Theory of hydrogen in dilute nitrides

At variance with the mechanisms responsible of the passivation of typical shallow dopants in GaAs, where H bonding to the impurity or to its neighbors permits a recovery of the ``natural'' chemical valence of the impurity itself, in the case of the N isoelectronic impurity in GaAs, H induces no changes in the N chemical valence. It breaks, instead, two Ga--N bonds to form complexes sharing a common, quite stable, NH$_2$ core characterized by a local C$_{2v}$-like symmetry, thus strongly perturbing the interactions of N with its four Ga neighbors. A theoretical picture is presented here, where the N effects on the GaAs electronic properties, as well as their passivation upon hydrogenation, are closely related to the local N--Ga interactions and to the perturbations induced by H bonding on the N environment. The formation of multiple-H complexes with N, predicted by theory, is also discussed together with the support given to their existence by recent experimental findings.