Ion Beam Mixing in Insulator Substrates
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adhesion are not understood, and models that invoke formation of one atom layer thick compounds to changes in surface energy due to defect formation and/or local composition variation have been proposed. Studies of ion beam mixing in metal/metal and metal/silicon bi- and multi-layer configurations have clarified many details of the process and detailed models for the atomic mechanisms and the influence of chemical factors have been proposed. However, the manner in which the chemical factors influence the process is still a manner of discussion. Ion-solid interactions in insulators are not well-understood in spite of many studies in recent years. This situation arises both from the complexity of an ion beam interacting with a multicomponent target and the restrictions placed on the defect population and motion by the chemical bonding of the target. Insulator substrates generally are compounds that contain at least two chemical species having different atomic masses and valence states and arranged on distinct sublattices. Displacement energies and efficiencies may be different for the different species, affecting the number and kinds of defects generated. For example, in sapphire (single crystal A12 0 3 ) the displacement energy, Ed, for Al is 16 or 24 eV and Ed for 0 is 76-78 eV [1,2]. There appears to be little attention given to the kinematics of defect production in polyatomic materials since the work of Parkin and Coulter [3]. The substrate becomes metal-rich near the interface of metal/insulator couples due to the nature of ballistic processes. Chemical bond-type and electrostatic neutrality requirements may be the dominant factors in determining the final disposition of implanted or recoiled "impurity" ions. Deviations from stoichiometry and solubility of impurities in oxides such as MgO and A1203 are below the limits of detection for most techniques. The addition of foreign atoms (ions) requires 83 Mat. Res. Soc. Symp. Proc. Vol. 396 0 1996 Materials Research Society
some type of charge compensation such as formation of F-type centers or agglomeration of the atoms into second phases. Defect Structure of Ion Irradiated Insulators Some details about the types of impurity-defect configurations have been determined. Many of the proposed mechanisms for ion beam mixing involve the migration of defects and foreign ions, and will be sensitive to the nature of these complexes. Implantation of MgO with light gas ions (Ne, He, H) appears to produce defects only in the anion lattice (F-, F+-, F2-center) [4]. Both F-type and V-type centers are produced by implantation of alkali metal ions, Fe, In and Au [5]. A study using optical absorption and M3ssbauer techniques showed that the implanted Fe was distributed among the valence states Fe 3+, Fe2 + (in two crystallographic sites), and Fe0 [6]. The metallic colloids (FeO) were less than 2 nm in size. The relative amounts of the valence states varied with implantation fluence. There appeared to be very specific local Mg-Fe-defect configurations associated with each charge s
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