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C.W. White, Guest Editor Ion beams are used extensively in materials research for processing and synthesis as well as for characterization. In the last few years, enormous advances have been made regarding the use of ion beams for processing or synthesis, and this issue of the MRS BULLETIN will review some of those advances. (The use of ion beams for materials characterization will be the subject of a future issue of the BULLETIN.) The areas covered in this issue are ion implantation, ion beam mixing, ion-assisted deposition, and direct ion beam deposition. For each area, recognized experts in the field prepared overview articles that should be very interesting to those who are not active in the field, and that should be useful to other experts in the field. The first large-scale use of ion beams for materials modification took place in the s e m i c o n d u c t o r i n d u s t r y m o r e t h a n 20 years ago when ion implantation began to be used to dope the near-surface region of silicon with Group III or Group V dopants. The use of ion implantation in the semiconductor industry has undergone explosive growth, and today almost all electronic devices are fabricated utilizing at lest one ion implantation step. In addition to the semiconductor area, research is being carried out using ion implantation in a multitude of other areas which include ceramics, metals and alloys, insulators, etc. The article on "Ion Implantation" by S.T. Picraux and P.S. Peercy provides an excellent overview of current research activities involving ion implantation of a wide spectrum of materials. Ion beam mixing is the name given to a technique which uses ion irradiation to cause the intermixing of surface layers. With this technique, in many systems it is possible to achieve impurity concentrations in the near surface that are significantly h i g h e r t h a n t h o s e t h a t can be achieved by direct ion implantation of the impurity. The article by D.A. Lilienfeld, L.S. Hung, and J.W. Mayer on ion beam mixing provides an excellent overview of the basic ideas which are important for ion beam mixing, and it discusses in some detail phase formation and stability during ion beam m i x i n g of a l u m i n u m - b a s e d alloys. In the last few years, it has been shown that the properties of deposited films can be strongly modified by concurrent ion bombardment during film deposition. This technique is referred to as ion-assisted deposition, and the article by S.M. Rossnagel and J.J. Cuomo provides an excellent summary of some of the structural and chemical c h a n g e s t h a t can be a c h i e v e d in deposited films by using simultaneous ion

bombardment during film deposition. The last article in this series describes direct ion beam deposition, a very recent ion beam technique which can be used to fabricate artificially structured materials and thin films of isotopic purity. With this technique, a mass-analyzed ion beam is decelerated to low energies (a few tens of eV) and then allowed to deposit onto a substrate. In this manner