Nonequilibrium Processing of Materials Using Beams of Particles and Clusters

M. Ghaly, K. Nordlund, H. Zhu and R. S. Averback, University of Illinois at Urbana-Champaign

This figure illustrates how a Cu cluster reorganizes as it hits a Cu surface. Before impact, the cluster is a perfect sphere; after the impact, the lower interface has reorganized to minimize the surface and interface energy. Note that the lowest surface has an almost perfect crystal structure, corresponding to the structure of the substrate.

Research Objectives

This research aims at clarifying how the properties of materials are changed during ion irradiation or cluster bombardment. This kind of non-equilibrium processing enables synthesis of materials which in many cases are difficult or impossible to produce by more conventional means. Using computer simulations in combination with experiments, we aim to obtain fundamental knowledge that enables production of new kinds of materials and improvements in the properties of existing materials.

Computational Approach

Phenomena of interest include sintering of nanoparticles, interactions of nanoparticles with surfaces, damage produced by shallow implants in semiconductors, electron-phonon coupling, and the role of surfaces on damage production in solids. We primarily use classical molecular dynamics simulations to study these phenomena on an atomistic level. The NERSC T3E enables direct simulation of experiments involving millions of atoms, corresponding to system sizes of tens of nanometers. This scale is sufficient to simulate collision cascades produced by irradiation in most materials.

Accomplishments

• We simulated the dynamics of energetic impacts of clusters on surfaces where the collision energy is negligible (see figure).
• We employed two types of computer simulations to provide a clearer picture of defect production in irradiated Si.
• We developed a combined experimental-simulation method to solve the problem of electron-phonon coupling. The results showed that in Ni, Pd and Pt, where coupling has traditionally been believed to be the strongest, it is in fact negligible, roughly an order of magnitude weaker than the most widely used models predict.

• We examined collision cascades between 30 and 100 keV in a variety of materials, including both metals and semiconductors, and looked at events where energetic recoils are initiated in the bulk and by ion implantation. The results showed that a variety of special damage mechanisms are possible in the presence of a surface.

Significance

Understanding the damage structures in silicon clarifies the mechanisms leading to amorphization of silicon during semiconductor processing. This knowledge may enable better control of implant processing in the industry. Understanding the large effect played by surfaces in metal irradiation may lead to new methods of modifying the properties of metal surfaces and thin films. Comprehending the nanocluster interactions of surfaces may enable the manufacture of nanocrystalline materials and surface coatings out of exotic combinations of materials such as normally immiscible metals.

Publications

K. Nordlund, M. Ghaly, R. S. Averback, M. Caturla, T. Diaz de la Rubia, and J. Tarus, "Defect production in collision cascades in elemental semiconductors and FCC metals," Phys. Rev. B 57, 7556 (1998).

K. Nordlund, L. Wei, Y. Zhong, and R. S. Averback, "Role of electron-phonon coupling on collision cascade development in Ni, Pd and Pt," Phys. Rev. B (Rapid comm.) 57, 13965 (1998).

M. Ghaly, K. Nordlund, and R. S. Averback, "Molecular dynamics investigations of surface damage produced by keV ion bombardment of solids," Phil. Mag. A 1998, in press.