Recoiling Target Atoms Research Articles

Monte Carlo study of incident-angle dependence of ion-induced kinetic electron emission from solids

The incident angle dependence of ion-induced kinetic electron emission (KEE) from solids is calculated using a Monte Carlo simulation of the transport of incident ions and recoiling target atoms, and a semi-empirical theory of KEE. The emphasis is put on the origin of the deviation from the inverse cosine law. The effect of the high-energy recoiling target atoms on the deviation is much greater than that of the trajectory distribution and backscattering of the incident ions, except for light and low-energy ion impact. The positive (negative) contribution of the recoiling target atoms to the incident angle dependence is dominant for high (low) impact energy. The deviation can be explained by the increase in the electron excitation by recoiling target atoms localized near the surface and the increase in the emission of the atoms, i.e., sputtering.

A Semiempirical Calculation of Ion-Induced Kinetic Electron Emission Statistics

Electron emission statistics (ES) of ion-induced electron emission from gold at keV energy are investigated using a Monte Carlo simulation of the transport of the projectile ion and recoiling target atoms and a semiempirical model of the kinetic emission. Apparent deviations of experimental ES from Poisson distributions can be explained to result from backscattering of ions and/or recoiling of target atoms.

Calculation of Incident Angle Dependence of Ion-Induced Kinetic Electron Emission from Aluminum

The Monte Carlo simulation of transport of incident ions and recoiling target atoms is combined with the semiempirical theory of ion-induced kinetic electron emission (KEE) for calculating the incident angle dependence of the KEE yields. The calculation was made for light and heavy ion impact on Al in the energy range from 100 eV to 1 MeV. The present results are consistent with the experimental data and have revealed the origin of the deviation from the inverse cosine law, which corresponds to the contributions from backscattering ions and recoiling target atoms.

Statistics of kinetic secondary electron emission due to ion-atom collisions in solids

Electron emission statistics of ion-induced kinetic secondary electron emission from solids in the energy range 1–4 keV are studied using a Monte Carlo simulation of the transport of the incident ion and recoiling target atoms and a semiempirical model of the kinetic emmisson. The emission statistics calculated for H + incident on Au are in good agreement with the experimental data obtained recently by Lakits et al. [Rev. Sci. Instr. 60 (1989) 315]; they show a larger width of the frequency distribution of emission than that of the Poisson statistics. The simulated calculateons not only for H + on Au but also for noble gas ions (He +, Ne +, Ar + Kr + and Xe +) on Mo show that large-angle elastic scattering which causes backscattering of the incident ion and the recoil of the target atoms broadens the distribution.

Influence of Recoiling Target Atoms on Kinetic Electron Emission from Molybdenum under keV Ion Bombardments

Ion-induced kinetic electron emission from molybdenum in the energy range 0.7-40 keV is investigated using Monte Carlo calculations of the electron yield and the emission statistics, taking into account the electron excitations by ions and recoil atoms. The factor f0/ε evaluated by fitting the calculations to experimental electron yields decreases at low energy, and with heavy ions (f0: the escape probability through the surface, ε: the energy deposited to excite one electron). If the mass of the ion is larger than that of the target atom, the kinetic emission is dominated by the electron excitation by the recoils. With increasing mass of the ion, there is a complete antithesis between the emission statistics of the ion and of the recoils because of the energy transfer from the ion to the target atoms through frequent elastic collisions.

A new method to calculate range and damage distributions by direct numerical solution of Boltzmann transport equations

We have developed a new method to determine depth distributions of implanted ions, recoiled target atoms and energy deposition in amorphous targets. Our procedure is based on the direct numerical solution of one-dimensional linearized Boltzmann transport equations for the scalar fluxes of the ions and the recoils. The profiles calculated by the new method are compared with range distributions obtained from TRIM Monte Carlo simulations. Our program BOTE is up to two orders of magnitude faster than the TRIM calculations.

Interaction of slow (100 ev/atom) carbon clusters with gold: Penetration properties and collision cascades

The interaction of carbon clusters of variable size with thin gold targets has been analyzed by molecular dynamics simulation. The initial energy was 100 eV per cluster atom. Only a minor difference was found between the mean energy loss of transmitted cluster atoms and individual bombarding carbon atoms. The energy spectrum of transmitted cluster atoms is much broader than for atomic bombardment. In particular, a substantial fraction of the cluster atoms is transmitted with energies higher than their initial energy. This effect has been traced to collisions between cluster atoms. Similar, yet less pronounced effects were also found for reflected cluster atoms. The angular distribution of transmitted cluster atoms shows a sharp dip in the forward direction while their average energy in the forward direction even exceeds the initial energy per atom. Recoiling target atoms may achieve kinetic energies up to more than twice the maximum recoil energy in a single elastic collision. This effect has been traced to mu ltiple hits by different cluster atoms.

The Effect of Ejection Angle in Ion-Beam Sputter Deposition of Superconducting Amorphous Beryllium Film

Ion-beam sputter-deposited amorphous Be films on silica substrates at different ejection angles are characterized by their superconducting properties. It is shown that the superconducting transition temperature of the film is varied by changing the ejection angle of the sputtered particles. To explain this, the role of incorporated oxygen and the contribution of highly energetic particles to the amorphous Be formation are discussed, taking account of both the anisotropic ejection of the direct recoiled target atoms measured by time-of-flight and the result of a Monte Carlo simulation of Be sputtering. The Monte Carlo simulation describes the angular distribution of the sputtered atoms with considerable success and is particularly helpful in revealing the contribution of energetic sputtered atoms of higher than 100 eV at low ejection angles.

Ion beam induced reactions in Fe-PVC thin film structures

Films of57Fe, 50–100 A thick, were evaporated onto PVC substrates. Samples were irradiated with 48 McV Br8+ ions. Recoiling target atoms, ejected from the sample, were collected in a time of flight-energy detector telescope to monitor effects induced by the impinging Br ions. Conversion electron Mossbauer spectra were recorded before and after ion irradiation. Substantial radiation induced alterations were observed both in recoil and Mossbauer spectra.

Influence of ionised electrons on heavy nuclei angular differential scattering cross sections

For the collision systems 1.4 MeV u-1 U32+ and 5.9 MeV u-1 U65+ on Ne, transverse (with respect to the beam axis) momentum distributions of recoiling target atoms have been measured applying a time-of-flight technique. In the case of isotropic electron emission, the transverse momenta of the recoil ion pR perpendicular to and the projectile pp perpendicular to after the collision are identical. This allows the transformation of measured pR perpendicular to distributions into projectile related differential scattering cross sections d sigma /d theta . Using such an analysis, the authors have measured differential cross sections in the scattering angle regime of 1*10-6 rad<or= theta >4*10-5 rad. The shape, as well as the absolute magnitude of the derived experimental d sigma /d theta is in reasonable agreement with Rutherford differential cross sections at large theta using a projectile nuclear charge of Zp=qP and a target nuclear charge of ZT=5. For theta <or approximately=1*10-5 rad, n-body classical trajectory Monte Carlo (nCTMC) calculations predict that the balance between pR perpendicular to and pP perpendicular to is strongly influenced by the momenta of anisotropically emitted ionised electrons. From the comparison between the derived experimental cross sections d sigma /d theta with the theoretical values, as well as from the agreement between experimental and theoretical transverse recoil-ion momentum distributions, they conclude that the momenta of the ejected electrons have a considerable influence on the scattering dynamics of the heavy nuclei. Furthermore, the calculations indicate that the projectile is scattered to negative angles at impact parameters of b>or approximately=3 alpha 0 due to anisotropic electron emission and strong polarisation of the target electron cloud by the Coulomb potential of the projectile. The possibility of rainbow scattering at theta approximately=1*10-6 rad is predicted.

A spatial damage energy distribution calculation for ion-implanted materials

A simple method allowing easy calculation of the spatial damage energy distributions for ion-implanted materials is presented. The direct procedure takes account of the variation with depth of the lateral spreading of implanted ions, as well as the effects of energy transport by the recoiling target atoms. The subsequent computer program LUPIN-3D provides three-dimensional damage distributions and allows the construction of damage energy mappings. Various substrates of technological interest are investigated and several fields of application of the calculation are envisaged. The density of cascades can therefore be determined and heterogeneous amorphization models can be implemented.

A direct and fast computing method for damage energy depth distribution in ion implanted materials

A theoretically simple and computationally fast method for calculating damage energy density distributions for ions implanted into noninsulating materials is presented. The effect of the energy transported by the recoiling target atoms is taken into account in order to extend the calculation to lower energies and the near-surface target region. It is found that the results obtained over energy and mass ranges of interest for ion implantation experiments are in good agreement with previously reported experimental and calculated “damage” distributions. Special emphasis is placed upon applications to surface studies, sputtering and amorphization of Si(111) surfaces, as recently reported in a REM study.

Ion Implantation Calculations in Two Dimensions Using the Boltzmann Transport Equation

A two-dimensional method based on numerical solution of the Boltzmann transport equation has been developed for calculating ion implantation profiles. The method is capable of treating arbitrarily contoured surfaces containing multiple layers of different materials. Calculations yield the concentration profile of the implanted ion, an estimate of the damage distribution, and concentration profiles from recoiled target atoms. Substantial differences are seen from one-dimensional and point-response two-dimensional methods when the interfaces are far from planar.

Ion damage calculations in crystalline silicon

Damage profiles in crystalline silicon produced by light (B) and heavy (Bi) ions with energies from 10 to 100 keV were studied using the computer program MARLOWE (version 12). The program follows not only the incident ion collision by collision, but also any Si target atom that is set into motion through an energetic collision. Thus, the transport effect of the complete cascade of recoiled target atoms is included in the damage profile. The influence of channeling was studied for Si(100) using beam tilt angles from the surface normal of 0°, 3° and 7° about the [001] or [011] axes. The effects of channeling on the damage profile are twofold: first, there is a large reduction of the central damage peak; second, there is a component of the damage profile that extends considerably deeper into the target than that found in conventional studies using a random target assemblage. The influence of amorphous overlayers of SiO 2 on the damage and implantation profiles in the Si(100) substrate has also been investigated.

Calculation of Recoil-Induced Ionization Distributions in Solids Bombarded by Ions

A new method for calculation of recoil-induced ionization distributions in solids bombarded by ions is presented. The method is based on a separation of the total ionization distribution into two components, one originating from the primary interaction between the incident ions and target electrons and one due to the interaction of recoiling target atoms with the electrons. The latter contributions, the so-called recoiled induced distribution, is extracted by taking the difference between the total and the primary distribution.Results from explicit calculations are shown for five different ion-target combinations in the energy range of 10 to 500 keV. A velocity proportional electronic stopping cross section is assumed and for each ion-target combination results are given for three different proportional constants. The method is shown to require a relatively small computational effort compared with more conventional methods for determination of energy deposition distributions.Classification: Radiation Damage and other irradiation effects (6180), Ions (6180J).

Auger electron emission from solids during bombardment with MeV ions

The production of secondary electrons during the impact of ions on thick solid targets is described and experimental investigation is presented. The main attention is directed to Auger electron emission induced by heavy ions in the MeV range. It is shown that in this range the spectra are representative of the direct interaction between the impinging ion and the solid, so that secondary effects like sputtering or excitation by recoiled target atoms can be neglected. A comparison with the Auger emission from gases in similar systems shows the relaxation phenomena due to the solid state.

Dependence of ion-electron emission from clean metals on the incidence angle of the projectile

We have studied the dependence of electron yields $\ensuremath{\gamma}$ from clean Cu and Au surfaces on the incidence angle $\ensuremath{\theta}$ of 5-50-keV ${\mathrm{He}}^{+}$, ${\mathrm{Ar}}^{+}$, and ${\mathrm{Xe}}^{+}$ projectiles, in the angular range 0\ifmmode^\circ\else\textdegree\fi{}-80\ifmmode^\circ\else\textdegree\fi{}, and under ultrahigh-vacuum conditions. We have found that, at small angles, $\ensuremath{\gamma}\ensuremath{\propto} {sec}^{f}\ensuremath{\theta}$, with $f$ generally different from unity. For ${\mathrm{Xe}}^{+}$ on Cu, $\ensuremath{\gamma}(\ensuremath{\theta})$ presents an energy-dependent maximum, similar to that obtained for sputtering. The results are explained in terms of the anisotropy of the electron cascade in the solid, and the depth distribution of the inelastic energy deposited by the projectile, and by rapidly recoiling target atoms in the near-surface region of the solid.

Electron emission from molybdenum under ion bombardment

Reports measurements of electron emission yields of clean molybdenum surfaces under bombardment with H+, H2+, D+, D2+, He+, N+, N2+, O+, O2+, Ne+, Ar+, Kr+ and Xe+ in the wide energy range 0.7-60.2 keV. The clean surfaces were produced by inert gas sputtering under ultrahigh vacuum. The results were compared with those predicted by a core-level excitation model. The disagreement found when using correct values for the energy levels of Mo is traced to wrong assumptions in the model. A substantially improved agreement with experiment is obtained using a model in which electron emission results from the excitation of valence electrons from the target by the projectiles and fast recoiling target atoms.

Ion induced electron emission from polycrystalline copper

Abstract The total yield of secondary electrons emitted from a polycrystalline copper target during bombardment by protons, noble gas ions and copper ions has been studied in the energy range 10–400 keV. Comparison is made with a new theory for secondary electron emission in which the energy dependence of the yield is explained in terms of energyy transferred to the electrons in a surface layer of the target. Both the energy transferred from the ion to the target electrons and the energy transferred from the recoiling target atoms to the electrons are taken into account. Within the validity region of the theory the agreement between the theory and the experiment is good. The experimental equipment and procedure are described.

Transport theory for kinetic emission of secondary electrons from solids

Kinetic secondary electron emission from a solid target resulting from incidence of keV electrons or keV and MeV ions is treated theoretically on the basis of ionization cascade theory. The energy and angular distribution and the yield of secondary electrons are calculated for a random target. These quantities are determined from the solutions to a system of Boltzmann transport equations. Input quantities are the cross sections for collisions between the involved particles and the surface barrier of the target. A general power cross section has been utilized in the analytical procedure. It is shown that liberated electrons of low energy move isotropically inside the target in the limit of high primary energy as compared to the instantaneous energy of the liberated electrons. The connection between the spatial distribution of kinetic energy of the liberated electrons and the secondary electron current from a solid is derived. To find the former, existing computations for ion slowing down and experimental and theoretical ones for electron bombardment can be utilized. The energy and angular distribution of the secondary electrons and the secondary electron yield are both expressed as products of the deposited energy at the surface of the target and a factor which depends only on the properties of the escaping secondary electrons. Corrections for energy transport away from the surface by energetic recoil electrons are partly included. Also the contribution from recoiling target atoms at heavy-ion bombardment in the keV region is largely taken into account. The predicted energy and angular distribution agree with absolute spectra for incident electrons, whereas the agreement with absolute spectra for incident protons is less satisfactory. Extrapolation of the energy distribution down to the vacuum level gives a spectrum which shows good agreement with experimental data. The electron- and proton-induced yields from aluminum are evaluated on the basis of existing low-energy-electron stopping-power data. The agreement with existing experimental data is good. Also, experimental yields from electrons, protons, and noble gas ions incident on copper agree within the accuracy of the treatment.