Hybrid functional investigations of substitutional Fe impurity in GaN

The search for semiconducting room-temperature (RT) ferromagnets for applications in spintronics continues. Predictions of above-RT Curie temperatures in diluted nitrides [1] has triggered work on transition-metal (TM) impurities in GaN. Hole-mediated exchange interactions are supposed responsible for magnetic ordering, and hence a prediction of the onset of ferromagnetism requires understanding the interactions between TM d-states and host anion-like p-states. Further, TM in nitrides are under intense theoretical investigation because of suspected effects of electron correlation, naturally expected due to the strong electronic localization of the d shell of the impurity and the valence states of the highly ionic hosts. Here we present spin-polarized density-functional-theory electronic structure calculations for neutral and charged Ga-substitutional Fe impurities in GaN, using the hybrid Heyd-Scuseria-Ernzerhof functional, with projector augmented waves in the implementation of the Vienna Ab-Initio Simulation Package. Impurity-induced states of three kinds appear: strongly Fe-localized majority states in the lower part of the valence band; dispersed Fe-N-hybrid majority states slightly below the valence top; empty minority states resonant with the Ga-like conduction band bottom. In the -1 charge state, occupied resonant minority state shifts down decidedly into the gap and the resulting transition level is in good agreement with the observed acceptor level of Fe:GaN. In the +1 charge state, an empty state appears just above the valence band, strongly localized on Fe-first-neighbor N's; correspondingly, a hyper-deep (+1/0) donor level at 0.3 eV is obtained, with a bound hole producing a 4+ionization state of Fe. In actual fact, the 4+ ionization state can only nominally be attributed to Fe in itself: the analysis of the orbital-projected density of states shows that Fe:GaN nicely follows the Raebiger-Lany-Zunger "charge self-regulation" rule: the sum of the population of band-resonant and gap states remains constant, so that the actual electronic charge residing on Fe does not change upon charging the impurity+host system. This invalidates any attribution of a well defined ionization state to the Fe impurity.