardly changed. However, by far most the inner bottom side of the samples is affected; it is considerably sputter etched. These unexpected results are explained by the shape of the cathode sheath from which the ions are accelerated and by the resulting ion trajectories.

Microstructure and composition of titanium nitride formed by ion beam enhanced nitrogen sorption of evaporated titanium under argon ion irradiation

Deposition of titanium nitride by ion beam-assisted evaporation (IBAD) is possible by two methods. One is evaporation of the metal under bombardment with nitrogen ions at a low gas pressure. The metal is converted to the nitride by implantation of the ions. The other method is evaporation of the metal in an atmosphere of nitrogen gas under bombardment with rare gas ions. The metal reacts with gas molecules from the ambient atmosphere. The features of the obtained TiN films strongly depend on the process mode. In the present study, TiN films prepared by gas sorption under argon ion bombardment are dealt with. The ion energies were in the medium-to-high energy regime of IBAD (>10 keV). Owing to the high mass of the bombarding species and a comparatively high energy, the TiN films are subject to heavy radiation damaging during deposition. This influences the microstructure and composition of the films. Argon ion irradiation leads to a reduction in contamination by residual gas. The oxygen content is reduced from 10–20 at.% down to below 5 at.%. On the other hand, argon is incorporated up to 6 at.%. The crystal orientation changes from 〈111〉 to 〈100〉, and a fiber texture develops. The grain size increases from ≈10 nm without ion bombardment up to 300 nm at high ion current densities. Without ion bombardment and at very low irradiation intensities, the material develops a fine-grained structure. With increasing ion irradiation intensity, the grains grow as large columns.

Ion beam assisted deposition of carbide and carbonitride of titanium at low substrate temperature

Ion beam assisted deposition was used to form titanium carbide and carbonitride films on glass, silicon and low alloy steel by electron beam evaporation of titanium in an atmosphere of ethene or a nitrogen/ethene gas mixture under bombardment with argon ions. Over a wide process parameter range, the films were formed as monophase material with varying composition. Oxygen contaminations from residual gas reactions decreased with increasing ion irradiation intensity. Hardness measurements showed a strong dependence of the hardness on the process parameters, particularly the ion-to-atom arrival ratio. Adhesion and corrosion behaviour were determined by scratch test and electrochemical measurements. It turns out that both features can be improved when a thin titanium metal film is deposited onto the substrate prior to formation of the ceramic film. These results look promising for application of the films as mechanical and chemical protection on temperature sensitive substrates such as heat-treated steels and aluminum alloys.

Microstructure evolution in amorphous Ge/Si multilayers grown by magnetron sputter deposition

Microstructure evolution in amorphous Ge/Si multilayers grown by dual-target dc magnetron sputtering was investigated by cross-sectional transmission electron microscopy, x-ray diffraction, and growth simulations. In films grown under low intensity ion-irradiation conditions, the structure is columnar with low-density regions along column boundaries where layer intermixing was observed. By increasing the low-irradiation intensity (controlled by an applied negative substrate-bias), structures with smooth and well-defined layers could be grown. This was achieved at bias voltages between 80 and 140 V, depending on the sputtering gas pressure. As the ion-irradiation intensity is further increased, ion-induced intermixing degrades the layer interfaces and finally an amorphous Si1−xGex alloy forms. The combination of x-ray diffraction measurements and reflectivity calculations reveals an asymmetry between the Ge/Si and Si/Ge interface widths due, primarily, to a corresponding asymmetry in incident particle energies during the growth of alternate layers.

Cubic nitrides of the sixth group of transition metals formed by nitrogen ion irradiation during metal condensation

Nitrogen-containing phases of chromium, molybdenum and tungsten were formed by evaporation of the metal under simultaneous nitrogen ion irradiation. With gradually increasing ion irradiation intensity, chromium forms initially Cr and Cr 2N phase mixtures, then additionally CrN appears, and at the highest intensities pure CrN films are formed. Molybdenum also forms pure nitride MoN under intense ion bombardment. However, in this case two different crystal structures are found, the stable hexagonal phase and the metastable cubic high-temperature phase. The latter is favoured under intense ion irradiation. In the case of tungsten, even at the highest intensities, only phase mixtures of W and W 2N were formed. These observed differences can be explained by the low reactivity of these metals towards nitrogen and the low chemical stability of the nitrides, particularly of WN. The metastable high-temperature structure of MoN is formed under the particular conditions of ion bombardment with rapid energy dissipation.

The influence of the ion flux density on the properties of molybdenum films deposited from the vapour phase under simultaneous argon ion irradiation

Molybdenum was deposited from the vapour phase onto fused quartz and alumina at room temperature under simultaneous irradiation with energetic argon ions with different ion flux densities. It was observed that increasing ion irradiation intensity lead to a slight increase in grain size, to a reduction in oxygen incorporation from the residual gas, and to argon ion implantation. As a result, a strong reduction in resistivity was observed. On iron substrates, electrochemical polarization measurements showed a reduction in microporosity.

On the mechanism of crystal growth orientation of ion beam assisted deposited thin films

The high number of degrees of freedom of the process parameters of ion beam assisted thin film deposition allows the formation of thin films with special features. Among these process parameters are ion energy, ion irradiation intensity, atom condensation rate and ion impact angle. By contrast to plasma deposition methods, these parameters are independent of each other and can be selected over a wide range. By carefully selecting the ratio I A of condensing atoms to impacting ions, the ion energy and the angle of ion incidence, films with a high degree of crystallographic orientation can be formed. To-date several mechanisms for the development of texture are discussed. Among them is the sputter theory which explains the growth of particular planes by sputtering and channeling. In a comparative study, this theory is applied to transition metal nitrides of group 4, 5 and 6 of the periodic system of elements. Based on results of structural measurements, the mechanisms which are involved in texture formation are discussed as a function of the process parameters arrival ratio I A , ion energy and ionic species. It turns out that it is not the ion energy deposited per atom but the momentum transfer per unit volume which correlates with the observed changes in preferred crystal orientation.

Low-temperature formation of metastable cubic tantalum nitride by metal condensation under ion irradiation

Nitrogen containing tantalum was grown on silicon by electron-beam evaporation of the metal under simultaneous irradiation with highly energetic nitrogen ions from an ion source. Phase analysis by means of x-ray diffraction revealed that at low irradiation intensities the deposited material is tantalum metal with dissolved nitrogen. At high intensities a phase transformation to tantalum nitride takes place. By contrast to the stable TaN in the phase diagram, which is hexagonal, the material grown under ion bombardment exhibits a metastable cubic structure of the NaCl type. The crystallographic orientation of the grains is directly dependent on the ion irradiation intensity. Initially, the films show a [111] texture. With increasing ion irradiation intensity the [100] orientation appears additionally. This orientation is independent of the substrate. The lattice spacings are also dependent on the irradiation intensity. With increasing ion irradiation the lattice is widened, then the spacings decrease again. The results are discussed in comparison to data from the literature on sputter deposition with low-energy ions.

Effect of ion and atom masses on the crystallographic orientation of nitride films prepared by ion-beam-assisted deposition

Abstract Ion-beam-assisted deposition of TiN leads to a change in crystal growth from a [111] orientation to [100] with increasing ion irradiation intensity. One of the reasons for this observation is radiation damage, which is higher for the close-packed (111) plane than it is for the open (100) plane where the ions are able to channel with much fewer atomic collisions. The magnitude of this effect is related to the mass of the ion. A comparison of TiN films deposited with nitrogen ion bombardment with those which were irradiated with argon ions shows that lower ion masses require higher irradiation intensities to achieve the same degree of preferred crystal orientation. This can be correlated with a higher degree of radiation damage and sputter etching of misaligned crystals for the heavier ions. The same relationship is observed for the mass of the metallic element in the film. Cubic TaN formed under nitrogen ion bombardment needs higher irradiation intensities than does TiN for a comparable crystal growth alignment. Radiation damage in the TaN lattice is smaller than that in the TiN lattice. Therefore, the crystal growth is less affected.