Summary of Research Interests for J.R. Sabin

Last updated 01.01.98

The characteristics of the deposition of energy in matter by swift, massive particles is of interest in areas ranging from design of inertial confinement fusion devices to radiation therapy. The understanding and prediction of the processes involved in such energy deposition provide the unifying theme of my research. The research program has three distinct but interrelated parts dealing with:

Each of these areas is summarized below.

Atomic and molecular stopping [collaborator: Jens Oddershede, Odense University]

It is the aim of this project to develop a scheme for calculation o"f the energy deposition of swift ions in molecules. The attack is two pronged. One prong, direct calculation and integration of the appropriate generalized oscillator strengths (GOS's) using the polarization propagator formalism, is presently feasible only for small molecules, but in principle provides an accurate method to calculate stopping. This scheme will be developed for study of small molecules, such as water, to provide a normative comparison for the large molecule scheme, and to gain insight on how the large molecule scheme can be improved. The second prong, a scheme based on the additivity and transferability of properties of chemical bonds, is not as accurate, but is not limited by the size of target molecules, and thus admits calculation of stopping properties for target molecules of a size big enough to be biologically significant. The large molecule part of this work will initially be done in the context of the kinetic theory of stopping, but it is hoped that experience in the small molecule regime will lead to a GOS based large molecule scheme as well. Thus with the two, eventually related methods, stopping by molecules of all sizes can be considered.

Among the problems currently under study are:

Stopping in ultra-thin films [collaborators: Sam Trickey, University of Florida, and Peter Apell, Chalmers University ]

One field of advanced materials research that becomes critically important in the context of endeavors such as the fabrication of microelectronic components and their deployment in space, is the interaction of these materials with penetrating radiation. In particular, swift, massive particles impinging on materials can both create impurity distributions in materials which are essential to device function, yet destroy the devices themselves, depending on the incident particles, their energies, and the target systems. It thus is essential to understand the energy deposition process so that damage can be prevented in extant devices, and proper dopants can be introduced in fabrication. We have embarked on a theoretical research program to attack this problem.

The research has several facets. First, we must understand the materials which will be the targets for the radiation. This understanding must relate structure and electronic properties in a reliable, predictive, computationally tractable way for real materials systems. Simultaneously, we must understand the energy deposition process in terms of a systematic theoretical treatment which is computationally implementable. The two bodies of theory must be mutually compatible, not rooted in fundamentally irreconcilable models.

Because forefront microelectronics devices depend upon the detailed quantum mechanical behavior of the electrons, the electronic structure of the materials systems is fundamental. The problem becomes more acute as the systems grow thinner, which is precisely the limit toward which the drive for speed in microelectronics is driving the multiple layers in devices. Thus, we use full potential, all-electron, local-spin density methods which do not impose any arbitrary symmetries on the system (e.g., supercells such as are sometimes used to mimic laminar interfaces and/or surfaces). The object here is to model the thin films structures encountered in microelectronics at a high enough level to describe the effects energy deposition without encountering artifacts introduced by the description of the material.

The problem that concerns us is to understand the energy deposition process of swift particles in ultra-thin films. The two standard approaches to the problem of energy deposition are those associated with Lindhard, which is based on a dielectric description of the weakly inhomogeneous electron gas, and with Bethe, which is an atomic description. Neither of these is sufficient in the regime which we wish to characterize, namely thin material films. The atomic interactions make the Bethe description inappropriate, while the atomic structure of a film and its two dimensional periodicity makes the Lindhard description prohibitively formidable (no truly first principles calculation of the frequency and wave vector-dependent dielectric function of a real material has ever been performed). Consequently an intermediate way must be found that admits a proper description of thin layered targets. Our generalization of the Kinetic Theory of Stopping (KTS ) in connection with Lindhard's Local Plasma Approximation (LPA) provides such a description. The description, although involving several approximations, is internally consistent as all phases of the calculation are based on the target density.

Among current problems under study are:

Trajectory dependent stopping of swift ions under channeling conditions [collaborator: Sam Trickey, University of Florida]

Recent advances in technology have made experiments possible that depend on the details of the trajectories of swift ions under axial channeling conditions. Such experiments probe, for example, the location of interstitial impurities in solids. Recently we have implemented a semiclassical treatment of channeling, where we time step integrate the classical force law for the particle trajectory. The force consists of the gradient of the electrostatic potential plus a dissipation part, the electronic stopping power. The first of these comes from an all electron, full potential density functional theory (LAPW) calculation, while we use the stopping function of Echenique et al. for the low velocity stopping. It should be noted that this procedure uses a proper, quantum mechanical potential with all atoms explicitly considered, and does not assume charge strings, statistical equilibrium, nor makes any of the other assumption usual to this sort of calculation. (That is not to say assumptions are not made!)

So far, the only test case that has been run is for a 0.15 keV proton along the (1,1,0) channel in Ge, but the results look very encouraging.

We intend to study:

Some recent representative publications dealing with energy deposition are:

Please send email to request reprints.