Kai Nordlund*,**, Juhani Keinonen*, M. Ghaly** and R. S. Averback**

The description of energetic ions slowing down in solids has evolved over the past two decades from one of a series of two-body elastic collisions, similar to loosely packed billiard balls colliding in three dimensions, to one of high energy localization, thermalization, and local melting [1]. The displacement of atoms in either description is chaotic in the sense that the slightest variation in the projectile's trajectory will lead to a completely different and unpredictable set of atomic displacements.

We found in molecular dynamics simulations of high-energy self bombardment of Cu and Ni that the high pressures developed in cascades centered well below the surface can lead to the coherent displacement of thousands of atoms over tens of atomic planes owing to shear-induced slip towards the surface [2]. We also find that the mechanism leads to both a dramatic increase in damage production and the formation of well-ordered adatom islands. These unexpected findings are very significant for understanding radiation damage, since ion irradiation always involves surfaces, and they are of particular concern when ion irradiations are employed to simulate fast neutron damage.

* Accelerator Laboratory, P.O. Box 43, 00014 University of Helsinki, Finland

** Materials Research Laboratory, University of Illinois, Urbana, IL 61801, USA

email kai.nordlund@helsinki.fi

1. T. Diaz de la Rubia, R.S. Averback, R. Benedek, and W.E. King, Role of Thermal Spikes in Energetic Collision Cascades, Phys. Rev. Lett. 59 (1987) 1930

2. K. Nordlund, J. Keinonen, M. Ghaly, and R.S. Averback, Coherent displacement of atoms during ion irradiation, Nature 398 (1999) 49



Kai Nordlund and F. Gao*

One of the central outstanding questions in radiation damage is how stacking fault tetrahedra (SFT's) can be formed below the vacancymigration temperature. Using molecular dynamics simulations of energetic collision cascades we now describe how a stacking fault tetrahedron can be created directly in a collision cascade. We also show that while the number of SFT's is small at low temperatures, at elevated temperatures the number will increase by rearrangement of complex damage clusters into SFT's, in good agreement with experiments.

Although it has long been clear that neutron and ion irradiation can lead to the formation of large defects like dislocation loops in metals [1], the microscopic mechanisms leading to the production of this damage have remained unclear. Yet this knowledge is central for understanding radiation damage in metals. In particular, it is a crucial issue in the control of fission reactor first wall material embrittlement, and important in selecting materials for future fusion reactors.

One especially interesting unanswered question is how stacking fault tetrahedra (SFT's) are produced during irradiation. These defects have the shape of four equilateral vacancy-type stacking faults on {111} planes intersecting along <110> edges to form a perfect tetrahedron, and are one of the most common vacancy-type defect clusters in metals of low stacking-fault energy.

Using molecular dynamics simulations of energetic collision cascades we have described how a stacking fault tetrahedron can be created directly in a collision cascade [2]. The SFT's form by direct collapse of a depleted atom region in the center of the cascade to a tetrahedron formed by four intersecting {111} vacancy stacking faults. We also showed that while the number of SFT's is small at low temperatures, at elevated temperatures the number will increase by rearrangement of complex damage clusters into SFT's, in good agreement with experiments.

* Department of Materials Science and Engineering, The University of Liverpool, Liverpool L69 3GH, UK

1. J. Silcox and P.B. Hirsch, Dislocation Loops in Neutron-irradiated Copper, Phil Mag 4 (1959) 1356

2. K. Nordlund and F. Gao, Formation of stacking fault tetrahedra in collision cascades, Appl Phys Lett 74 (1999) 2720



Marcus Gustafsson and Juhani Keinonen

As an independent continuation on the previous work [1] done on a-SiO2 in collaboration with others*,**, recrystallization of irradiated a-SiO2 has been studied.

The physics of the amorphisation/recrystallization of SiO2 is an important issue in the semiconductor industry and the increased use of opto-electronics and high speed device technology today has made it even more so.

The previous work focused on alkaline-ions and annealing of a-SiO2 in oxygen-18 atmosphere. This study focuses on implanting lithium and sodium in combination with oxygen-18 and nitrogen ions to study the recrystallization mechanism and intends to use this information to attain enhanced recrystallization at lower annealing temperature.

Different energies were chosen so that the implanted ions were either at the same depth, deeper or closer to the surface than the implanted alkali ions. Nitrogen was chosen because it diffuses easily, while oxygen-18 replaces the oxygen in the SiO2-network and is very difficult to get out by annealing. The annealing temperatures ranged from 500°C to 900°.

The samples were measured both with time-of-flight recoil detection analysis (TOF-ERDA) and Rutherford backscattering using channeling geometry (RBS-C).

No recrystallization was observed with the oxygen-18, but in the case of the nitrogen some recrystallization did occur. During the measurements a large amount of data was accumulated which will be used to thoroughly study the diffusion mechanisms at work.

* Zweites Physikalisches Institut, Universität Göttingen, Bunsenstr. 7-9, D-37073 Göttingen, Germany

** Institut für Strahlenphysik, Universität Stuttgart, Allmandring 3, D-70569 Stuttgart, Germany

1. M. Gustafsson, F. Roccaforte, J. Keinonen, W. Bolse, L. Ziegler, K.P. Lieb, Phys Rev B 61 (2000) 3327-3332



Arto Nurmela, Petteri Pusa, Eero Rauhala and Jyrki Räisänen*

The series of elastic scattering cross section experiments near the Coulomb barrier has been continued. Elastic scattering cross section measurements and optical model calculations of Ni(7Li, 7Li)Ni through scattering angles of 115° and 135° and Ni(11B, 11B)Ni through scattering angles of 89°, 110° and 132° have been completed. By measuring the backscattering cross sections and kinematically reversing the reaction, the recoil cross sections were determined. Scattering angles were selected to correspond the reversed recoil angles of 20°, 30° and 40°. The non-Rutherford threshold energies where the scattering cross section deviates 4% from Rutherford value have been extracted. The results have been presented in Ion Beam Analysis 14-conference and published in Ref. [1].

Elastic scattering cross section measurements of 4He scattering by nickel have also been completed. The measurements were performed at three scattering angles: 96°, 117° and 137° for energies below 14.3 MeV. Angular distribution for cross sections at 14 MeV was measured. Optical model analysis was performed on the basis of the measured data. Again the recoil cross sections were determined by kinematically reversing the reaction. The threshold energies for the forward and backscattering reactions were also extracted.

The scattering cross sections studies of protons by helium have been continued. At the proton energy range of 1.2 to 5.2 MeV through scattering angles of 85°, 106° and 128° the 4He(p,p)4He scattering cross sections were measured and kinematically reversed to p(4He,p) 4He recoil scattering.

* University of Jyväskylä, Department of Physics, P.O.Box 35, 40351 Jyväskylä, Finland

1. A. Nurmela, P. Pusa, E. Rauhala and J. Räisänen, Nucl Instr and Meth in Phys Res B 161-163 (2000) 130



C. Zimmermann*, M. Yeadon*, M. Ghaly*, Kai Nordlund*,**, J.M. Gibson*, R.S. Averback*, U. Herr*** and K. Samwer***

Metal nanoparticles display a unique behavior when deposited on substrates with a significantly lower surface energy. Co nanoparticles in the 10 nm size regime were deposited on clean Cu and Ag(100) substrates [1]. Randomly oriented at room temperature, the particles assume the substrate orientation upon deposition at 600 K and then spontaneously burrow beneath the surface.

This behavior can be understood in terms of the surface energies and the tension associated with a nanoparticle. Upon deposition the Co cluster keeps its shape and gets coated with substrate atoms almost immediately. Subsequently diffusion of substrate atoms from underneath the cluster along the interface onto the surface facilitates the sinking of the cluster. The diffusion is driven by the enhanced chemical potential caused by the surface curvature. The sinking process should happen in all systems where nanoparticles with a significantly higher surface energy than the substrate are deposited.

The effect observed has implications on the mobilities of clusters with potential applications in the engineering of rough surfaces.

* Materials Research Laboratory, University of Illinois, Urbana, IL 61801

** Accelerator Laboratory, P.O. Box 43, 00014 University of Helsinki, Finland

*** Institut für Physik, Universität Augsburg, D-86135 Augsburg

1. C. Zimmermann, M. Yeadon, M. Ghaly, K. Nordlund, J.M. Gibson, R.S. Averback, U. Herr and K. Samwer, Burrowing of Co nanoparticles after soft landings on clean Co and Ag surfaces, Phys Rev Lett 83 (1999) 1163



Elizaveta Vainonen-Ahlgren, Timo Sajavaara, Walter Rydman, Tommy Ahlgren, Kai Nordlund, Juhani Keinonen, J. Likonen*, S. Lehto*, C.H. Wu**

Deuterium retention, solubility and out-diffusion have been studied in silicon doped carbon films produced by physical vapor deposition. The deuterium concentration profiles were measured by the time-of-flight elastic recoil detection analysis technique and secondary ion mass spectrometry. The D retention and solubility were measured in D implanted carbon samples. The out-diffusion of D was investigated in D co-deposited samples. The solubility of D was shown to increase as a function of Si concentration in the co-deposited samples while in the implanted samples no dependence of the Si content was observed. It was proposed that annealing behavior of deuterium has a trapping-like character.

* Technical Research Centre of Finland, Chemical Technology, P.O.Box 1404, FIN-02044 VTT

** The NET Team, Max-Planck-Institute für Plasmaphysik, D-85748 Garching bei München



Emppu Salonen, Kai Nordlund, Jura Tarus, Tommy Ahlgren, Juhani Keinonen and C.H. Wu*

Standard models describing the sputtering and erosion of materials by ion bombardment assume that the sample composition remains constant during the bombardment, implying that the erosion yield per incoming ion is independent of the ion flux. However, the erosion of carbon by intensive hydrogen bombardment has been recently shown to decrease sharply at very high fluxes (~ 1019 ions/cm2s) [1,2]. Understanding this effect is central for selection of carbon based fusion reactor divertor materials and formulation of sputtering models for high-flux conditions.

Using molecular dynamics simulations, we have shown that the experimentally observed decrease in carbon erosion from amorphous hydrogenated carbon (a-C:H) samples during very high-flux low-energy H bombardment is due to the buildup of a high H content at the surface [3]. The high H content leads to the shielding of carbon atoms from new incoming H ions, and thus a decrease of roughly an order of magnitude in the C erosion yield. Our results also demonstrate that for extremely high fluxes standard sputtering models are not necessarily reliable since they do not account for temporary supersaturation of material at the surface.

* The NET Team, Max-Planck-Institut für Plasmaphysik, D-85748 Garching bei München

email msalonen@beam.helsinki.fi

1. J. Roth and R.-C. Garcia, Nucl Fusion 36 (1996) 1647

2. A. Kallenbach, A. Thoma, A. Bard, K. Behringer, K. Schmidtmann, M. Weinclich and the ASDEX upgrade team, Nucl.Fusion 38 (1998) 1097

3. E. Salonen, K. Nordlund, J. Tarus, T. Ahlgren, J. Keinonen and C.H. Wu, Phys Rev B (Rapid Communications) 60 (1999) 14005



Kai Arstila, J. Forster*, K. Mizohata, Erik Edelmann, Pertti Tikkanen, Timo Sajavaara and Juhani Keinonen

Elastic recoil detection analysis (ERDA) has become an important technique for thin film analysis, especially for depth profiling and identification of light elements within a heavy matrix. A variety of detection methods like time-of-flight (TOF) and magnetic spectrometers have been applied in ERDA. These techniques are often restricted to a small acceptance solid angle. Therefore, in order to obtain reasonable statistics, long exposure times or high beam currents are required, which often cause radiation damage in the sample. The solid angle of the system can be increased by using a position sensitive gas-ionisation detector. In such a system the position information is used to correct the kinematic energy spread of recoils over the acceptance angle.

Our ERDA detector is an advanced version of the gridded ionisation chamber. The length of active gas volume is 30 cm. Isobutane is used at 10-100 mbar pressure as a fill gas. The anode is subdivided into three parts to get D E1, D E2 and Erest signals for particle identification. To measure the position in the x-direction D E2 is divided in two separated parts with a backgammon shape. The position in y-direction is obtained by measuring the signal at the cathode, which is propotional to the distance from grid.

In first test measurements an energy resolution of 2% for the 4.5 MeV a-particles was measured. By the use of a 72 MeV 127I14+ beam different elements in SrTiO sample were clearly separated and depth profiles obtained.


Fig. A measured spectrum from SrTiO3 sample.


* Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada


Timo Sajavaara, Kai Arstila and Juhani Keinonen

A Monte Carlo simulation code was written for elastic recoil detection analysis which can be used in the simulation of the multiple and plural scattering effects both in target and detector in an elastic recoil experiment. A fully realistic Monte Carlo treatment would be extremely time consuming. In our code we introduce several methods to enhance the efficiency of the computer simulation of the elastic recoil detection process while maintaining the accuracy of the results.

The energy spectra of the recoiled atoms simulated with the Monte Carlo approach are compared with the experimental spectra measured with a large variety of primary beam ions, recoiled atoms and target materials.

The comparisons show that a good agreement between experimental and simulated results can be obtained. Thus, simulations can be used to obtain detailed information of the target composition.



Timo Sajavaara, Kai Arstila, A. Laakso*, and Juhani Keinonen

In order to study surface roughness effects on the elastic recoil detection analysis series of stainless steel samples were prepared to four different roughnesses by wet grinding and polishing. A multilayer film was electron beam evaporated on stainless steel samples and a flat silicon wafer. The time-of-flight elastic recoil detection analysis (TOF-ERDA) method was used to measure energy spectra of the recoiling sample elements. The topography of the samples was determined by atomic force microscopy and the film thickness by Rutherford backscattering spectrometry.

A Monte Carlo simulation program which uses a measured surface structure and takes fully account of multiple scattering was written to simulate the elastic recoil measurements. Effects observed in the experimental energy spectra, like broadening of the peaks of deep lying layers, were reproduced in the simulations. Multiple scattering is the dominant factor behind the broadening for flat surfaces. It enhances strongly also the broadening due to the surface roughness.

* Laboratory of Physics, Helsinki University of Technology, 02150 Espoo, Finland

1. T. Sajavaara, K. Arstila, A. Laakso, and J. Keinonen, Nucl Instr and Meth in Phys Res B 161-163 (2000) 235