We present a theory of interstitial Mn in Mn-doped ferromagnetic semiconductors. Using density-functional theory, we show that under the non-equilibrium conditions of growth, interstitial Mn is easily formed near the surface by a simple low-energy adsorption pathway. In GaAs, isolated interstitial Mn is an electron donor, each compensating two substitutional Mn acceptors. Within an impurity-band model, partial compensation promotes ferromagnetic order below the metal-insulator transition, with the highest Curie temperature occurring for 0.5 holes per substitutional Mn.
@article{arxiv.cond-mat/0209329,
title = {Self-compensation in manganese-doped ferromagnetic semiconductors},
author = {Steven C. Erwin and A. G. Petukhov},
journal= {arXiv preprint arXiv:cond-mat/0209329},
year = {2009}
}