The inter-solid diffusion can occur by two types of movements: 1. substitutional and 2. interstitial. In the substitutional movement, the dopant atom occupies the crystal lattice space normally occupied by the silicon. Hence, it can move from one location to another only if vacant sites are present. In this, it has to break old bonds and form new bonds for each movement and this is inherently a slow process. Elements such as B, P, Ga, As, In and Al diffuse primarily through substitutional mechanism.
In the interstitial diffusion, the dopant atoms are found in the space between the regular crystal lattice sites. Some elements such as Au, Cu, Fe, Na and Li diffuse primarily by interstitial diffusion. They do not form strong bond with the silicon atoms and do not need vacancies to move. Thus, interstitial diffusion is a relatively fast process. Since it is a fast process, it is difficult to control it
