Ion-beam mixing and the formation of amorphous alloys

Abstract

Ion beam interactions with solids have been of significant interest to both academic and industrial researchers for more than half a century. Looking back into the history of its development, one finds prominent names such as Bohr, Fermi, and Bethe, closely associated with the early development of this area of scientific research. Their efforts in understanding the interatomic potential of ions and atoms have led to the knowledge of ion penetration depth and energy loss in solids. It was also one of the early successful applications of the newly born quantum mechanics. The subsequent development of the range theory by Lindhard, Scharff, and Schioff in the early sixties (LSS theory) enables one to calculate the ion penetration depth in elemental solids to even greater accuracy and provides the basis for the realization of ion implantation technology. Swelling of reactor materials, caused by ion irradiation, was first observed in austenitic steel in 1967 by a group of British scientists. Such radiation damage drew unprecedented attention of people from both academia and industry to the problem of ion-solid interactions. Irradiation-induced structural and phase changes in materials have since become a major part of the active research in ion beam interactions with solids. Ion implantation, which originated in the early 1960s, revolutionized the microelectronics industry. Because of the development of the range theory, ion implantation offered precise control over the number and depth of dopant atoms in semiconductor materials, making possible the miniaturization of electronic devices. Meanwhile, various ion beam analysis techniques, such as RBS (Rutherford Backscattering Spectrometry), developed rapidly and were applied to study problems ranging from Moon rocks to electronic devices. Around 1972, while studying ion implantation in Si through an Al overlayer, D. H. Lee, O. J. Marsh, and R. R. Hart of the Hughes Research Laboratories observed coloration of the surface layer. Using RBS and optical reflectance measurements, they determined that the coloration was due to the migration of Si to the surface of the sample under the influence of ion bombardment. In another case, they observed Pt2Si formation due to mixing of the Si and the Pt overlayer caused by ion implantation. Soon it was found that metastable phases as well as equilibrium phases could be obtained by this type of ion beam-induced mixing phenomena, more commonly called ion mixing processes. Today, ion implantation has found further applications in areas such as corrosion resistance, reduction of wear and friction, and improvement of adhesion. Ion mixing has been viewed as a powerful extension of the traditional ion implantation technology. It provides new challenges to fundamental research in ion-solid interactions and offers new promises to further industrial applications. Two issues are considered of fundamental importance and are treated in this thesis: I. Mechanisms of ion-beam-induced mixing in solids. II. The formation of amorphous alloys. In Part I, several aspects of the mixing mechanisms, such as the influence of chemical driving force, the influence of cohesive energy, a newly developed phenomenological model of ion mixing, and a correlation between the cohesive energy and the onset of radiation-enhanced diffusion will be discussed. Also in this part, the evolution of collisional cascades will be studied by using a fractal geometry approach. In Part II, mechanisms of amorphous alloy formation are discussed. Comparisons between amorphous alloy formation by ion mixing and by solid-state reaction illustrate various aspects of glass formation.