Theoretical prediction of a high Curie temperature in ZnO doped with Mn, Fe, and other transition metals has stimulated the investigation of these materials by many research groups. Although charge-compensated Fe(3+) centers in ZnO:Fe have been observed by means of EPR and have been known for decades, conclusions on the chemical nature of these defects are still contradictory. Originally, these centers were treated as Fe(3+)-Li(+) complexes with both ions occupying adjacent cationic sites. Recently, however, the centers were interpreted as a substitutional Fe(3+) ion with a vacancy at an adjacent zinc or oxygen site (Fe-VZn or Fe-VO). In order to determine the chemical nature of the impurity associated with Fe(3+), electron-nuclear double resonance (ENDOR) spectroscopy was used. ENDOR measurements reveal NMR transitions corresponding to nuclei with g-factor gN=2.171 and spin I=3/2. This unambiguously shows presence of Li as a charge compensator and also resolves contradictions with the theoretical prediction of the Fe-VO formation energy. The electric field gradients at the (7)Li nuclei (within the Fe(3+)-Li(+) complexes) were estimated to be significantly lower than the gradient at undistorted Zn sites.
Keywords: Charge-compensated centers; Complex; ENDOR; High-frequency EPR; Iron; Lithium; Quadrupole interaction; W-band; ZnO.
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