Evidence of substitutional Co ion clusters in Zn(1-x)Co(x)O dilute magnetic semiconductors

Structural studies become increasingly important for understanding the nature of magnetic properties in dilute magnetic semiconductors (DMSs) [K. Ando, Science 312, 1883 (2006)]. In this work, x-ray absorption fine structure (XAFS) at both Co and O K edges is used to investigate the Co occupation sites and distributions in Zn(1-x)Co(x)O thin films with low Co concentrations (x=0.005,0.02). Co K-edge XAFS analysis indicates unambiguously that the doped Co ions are substantially incorporated into the ZnO host lattice. However, the O K-edge XAFS spectral features are strikingly different from those of ZnO and Zn(1-x)Co(x)O reported in the literature so far, and can be reproduced by the assembling of substitutional Co ions around intervening oxygen atoms. This provides experimental evidence for numerous theoretical predications that the substitutional Co ions in ZnO tend to gather together. It is expected that similar phenomenon can also be observed by XAFS in a number of other ZnO-, GaN-, and TiO(2)-based DMSs for which the clustering of substitutional dopants have been previously predicted.