Modelling of long-range damage in ion-bombarded GaAs

A complex panorama of ion-induced processes in GaAs is studied by model calculations. The main goal was to explain the unusual behavior of the altered layer in GaAs under high-fluence 5 keV Ar+ bombardment. Recently, dramatic changes of microstructure have been found by TEM, with the depth of the altered layer increasing during the ion bombardment and exceeding 200 nm after 20 h of the irradiation. Further, complicated composition-depth profiles with two maximums of Ga content have been observed in the altered layer. In this paper, the role of collision mixing, metastable structural transformations and segregation processes in the formation of the altered layer in GaAs is discussed and illustrated by results of calculations. The anomalous expansion of the altered layer in GaAs is explained in the framework of a reaction-diffusion model which involves an autocatalytic process of amorphization followed by a partial recrystallization. It is shown that the composition changes observed in GaAs experimentally are most probably to arise during the post-bombardment relaxation, and result from segregation on internal grain boundaries.