Energy loss of H and He in the semiconductors GaAs, ZnSe, InP and SiC

We have theoretically studied the electronic stopping cross section and the energy loss straggling of swift light ions (H⁺ and He⁺) moving through several compound semiconductors (GaAs, ZnSe, InP and SiC) as a function of the incident projectile energy. The calculations have been done using the dielectric formalism, in which the electronic structure of the projectile is described by the modified Brandt–Kitagawa model and the energy loss function (ELF) of the semiconductors is obtained using a linear combination of Mermin-type ELF to describe the outer electron excitations and generalized oscillator strengths to take into account the excitations of the inner-shell electrons. The different charge states that the projectile can acquire during its travel through the solid, as a result of electronic capture and loss processes, has been also considered. The contributions to the projectile energy loss from both the outer- and the inner-shell electron excitations are analyzed. The comparison of our calculated stopping cross sections with available experimental data shows a good agreement in a wide range of incident projectile energies.     

Negative ions, energy loss, and electron emission during grazing scattering of fast H and He atoms from a clean and oxidized NiAl(1 1 0) surface

Negative ion fractions, projectile energy loss, and the emission of electrons is studied for grazing scattering of hydrogen and helium atoms/ions from a clean and oxidized NiAl(1 1 0) surface. Making use of translation energy spectroscopy and the coincident detection of the number of emitted electrons we have studied the electronic interaction mechanisms for the change from a clean metal target to an insulator surface via the preparation of a well defined ultrathin alumina film on top of the metal substrate. We find that already for a monolayer thick oxide film the characteristic different features of electronic processes for the surface of an insulator crystal are present.     

An intranuclear cascade model for inelastic scattering and breakup reactions involving deuterons and alpha particles

The intranuclear cascade model is extended to cluster-induced (deuteron and alpha particle) nuclear reactions involving inelastic scattering and breakup reactions. The proposed model explains the projectile breakup process by describing the projectile cluster as a superposition of several states. The incident cluster and the produced cluster are assumed to be collections of independent particles and may undergo nuclear interaction through nucleon–nucleon interaction with the target nucleus. Trajectory deflections for the projectile and ejecta are incorporated in the model to account for angular distributions. Calculations with the proposed model followed by the generalized evaporation model are performed for validation by comparing with experimental double-differential cross-section spectra produced by bombarding an ²⁷Al target separately with 80-MeV and 99.6-MeV deuterons and 140-MeV alpha particles. The calculation results show good agreement with experimental spectra.     

Extending the inelastic thermal spike model for semiconductors and insulators

The inelastic thermal spike framework was extended to incorporate an additional balance equation for the carrier density. Temporal and spatial evolution of carrier density, electronic and lattice temperatures were solved for silicon using a finite difference method. Calculated track radii for a range of electronic stopping powers are presented. The model allows us to fit the electron-phonon coupling to experimental data of amorphised track radii. We compare the methodology of this framework to an earlier inelastic thermal spike model, which is based on the two-temperature model for non-equilibrium processes in metals, and discuss its contribution to the understanding of microscopic processes following a swift ion irradiation event in band gap materials.     

Single electron loss in the collisions of lowly charged ions and atomic hydrogen

In this paper, the projectile electron loss cross sections of He⁺, Li⁺, Li²⁺ and C²⁺ colliding with atomic hydrogen are studied in the frame work of the extended over-barrier model at intermediate velocities (25–600 keV/u). The electron loss cross sections are calculated in terms of the interaction between the screened target nucleus and the active projectile electron and of the interaction between the projectile electron and the target electron. Compared with the measurements, this model satisfactorily reproduces the experimentally obtained energy dependence of the single electron loss cross sections over the energy range investigated here.     

Charge-transfer and electron emission in the slow (v < v0) multicharged-ion-surface interaction

Charge transfer and electron emission in the slow (v < v₀) multicharged-ion surface interaction is examined. We discuss the form of the total electronic potential, the influence of image and screening interactions on the projectile state energies, and calculate Auger transition rates and Auger electron emission yields for KVV transitions for the oxygen-copper and oxygen-gold interactions. It is shown that efficient transfer of charge from the valence band of metals occurs within projectile-surface separations of order 2q+7 a.u., where q is the projectile charge.     

Coincident Rutherford Backscattering Spectrometry: A novel technique for measuring charge state distributions in violent ion-atom collisions

Charge-state distributions in violent ion-atom collisions were investigated using a novel combination of traditional Rutherford backscattering spectrometry (RBS), time-of-flight (TOF) coincidence, and position-imaging techniques. The combination is termed Coincident Rutherford Backscattering Spectrometry (CRBS). A special apparatus was built in which the backscattered and recoil ions are time and charge state correlated. CRBS measurements for 0.5 and 0.6 MeV He⁺-Ar collisions are presented. From the recoil ion-projectile ion coincidence measurements of the charge state distributions, it was observed that backscattered projectile ions of the same charge state correlate with different recoil ion charge states and vice versa, indicating that any particular charge state may result from different reaction channels. Moreover, the Ar recoil-ion and He projectile-ion correlation exhibits a strong dependence on the projectile beam energy. An energy deposition model was attempted to account for some of the recoil ion charge state distributions. The model qualitatively accounts for the distributions and confirms that energy loss of a backscattered projectile due to its interaction with the target electrons is very small compared to that due to its interaction with the target nucleus.     

Influence of the projectile charge state on the ionization probability of sputtered particles

We present a computational study of the effect of the projectile charge state on secondary ion formation in sputtering. A molecular dynamics simulation of an atomic collision cascade is combined with a kinetic excitation model including electronic friction and electron promotion in close atomic collisions. The model is extended to account for potential excitation following the bombardment with a highly charged ion (HCI). The spatial spreading of the excitation generated in the cascade is treated in an diffusive approach. The excitation energy density profile obtained this way is parametrized via an effective electron temperature, which is then used to calculate the ionization probability of each sputtered atom in terms of a simple charge exchange model. The results obtained for the impact of a 5 keV Ag atom onto a solid silver surface show that the average ionization probability increases from 4.7×10^-4 for a neutral projectile to 5.4×10^-4 for a highly charged projectile ion with a total ionization energy of 576 eV.     

Repair by radiation itself or not? A study of threonine irradiated by keV ion

A saddle type dose-survival rate curve of threonine was confirmed in low energy ion irradiation. Mass spectrometry has been used as an analytical tool in determining the species and quantities of volatile reaction products generated during the decomposition of threonine by fast atom bombardment. The radiation chemistry was related to the radiation physics of fundamental interaction between the projectile and target molecule. The nuclear collision process was proposed as the main reason of the particular dose effects curve, based on the analysis of the interaction process between the projectile and target molecule. A new repair model was assumed.     

Polarization Bremsstrahlung from fast ions with the core in polycrystal

This paper is devoted to the theoretical investigation of polarization Bremsstrahlung (PB) from fast ions with electronic core scattering in a polycrystalline medium. There are two channels of radiation in this case, namely, projectile PB and target PB. Projectile PB is a new radiation mechanism due to the scattering of the own target electric field into a real photon on a bound electrons of the projectile. The main features of projectile PB from hydrogen-like multi-charged incident ions are studied, including spectral-angular distribution of PB photons and its dependence upon the projectile kinetic energy.     

Dynamic interactions of fast ions with carbon nanotubes

We used the hydrodynamic model to describe the dielectric response of a multi-walled carbon nanotube to a fast point charge moving paraxially, either inside or outside the nanotube. Calculations are performed for a two-walled nanotube, giving rise to a splitting of the plasmon frequencies due to the interaction between the electron fluids on the two cylinders. The dependences of the projectile stopping power and the self-energy (image potential) on the velocity and the distance from the nanotube axis show interesting features when the projectile velocity matches the phase velocity of the plasmon mode with a quasi-linear dispersion.     

Orbital and whole-atom proton stopping power and shell corrections for atoms with Z ⩽ 36

Stopping cross sections and shell corrections for atoms with 1 ? Z ? 36 have been evaluated using a technique based on Sigmund's kinetic theory of electronic stopping. Results are tabulated for projectile velocities from 1 to 60 atomic units both for the whole atom and for the individual subshells.     

Target dependence for low-Z ion charge state fractions

Equilibrium charge state fractions have been measured for 3-7 MeV lithium, boron and carbon ions passing through thin foils of copper, silver, and gold. The current results are combined with other low-Z ion data from the literature to give the relative influence of different target materials on charge exchange processes. The mean charge of the projectile, the functional form of the charge state distribution, and the charge state distribution width are parameters used to examine the effects of the electronic structure on charge exchange for various target-projectile combinations. Projectile shell structure is found to have a large influence on the widths of the charge state.     

Charge transfer and excitation in high-energy ion-atom collisions

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.     

Shell effect on stabilization processes following multi-electron transfer in slow Arq+–Ar (q = 6–9, 11) collisions

We experimentally investigate the shell effect on the stabilization processes following the multi-electron transfer in slow collisions of Ar^q+–Ar (q = 6–9, 11). The relative cross-section ratios of multi-electron transfer and of the subsequent stabilization with respect to single-electron capture are measured meanwhile compared with the theoretical results predicted by the classical over-barrier model. Our result indicates that the multi-electron transfer is dominant when the projectile charge is large and the subsequent stabilization shows a dramatic variation if the projectile L-shell configuration becomes open. It shows that the subsequent stabilization processes of multiply excited scattering ions have a strong dependence on the projectile shell.     

离子在近电子能损阈值能区诱发云母表面小丘形成

利用0.65 MeV的He⁺离子轰击白云母膜,并在大气环境下用原子力显微镜(AFM)的轻敲模式分析了辐照后的膜表面。实验结果显示,在不同温度下离子诱导的小丘高度在小于1 nm到几nm之间,且室温条件下能诱发小丘生成的He⁺离子电子能损阈值在0.44 keV/nm以下。此外,升高温度至973 K并在其中选取不同温度进行表面辐照来验证观测到的小丘结构。实验发现,相比于室温,小丘直径和高度的统计分布在更高温度下表现出了更大的歧离。分别利用分析热峰模型和双温热峰模型计算了辐照过程中的核能损与电子能损,并选取了用能损在阈值附近的离子辐照所产生的小丘的实验数据与模拟结果相比较,发现实验结果与双温热峰模型吻合较好。     

Double resonant neutralization in hyperthermal energy alkali ion scattering at clean metal surfaces

Resonant neutralization of hyperthermal Na⁺ ions impinging on clean surfaces is studied, focussing on long lived electronic interactions involving the projectile and a target atom. Specific trajectories are considered where the incident particle undergoes multiple collisions within the first surface layers, interacting simultaneously with several target atoms, which leads to single emission of a surface atom that can resonantly exchange charge with both the solid and the projectile. The system is described via a semi-empirical, one-electron potential that includes the effect of a plane metal surface, with projected band gap, the projectile, whose charge state will be eventually investigated, and the substrate atom. On this basis, a model Hamiltonian of the Anderson-Newns type is constructed and the calculated neutralization probability is compared with the angle resolved neutral fraction measured by Keller et al. [C.A. Keller, C.A. DiRubio, G.A. Kimmel, B.H. Cooper, Phys. Rev. Lett. 75 (1995) 1654].     

Fast atom diffraction for grazing scattering of Ne atoms from a LiF(0 0 1) surface

Angular distributions of fast Ne atoms after grazing collisions with a LiF(0 0 1) surface under axial surface channeling conditions are experimentally and theoretically studied. We use the surface eikonal approximation to describe the quantum interference of scattered projectiles, while the atom-surface interaction is represented by means of a pairwise additive potential, including the polarization of the projectile atom. Experimental data serve as a benchmark to investigate the performance of the proposed potential model, analyzing the role played by the projectile polarization.     

Molecular dynamics study of kinetic electron emission induced by slow sodium ions incident on gold surfaces

Electron excitation and emission phenomena, due to Na⁺ ion impact on Au (1 0 0) surfaces, are studied at incident projectile energies below the threshold for kinetic electron emission. The trajectories and velocities of the projectile and the target atoms are simulated with molecular dynamics. This information are used to calculate the energy loss by electronic stopping as a series of discrete events, localized in space and time, that are treated as sources of excitation energy. The diffusion of the energy deposited by the projectile into the solid is converted into electron yield as proposed by Duvenbeck and coworkers [14]. The results show similar trends to available experimental data.     

Transient Annealing in Sekiconductor Devices Following Pulsed Neutron Irradiation

Transient annealing following pulsed neutron exposure has been investigated in silicon transistors and solar cells as a function of both irradiation temperature and injection level. In addition, experiments incorporating both X and gamma ray irradiation have demonstrated that transient annealing is a bulk, not a surface, effect. The observed annealing following neutron irradiation is considerably slower at low temperatures than at room temperature. At 213°K thg density of annealable defects remaining at 10-10 seconds after exposure is approximately three times that observed at this same time for a 300°K irradiation. The injection studies show that at any temperatures from 76°K to 300°K the presence of minority carrier injection considerably speeds the transient annealing processes. A model is presented which explains many of the features observed in transient annealing. Various kinetic processes are discussed in the derivation of this model, including diffusion-limited and generation-limited processes, and the results of the model are compared with experimental data.