Publications of Kai Nordlund

Note: the rest of this page is no longer maintained. The link above has an up-to-date list of all of my publications and links to copies of many of them.

Highlights (some of my own favourites)

This is a collection of abstracts and preprints of some of my scientific refereed publications (a complete list of publications is found here ).

Note that the material in all publications are copyright either to some publishing company, to the authors, or both. you are not allowed to republish any part of this material in any context without permission. For more legalese, see the end of this page.

I have tried to put here some sort of postscript and PDF version of as many publications as possible; if one is missing, and you can not find the paper in your library, feel free to mail me and ask for a reprint of it. All the full-text files are preprints, and hence differ from the final published version in the layout and possibly slightly in the text. The scientific content is the same in all cases.

Of course, the papers not yet published may still be subject to changes. Furthermore, the layout in the papers here is probably not identical to that in the published paper.

A low-level detection system for hydrogen analysis
with the reaction 1H(15N,\alpha\gamma)12C

P. Torri, J. Keinonen and K. Nordlund

A spectrometer has been constructed for measurements of low hydrogen concentrations by detecting 4.43 MeV \gamma -rays from the reaction 1H(15N,\alpha\gamma) 12C at the 6.39 MeV resonance. It consists of an annular large-volume (2600 cm3) BGO detector, an annular plastic-detector anticoincidence shield around it, and Pb and Cd shields. The spectrometer is placed in the target hall of the laboratory located 20 m deep inside the granite bedrock. The background counting rate arising mainly from impurity of the BGO materials is 0.023 Hz in the energy range 3.9 - 4.9 MeV. The detection sensitivity of 10 at. ppm H in silicon substrates was determined by measurements of concentration distributions of 2 \times 1013 - 1 \times 1017 40 keV protons cm-2 in single-crystal silicon substrates. The range profiles of 40 keV protons simulated by molecular dynamics calculations resulted in the correction factor of 1.21 for the commonly used electronic stopping power given by Ziegler et. al. [The Stopping Power and Ranges of Ions in Matter (Pergamon, New York, 1985) vol. 1].

Nucl. Instr. Meth. Phys. Res. B 84 (1994) 105-

First-Principles Simulation of Collision Cascades in
Si to Test Pair-Potential for Si-Si Interaction at 10 eV -- 5 keV

J. Keinonen, A. Kuronen, K. Nordlund, R. M. Nieminen and A. P. Seitsonen

Interatomic potentials for Si-Si interaction are tested at energies 10 eV--5 keV for Si ions in ion-beam amorphized Si by simulating range distribution data with the molecular dynamics method. The range profile of 1\times1016 10-keV 30Si+ cm-2 implanted into originally crystalline silicon were measured using a nuclear reaction technique. An interatomic repulsive potential from first-principles calculations is proposed for the Si-Si interaction. The dependence of the number of vacancies produced in low-energy collision cascades on the potential is demonstrated by simulations of the cascade dynamics.

Nucl. Instr. Meth. Phys. Res. B 88 (1994) 382-

Lifetimes in 15N from gamma-ray lineshapes produced in the
2H(14N, p\gamma ) and 14N(thermal n, \gamma) reactions

S. Raman, E. T. Jurney, J. W. Warner, A. Kuronen, J. Keinonen, K. Nordlund, and D. J. Millener

Mean lifetimes of bound states in 15N were inferred from the Doppler-shift-attenuation (DSA) of \gamma rays produced in the inverse reaction 2H(14N, p\gamma) and from the primary gamma-ray-induced Doppler broadening (GRID) of secondary \gamma rays in the thermal-neutron capture reaction 14N( n, \gamma ). Targets for the DSA measurements were prepared by implanting first neon into a gold backing and then deuterium into the same region such that deuterium was trapped at the neon precipitates. To find out experimentally the initial velocity distributions of the recoiling 15 N nuclei, another target was prepared by implanting deuterium into silicon, which is a slow stopping-power medium. Computer simulations with the Monte Carlo method and experimental stopping powers were used in the DSA analysis of the \gamma -ray lineshapes. This analysis yielded the following lifetimes for eight bound levels in 15 N: 43 ± 4 fs (5.30 MeV), < 12 fs (6.32 MeV), 11 ± 2 fs (7.16 MeV), < 3 fs (7.30 MeV), 129 ± 6 fs (7.57 MeV), < 3 fs (8.31 MeV), < 10 fs (8.57 MeV), and < 4 fs (9.05 MeV). GRID measurements were made with melamine (C3H6N6) and with silicon nitride (Si3 N4) as both targets and slowing-down media. Measurements were also made with air (80\% nitrogen). The Doppler-broadened \gamma -ray lineshapes were analyzed by molecular-dynamics simulations of the slowing-down process to obtain the lifetime values of 40 ± 3 fs (5.30 MeV), < 2 fs (6.32 MeV), < 3 fs (7.30 MeV), and < 3 fs (8.31 MeV) for levels in 15N. The extent to which the mirror symmetry of levels in 15N and 15O is valid is examined in some detail.

Phys. Rev. C 50 (1994) 682-

Effect of the Interatomic Si-Si-potential
on Vacancy Production during Ion Implantation of Si

K. Nordlund, J. Keinonen, and A. Kuronen

Collision cascades in crystalline silicon due to impinging 10 eV - 1 keV Si atoms are simulated using molecular dynamics methods. The simulations are carried out for 30 - 100 events to obtain representative statistics for production of different types of vacancies. The results are used to examine the dependence of vacancy production on the interatomic Si-Si potential between the colliding atoms. The dependence of the number of vacancies was found to be sensitive to the form of the potential well but not to the repulsive potential. The results suggest that within the heavily damaged volume of the collision cascade an interatomic potential with somewhat narrower well than that of the commonly used Stillinger-Weber potential should be used to simulate the vacancy production in silicon.

Physica Scripta T54 (1994) 34-

Molecular dynamics simulation of ion ranges in the post-keV energy region

K. Nordlund

Binary collision approximation methods have been conventionally used to describe the slowing down of recoiling ions. In order to better understand the slowing-down process molecular dynamics methods are more and more used in the literature. However, the computer capacity limits the usefulness of the methods in most practical cases where ion implantation in the 1-100 keV energy range is used. We present an efficient molecular dynamics method for calculating ion ranges and deposited energies in the recoil energy region 100 eV to 100 keV. By taking into account only the interactions that are involved in the slowing-down process, range and deposited energy distributions at higher energies can be simulated. The method is demonstrated by range calculations of 40 keV H atoms in Si, 40 keV He atoms in Ta and 100 eV to 10 keV Si atoms in Si.

Comp. Mat. Sci. 3 (1995) 448

Molecular dynamics simulation of ion ranges at keV energies

K. Nordlund and A. Kuronen

We present an efficient molecular dynamics method for calculating ion ranges and deposited energies in the recoil energy region 100 eV to 100 keV. The method overcomes some of the drawbacks of the binary collision approximation methods conventionally used to calculate ion ranges. We describe principles by which one can simulate implantation into polycrystalline materials, and study the effect of the crystal structure on the ion range. Application of the simulation method to practical cases is demonstrated by analyzing experimental range results of 50 keV silicon self-ion-implantation measured at our laboratory.

Nucl. Instr. Meth. Phys. Res B. 115 (1995) 528 (conference paper at ICACS-16).

Range profiles in self-ion-implanted crystalline Si

K. Nordlund, J. Keinonen, E. Rauhala and T. Ahlgren

Range profiles of 50- and 100-keV 30Si+ ions implanted into Si(100) at room temperature with doses from 2 \times 1016 to 1 \times 1018 ions cm-2 , and of 2 \times 1016 50-keV 30Si+ ions cm-2 in Si(100) preimplanted with 50-keV 28Si+ ions with doses from 5 \times 1013 to 1 \times 1018 ions cm-2 , have been studied with nuclear reaction techniques. The structural transformation of the samples was studied by Rutherford backscattering and channeling. The dependence of the range profile on the damage structure of the sample has been examined by comparing measured range profiles with those obtained in molecular dynamics simulations. The dependence of the range profile on the polycrystalline sample structure is shown.

Phys. Rev. B 52, 15170 (1995)

Stopping of 5 -- 100 keV helium in tantalum,
niobium, tungsten, and AISI 316L steel

P. Haussalo, K. Nordlund and J. Keinonen

The stopping power of Ta, Nb, W, and AISI 316L stainless steel for He ions at velocities below the Bohr velocity has been deduced by comparing the ranges of 5 to 100 keV He + -ions determined with the elastic-recoil-detection-analysis method with those obtained in molecular dynamics simulations. The nuclear slowing down was treated through the use of molecular dynamics calculations and a potential obtained from density-functional theory calculations. The comparisons showed that the electronic stopping powers given by J. F. Ziegler, J. P. Biersack, and U. Littmark [ The Stopping Powers and Ranges of Ions in Matter (Pergamon, New York, 1985), Vol. 1], had to be multiplied with a factor of 1.4 for Ta, 1.0 for Nb, 1.1 for W, and 1.0 for AISI 316L. The uncertainty of the factors and hence the scaled stopping power values is ± 5 \% . The total stopping powers were obtained from molecular dynamics simulations in which the modified electronic stopping powers were used.

Nucl. Instr. Meth. Phys. Res. B 111 (1996) 1

Formation of ion irradiation-induced small-scale defects on graphite surfaces

K. Nordlund, J. Keinonen, and T. Mattila

Hillock formation on ion-irradiated graphite surfaces

K. Nordlund and T. Mattila

Scanning probe microscopy experiments show that ion irradiation of (1000) graphite results in the formation of isolated defects comprising of a few tens of atoms. We use molecular dynamics simulations and density-functional theory calculations to study the formation probabilities of these defects. We identify different defect structures which correspond to experimentally observed hillocks on graphite surfaces. We find that the predominant source of defects are vacancies and interlayer interstitials, and identify a three-atom carbon ring defect on the graphite surface.

COSIRES '96 conference paper, Rad. Eff. & Def. in Sol. 142 (1996) 450.

Comparison of TOF-ERDA and nuclear resonance reaction techniques for range profile measurements of keV-energy implants

J. Jokinen, J. Keinonen, P. Tikkanen, A. Kuronen, T. Ahlgren and K. Nordlund

A comparative study on the range measurements of keV-energy implants by the Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) and conventionally used nuclear resonance reaction methods has been performed for 20 -- 100-keV 15N+ ions implanted into crystalline silicon. Range profiles of 15 N atoms were chosen because they can be measured accurately using a very strong and narrow resonance at E_p = 429.6 keV in the reaction 15N(p, \alpha \gamma)12C which provides a challenging test for other methods. The measured range profiles were simulated by molecular dynamics calculations where the interatomic N-Si pair potential is deduced from first principles calculations. The electronic stopping power for 20 -- 100 keV nitrogen ions in silicon is deduced from the comparison of the measured and simulated range profiles. The results are discussed in the framework of the applicability of the TOF-ERDA technique for range measurements of keV-energy ions.

Nucl. Instr. Meth. Phys. Res. B 119 (1996) 533

Point defect movement and annealing in collision cascades

K. Nordlund and R. S. Averback

The effect of collision cascades on pre-existing point defects in crystalline materials was studied by simulating 5 keV collision cascades in gold, copper, aluminum, platinum and silicon. The results indicate that collision cascades do not significantly affect interstitials or vacancies outside the liquid core of the cascade, although in the FCC metals, the heating of the crystal due to the cascade causes some thermal migration of the interstitials. Within the liquid cascade core, both interstitials and vacancies move towards the center of the molten region when it resolidifies and recombine or cluster there. At elevated temperatures, random jumps of interstitials during the thermal spike phase can cause significant additional trapping of interstitials in the liquid. In contrast to the annealing effects of pre-existing damage in the FCC metals, in silicon the amount of new damage created by a cascade is roughly independent of the number of initial point defects. The difference is attributed to the nature of the bonding in the materials.

Phys. Rev. B. 56 (1997) 2421

Repulsive interatomic potentials calculated using Hartree-Fock and density-functional theory methods

K. Nordlund, N. Runeberg and D. Sundholm

The repulsive part of the interatomic potential affects the outcome of computer simulations of many irradiation processes of practical interest, like sputtering and ion irradiation range distributions. The accuracy of repulsive potentials is studied by comparing potentials calculated using commonly available density-functional theory and Hartree-Fock methods to highly accurate fully numerical Hartree-Fock and Hartree-Fock-Slater calculations. We find that density-functional-theory calculations utilizing numerical basis sets and Hartree-Fock calculations using decontracted standard basis sets provide repulsive potentials which are significantly improved compared to the standard universal ZBL potential. The accuracy of the calculated potentials is almost totally governed by the quality of the one-particle basis set. The use of reliable repulsive potentials open up new avenues for analysis of ion irradiation experiments.

Nucl. Instr. Meth. Phys. Res. B 132 (1997) 45

Atomic displacement processes in irradiated amorphous and crystalline silicon

K. Nordlund and R. S. Averback

Ion beam mixing was investigated in crystalline and amorphous Si using molecular dynamics simulations. The magnitude of mixing was found to be larger in amorphous Si by a factor of about two. The difference is attributed to local relaxation mechanisms occurring during the cooling down phase of the cascade. Comparison of mixing between Si and Al shows that short range structural order also has a significant influence on mixing.

Appl. Phys. Lett. 70 (1997) 3103

Fully atomistic analysis of diffuse X-ray scattering spectra of silicon defects

K. Nordlund, P. Partyka and R. S. Averback

Diffuse X-ray scattering is a useful method for studying defects in silicon and metals. Although the traditional approaches of analyzing experimental diffuse X-ray scattering data have given much information about the size of defects and defect clusters, they are not very well suited for determining the atomic configuration. We present a fully atomistic computational method to calculate the diffuse X-ray scattering line profile of an arbitrary atomic configuration, and compare line profiles of point defects and Frenkel pair configurations with experiment.

MRS Spring meeting paper, to be published in Mat. Res. Soc. Symp. Proc. (1997)

Effect of atomic bonding on defect production in collision cascades.

K. Nordlund, R. S. Averback and T. Diaz de la Rubia

We study the mechanisms of damage production during ion irradiation using molecular dynamics simulations of 400 eV - 10 keV collision cascades in four different materials. The materials Al, Si, Cu and Ge are contrasted to each other with respect to the mass, melting temperature and crystal structure. The results show that the crystal structure clearly has the strongest effect on the nature of the damage produced, and elucidate how the open crystal structure affects the nature of defects produced in silicon.

MRS Spring meeting paper, to be published in Mat. Res. Soc. Symp. Proc. (1997)

Molecular dynamics investigations of surface damage produced by keV self-bombardment of solids

M. Ghaly, K. Nordlund and R. S. Averback

Molecular dynamics computer simulations were employed to study damage production mechanisms at solid surfaces during bombardment with keV ions. Three separate mechanisms are identified: ballistic damage, viscous flow, and microexplosions. Ballistic damage is created by the direct knock-on of atoms onto the surface as described within the binary collision approximation. Viscous flow refers to local melting and convective mass flow onto the surface, and microexplosions occur when the high pressures developed in cascades lead to rupturing of the nearby surface. The relative importance of each mechanism depends on several parameters: atomic mass, melting temperature, atomic density, structure and atomic bonding of the target, and the mass and energy of the projectile. The simulations were performed for Pt, Au, Cu, Ni and Ge targets. Cascades in the interior of the targets were also examined to provide a comparison for the surface events. In addition several events of 4.5 keV Ne and Xe bombardment of Pt(111) were simulated for comparison with experimental studies of these same bombardments using scanning tunneling microscopy.

Phil. Mag. A 79 (1999) 795

• Phil. Mag. A issue

Preprint of publication not available

Mechanisms of ion beam mixing in metals and semiconductors

K. Nordlund, M. Ghaly and R. S. Averback

Ion beam mixing was investigated in crystalline and amorphous semiconductors and metals using molecular dynamics simulations. The magnitude of mixing in an amorphous element compared to its crystalline counterpart was found to be larger by a factor of two or more. Mixing in semiconductors was found to be significantly larger than in an FCC metal of corresponding mass and atomic density. The difference in mixing between amorphous and crystalline materials is attributed to local relaxation mechanisms occurring during the cooling down phase of the cascade. Comparison of mixing in semiconductors and metals shows that short range structural order also has a significant influence on mixing. The mixing results in FCC metals indicate that the role of the electron-phonon coupling in the evolution of collision cascades may be less significant than previously thought.

J. Appl. Phys. 83 (1998) 1238

Molecular Dynamics Simulations of LATID Implants into Silicon

Phil Oldiges, Huilong Zhu and Kai Nordlund

Molecular dynamics simulations of Large-Angle-Tilt Implanted Drain technology are shown. Calculation results are shown of ion range variation as a function of implant angle over the entire spectrum of possible implant angles. Through this calculation, it is possible to determine the optimal angles for large tilt angle implants. The simulator also allows for the definition of amorphous layers over a crystalline substrate. Results of these calculations accurately predict effects such as paradoxical profile broadening.

Proceedings of the 1997 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), p. 245

Defect production in collision cascades in elemental semiconductors and FCC metals

K. Nordlund, M. Ghaly, R. S. Averback, M. Caturla, T. Diaz de la Rubia and J. Tarus

A comparative molecular dynamics simulation study of collision cascades in two elemental semiconductors and five FCC metals is performed to elucidate how different materials characteristics affect primary defect production during ion irradiation. By using simulations of full 400 eV -- 10 keV collision cascades and contrasting the results on different materials to each other, we probe the effect of the mass, melting temperature, material strength and crystal structure on the modification of the material due to the cascade. The results show that the crystal structure has a strong effect on most aspects of damage production, while other material characteristics are of lesser overall importance. In all materials studied, isolated point defects produced by the cascade are predominantly interstitials. In semiconductors, amorphous clusters are produced in the cascade core, whereas in metals most of the crystal regenerates leaving only small vacancy-rich clusters. Large interstitial clusters found in a few events in the heavy metals were observed to form by the isolation of a high-density liquid zone during the recrystallization phase of a cascade.

Phys. Rev. B 57 (1998) 7556.

Defect Production in Tungsten: A Comparison Between Field Ion Microscopy and Molecular Dynamics Simulations

Y. Zhong, K. Nordlund, M. Ghaly and R. S. Averback

Molecular dynamics (MD) computer simulations of 20 - 30 keV self-ion bombardment of W were performed and compared to past Field Ion Microscopy (FIM) studies [M. I. Current et al., Phil. Mag A 47, 407 (1983)]. The simulations show that the unusually high defect production efficiencies obtained by FIM is a consequence of a surface effect, which greatly enhances defect production compared to that in the crystal interior. Comparison of clustering of vacancies and the formation of interstitial atoms found in the FIM experiments and MD simulations shows overall good agreement.

Phys. Rev. B (brief reports) 57 (1998) 13965.

The role of self-interstitial atoms on the high temperature properties of metals

K. Nordlund and R. S. Averback

Equilibrium concentrations of self-interstitial atoms and divacancies have been determined in Cu by molecular dynamics computer simulations using embedded atom potentials. Near the melting temperature these concentrations are both $\sim 10^{-6}$. Owing to the higher mobility of the interstitial atoms, however, they contribute more to diffusion. In perfect, or pulse-heated crystals, spontaneous Frenkel pair production results in even higher interstitial concentrations.

Phys. Rev. Lett. 80 (1998) 4201

Role of electron-phonon coupling on collision cascade development in Ni, Pd and Pt

K. Nordlund, L. Wei, Y. Zhong and R. S. Averback

Electron-phonon coupling in energetic collision cascades is believed to greatly enhance the cooling rate of heat spikes in many metals. Previous studies have not been able to conclusively determine the magnitude of the coupling, however. By directly comparing ion beam mixing experiments and molecular dynamics simulations of collision cascades in Ni, Pd and Pt, metals in which the coupling is believed to be most important, we show that the influence of electron-phonon coupling on mixing can be no more than about 30 %, roughly an order of magnitude less than the most widely used models predict.

Phys. Rev. B. (Rapid comm.) 57 (1998) 13965

Effect of Surface on Defect Creation by Self-ion Bombardment of Si (001)

J. Tarus and K. Nordlund and A. Kuronen and J. Keinonen

We have studied defect formation and defect distributions in silicon under low-energy (25 -- 800 eV) self-bombardment of the $2\times1$ terminated Si (001) surface. We applied the classical molecular dynamics technique and collected statistically significant averages to be able to detect defect production trends in the energy dependence. The number of defects created in implantations was found to be a superlinear function of energy at low energies ($\le$ 200 eV), and larger than the defect production in the bulk up to about 1 keV. We have also examined the depth dependence of close-to-surface damage, explored the energy and time dependence of the defect creation mechanisms and the sensitivity of the results to the choice of the model potential.

Phys. Rev. B 58 (1998) 9907

Channeling in Manufacturing Sharp Junctions: a Molecular Dynamics Simulation Study

J. Sillanpää, K. Nordlund and J. Keinonen

We examine how channeling effects affect ion implantation depth profiles in silicon and whether channeling can be beneficial for achieving a sharp end-of-range in the profile. Accurate modeling of channeling requires a molecular dynamics program with a realistic electronic stopping model. We have implemented the local electronic stopping model originally proposed by Cai et al. and the Firsov model to describe energy loss in inelastic collisions between recoiling ions and target atoms. Simulations of 0.25 - 15 keV B and As ions in the <100>, <110>, <111> and off-channel directions in silicon show that channeling effects become unimportant at keV energies in the <100> and <111> directions, whereas in the <110> channel they persist down to the very lowest energies. Implantation into the <100> channel gives optimal sharpness of the profiles down to implantation depths of roughly 400 Å, indicating that no tilt is necessary for the very shallow implants needed in next-generation semiconductors.

Nordic semiconductor conference paper. Physica Scripta T79 (1998) 272-274.

Glancing Incidence Diffuse X-ray Scattering Studies of Implantation Damage in Si

K. Nordlund, P. Partyka, Y. Zhong, I. K. Robinson, R. S. Averback and P. Ehrhart

Diffuse X-ray scattering (DXS) at glancing incidence is a potentially powerful means for elucidating damage structures in irradiated solids. Fundamental to the analysis of diffuse X-ray scattering data is a knowledge of the atomic displacement field around defects, which for implantation damage in crystals like Si has been difficult to obtain using analytical solutions of elastic continuum theory. We present a method for predicting the diffuse scattering pattern by calculating the displacement field around a defect using fully atomistic simulations and performing discrete sums for the scattering intensity. We apply the method to analyze experimental DXS results of defects produced by 4.5 keV He and 20 keV Ga irradiations of Si at temperatures of 100~-~300~K. The results show that the self-interstitial in ion-irradiated Si becomes mobile around 150 K, and that amorphization of silicon by light and medium-heavy projectiles occurs homogeneously through the buildup of interstitial clusters, and not within single cascade events.

EMRS-98 conference paper. Nucl. Instr. Meth. Phys. Res. B 147 (1999) 399

Inverse Kirkendall mixing in collision cascades

K. Nordlund and R. S. Averback

We study ion beam mixing of metallic bilayer interfaces using classical molecular dynamics simulations of 5 keV collision cascades in the vicinity of Co/Cu and Ni/Cu (111) interfaces. We find that the production of vacancies and interface roughening is asymmetrical. On average, more Cu is introduced into the Co or Ni parts than vice versa, and more vacancies are produced in the Cu, indicative of an inverse Kirkendall effect in collision cascades. The effect is explained by the difference in melting points leading to different recrystallization rates of the two materials.

Phys. Rev. B 59 (1999) 20.

Recoils, flows and explosions: surface damage mechanisms in metals and semiconductors during 50 eV -- 50 keV ion bombardment

K. Nordlund, J. Keinonen, M. Ghaly and R. S. Averback

We review some recent simulation results on mechanisms of damage production close to a surface during ion irradiation. The simulation work encompasses studies of several metals and semiconductors at irradiation energies ranging from a few tens of eV:s to 50 keV. The results show that in dense metals the presence of a surface can dramatically enhance the damage production up to energies of at least 50 keV. The added damage is mostly in the form of vacancy clusters, which can extend quite deep, ~ 10 nm, in the sample. In semiconductors, by contrast, the surface in general has little effect on the damage production in bulk.

IBMM-98 conference paper. Nucl. Instr. Meth. Phys. Res. B 148 (1999) 74.

Defect creation by low-energy ion bombardment on GaAs (001) and Ge (001) surfaces

A. Kuronen, J. Tarus and K. Nordlund

We have studied the ion-beam - surface interactions with the classical molecular dynamics simulation method. The properties of the GaAs (001) surface predicted by the potential model were investigated. The structure and amount of defects created on the GaAs (001) and Ge (001) surfaces 50 eV Ga and Ge ions, respectively, were investigated and compared. The defect creation for the GaAs system was found to differe considerably from that of the Ge system. Since Ga, As and Ge have similar masses this illustrates the importance of chemical effects on damage production in low-energy ion irradiation.

COSIRES '98 conference paper, Nucl. Instr. Meth. Phys. Res. B. 153 (1999) 209

Heat spike and ballistic phase contribution to mixing in Si

J. Tarus, K. Nordlund, J. Sillanpää and J. Keinonen

We have studied atomic mixing in silicon by the classical molecular dynamics method, and directly compared the simulated data to experimental measurements. The relative importance of ballistic collisions and heat spike to the mixing is considered. We obtain a fairly good agreement between experiments and simulations. The heat spike contribution to the total mixing seems to be much lower than that of ballistic collisions.

COSIRES '98 conference paper, Nucl. Instr. Meth. Phys. Res. B. 153 (1999) 378-382

Collision cascades in metals and semiconductors:
defect creation and interface behaviour

K. Nordlund and R. S. Averback

Using molecular dynamics simulations of collision cascades, we examine point defect and defect cluster formation mechanisms in metals and semiconductors. In metals we find that the primary mechanism causing separation of interstitials and vacancies is the pushing of vacancies toward the cascade center during the cooling phase of the cascade. We further describe how the isolation of a part of the liquid formed in the cascade can lead to the formation of interstitial clusters in metals. By comparing ballistically similar pairs of metals and semiconductors like Al and Si and Cu and Ge, we deduce how the cascade behaviour depend on the nature of interatomic bonding and crystal structure. We also find that close to sharp interfaces of metals with different melting points the vacancy push'' mechanism can lead to most vacancies being pushed to one of the materials, and an asymmetry in the impurity introduction over the interface owing to an inverse Kirkendall effect.

Spain metals irradiation workshop paper, J. Nucl. Mater. 276 (2000) 194.

Formation of stacking fault tetrahedra in collision cascades

K. 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 vacancy migration 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.

Appl. Phys. Lett. 74 (1999) 2720

Burrowing of Co nanoparticles on clean Cu and Ag surfaces

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

Metal nanoparticles can display a unique behavior when deposited on substrates with a significantly lower surface energy . Co nanoparticles in the 10 nm size regime burrow into clean Cu(100) and Ag(100) substrates when deposited at 600 K and also assume the substrate orientation. Deposition at room temperature fails to show either burrowing or reorientation. Crucial in understanding these results are the capillary forces and surface tension associated with a nanoparticle: they must be high enough to drive atoms away from underneath the cluster.

Phys. Rev. Lett. 83 (1999) 1163.

Coherent displacement of atoms during ion irradiation

K. NORDLUND, J. KEINONEN, M. GHALY & R. S. AVERBACK

Ion irradiation is a common technique of materials processing, as well as being relevant to the radiation damage incurred in nuclear reactors. Early models of the effects of ion irradiation typically assumed that particles undergo two-body elastic collisions, like billiard balls colliding in three dimensions. Later descriptions invoked such phenomena as localization of kinetic energy, thermalization and localized melting. In all these descriptions, the displacement of atoms is chaotic in that slight variations in the ion's trajectory produce completely different, unpredictable sets of atomic displacements. Here we report molecular-dynamics simulations of high-energy self-bombardment of copper and nickel, in which we see collective displacements of atoms. The high pressures developed in collision cascades centred well below the surface can cause a coherent displacement of thousands of atoms, over tens of atomic planes, in a shear-induced slip motion towards the surface. The mechanism leads to a significant increase in damage production near the surface, characterized by well-ordered islands of adsorbed atoms. Our findings suggest an explanation for some features of radiation damage, as well as for differences between ion and neutron irradiation.

Nature 398, 49 - 51 (1999) © Macmillan Publishers Ltd.

Preprint of publication not available due to copyright restrictions.

Electronic stopping of Silicon from a 3D Charge Distribution

J. Sillanpää, K. Nordlund and J. Keinonen

For an accurate prediction of penetration depths of energetic ions in materials, especially in crystal channels, a realistic electronic stopping model is needed. Usually simulation codes employing the binary collision approximation are used and the electronic stopping is calculated from either a spherically symmetric or uniform charge distribution. By using molecular dynamics and calculating the electronic stopping from a 3D charge distribution without using any free parameters, we obtain accurate range distributions on a realistic physical basis. Our electronic stopping model is based on the Brandt--Kitagawa (BK) [W. Brandt and M. Kitagawa, Phys. Rev. B 25 5631(1982)] theory, in which the electronic stopping of a heavy ion is the electronic stopping of a proton scaled by the square of the effective charge. We first test the model for hydrogen, to determine whether or not a basis exists for the scaling hypothesis, and then for other ions. The results are compared with experimental range profiles and show good agreement, much better than that achieved by using standard (nonlocal) electronic stopping models. Our model has a sound physical basis, no free parameters and can be used for all ion-target--combinations.

Phys. Rev. B 62 (2000) 3109.

Heat spike effects on ion beam mixing

K. Nordlund, J. Tarus, J. Keinonen, M. Ghaly and R. S. Averback

Although ion beam mixing has been studied intensively over the last 20 years, many questions about the fundamental mechanisms involved during mixing remain unresolved. We review here recent simulation and experimental work which provides answers to some of the lingering questions about mixing in elemental materials. The results make clear the specific role which thermodynamic material properties, the nature of atomic bonding and electron-phonon coupling can have on ion beam mixing. Agreement obtained by direct comparison of simulated and experimental mixing coefficients gives confidence in our results, indicating that the experimental mixing values in heavy metals can be understood predominantly on the basis of atomic motion in liquid-like zones, and that the role of the electron-phonon coupling on ion beam mixing is much smaller than previously thought.

ICACS-18 conference paper, Nucl. Instr. Meth. Phys. Res. B 164-165 (2000) 441

Surface Defects and Bulk Defect Migration Produced by Ion Bombardment of Si(001)

K. Kyuno, David G. Cahill, R. S. Averback, J. Tarus and K. Nordlund

Variable-temperature scanning tunneling microscopy is used to characterize surface defects created by 4.5 keV He ion bombardment of Si(001) at 80-294 K; surface defects are created directly by ion bombardment and by diffusion of bulk defects to the surface. The heights and areal densities of adatoms, dimers, and adatom clusters at 80 and 130 K are approximately independent of temperature and in reasonable agreement with molecular dynamics calculations of adatom production. At 180 K, the areal density of these surface features is enhanced by a factor of ~3. This experimental result is explained by the migration and surface trapping of bulk interstitials formed within ~2 nm of the surface.

Phys. Rev. Lett. 83 (1999) p. 4788-4791

Suppression of carbon erosion by hydrogen shielding during high-flux hydrogen bombardment

E. Salonen, K. Nordlund, J. Tarus, T. Ahlgren, J. Keinonen and C. H. Wu

The erosion of carbon by intensive hydrogen bombardment has been recently shown to decrease sharply at very high fluxes (~ 1019 ions/cm2s). This effect can not be explained by standard sputtering or erosion models, yet understanding it is central for selection of fusion reactor divertor materials, and formulation of sputtering models for high-flux conditions. Using molecular dynamics computer simulations we now show that the effect is due to the buildup of a high hydrogen content at the surface, leading to a shielding of carbon atoms by the hydrogen.

Phys. Rev. B (Rapid commun.) 60 (1999) 14005.

Diffuse X-ray streaks from stacking faults in Si analyzed by atomistic simulations

K. Nordlund, U. Beck, T. H. Metzger and J. R. Patel

Since extrinsic stacking faults can form during post-implantation annealing of Si, understanding their properties is important for reliable control of semiconductor manufacturing processes. We demonstrate how grazing-incidence X-ray scattering methods can be used as a non-destructive means for detecting extrinsic stacking faults in Si. Atomistic analysis of diffuse intensity streaks is used to determine the size of the faults, the minimum size at which the streak pattern in the scattering will be visible, and the magnitude of atomic displacements in the center of the stacking fault.

Appl. Phys. Lett. 76 (2000) 846

Atomistic simulation of diffuse X-ray scattering from defects in solids

K. Nordlund, P. Partyka, R. S. Averback, I. K. Robinson and P. Ehrhart

Diffuse X-ray scattering is a powerful means to study the structure of defects in crystalline solids. The traditional analysis of diffuse X-ray scattering experiments relies on analytical and numerical methods which are not well suited for studying complicated defect configurations. We present here an atomistic simulation method by which the diffuse X-ray scattering can be calculated for an arbitrary finite-sized defect in any material where reliable interatomic force models exist. We present results of the method for point defects, defect clusters and dislocations in semiconductors and metals, and show that surface effects on diffuse scattering, which might be important for the investigation of shallow implantation damage, will be negligible in most practical cases. We also compare the results with X-ray experiments on defects in semiconductors to demonstrate how the method can be used to understand complex damage configurations.

J. Appl. Phys. 88 (2000) 2278-.

Strain effects in Ge surface cascades

J. Tarus, K. Nordlund, J. Keinonen and R. S. Averback

By using classical molecular dynamics technique we have simulated the effects of 5 keV Xe atoms impinging on the strained Ge(100)2x1 surface. We found that large adatom islands are formed on top of the amorphous zones created by the cascades. We also found that lattice atoms around the molten zone move radially inwards and thus cause strain relief in the sample.

ICACS-18 conference paper, Nucl. Instr. Meth. Phys. Res. B 164-165 (2000) 482

Strain-induced Kirkendall mixing at semiconductor interfaces

K. Nordlund, J. Nord, J. Frantz and J. Keinonen

We use molecular dynamics computer simulations to study the damage production by collision cascades at Si/Ge and AlAs/GaAs and InAs/GaAs interfaces. For the arsenide systems we find that present interatomic potentials have troubles in describing even the basic elastic and melting properties. We report parameter refinements which give a significantly better description of these properties. Our results for collision cascades at strained semiconductor interfaces show a strong asymmetry in the distribution of vacancies and impurities produced at the interface. The effect is explained as a strain-induced effect analogous to the classical Kirkendall effect. We also show that although the chemical composition of compound semiconductors does not strongly affect the overall evolution of collision cascades, the composition may in some cases have a significant effect on the final distribution of defects.

Comput. Mater. Sci. (2000) 18 (2000) 283.

Sputtering of hydrocarbons by ion-induced breaking of chemical bonds

K. Nordlund, E. Salonen, J. Keinonen and C. H. Wu

While the physical sputtering of atoms caused by keV and MeV ions has been studied extensively both by MD simulation and experiments, the mechanisms leading to atom and molecule erosion at energies $\sim 1 - 100$~eV are not very well understood. We now describe how low-energy hydrogen ions can cause erosion of carbon atoms and hydrocarbon molecules by entering the region of a carbon-carbon chemical bond and hence breaking it, a process we call swift chemical sputtering''. In the particular case of hydrogen bombardment of carbon-based materials, we further show that this can lead to erosion yields far exceeding those expected for a physical sputtering process.

COSIRES 2000 conference paper, Nucl. Instr. Meth. Phys. Res. B 180 (2001) 77.

Bond-breaking mechanism of sputtering

E. Salonen, K. Nordlund, J. Keinonen and C. H. Wu

Energetic ions are known to be able to erode atoms from a solid even when a collisional mechanism can not transfer enough kinetic energy from the impinging ions to substrate atoms to overcome the surface binding energy. We now describe how low-energy ions can cause erosion of atoms and molecules by colliding with and breaking chemical bonds between the atoms. In the particular case of hydrogen bombardment of amorphous carbon networks, we further show that this can lead to erosion yields far exceeding those expected for a collisional process alone.

Europhys. Lett. 52 (2000) 504.

Properties of intrinsic point defects in irradiated Si

P. Partyka, K. Nordlund, R.S. Averback, I. M. Robinson and P. Ehrhart

The properties of self-interstitial defects in ion-irradiated Si were investigated using in situ grazing incidence diffuse x-ray scattering. Comparison of defect structures produced by 4.5 keV He ions with those produced by MeV electrons reveals that the migration of interstitials below 10 K is induced by electron excitation. Annealing studies showed, moreover, that the thermal migration of interstitials occurs between 80 K and 170 K, while vacancies become mobile at higher temperatures.

Phys. Rev. B 64 (2002) 235207

Swift chemical sputtering of amorphous hydrogenated carbon

E. Salonen, K. Nordlund, J. Keinonen and C. H. Wu

Ion bombardment of carbon materials is known to cause erosion with energies far below the threshold energy of physical sputtering, as well as at temperatures below the threshold of thermal desorption. Generally regarded as chemical sputtering, this effect and factors contributing to it, are not well understood. We use classical molecular dynamics simulations, capable of realistically describing bond formation and breaking, to study amorphous hydrogenated carbon surfaces under low-energy hydrogen bombardment. We present a swift chemical sputtering mechanism which can explain the experimentally observed characteristics of erosion by low-energy ion irradiation. We also show how the difference in the surface hydrogen concentration and carbon coordination fractions at various temperatures affect the carbon sputtering yield.

Phys. Rev. B. 63 (2001) 195415

Electronic stopping calculated using explicit phase shift factors

J. Sillanpää, J. Peltola, K. Nordlund, J. Keinonen and M. J. Puska

Predicting range profiles of low-energy (0.1 - 10 keV/amu) ions implanted in materials is a long-standing problem of considerable theoretical and practical interest. We combine here the best available method for treating the nuclear slowing down, namely a molecular dynamics range calculation method, with a method based on density-functional-theory to calculate electronic slowing down for each ion-target atom pair separately. Calculation of range profiles of technologically important dopants in Si shows that the method is of comparable accuracy to previous methods for B, P and As implantation of Si, and clearly more accurate for Al implantation of Si.

Phys. Rev. B. 63 (2001) 134113

Self-interstitial atoms at high temperatures in dense metals

K. Nordlund and R. S. Averback

It has long been clear that vacancies are usually the most abundant defect in equilibrium in most dense, close-packed metals. However, some important effects, such as the observed deviation of the diffusion coefficient from pure Arrhenius behaviour, can not be explained by invoking only a single vacancy. The most common explanation to the deviations is that also divacancies, and possibly even trivacancies, play a significant role. However, some evidence also indicate that interstitials could play a significant role. In this article, we review some recent work and our computer simulation results on this issue. Both the reviewed work and simulations point in the direction that the interstitial could be of major importance close to the melting temperature and during rapid heating. The results also strongly support the Granato model of liquids and solids.

Defect and Diffusion in Metals - Annual Retrospective 2000 ( invited review paper )

Preprint of publication not available, but see the PRL on the same topic.

Defect clustering during ion irradiation of GaAs: insight from molecular dynamics simulations

K. Nordlund, J. Peltola, J. Nord, J. Keinonen, and R. S. Averback

Defect formation in compound semiconductors such as GaAs under ion irradiation is not as well understood as in of Si and Ge. We show here how a combination of ion range calculations and molecular dynamics computer simulations can be used to predict the atomic-level damage structures produced by MeV ions. The results show that the majority of damage produced in GaAs both by low-energy self-recoils and 6 MeV He ions is in clusters, and that a clear majority of the isolated defects are interstitials. Implications of the results to suggested applications are also discussed.

J. Appl. Phys. 90 (2000) 1710.

Formation of ion irradiation-induced atomic-scale defects on walls of carbon nanotubes

A. V. Krasheninnikov, K. Nordlund, M. Sirviö and J. Keinonen

Recent experiments on irradiated carbon nanotubes evidence that ion bombardment gives rise to their amorphization and dramatic dimensional changes. Using an empirical potential along with molecular dynamics, we study structure and formation probabilities of atomic-scale defects produced by low-dose irradiation of nanotubes with Ar ions. For this, we simulate impact events over a wide energy range of incident ions. We show that the maximum damage production occurs for a bombarding ion energy of about 600 eV, and that the most common defects produced at all energies are vacancies, which at low temperatures are metastable but long-lived defects. Employing the tight-binding Green's function technique, we also calculate STM images of irradiated nanotubes. We demonstrate that irradiation-induced defects may be detected by STM and that isolated vacancies may look like bright spots in atomically-resolved STM images of irradiated nanotubes.

Phys. Rev. B. 63 (2001) 245405

Mechanism of electron-irradiation induced recrystallisation in Si

J. Frantz, J. Tarus, K. Nordlund and J. Keinonen

It has recently become clear that electron irradiation can recrystallise amorphous zones in semiconductors even at very low temperatures, and even when the electron beam energy is so low that it can not induce atomic displacements by ballistic collisions. We study the mechanism of this effect using classical molecular dynamics augmented with models describing the breaking of covalent bonds induced by electronic excitations. We show that the bond-breaking allows geometric rearrangement at the crystal-amorphous interface which can induce recrystallisation in silicon without any thermal activation.

Phys. Rev. B 64 (2001) 125313

Amorphization mechanism and defect structures in ion beam amorphized Si, Ge and GaAs

J. Nord, K. Nordlund and J. Keinonen

We are studying ion irradiation induced amorphization in Si, Ge and GaAs using molecular dynamics simulations. Although high-energy recoils produce defects and amorphous pockets, we show that low-energy recoils (about 5 - 10 eV) can lead to a significant component of athermal recrystallization of pre-existing damage. For typical experimental irradiation conditions this recrystallization is, however, not sufficient to fully recrystallize larger amorphous pockets, which grow and induce full amorphization. We also examine the coordination and topological defect structures in Si, Ge and GaAs observed in the simulations, and find that these structures can explain some experimentally observed features found in amorphous semiconductors. For irradiated amorphous GaAs, we suggest that long (about 2.8 Å) and weak Ga-Ga bonds, also present in pure Ga, are produced during irradiation.

Phys. Rev. B 65 (2002) 165329

Cratering-energy regimes: From linear collision cascades to heat spikes to macroscopic impacts

E. M. Bringa, K. Nordlund and J. Keinonen

Using classical molecular-dynamics simulations we examine the formation of craters during 0.4-100-keV Xe bombardment of Au. Our simulation results, and comparison with experiments and simulations of other groups, are used to examine to what extent analytical models can be used to predict the size and properties of craters. We do not obtain a fully predictive analytical model (with no fitting parameters) for the cratering probability, because of the difficulty in predicting the probability of cascades splitting into subcascades, and the relation of the heat spike lifetime and energy density. We do, however, demonstrate that the dependence of the crater size on the incident ion energy can be well understood qualitatively in terms of the lifetime of the heat spike and the cohesive energy of the material. We also show that a simple energy density criterion cannot be used to predict cratering in a wide ion energy range because of the important role of the heat spike lifetime in high-energy cascades. The cohesive energy dependence differs from that obtained for macroscopic cratering (observed, e.g., in astrophysics) because of the crucial role of melting in the development of heat spikes.

Phys. Rev. B, 64, 235426 (2001)

Stability of irradiation-induced point defects on walls of carbon nanotubes

A. V. Krasheninnikov and K. Nordlund

Using empirical-potential and tight-binding models, we study the structure and stability of atomic-scale irradiation-induced defects on walls of carbon nanotubes. Since atomic vacancies are the most prolific but metastable defects which appear under low-dose, low temperature ion irradiation, we model the temporal evolution of single vacancies and vacancy-related defects (which isolated vacancies can turn into) and calculate their lifetimes at various temperatures. We further simulate scanning-tunneling microscopy (STM) images of irradiated nanotubes with the defects, employing for this the tight-binding Green's function technique. Our simulations demonstrate that the defects live long enough at low temperatures to be detected by STM and that different defects manifest themselves in STM images in different ways, all of which makes it possible to detect and distinguish the defects experimentally.

J. Vac. Sci. Techn. 20 (2001) 728.

Diffuse x-ray scattering from 311 defects in Si

K. Nordlund

311 defects are extended, rod-like defects which play a central role in the processing of Si during integrated circuit manufacturing. Diffuse x-ray scattering techniques provide a non-destructive means to detect defects in solids. However, to date there has been no knowledge of what the x-ray scattering pattern from 311 defects looks like. Using a recently introduced fully atomistic modeling scheme, I calculate the diffuse x-ray scattering patterns from 311 defects. The results demonstrate how 311 defects can be detected, how the main varieties of 311 defect can be distinguished, and how both the defect width and length can be derived from the scattering.

J. Appl. Phys. 91 (2002) 2978

Effects of damage build-up in range profiles in crystalline Si; molecular dynamics simulations

J. Peltola, K. Nordlund and J. Keinonen

Molecular dynamics has been successful in predicting range profiles for ions implanted into crystalline materials in the dose regime where it can be approximated that changes in the sample structure do not affect the profiles. Many experimental distributions are, however, strongly dose-dependent due to the amorphization of the crystalline material. This has so far been taken into account only in some binary-collision-approximation calculations with a damage build-up model that depends on the probability for the amorphization to occur at a certain depth. We present here a fast molecular dynamics model for predicting range profiles of ions in crystalline Si. The model includes cumulative damage build-up, where the amorphization states are taken from molecular dynamics simulations of cascade damage. The method can be used to predict profiles for any material. We used silicon because of the large amount of experimental data available. No free parameters were used. Comparison of results to a wide range of experiments show very good agreement.

Nucl. Instr. Meth. Phys. Res. B. 195 (2002) 269-280

Production of defects in supported carbon nanotubes under ion irradiation

A. V. Krasheninnikov, K. Nordlund and J. Keinonen

Ion irradiation of individual carbon nanotubes deposited on substrates may be used for making metallic nanowires and studying effects of disorder on the electronic transport in low-dimensional systems. In order to understand the basic physical mechanisms of radiation damage production in supported nanotubes, we employ molecular dynamics and simulate ion impacts on nanotubes lying on different substrates, such as platinum and graphite. We show that the defect production depends on the type of the substrate and the damage is higher for metallic heavy-atom substrates than for light-atom substrates, since in the former case sputtered metal atoms and backscattered recoils produce extra damage in the nanotube. We further study the behavior of defects upon high-temperature annealing and demonstrate that, although ions may severely damage nanotubes in a local region, the nanotube carbon network has a unique ability to heal such a strong localized damage due to defect migration and dangling bond saturation. We also show that after annealing the residual damage in nanotubes is independent of the substrate type. We predict pinning of nanotubes to substrates through nanotube-substrate bonds which appear near irradiation-induced defects.

Phys. Rev. B 65 (2002) 164523.

Molecular dynamics simulations of CH3 sticking on carbon surfaces

P. Träskelin, E. Salonen, K. Nordlund, A. V. Krasheninnikov, J. Keinonen and C. H. Wu

Employing both quantum-mechanical and empirical force models, we use molecular dynamics simulations to obtain sticking cross-sections for CH3 radical chemisorption on unsaturated sites on carbon surfaces. Effects of the local atomic neighbourhood on the chemisorption were examined in order to compare the results with experiments. Our results show that the chemisorption of a CH3 radical onto a dangling bond is highly affected by the neighbourhood of the unsaturated carbon atom sites. Notably, sticking cross sections of totally bare dangling bond sites at the surface and sites partly shielded by neighbouring methyl groups were observed to differ by two orders of magnitude, (15.3 ± 1.7) Å2 and (0.2 ± 0.1) Å2, respectively.

J. Appl. Phys. 93, 1826 (2003)

Reduced chemical sputtering of carbon by silicon doping

E. Salonen, K. Nordlund, J. Keinonen, N. Runeberg and C. H. Wu

Doping is a widely used method to enhance the properties of materials. Despite the recently increased understanding of the mechanisms of chemical erosion by low-energy hydrogen ions, the effect of doping on these types of processes is still not well understood. We study the erosion of Si-doped (0 -- 30 at.\%) carbon under 20 eV deuterium irradiation using molecular dynamics simulations. We show that the chemical sputtering of carbon decreases with increasing Si concentration. The reasons for the reduced sputtering yield lie in the longer Si--C interaction lengths and efficient dynamic rebonding of hydrocarbon species.

J. Appl. Phys. 92 (2002) 2216

Molecular dynamics simulation method for calculating fluence-dependent range profiles

J. Peltola, K. Nordlund and J. Keinonen

Molecular dynamics has proven to be succesful in calculating range profiles for low energy (keV) ions implanted into crystalline materials. However, for high fluences the structure of the material changes during the impl antation process. The crystalline material becomes amorphized, which changes the range profiles. This damage build-up process has usually been taken into account with probabilities for changing the crystal structure during the simulation, and typically only BCA methods have been used. We present a fast MD method that simulates the damage build-up process in silicon, without bringing any free parameters to the simulation. Dama ge accumulation during the implantation is simulated by changing the material structure in front of path of the incoming ion. The amorphization l evel at each depth is proportional to the nuclear deposited energy in that depth region. The amorphization states are obtained from MD simulations of cascade dam age. Silicon was used as a target material because of the large amount of experimental data available. Comparison with the simulations show a go od agreement for a wide range of experiments.

Nucl. Instr. Meth. Phys. Res. B 202 (2003) 132.

Mechanism of Co nanocluster burrowing on (100) Cu surface

J. Frantz and K. Nordlund

Using classical molecular dynamics method we have studied the burrowing mechanis m of Co nanoclusters on a Cu substrate. We found that there are primarily two different mechanisms for the burrowing, depending on the configuration of the cluster after thermal deposition. Deposited clusters with an epitaxial configuration will burrow through vacancy migration a long the Co-Cu interface, and non-aligned clusters burrow through disordered motion of atoms. The re-alignment of the non-aligned c lusters was found to be due to a collective rotational movement of the whole cluster during the burrowing process. We discuss these res ults and perform a comparison with experimental and previously simulated results.

Phys. Rev. B 67 (2002) 075415

Heat spike effect on the straggling of cluster implants

J. Peltola and K. Nordlund

Recent experiments have shown that when gold atom clusters bombard copper with an energy of 10 keV/atom, the mean range of the gold atoms is independent of the cluster size, but the straggling (broadening) of the depth distribution is an increasing function of the cluster size. The same set of experiments did not show this effect when the target was amorphous Si. Using molecular dynamics computer simulations we have studied this effect by simulating Au cluster bombardment of Cu and Si with energies 1 -- 10 keV/atom. We found that in Cu, the mean range is not fully independent of the cluster size, but the dependence on cluster size is so weak it is hard to observe experimentally. On the other hand, we found a strong enhancement of the straggling in Cu, but not Si, in agreement with the experiments. By following the time dependence of the straggling we show that this is due to the massive heat spike effects which are present in Cu but not Si.

Phys. Rev. B 68 (2003) 035419

Atomic fingers, bridges and slingshots: formation of exotic surface structures during ion irradiation of heavy metals

K. Nordlund, J. Tarus, J. Keinonen, S. E. Donnelly and R. C. Birtcher

Recent experiments on ion irradiation of heavy metals such as gold and silver have shown that very unusual surface configurations can be produced by the irradiation. Typically, the surface damage has the shape of a crater, similar to those produced by meteorite impacts. The crater shapes are, however, often highly asymmetric and can show extended adatom ridges extending far from the crater well. Using molecular dynamics simulations we show how such exotic atom arrangements are produced. We describe atomic bridges over a crater and illustrate a slingshot-like effect which can propel atom clusters far from an impact position to produce isolated adatom islands.

Nucl. Instr. Meth. Phys. Res. B 206, 189 (2003).

Modelling of compound semiconductors: Analytical bond-order potential for gallium, nitrogen and gallium nitride

J. Nord, K. Albe, P. Erhart and K. Nordlund

An analytical bond-order potential for GaN is presented that describes a wide range of structural properties of GaN as well as bonding and structure of the pure constituents. For the systematic fit of the potential parameters reference data are taken from total energy calculations within the density functional theory if not available from experiments. Although long-range interactions are not explicitly included in the potential, the present model provides a good fit to different structural geometries including defects and high pressure phases of GaN.

Journal of Physics: Condensed Matter 15 (2003) 5649.

Structure and stability of non-molecular nitrogen at ambient pressure.

K. Nordlund, A. Krasheninnikov, N. Juslin, J. Nord and K. Albe

Non-molecular solid nitrogen, metastable even at zero pressure and low temperature, was recently manufactured by Eremets et al. [Nature 411 (2001) 173] and investigated with respect to its pressure stability. In this study we present analytical potential molecular dynamics simulations that allow reproducing all the stages of the experiment. The bonding structure and energetics of the N polymeric state obtained from the dynamical simulations are verified to be reasonable using a plane wave {\it ab initio} method. We predict that the metastable low-pressure phase has predominantly 3-folded bonded atoms in a disordered configuration. The energy accumulated in the metastable phase is about 1 eV/atom.

Europhysics Letters 65 (2004) 400

Heat spike effect on the straggling of cluster implants

J. Peltola and K. Nordlund

Recent experiments have shown that when gold atom clusters bombard copper with an energy of 10 keV/atom, the mean range of the gold atoms is independent of the cluster size, but the straggling (broadening) of the depth distribution is an increasing function of the cluster size. The same set of experiments did not show this effect when the target was amorphous Si. Using molecular dynamics computer simulations we have studied this effect by simulating Au cluster bombardment of Cu and Si with energies 1 -- 10 keV/atom. We found that in Cu, the mean range is not fully independent of the cluster size, but the dependence on cluster size is so weak it is hard to observe experimentally. On the other hand, we found a strong enhancement of the straggling in Cu, but not Si, in agreement with the experiments. By following the time dependence of the straggling we show that this is due to the massive heat spike effects which are present in Cu but not Si.

Phys. Rev. B 68, 035419 (2003).

A molecular dynamics study of damage accumulation in GaN during ion beam irradiation

J. Nord, K. Nordlund, and J. Keinonen

We have used molecular dynamics methods to study the accumulation of damage d uring ion beam irradiation of GaN. First we analyzed individual recoils between 200 eV and 10 keV. We found that the spatial average of the threshold displacement energy was high and much less damage was produced in GaN than in Si, Ge or GaAs cascades. Most of the damage was in isolated point defects or small clusters, which enhanc es the damage recombination probability. Ion beam amorphization was simulated by starting successive 400 eV or 5 keV reco ils in initially perfect crystal. The development of volume, energy and ring statist ics, as well as segregation of compounds was followed through the process. The simul ations show that the amorphization begins with single defects and formation of long weak Ga-Ga bonds in the distort ed lattice. We also observe that nitrogen gas is produced during prolonged irradiation, in agreement with experimental observations. We recognize two reasons for the high amorphization dose of GaN, the high threshold displacement energy (about a factor of 5) and in-cascade recombin ation (about a factor of 1-2).

Phys. Rev. B 68, 184104 (2003)

Evolution of Cu nanoclusters on Cu(100)

J. Frantz, M. Rusanen, K. Nordlund, and I. T. Koponen

Using molecular dynamics simulations combined with kinetic Monte Carlo methods w e have studied the evolution of copper nanoclusters on a copper (100) surface. We have developed a method for relaxing the clusters into a suitable configurati on for input into the kinetic Monte Carlo method using molecular dynamics. Using kinetic Monte Carlo methods we have simulated the evolution of clusters wi th sizes of 22 -- 2045 atoms at temperatures of 220 -- 1020 K. We found that the Cu clusters on the surface will be reduced to one monolayer if given enough time to relax, and that this process shows an Arrhenius behaviour. In this article we present the relaxation method we developed and our observatio ns for the evolution of the clusters.

Journal of Physics: Condensed Matter (2004), accepted for publication

Fragmentation of clusters sputtered from silver and gold

K. O. E. Henriksson, K. Nordlund, and J. Keinonen

Using molecular dynamics simulations and the Embedded Atom Method (EAM) potential we have investigated the sputtered atom clusters produced by 15 keV xenon impacts on silver and 20 keV xenon impacts on gold. Ejected clusters were simulated for long times, up to 0.01 - 1 microseconds, in order to investigate the fragmentation of nascent clusters. The size distributions of nascent and final clusters were calculated and fitted to an inverse power law, resulting in exponents close to 2 and 3, depending on the range of the cluster sizes used. These values are in agreement with other simulations and experiments. The results show that clusters are subject to a dramatic breakup, which makes the size of the largest sputtered cluster go down by a factor of 2-4. Despite this, the exponent in the power law does not change very much from the size distribution of nascent to that of final clusters. Considering the uncertainties, the exponent of the final size distribution is 1.0-1.7 times the exponent of the nascent size distribution.

Physical Review B, accepted for publication

Carbon nanotube mats and fibers with irradiation-improved mechanical characteristics: a theoretical model

J. A. Åström, A. V. Krasheninnikov, and K. Nordlund

We employ a theoretical model to calculate mechanical characteristics of macroscopic mats and fibers of single-walled carbon nanotubes. We further investigate irradiation-induced covalent bonds between nanotubes and their effects on the tensile strength of nanotube mats and fibers. We show that the stiffness and strength of the mats can be increased at least by an order of magnitude, and thus small-dose irradiation with energetic particles is a promising tool for making macroscopic nanotube materials with excellent mechanical characteristics.

Physical Review Letters 93, 215503 (2004)

Analytical interatomic potential for modeling non-equilibrium processes in the W--C--H system

N. Juslin, J. Nord, K. O. E. Henriksson, P. Träskelin, E. Salonen, K. Nordlund, P. Erhart and K. Albe

A reactive interatomic potential based on an analytic bond-order scheme is devel oped for the ternary system W-C-H. The model combines Brenner's hydrocarbon potential with parameter sets for W-W, W-C and W-H interactions and is adjusted to materials properties of reference structures with different local atomic coor dinations including tungsten carbide, W-H molecules as well as H dissolved in bulk W. The potential has been tested in various scenarios, like surface, defect, and me lting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon inte ractions with tungsten, that have a crucial role in fusion reactor plasma-walls. Furthermore, this study shows that the angular dependent bond-order scheme can b e extended to second-nearest neighbour interactions, which are relevant in bcc metals. Moreover, it provides a possibly general route for modeling metal carbides.

Journal of Applied Physics, accepted for publication (2005)

Two-band model of alpha-prime phase formation in Fe-Cr.

Pär Olsson, Janne~Wallenius, Christophe~Domain, Kai Nordlund and Lorenzo Malerba

We have developed a two-band model of Fe-Cr, fitted to properties of the ferro-magnetic alloy. Fitting many-body functionals to the calculated mixing enthalpy of the alloy and the mixed interstitial binding energy in iron, our potential reproduces changes in sign of the formation energy as function of Cr concentration. When applied in Kinetic Monte Carlo simulations, the potential correctly predicts decomposition of initially random Fe-Cr alloys into the $\alpha$-prime phase as function of Cr concentration.

Physical Review B 72 (2005) 214119.

Strings and interstitials in liquids, glasses and crystals

K. Nordlund, Y. Ashkenazy, R. S. Averback and A. V. Granato

Recent work has shown that atom motion in liquids is not completely homogeneous, but that stringlike cooperative motion can be used to explain several properties of liquids. We now show that with increasing concentration the properties of interstitial atoms in crystalline materials smoothly approach and eventually completely match the properties of the string atoms in liquids. In terms of the interstitialcy theory of liquids and solids, the strings are direct manifestations of interstitials.

Europhys. Lett. 71 (2005) 625.

Swift chemical sputtering of covalently bonded materials

K. Nordlund, E. Salonen, A. V. Krasheninnikov and J. Keinonen

Numerous experiments have shown that low-energy H ions and neutrals can erode amorphous carbon at ion energies of 1-10 eV, where physical sputtering is impossible, but at erosion rates which are clearly higher than those caused by thermal ions. In this paper we will first review our computer simulation work providing an atom-level mechanism for how this erosion occurs, and then present some new results for H and He bombardment of tungsten carbide and amorphous hydrogenated silicon, which indicate the mechanism can be of importance in a wide range of covalently bonded materials. We also discuss how the presented mechanism relates to previously described abstraction and etching mechanisms.

Pure and Applied Chemistry 78 (2005) 1203-1212

Major elemental assymetry and recombination effects in irradiated WC

C. Björkas, K. Vörtler, and K. Nordlund

We study the initial state of irradiation damage in WC, an alloy with a large mass difference between the constituents, using molecular dynamics computer simulations. We find that a vast majority of the resulting isolated defects are carbon. Moreover, an in-cascade defect recombination effect even more pronounced than that known previously to occur in metals is observed. Both effects are shown to be related to the high formation energy of W defects.

Phys. Rev. B (Rapid comm.) 74 (2006) 140103

Atomistic simulation of the transition from atomistic to macroscopic cratering

J. Samela and K. Nordlund

Using large-scale atomistic simulations, we show that the macroscopic cratering behavior emerges for projectile impacts on Au at projectile sizes between 1,000--10,0000 Au atoms at impact velocities comparable to typical meteoroid velocities. In this size regime, we detect the compression of material in Au nanoparticle impacts similar to that observed for hypervelocity macroscopic impacts. The simulated crater volumes agree with the values calculated using the macroscopic crater size scaling law, in spite of a downwards extrapolation over more than fifteen orders of magnitude in terms of the impactor volume. The result demonstrate that atomistic simulations can be used as a tool to understand the strength properties of materials in cases where only continuum models have been possible before.

Phys. Rev. Lett. 101 (2008) 027601, and cover of issue 2.

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