Properties and applications of helium implanted gold

Abstract

Previous studies have shown that helium implantation of metals can result in highly cavitated structures in a narrow band near the surface. At low doses, an ordered gas bubble superlattice appears to be a universal response to helium implantation. Typically the bubbles are ~ 2nm in diameter and spaced ~ 6nm apart. The superlattice has the same structure and symmetry as the host metal. The highly cavitated layers offer the potential for enhanced ingress of chosen dopants. This potential has already been demonstrated for two cases, the filling of cavities in silicon with gold, and the oxidation of implanted titanium and vanadium. The objective of this project was to assess the feasibility of forming an ordered cavity structure in gold by helium implantation, and then filling the cavities with cobalt, a metal with a large magnetic dipole moment. Evidence that cobalt was precipitating out into the cavities was obtained, suggesting that it may be possible to produce cobalt-filled, ordered cavities in gold. A material containing an ordered array of ferromagnetic nanoparticles set in an inert, non-ferromagnetic material like gold has potential as a high capacity magnetic data storage medium for applications such as hard disk drives. The ultimate goal is an ordered array of 2nm diameter ferromagnetic particles in gold or copper. At this scale, pure cobalt will not suffice as the magnetic material. An alloy more resistant to the superparamagnetic effect, such as CO5Sm, will be required. Previous work on ion implantation is reviewed, and the results of new work on the cavity structures and electronic properties of helium implanted gold is presented. TEM investigation of helium implanted gold confirmed the formation of an fcc superlattice of small helium bubbles, and subsequent evolution of the cavity structure with implanted helium dose, as seen in previous studies of gold and other fcc metals. Thin gold films have been prepared by vapour deposition onto glass substrates. Helium implanted thin film samples were used for optical and electrical resistance experiments. Spectrometry of helium implanted gold in the visible region of the spectrum showed spreading of the interband transition, a drop in reflectivity below the interband transition, and an increase in the reflectivity at short wavelengths in samples implanted with a high dose of helium. The spreading of the interband transition is attributed to compression and disruption of the gold matrix, changing the d-band and conduction band energy levels. Reduced reflectivity below the interband transition is attributed to a combination of absorption by surface plasmons and a reduction in the relaxation time of conduction electrons. The anomalous reflectivity at short wave-lengths is attributed to a lower density of d-electrons in helium implanted gold. The infrared spectra obtained from helium implanted gold were not reproducible, and no indications were found that helium implantation had altered the reflectivity of the gold in the mid-infrared region. Improvements on the equipment and techniques used here will be needed for further infrared work. The results of new experiments on the electrical resistance of thin films during ion implantation are reported. With the refinements made to the measuring system, the performance and stability of the particle accelerator is now the limiting factor in these measurements. The resistance of a thin film gold sample can be measured during helium implantation, and the component of the sample resistance caused by the implantation can be isolated from thermal components.