Dopants of transition metal ions in II-VI semiconductors exhibit native 2+ valency. Despite this, 3+ or mixed 3+/2+ valency of iron ions in ZnO was reported previously. Several contradictory mechanisms have been put forward for explanation of this fact so far. Here, we analyze Fe valency in ZnO by complementary theoretical and experimental studies. Our calculations within the generalized gradient approximation (GGA+U) indicate that the Fe ion is a relatively shallow donor. Its stable charge state is Fe2+ in ideal ZnO, however, the high energy of the (+/0) transition level enhances the compensation of Fe2+ to Fe3+ by non-intentional acceptors in real samples. Using several experimental methods like electron paramagnetic resonance, magnetometry, conductivity, excitonic magnetic circular dichroism and magneto-photoluminescence we confirm the 3+ valency of the iron ions in polycrystalline (Zn,Fe)O films with the Fe content attaining 0.2%.We find a predicted increase of n-type conductivity upon the Fe doping with the Fe donor ionization energy of 0.25 +/- 0.02 eV consistent with the results of theoretical considerations. Moreover, our magnetooptical measurements confirm the calculated non-vanishing s,p-d exchange interaction between band carriers and localized magnetic moments of the Fe3+ ions in the ZnO, being so far an unsettled issue.
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