Ion Beam Mixing: Amorphous, Crystalline, and Quasicrystalline Phases

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 amorphous 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-noble-metal alloys, demonstrate nearly all the features associated with ion-induced phase formation. Further, Al-rich refractory metal alloys form quasicrystalline icosahe-dral alloys. Ion-beam mixing results parallel those of splat-quenching, the technique first used to produce the fivefold symmetric structure.