Charbel S. Madi

Molecular dynamics of single-particle impacts predicts phase diagrams for large scale pattern formation

Energetic particle irradiation can cause surface ultra-smoothening, self-organized nanoscale pattern formation or degradation of the structural integrity of nuclear reactor components. A fundamental understanding of the mechanisms governing the selection among these outcomes has been elusive. Here we predict the mechanism governing the transition from pattern formation to flatness using only parameter-free molecular dynamics simulations of single-ion impacts as input into a multiscale analysis, obtaining good agreement with experiment. Our results overturn the paradigm attributing these phenomena to the removal of target atoms via sputter erosion: the mechanism dominating both stability and instability is the impact-induced redistribution of target atoms that are not sputtered away, with erosive effects being essentially irrelevant. We discuss the potential implications for the formation of a mysterious nanoscale topography, leading to surface degradation, of tungsten plasma-facing fusion reactor walls. Consideration of impact-induced redistribution processes may lead to a new design criterion for stability under irradiation.

Linear Stability and Instability Patterns in Ion-Sputtered Silicon

We study the patterns formed on Ar(+) ion-sputtered Si surfaces at room temperature as a function of the control parameters ion energy and incidence angle. We observe the sensitivity of pattern formation to artifacts such as surface contamination and report the procedures we developed to control them. We identify regions in control parameter space where holes, parallel mode ripples and perpendicular mode ripples form, and identify a region where the flat surface is stable. In the vicinity of the boundaries between the stable and pattern-forming regions, called bifurcations, we follow the time dependence from exponential amplification to saturation and examine the amplification rate and the wavelength in the exponential amplification regime. The resulting power laws are consistent with the theory of nonequilibrium pattern formation for a type I (constant wavelength) bifurcation at low angles and for a type II (diverging wavelength) bifurcation at high angles. We discuss the failure of all sputter rippling models to adequately describe these aspects of the simplest experimental system studied, consisting of an elemental, isotropic amorphous surface in the simplest evolution regime of linear stability.

Nanoscale topographic pattern formation on Kr+-bombarded germanium surfaces

The nanoscale pattern formation of Ge surfaces uniformly irradiated by Kr+ ions was studied in a low-contamination environment at ion energies of 250 and 500 eV and at angles of 0° through 80°. The authors present a phase diagram of domains of pattern formation occurring as these two control parameters are varied. The results are insensitive to ion energy over the range covered by the experiments. Flat surfaces are stable from normal incidence up to an incidence angle of θ = 55° from normal. At higher angles, the surface is linearly unstable to the formation of parallel-mode ripples, in which the wave vector is parallel to the projection of the ion beam on the surface. For θ ≥ 75° the authors observe perpendicular-mode ripples, in which the wave vector is perpendicular to the ion beam. This behavior is qualitatively similar to those of Madi et al. for Ar+-irradiated Si but is inconsistent with those of Ziberi et al. for Kr+-irradiated Ge. The existence of a window of stability is qualitatively inconsisten...

Ion-sculpting of nanopores in amorphous metals, semiconductors, and insulators

We report the closure of nanopores to single-digit nanometer dimensions by ion sculpting in a range of amorphous materials including insulators (SiO2 and SiN), semiconductors (a-Si), and metallic glasses (Pd80Si20)—the building blocks of a single-digit nanometer electronic device. Ion irradiation of nanopores in crystalline materials (Pt and Ag) does not cause nanopore closure. Ion irradiation of c-Si pores below 100 °C and above 600 °C, straddling the amorphous-crystalline dynamic transition temperature, yields closure at the lower temperature but no mass transport at the higher temperature. Ion beam nanosculpting appears to be restricted to materials that either are or become amorphous during ion irradiation.

Excimer Laser Processing of Novel Materials for Optoelectronic and Spintronic Applications

The interaction of the highly energetic pulsed excimer laser beam with a target material induces non-equilibrium physico-chemical processes which could be harnessed to synthesize a variety of novel and technologically attractive materials that are difficult to grow using more conventional thin film deposition techniques. In this paper, recent advances on two excimer laser based techniques that we have used in the processing of thin films and surfaces will be presented. First, we demonstrate the synthesis, by Pulsed Laser Melting (PLM), of silicon supersaturated with sulfur at concentrations several orders of magnitude greater than the solubility limit of silicon alloys, with strong sub-bandgap optical absorption. This material has potential applications in the fabrication of Si-based opto-electronic devices. Second, the capability of Remote Plasma Pulsed Laser Deposition (RP-PLD) in synthesizing the meta-stable half-metallic CrO2 compound that is of great interest in the field of spintronics was assessed. Infra-Red spectroscopy and Magnetic Force Microscopy indicate that the use of the remote plasma is beneficial to the formation of the CrO2 phase, at a deposition pressure of 30 mTorr and for deposition temperature below 350 °C. Atomic Force Microscopy and Magnetic Force Microscopy studies respectively show that films containing the CrO2 phase have significantly different surface topography and magnetic characteristics from those in which the Cr2O3 phase is dominant.