Structural and electronic properties of ZnO polycrystals doped with Co

Zn(1-x)Co(x)O samples were prepared by a standard solid- state reaction method. Zn(1-x)Co(x)O crystals in the wurtzite structure were obtained with a Co composition of up to 22.1%. The a- and c-axis lengths increased and decreased, respectively, with an increase in Co composition. Raman spectra showed systematic broadening of the E(2) (high) phonon mode associated with the increase in Co composition, and electronic transitions of Co in the oxygen tetrahedron were observed in optical absorption measurement. These results indicated systematic substitution of Co into the Zn sites. Furthermore, an additional broad absorption band at 2.4-3.3 eV corresponding to the charge transfer (CT) process (Co(2+) -> Co(1+)) was also observed. The Raman spectra showed strong enhancement of the LO phonon due to a resonant Raman process induced with the coupling of the LO phonon and a photo-excited carriers mediated CT gap. These results suggest the possibility of carrier-induced ferromagnetism based on double exchange interaction in Zn(1-x)Co(x)O by visible light irradiation.