Ion-beam-induced epitaxial crystallization and amorphization in silicon

The ion-beam-induced epitaxial crystallization (IBIEC) and planar amorphization of amorphous Si (a-Si) layers onto single-crystal Si substrates is reviewed. In particular, the dependence of the process on substrate temperature, on substrate orientation and on the energy deposited by the impinging ions into electronic and elastic collisions is treated in detail and discussed. Emphasis is also given to the influence of impurities on IBIEC, where a variety of different phenomena are observed. For instance, fast diffusers, such as Au, are seen to be swept by the moving c-a boundary and present intriguing segregation profiles. Slow diffusers such as As or O, on the other hand, have not enough mobility to move over long-range distances even in the presence of irradiation, but they can strongly modify the kinetics of IBIEC. Dopants such as B, P and As, for example, enhance the ion-induced growth rate by a factor of 2–3, while O retards it. It is also shown that by decreasing the substrate temperature (or by increasing the ion flux) IBIEC can be reversed resulting in a planar layer-by-layer amorphization. This phenomenon evidences the unique non-equilibrium features of ion-assisted phase transitions in silicon which are the result of a dynamic balance between defect production rate and defect annihilation rate. These data are discussed, mainly in comparison with the purely thermally activated growth of a-Si and a possible explanation of the observed phenomena is presented in terms of a simple model. Finally, new possible applications of the phenomenon, such as the ion-induced regrowth of deposited Si layers and of deposited GeSi heterostructures, are illustrated, demonstrating the high potentialities of ion-beam processing in producing epitaxial layers in a non-conventional manner.