# Excitons bound to neutral impurities

1. Jan 4, 2012

### mendes

Hi all,

I would like to understand the mechanism by which a neutral impurity can bind an exciton. Because the impurity is neutral the attracation can not be simply electrostatic. I know that there must be a "neutralising electyron (or hole)" in the machanism but things are not clear enough for me.

2. Jan 6, 2012

### Cthugha

In some cases you can imagine the impurity as having a short range potential which can trap one of the carriers and the resulting charged complex then has a long range potential that will trap the other carrier.

More generally speaking the situation is analogous to a hydrogen molecule. Only the masses of the constituents differ. This similarity has been pointed out in Murray A. Lampert, "Mobile and Immobile Effective-Mass-Particle Complexes in Nonmetallic Solids", Phys. Rev. Lett. 1, 450–453 (1958).

The exact ratio of the effective masses of the electrons and holes in a certain material define the details. An estimate on whether such complexes are stable or not is given by Hayne's rule. See J. R. Haynes, "Experimental Proof of the Existence of a New Electronic Complex in Silicon", Phys. Rev. Lett. 4, 361–363 (1960). Check the footnote on his personal communication with Kohn.