Ion Beam Mixing: Amorphous, Crystalline, and Quasicrystalline Phases
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Ion Beam Mixing: Amorphous, Crystalline, and Quasicrystalline Phases
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D.A. Lilien feld, LS. Hung, and J.W. Mayer Introduction
In the last quarter of a century, modification of the near-surface region of materials has become of major technological importance. The principal surface modification technique utilized in integrated circuit technology is ion implantation, a technique which has more recently been applied in the metal-processing industry as well. The very high doses required for applications such as increasing the hardness of steel or forming buried oxide layers in silicon have pushed ion implantation to its limits. Ion beam mixing, the intermixing of surface layers by the penetration of energetic ions through them, was developed to overcome these limits. Additionally, ion beam mixing has been able to produce new phases, amorphous and crystalline, which have technologically and scientifically interesting properties. Ion beam mixing was studied extensively in silicide forming systems, due partly to applications to electrical contacts for silicon devices. In intermetallic alloy systems, research has concentrated on determining the interplay between the formation of a m o r p h o u s and crystalline structures and that between equilibrium and metastable phases. Although over 50 alloy systems have been studied, this article will concentrate on the Al-based alloys. These alloys, particularly the near-noblemetal alloys, demonstrate nearly all the features associated with ion-induced phase formation. Further, Al-rich refractory metal alloys form quasicrystalline icosahedral alloys. Ion-beam mixing results parallel those of splat-quenching, the technique first used to produce the fivefold symmetric structure. The topics in this article cover: (1) an introduction to ion beam mixing from a historical perspective; (2) discussion of the formation of metastable solid solutions by ion beam mixing of multilayered thin films; (3) comparison of thermal annealing and ion beam mixing results in the metal-Al; (4) discussion of the formation of amorphous and crystalline phases by ion beam mixing using the Al-Pd and Al-Ni systems as examples; and (5) discussion of the formation of icosahedral and ordered cubic structures in the Al-transition elements systems, which demonstrates the diversity of the metastable phases that can be formed by ion beam mixing. The research in ion beam mixing is quite extensive and we do not attempt to review the field thoroughly. We will concentrate on our own recent work in aluminum-
based alloys to illustrate applications of ion beam mixing. The general references at the end of the reference section provide a good introduction to this diverse field. Ion Beam Mixing —A Historical Perspective
This section will discuss the mechanism of ion beam mixing in which energetic ions cause the intermixing of layers during ion implantation, limitations of ion implantation in achieving the necessary high doses for strengthening steels or producing buried oxide layers, and sputtering effects, which remove m
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