Ion bombardment and implantation

The scope and importance of ion bombardment studies has in recent years been considerably extended by the realization that the surface properties of materials, and in particular the semiconductors, may be usefully modified by means of the implantation of suitable ions in the form of an accelerated beam. Whereas earlier studies of radiation damage and sputtering were confined mostly to low ion energies, interest has now grown in the behaviour at higher energies, up to the Mev region. Often the bombarded specimens are crystalline, and the so-called channelling phenomenon is important in determining the range distribution of implanted material and the degree of lattice disruption created.

After a summary of the various commonly used approximations to the interatomic potential, the energy loss mechanisms of ions in solids are discussed and experimental results are compared with theoretical predictions for both the amorphous and crystalline cases. In the next section, attention is focused upon the radiation damage produced by ion bombardment of crystalline targets, and the migration and clustering of radiation-induced defects is discussed.

The sputtering process is described and recent work is summarized. In this, and the following section on secondary electron emission, the behaviour of single-crystal targets has proved important in understanding the physical nature of the process. Modern developments in ultra-high vacuum technology have aided both these studies.

A lengthy account is given, in the final section, of the physical processes involved in ion implantation. The advantages of this method of doping semiconductor crystals are discussed, as well as some attendant problems. Examples of the practical application which has already been made are given, and the equipment required for such work is described. A certain degree of speculation is hazarded regarding the future prospects of the ion-implantation technique in research and industrial applications.