Effect of vacancies on ferromagnetism in GaN:Mn dilute magnetic semiconductors from first-principles

In spite of considerable interest in ferromagnetism of the dilute magnetic semiconductor GaN:Mn, the nature of the ferromagnetism is still quite controversial. Experimental values for the Curie temperature T-C vary widely depending upon the details of the growth conditions which might affect the impurity concentrations and possibly the magnetic properties. In order to gain insight into the effects of the impurities, we have performed ab initio density functional studies of the magnetic interactions in GaN in the presence of vacancies. Both nitrogen and gallium vacancies have been considered. The nitrogen vacancy releases electrons in the system which changes the Mn d(4) state to a half-filled Mn d(5) state, so that the antiferromagnetic superexchange becomes dominant. Previous studies have found the nitrogen vacancy has the lowest formation energy, so the presence of these vacancies is predicted to lower T-C. The naive picture of Ga vacancies is the release of holes into the system which should increase ferromagnetism due to the increased hole concentration compared to the vacancy-free material. However, we find an antiferromagnetic interaction for the Ga vacancy as well, in agreement with Mahadevan's work. This can be attributed to the localized nature of the hole states which do not participate in the transport. The effects of localization of the holes from the Ga vacancy has been demonstrated using the virtual crystal approximation. Thus both the nitrogen and gallium vacancy are found to impede ferromagnetism.