CHEMICAL AND COMPOSITIONAL CHANGES INDUCED BY ION-IMPLANTATION IN SIC AND RESULTING HYDROGEN PERMEATION PROPERTIES

Crystalline SiC films are of technological interest because of potential applications in optoelectronics, in high-temperature semiconducting devices, as a fusion first-wall material (coated onto steel), and as a material with high wear resistance (again coated onto steel). Much of the usefulness of crystalline SiC is preserved in the amorphous state, a-SiC. Many investigations have been directed towards the study of the structure and mechanical properties of this kind of amorphous film; nevertheless, little is known about bombardment-induced chemical and compositional changes. Here we present an overview of observations and interpretations relevant to bombardment-induced structural and compositional changes in SiC and other carbides. Particular emphasis is placed on N+-implantation, which leads to SiC(x)N(y) compounds with C being gradually substituted with N. It is shown that N-implantation strongly enhances the a-SiC-stainless steel adhesion by promoting the formation of new chemical bonds at the ceramic-metal interface. By examining data from the literature, we find more or less clear evidence for a number of radiation-induced transport processes. These include (a) Gibbsian segregation, (b) vaporization, (c) long-range forces such as that provided by unbalanced charges, (d) defect-induced transport, and (c) preferential loss of C from the surface. These phenomena help to explain composition changes variously at or beneath the surface of SiC. When low to medium energy hydrogen collides with a carbide surface both chemical and transport-related processes occur. In the future generation of fusion reactors very high levels of tritium could be present in either a trapped or mobile form in walls and other internal parts. Thus tritium permeation even through thick wall materials may present problems. The permeation of hydrogen through a-SiC films is discussed showing that it can be strongly affected by surface changes.