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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 wasreported previously. Several contradictory mechanisms have been put forward forexplanation of this fact so far. Here, we analyze Fe valency in ZnO bycomplementary theoretical and experimental studies. Our calculations within thegeneralized gradient approximation (GGA+U) indicate that the Fe ion is arelatively shallow donor. Its stable charge state is Fe2+ in ideal ZnO,however, the high energy of the (+/0) transition level enhances thecompensation 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 andmagneto-photoluminescence we confirm the 3+ valency of the iron ions inpolycrystalline (Zn,Fe)O films with the Fe content attaining 0.2%.We find apredicted increase of n-type conductivity upon the Fe doping with the Fe donorionization energy of 0.25 +/- 0.02 eV consistent with the results oftheoretical considerations. Moreover, our magnetooptical measurements confirmthe calculated non-vanishing s,p-d exchange interaction between band carriersand localized magnetic moments of the Fe3+ ions in the ZnO, being so far anunsettled issue.

Fe dopant in ZnO: 2+ vs 3+ valency and ion-carrier s,p-d exchange interaction