Etude théorique du pouvoir d'arrêt électronique en canalisation planaire des ions

A new theory which is a generalization of the wave packets method (already used in a dynamic treatment of Auger process previously given in this review) enables us to treat the electronic stopping power of ions in the case of planar channeling. This theory undertaken in the frame of optical potential shows that continuum potential and Born-Mayer form can be uniquely interpreted by a mechanism of tunneling of electrons through the walls of the channel. These walls have in fact at the vicinity of ions the behaviour of a surface. A formulation of the electronic stopping power is given and discussed regarding the experimental evidence.     

Stopping power of fast channeled protons in Hartree-Fock approximation

The electronic energy loss of MeV-protons in crystals is investigated in the Hartree-Fock approximation for the core electrons of the target. It turns out that each core electron can be regarded as stopping the proton independent of all other electrons without restriction by the Pauli principle. For the channeling stopping power the impact parameter dependent energy loss of a proton moving rectilinearly past a crystal ion is calculated in first Born approximation. Low excitations of the core electrons lead to a long range impact parameter dependence, whereas high excitations contribute to the energy loss proportional to the electron density sampled along the proton trajectory. The results are applied to 4 MeV proton channeling along the main channels of Si using Clementi wave functions for the core electrons and a free electron approximation for the valence electrons. The comparison with the experimental results of Clarket al. yields good agreement. In the high velocity limit a reduction of the channeling stopping power to 0.64(0.83) of the random value is predicted for Si(C).     

Electronic stopping power calculation method for molecular dynamics simulations using local Firsov and free electron-gas models

Molecular dynamics simulations have proven to be accurate in predicting depth distributions of low-energy ions implanted in materials. Free parameters adjusted for every ion-target combination are conventionally used to obtain depth profiles in accordance with the experimental ones. We have previously developed a model for predicting depth profiles in crystalline Si without free parameters. The electronic stopping power was calculated using local total electron density. The model underestimated the stopping in the ?1 1 0? channeling direction. We have now taken a new approach to calculate the electronic stopping power. We use the local valence (3p²) electron density to account for the electronic energy loss between collisions and the Firsov model to account for the electronic energy loss during collision. The lowest electron densities are adjusted with a parametrization that is same for all ions in all implanting directions to correct the problems in the ?1 1 0? channeling direction.     

Resolution of the frozen-charge paradox in stopping of channeled heavy ions

The long-standing problem of the lacking signature of a Barkas effect in the stopping of swift heavy ions under channeling conditions has been analyzed theoretically. The stopping model provides explicit dependences on impact parameter and allows for projectile screening and higher-order Z(1) corrections. The analysis differentiates between principal target shells. A distinct Barkas correction is found in accordance with standard theory. It is less pronounced for channeled than for random stopping because of the dominance of outer target shells. Varying contributions from different target shells to the stopping force may give rise to an inversion of the commonly observed variation with ion energy and charge state of the Barkas correction.     

4He+在Al和Si单晶晶面沟道中的半波长及阻止本领

采用高分辨探测系统,在不同的入射角θ_in和出射角θ_out的条件下,测量了1—2MeV⁴He⁺入射在Al(100)面和Si(110),(100)面中的背散射能谱。得到了背散射能谱振荡峰的间距ΔE与cosθ_in/cosθ_out的直线关系,从而获得了1—2MeV⁴He⁺在这三个晶面中的振荡半波长及阻止本领。实验得到的半波长与理论计算值在误差范围内一致,晶面沟道方向的阻止本领略大于随机方向的阻止本领。 关键词：     

Sputtering of compound semiconductor surfaces. I. Ion-solid interactions and sputtering yields

Several phenomena occur on the surface of a solid when being bombarded by energetic ions. A short general review is given of the major ion-solid interactions on compound semiconductor surfaces. An in-depth discussion is presented of the total sputtering yields of component semiconductors. For this discussion, GaAs is assumed to be the prototype compound semiconductor because most experimental measurements exist for GaAs. To exclude any chemical effects in the sputter yields, only the total sputtering yield data for argon ion bombardment of GaAs are compared with the predictions of the major sputtering theories, with particular attention to the Sigmund theory for linear cascade sputtering. Different proposals of each of the parameters in this theory are presented and compared with the GaAs data. These parameters are the surface binding energy, the nuclear stopping power, and the factor α, which represents the fraction of energy available for sputtering. Use of the different parameters results in a large variation in the predictions. Topics also considered are the angle dependence of the sputtering yields, sputter threshold energy, and channeling effects in the sputter yields of compound semiconductors. Spike sputtering effects are evident in the sputtering yields of GaAs by krypton and xenon ions.     

Das elektronische Bremsvermögen von Stickstoff und Sauerstoff für niederenergetische Protonen

The electronic stopping power of molecular oxygen and nitrogen for protons with energies between 1 keV and 30 keV has been measured using a differentially pumped stopping cell. Our results give a surprisingly good confirmation of the Lindhard-Scharff statistical theory which predicts a linear velocity dependence of the electronic stopping power at low projectile energies. Moreover our data are in fair agreement with earlier high energy (E ≧ 20 keV) measurements in other laboratories. The combination of the present measurements with theoretically calculated nuclear stopping powers yields an estimate of the atomic stopping power. This estimate leads to substantially lower atomic stopping powers at low energies compared to values derived from range measurements. This result may have interesting implications on auroral hydrogen emissions.     

Experimental setup for transmission channeling of MeV ions to study edge dislocations

An experimental facility was built where thin metal (Ta) crystals, with known thickness, could be studied by transmission channeling of MeV ions. The details of obtaining a faint beam (1.5–3.6 MeV) of constant flux (±1%) on the target and normalization of the spectrum of forward scattered particles have been discussed with merits and demerits of various setups used. Preliminary experimental results are reported for measuring dechanneling coefficients from edge dislocations produced by cold work, using H⁺ ions. The H⁺ stopping power for Ta in various planar channels and random direction are estimated and using those, it is shown possible to use the setup for thickness measurements of thin crystal films.     

Calculation of helium stopping powers in the intermdiate energy region

A model for calculation of helium stopping powers is developed which takes into account different charge fraction contributions. The neutral part contribution is treated according to the ordinary Firsov theory, whereas for the charged part of the ion beam earlier classical stopping power calculations for point charges by Kührt and Wedell are used.     

重离子束在热靶中的电子阻止本领与有效电荷数

采用线性介电响应理论,研究了重离子束在热靶中的有效电荷数和电子阻止本领。为了考虑入射离子的束缚电荷分布,我们将Brandt-Kitagawa的有效电荷理论推广到热靶。在低速和高速情况下,分别得到了有效电荷数和电子阻止本领的解析表示式。数值结果表明,对于低速离子,有效电荷数随电子气的温度增加而增加;在一定的温度范围内,低速离子的阻止本领与冷靶相比有明显增加。对于高速离子,我们的理论结果与实验值符合得较好。 关键词：     

A comparison of proton channeling in the <111> direction for BaF2 and CaF2

Protons in the region of 400 KeV have been used to study the channeling process in BaF₂ and CaF₂. The rate of loss of protons from the 111 linear channel, stopping power, and critical angle for the channel were found. Two techniques were employed, namely observationof elastic scattering by the lattice ions, and alpha particles produced by the reaction ¹⁹F(p,αy)¹⁶O, from the sharp resonance at 340 keV. Both techniques gave results in good agreement. The channel parameters were much closer together for the two crystals than had been anticipated on the basis of the large difference in charge between Ba and Ca.     

Thermal properties of electron stopping in solids

Thermal effects in the stopping power of a solid target are analysed in the framework of the Bethe-Bloch theory. These effects due to the coupling of the electron dynamics with lattice phonons can be important in understanding the scatter of stopping cross section data.     

Energy losses of muons, pions, protons, and deuterons channeled in Si

The energy loss of positive muons, pions, protons, and deuterons channeled in Si crystals has been investigated by computer simulation. The model considers the individual trajectories of particles inside the target and calculates the electronic energy loss for each particle history. The results show important differences in the energy loss distributions, stopping powers, and straggling values for low and intermediate energies, as a consequence of channeling effects.     

Kinetic Study on Channelling of Protons in Metallic Carbon Nanotubes

Based on the kinetic model and the dielectric response theory, a theoretical model is put forward to describe the transport of protons along nanotube axes. With the introduction of electron band structure for different nanotubes like zigzag and armchair nanotubes of metallic properties, the collective excitation of electrons on the cylinders induced by the incident ions is studied, showing several distinct peaks in the curves of the energy loss function. Furthermore, the stopping power and the self-energy are calculated as functions of ion velocities, especially taking into account the influence of damping coefficients. It is conceivable from the results that, in the kinetic formulation, plasmon excitation plays a major role in the stopping. And as the damping increases, the peaks of the stopping power shift to the lower velocities, with the broadening of the plasmon resonance.     

Density functional calculation of stopping power of an electron gas for slow ions

We describe the first calculation of the stopping power of an electron gas for slow ions using the density-functional formalism. We evaluate the nonlinear self-consistent potential around the ion and from scattering theory determine the energy loss directly. Comparison with the results of linear theory is made.     

The channeled stopping powers of Boron ions and range calculations in Si

An analytical fitting expression is obtained for the channeled stopping power of B ions in crystalline silicon. Ions incident in the Si 〈100〉 direction at energies from 50 keV/amu to 900 keV/amu are considered. The mean projected ranges of ions have been determined within the framework of the transport theory formalism where the fitting relation obtained for the stopping power was used. For this purpose, a first order ordinary differential equation including second order stopping coefficients is solved numerically by usingFehlberg fourth-fifth order Runge-Kutta method. By using the results for the channeled ion ranges, a transformation relation is proposed for the conversion of channeled/random ion ranges for the ion target system under consideration. The obtained results are compared with widely used standard programs such as Crystal-TRIM, SRIM and PRAL and our program applied to crystalline targets is in good agreement with Crystal-TRIM.