Stopping Power Of Channeled Ions Research Articles

Further improved technique for channeled stopping power measurements

A few years ago, an ingenious method was proposed to quickly and accurately measure the difference between random and aligned stopping powers without the need for free-standing mono-crystalline thin films; the main idea was to increase the energy of the incident ion beam so that the channel numbers for a characteristic feature in the backscattering spectrum in random and aligned direction coincide, thus compensating for the difference in stopping power endured by the outgoing ions (Greco et al. 2007) [7]. A small drawback of this method is that the total length of the path that the ion travels through the thin film is slightly different in random and aligned geometries. We have improved this method by changing the measurement geometry such that the aligned and random ions travel the exact same distance through the thin film. Representative results for 0.5–3MeV He ions channeled in <011> Si are presented and show that earlier measurements appear to overestimate slightly the reduction in stopping power for channeled ions.

Position-dependent stopping power for channeled ions

A method is developed for predicting the stopping power of crystals toward swift channeled ions, based on a semiempirical treatment of the dielectric response of the medium. Random stopping power is also obtained. Theoretical results are compared with the experimental channeled stopping power of gold as a function of ion velocity and position within channels. It is shown that the theory is accurate when the velocity of the ion equals at least twice the root-mean-square velocity (estimated from binding energies) of most of the electrons near its path.

Z-oscillations in channeling stopping power

A systematic study of velocity dependence of channeling stopping power in the low velocity region has been made. The model used is the one first developed by Briggs and Pathak in which the target electrons are assumed to scatter from the incoming projectile and the energy loss is proportional to the momentum transfer (or transport or diffusion) cross-section. The results for this cross-section for the entire range of Z 1 from 2 to 80 and projectile velocity from 0.75 a.u. to 2.0 a.u. show that the positions of minima gradually shift to the higher Z 1 side and tend to merge with the maxima resulting in a systematic decrease of magnitude of the oscillations. First, maxima and minima are washed out almost completely by 2 a.u. velocity. This study underlines the need and importance of systematic experimental investigations over this velocity region, especially to check the variation in phase and cut-off velocity for the existence of first, second, third and fourth peaks in electronic stopping power for channeled ions.

A method for measuring stopping powers of channeled ions: Boron in Si

Abstract A method for measuring the stopping power of channeled ions, which does not require the use of thin samples is described. Heavy ions are first implanted into a single crystal under such conditions as to create a burried layer of non-substitutional atoms, leaving the sample relatively undamaged. Light ions, whose stopping power is to be determined, are backscattered from the heavy impurity atoms and their energy spectra are recorded for both channeling and non-channeling incidence. The ratio α = (dE/dx)ch/(dE/dx)tan is determined from the difference between these spectra. The use of the technique is demonstrated for 700 keV boron ions channeled in silicon, with thalium implants serving as the heavy scatterers. Values of α<110> = 0.81 ± 0.02 and α<110> = 0.84 ± 0.02 were determined.

19F range-energy curve in Si from 100 to 550 keV

The range statistics for C, F, Ne, Al, and Si in Si recently published by Gibbons, Johnson, and Mylroie (GJM) are significantly different from the predictions of the LSS theory. This difference exists because in their calculation GJM made use of measured electronic stopping powers (Se) for channeled ions. The scaling procedure employed by GJM builds into their calculation the pronounced Z1 oscillations observed in the Se of channeled ions. Use of the measured electronic stopping powers was limited to N, O, P, Cl, Ar, and the above mentioned ions in Si. In order to make an experimental check of the scaling procedure employed the range-energy curve for 19F in Si has been measured between 100 and 550 keV. The following observations can be made: (1) The experimental range-energy curve for 19F is in good agreement with LSS calculations and (2) the measured ranges are significantly less than the predictions of Gibbons, Johnson, and Mylroie. These results are interpreted as strong evidence against the validity of scaling Eisen’s measurements of the electronic stopping power for channeled ions over to random trajectories.

Deduction of Interaction Potentials from Planar-Channeling Experiments

A model is presented which allows planar-channeling data to be used to deduce the parameters of the interaction potential between the channeled ions and the atoms of the solid and which relates the stopping power of channeled ions to that of randomly directed ones. The analysis is based on the experimental observation of the proportionality between the stopping power and the transverse oscillation frequency of ions traversing the planar channels of thin crystal targets. It is applied to data on He, O, and I ions transmitted through the {111} and {100} channels of Au crystals and on H ions transmitted through the {111} and {110} channels of Si crystals. For both targets, the screening constants of the interaction potentials are in excellent agreement with recent Hartree calculations. The same potentials allow the calculation of random stopping powers from the channeling data which are in good agreement with observation, especially when account is taken of thermal vibrations of the lattice atoms.

Interpretation of Channeled-Ion Energy-Loss Spectra

The energy-loss spectra produced in beams of energetic He and I ions transmitted through planar channels in thin Au monocrystals have been explained by a simple model in which the ions execute transverse anharmonic oscillations in the channel between a pair of adjacent planes of target atoms and in which the stopping power of the medium is a function of the distance of the ion from the crystal planes. A quantitative comparison of this model with recent experiments is made using a plane-averaged potential based on Moliere's approximation to the Thomas-Fermi screening function to describe the oscillatory motion. The stopping power is found to obey the same functional dependence on the coordinates, but with a doubled screening length. The parameters of the model may be deduced directly from the experimental data. For He in Au, these parameters agree well with those expected from Firsov's treatment of the interaction potential between neutral atoms. For I in Au, the high charge of the ion leads to parameters which differ significantly from Firsov's theory. The data also suggest that the energy dependence of the stopping power for channeled ions may be somewhat different from that for randomly directed ones.