Why is interstitial diffusion faster than vacancy

Advertisement Hide. Authors Authors and affiliations Donald R. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Donald R. Askeland 1 1. Interstitial diffusion is generally faster and there are many more interstitial sites than vacancy sites to jump to. At high temperature many atoms in the semiconductor move out of their lattice site, leaving vacancies into which impurity atoms can move.

The impurities, thus, diffuse by this type of vacancy motion and occupy lattice position in the crystal after it is cooled. Thus, substitutional diffusion takes place by replacing the silicon atoms of parent crystal by impurity atom. In other words, impurity atoms diffuse by moving from a lattice site to a neighboring one by substituting for a silicon atom which has vacated a usually occupied site as shown in the figure below. Substitutional diffusion mechanism is applicable to the most common diffusants, such as boron, phosphorus, and arsenic.

These dopants atoms are too big to fit into the interstices or voids, so the only way they can enter the silicon crystal is to substitute for a Si atom.

Now study on-the-go. If a vacancy is present, one of the adjacent atoms can move into the vacancy, creating a vacancy on the site that the atom has just left. In the same way that there is an equal probability of an atom moving into any adjacent atomic site, there is an equal probability that any of the adjacent atoms will move into the vacancy.

It is often useful to think of this mechanism as the diffusion of vacancies, rather than the diffusion of atoms. The diffusion of an atom is therefore dependent upon the presence of a vacancy on an adjacent site, and the rate of diffusion is therefore dependent upon two factors: how easily vacancies can form in the lattice, and how easy it is for an atom to move into a vacancy.

The dependence upon the presence of vacancies makes substitutional diffusion slower than interstitial diffusion, which we will look at now. In this case, the diffusing atom is not on a lattice site but on an interstice. The diffusing atom is free to move to any adjacent interstice, unless it is already occupied.

The rate of diffusion is therefore controlled only by the ease with which a diffusing atom can move into an interstice.