Amorphous Carbon Targets Research Articles

Order improvement of surface nanopatterns via substrate rocking under ion bombardment: Experiments and nonlinear models

Surface nanopatterns formed by ion beam sputtering (IBS) frequently include a high density of structural defects, which seriously limit their practical applications. Recent theoretical work [M. P. Harrison and R. M. Bradley, Phys. Rev. E 93, 040802(R) (2016)], based on the anisotropic Kuramoto-Sivashinsky (aKS) equation, proposes that rocking a substrate during IBS can produce defect-free patterns under certain requirements. We find experimentally, via low-energy ${\mathrm{Kr}}^{+}$ irradiation of Si and amorphous carbon targets, that rocking the substrates generally improves the order of the ripple patterns. Indeed, order is improved even when conditions required in the aKS model may not be satisfied. Through numerical simulations, we show that a suitable generalization of this equation, in which conserved nonlinear effects are incorporated, reproduces satisfactorily the pattern order in our experimental conditions.

Reflection of Hydrogen Isotopes and Helium Atoms from the Surface of the First Wall of the ITER Tokamak

The reflection coefficients were calculated for the scattering of hydrogen isotopes and helium atoms from the surface of amorphous beryllium, carbon, and tungsten targets, which are of interest in the physics of thermonuclear plasma. The results demonstrate good agreement with the available experimental data. The isotope effect in the reflection of particles from the wall was analyzed. For the reflection of He atoms, a similarity in reflection coefficient versus Lindhard parameter curves was observed.

Wake potential of swift ion in amorphous carbon target

The wake potential and wake phenomena for swift proton in an amorphous carbon target were studied by utilising various dielectric function formalisms, including the Drude dielectric function, the Drude–Lorentz dielectric function and quantum dielectric function. The Drude model results exhibited a damped oscillatory behaviour in the longitudinal direction behind the projectile; the pattern of these oscillations decreases exponentially in the transverse direction. In addition, the wake potential extends slightly ahead of the projectile which also depends on the proton coordinate and velocity. The effect of electron binding on the wake potential, characterised by the ratio ωp2/ω02=10 to 0.1, has been studied alongside the Drude–Lorentz dielectric function and quantum dielectric function formalisms; the results evidently show that the wake potential dip depth decreases with more oscillations when the electron density ratio ωp2/ω02 decreases from 10 to 0.1. One of the primary objectives of the present work is to construct a reasonably realistic procedure for simulating the response of target to swift ions by combining an expression for the induced wake potential along with several important dielectric function models; the aim of this research is to reduce computational complexity without sacrificing accuracy. This is regarded as being an efficient strategy in that it creates suitable computer simulation procedures which are relevant to actual solids. After comparing this method with other models, the main differences and similarities have been noted while the end results have proved encouraging.

Lifetime dependence of nitrided carbon stripper foils on sputter angle during N+ ion beam sputtering

We fabricated high-lifetime thin nitride carbon stripper (NCS) foils with high nitrogen contents using ion-beam sputtering with reactive nitrogen gas and investigated the dependence of their lifetimes on the sputter angle. The nitrogen in carbon foils plays a critical role in determining their lifetime. Therefore, in order to investigate the effects of the nitrogen level in NCS foils on foil lifetime, we measured the sputtering yield for different sputter angles at a sputtering voltage of 10kV while using carbon-based targets. We also measured the nitrogen-to-carbon thickness ratios of the foils using Rutherford backscattering spectrometry. The foils made at a sputter angle of 15° using a glassy amorphous carbon target exhibited an average increase of 200-fold in lifetime when compared to commercially available foils.

Capacity of graphite's layered structure to suppress the sputtering yield: A molecular dynamics study

20–120keV C60 bombardment on graphite and 20keV C60 impact on diamond are studied by classical molecular dynamics (MD) simulations. The number of atoms ejected from graphite after a 20keV C60 impact is found to be much smaller than that from diamond. By analyzing the microscopic sputtering process, we find this difference is due to the combined effects of graphite's low number density and layered structure. These two features of graphite make the pressure waves during the spike stage much weaker and the crater rim much more stable, compared to the case of diamond. While the role of atomic density on sputtering has been discussed in previous studies, effect of layered structure has not gained much attention yet. To affirm this effect and exclude the influence of density, we have also simulated C60 impact on an amorphous carbon (a-C) target whose density is very close to that of graphite. The yield of a-C is higher than that of graphite, certifying the capacity of graphite's layered structure to suppress the sputtering yield.

Generation of an amorphous graphite substrate by cumulative deuterium bombardment using molecular dynamics with full nonbonded interactions

A deuterated amorphous carbon target is generated from a fresh graphite layer via hyperthermal irradiation with deuterium ions using molecular dynamics (MD) simulations. We use interatomic potentials that include nonbonded (long-range) interactions for maximum accuracy and simulate cumulative bombardment up to doses of 5.80 × 1016 ions cm−2 at 1000 K. The graphite target goes through several stages of erosion and swelling, leading to complete amorphization and significant density loss. The calculations show a transition from graphitelike hybridization to a mixture of diamondlike and linear hybridizations with dose. It is concluded that the current sample sizes obtained directly by cumulative irradiation affordable with MD are not sufficiently large to be used for sputtering calculations under steady-state conditions.

Measurements of the highest acceleration gradient for ions produced with a long laser pulse

Ultrafast plasma light ion streams have been produced using the 300 ps, kJ-class iodine laser, operating at PALS Centre in Prague. Ion detection was performed through standard ion collectors (IC) in time-of-flight configuration (TOF), shielded by thin metallic absorbers. This new diagnostics technique has been theoretically studied and experimentally tested in order to cut the long photopeak contribution and to analyze the ultrafast particle signal. Processing the obtained experimental IC-TOF data, including deconvolution processes of the TOF signals, UV/soft-x-ray photopeak absorption, and ion transmission calculations for different metallic filters, is shown. Mainly amorphous carbon (graphite) targets have been irradiated in order to limit the maximum number of ion charge states and to focus our study on demonstrating the validity of the proposed investigation technique. Maximum ion energy and acceleration gradient estimations as a function of the laser energy and focal spot diameter are reported.

Dynamic screening and charge state of fast ions in plasma and solids

AbstractThis paper addresses the effect of target plasma electrons on the charge state of energetic ions, penetrating a target composed of bound as well as plasma electrons. Dynamic screening of the projectile Coulomb potential by the plasma electrons brings about a depression in the ionization energy of the ionic projectiles, as has been verified experimentally. This in turn makes the ionization cross-sections larger, while making the recombination cross-section smaller, thereby causing an increase in the ion charge state compared to the case of a gas target. The effect of the plasma environment, where the valence electrons are treated as plasma, is illustrated here for a 2 MeV carbon beam penetrating amorphous carbon targets of varying densities.

Reduction of the energy loss of swift molecular ions in solids due to vicinage effects in the charge state

We have calculated the energy loss of swift O{sub 2}{sup +}, N{sub 2}{sup +}, and C{sub n}{sup +} (n=2-60) molecular ions moving through an amorphous carbon target. The dielectric formalism is used to evaluate the vicinage effects in the energy loss of the atomic ions that form the molecular projectile, but we take into account that the charge state of these atomic ions is affected by their correlated motion through the target and by the screened Coulomb potential between them. When vicinage effects in the charge state are taken into account, the Coulomb repulsion is weakened, leading to a reduction in the interatomic separations ({approx}3% for N{sub 2}{sup +} and {approx}9% for C{sub 60}{sup +}, both having similar velocities). These charge state effects can be neglected for diatomic molecular ions, but they give rise to a reduction of {approx}8% in the vicinage effects in the energy loss of larger molecular ions with cage like geometrical structures, like C{sub n}{sup +} (n=20,60) projectiles.

Charge state of C10 and C5 energetic cluster ions in amorphous carbon targets: simulations

We present here detailed simulations of the interaction of energeticC10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbontarget. The spatial evolution of the cluster components is simulated accounting for bothscattering and Coulomb explosion. The former is calculated by means of the Monte Carlomethod while the latter is computed by means of molecular dynamics. The charge state ofthe individual cluster components is calculated as a function of penetration depth, and isdetermined by the competition between electron ionization and recombination. Theresults of calculations of the effect of the neighbouring cluster components on thesuppression of the values of the charge state are presented and compared to theexperimental values of Brunelle et al. Charge state suppression calculations for the2 MeV/C clustersfor both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and250 Å respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons.At 4 MeV/C the results are similar although less screening is required; a possible explanation is theinability of the plasma to completely screen the higher velocity projectiles. The1 MeV/C calculated results however differ in their behaviour from the2 and 4 MeV/C cases.

Characterization of [formula omitted] clusters produced by electronic sputtering

The structures and abundances of C n = 2 – 16 + clusters produced by electronic sputtering from the impact of 252Cf fission fragments on CO ice and from the UV laser desorption of amorphous carbon and graphite targets are analyzed. A systematic search for the more stable conformers at the DFT/B3LYP level of calculation and their contribution to the mass spectra is reported. A new methodology for a proper taxonomic description of the cluster isomers is proposed. Besides the known linear and monocyclic clusters series, new members of these series and two new series are described: the bi-cyclic and/or tri-cyclic and the rhombic core series.

Experimental study and Monte Carlo simulation of the correlation between electron emission and stopping power for swift proton impact on amorphous carbon target

It is usually well accepted that for swift protons, the induced backward and forward electron emission yield is proportional to the projectile electronic stopping power. This was observed in particular for thin amorphous carbon foils. However, this law was established from a non extensive set of experimental data and somewhat confirmed by rough macroscopic theories. We then developed a standard Monte Carlo simulation to predict the yield dependence on proton energy [0.5-10 MeV] and for a wide range of foil thickness. After evaluating the reliability of this simulation, we showed and explained why the law of proportionality cannot generally hold for forward electron emission. In particular, the ratio between forward yield and stopping power generally depends on foil thickness and proton energy. We performed a new experiment that confirmed our theoretical predictions.

Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part I. Optical emission spectroscopy

A comparative study of ablation plasma plumes originated from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum, the emission lines of carbon neutral C and ions of C+, C2+, and C3+ can be observed from both the SCG and a-C plasma plumes. The emission intensity from C atoms increases with laser energy density (EL) increase for both cases. The C2 emission intensity from the SCG plasma plume changes drastically with EL, while that from the a-C plasma plume is almost constant. The C2/C emission intensity ratio for the a-C case decreases with EL increase. As for the SCG case, the C2/C ratio decreases with EL increase up to 3.0 J/cm2, and increases slightly with further EL increase. Nanohardness of the deposited films decreases with the increase of the C2/C emission intensity ratio. It is suggested that for both the SCG and a-C target cases, the C2 molecule in the ablated plasma plume may not play an important role in producing high quality DLC films. It is further proposed that the threshold of laser fluence for the formation of diamond-like character film using KrF excimer PLD is 2.1 J/cm2(0.84×108 W/cm2) for the a-C target and 3.0 J/cm2(1.2×108 W/cm2) for the SCG target. The C2 vibrational temperature of the SCG and the a-C plasma plumes show different features on both the laser energy density and nitrogen pressure dependencies. Through optical emission spectroscopy and Langmuir probe measurements in vacuum and nitrogen background, it is concluded that there are many particles with higher mass in the SCG plasma plume, especially at relatively lower laser energy density below 3.0 J/cm2.

Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part II. Electrostatic probe measurements

In an ongoing effort to investigate plasma plume features yielding high quality diamond-like carbon films, we have applied plasma plume diagnosis and film characterization to examine plume character distinction from KrF laser ablation of both amorphous carbon (a-C) and single crystal graphite (SCG) targets. The advancing plasma plume produced by these structurally different targets are observed to possess quantitatively similar total heavy particle inventory, ionized fraction, and electron thermal content, yet quite different ion kinetic energy, plume profile, C2 formation mechanism, and concentration of complex molecules. Plume electron temperatures are observed to reside in the range 1–3 eV, with those in SCG plumes ∼10%–30% greater than a-C at all spatial positions downstream of the target. For both target cases, we find Te drop off with position away from the target with radiation as the most likely loss mechanism for these noninteracting plumes propagating in vacuum. Electron density is found to be ∼10%–12% lower near the target in SCG than a-C plumes consistent with mass loss inventory measurements, whereas ion fractions are estimated in the range ∼10%–15% for both target cases. All recorded data support the conclusion that the SCG target plasma plume is populated with heavier, more complex molecules than those in a-C which have been shown to be predominantly comprised of C and C+ under vacuum conditions with the addition of C2 at high fill pressure. A significantly smaller profile peaking factor for SCG plumes supports this conclusion. Less energetic and slightly lower temperature SCG plume conditions are consistent with reduced peaking and more massive plume species. Plasma plumes from SCG targets exhibit laser energy (El) dependent peaking, again consistent with more complex molecules increasingly disassociated with El increase. The El dependence further suggests the potential for control of particle size distribution and plume profile peaking, though not independently. Consistent with this scenario is the observation of harder films produced from SCG targets at lower El. Micro-Raman results indicate strongly heterogeneous films deposited by SCG target ablation even under vacuum conditions further supporting the case for more complex structures with greater hardness. Energy balance estimates indicate that ion kinetic energy dominates the balance and that SCG ablation liberates about twice the number of C12 atoms from the target per unit El. As well, high pressure background fill indicates lesser plume energy attenuation for SCG plumes, again suggesting the presence of higher mass particles.

Charge dependence of electron emission in swift heavy-ion collisions with carbon

We report on the charge dependence of electron yields from sputter-cleaned amorphous carbon targets bombarded with an isotachic set of swift ions. The experiments were performed in a UHV setup at the heavy-ion accelerator GANIL in Caen. The ion velocity was 19 a.u. (corresponding to a kinetic energy of 9.2 MeV/nucleon) and the projectile charge QP was varied from 6 to 39. As observed for ion-atom collisions, the electron yield exhibits a reduction with respect to a Q2P law. We show that this projectile charge dependence is consistent with a strong saturation of low-energy primary electron ejection. This effect is related to the primary projectile-target interaction itself and not to high-charge effects affecting the electron transport before escape from the target.

Molecular structure effects inthe energy loss of swift boron molecular ions in solids

We analyse the effects on the stopping power due to the geometricalstructure of swift Bn+ (n = 2-6) ions incident on amorphouscarbon targets. We use the dielectric formalism to describe boththe interaction of the projectile with the stopping medium and thevicinage effects on the atomic ions resulting from thedissociation of each Bn+ ion. The variation of the energy losswith the temporal evolution of the relative positions of the atomicions, due to Coulomb explosion, has also been taken into account.The calculated stopping power ratio for each Bn+ ion reproducesfairly well the main trends, as a function of the dwell time,observed in the available experimental data. The differences among thestopping power ratios of the Bn+ ions disappear as the number ofatomic ions, n, increases.

Electronic Stopping Power of Amorphous Carbon for H2+ and H3+ Beams

The electronic stopping power of amorphous carbon targets for H2+ and H2+ molecular beams has been analysed within the framework of the dielectric formalism. Coulomb explosion between the partners of the molecule has been taken into account to evaluate the average stopping power ratio during the dwell time. The charge state of the fragments is assumed fully stripped. Analysis and comparison of the calculated stopping power ratio with experimental data show a reasonable agreement.

Exit angle, energy loss and internuclear distance distributions of H2+ ions dissociated when traversing different materials

We have performed computer simulations of the trajectory followed by each proton resulting from the dissociation of H2+ molecules when traversing a thin solid target. We use the dielectric formalism to describe the forces due to electronic excitations in the medium, and we also consider the Coulomb repulsion between the pair of protons. Nuclear collisions with target nuclei are incorporated through a Monte Carlo code and the effect of the coherent scattering is taken into account by means of an effective force model. The distributions of exit angle, energy loss and internuclear separations of the protons fragments are discussed for the case of amorphous carbon and aluminum targets.

Plasma Diagnostics During Pulsed Laser Deposition of Diamond-Like Carbon Using Single Crystal Graphite and Amorphous Carbon

ABSTRACTOptical emission study of ablation plasma plumes from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been investigated. The C I emission intensity increases with laser energy density (EL) increase, while the C2 emission changes drastically with EL for both ablated plasma plumes. The C2/C emission intensity ratio for the a-C plume decreases with EL increase, while the C2/C ratio for the SCG plume decreases with EL increase up to 3.0 J/cm2, then increases slightly with further EL increase Nanohardness of the DLC films decreases with the C2/C ratio increase. It is suggested that the C2 molecule in the plasma plume does not play an important role in producing high quality DLC films, and that the C2/C ratio is a good parameter to monitor the PLD process.

Multiple Ionisation and High Charge Effects in Electron Spectra from Impact of 9 MeV/u Ions on Carbon

We measured energy spectra of electrons emitted from sputter-cleaned amorphous carbon targets bombarded with swift ions of constant velocity (9 MeV/u, vP = 19v0) with a new UHV set-up at the heavy ion accelerator GANIL. The charge of the projectile qp varied from 6 to 45. At large electron energy, a qp2 scaling law is observed. For energies below the 1s ionisation threshold, strong reduction effects with increasing qp occur. A carbon Auger hypersatellite line due to double ionisation of the K shell appears for high projectile charge only. From these Auger spectra, the ratio of double and single ionisation was deduced as a function of qp and found to increase from a threshold up to a saturation.