Analysis of the vibrational properties of Zn1-xCoxO by Raman spectroscopy

Currently, ZnO-based diluted magnetic semiconductors (DMS) attract much attention, mainly because of the predicted possibility of ferromagnetism at 300 K. Among these materials, Zn1-xCoxO takes a prominent position. In spite of numerous investigations, a general understanding of its magnetic properties is still missing. Moreover, its detailed magnetic behavior depends on the sample fabrication and after growth treatment. We present a systematic analysis of structural Zn1-xCoxO properties by micro-Raman scattering for a wide variety of different sample parameters. Bulk mixed crystals (x < 0.05) and thin layers (0.03 < x < 0.12) were investigated between 1.6 K and 295 K. Beside the common eigenmodes of the ZnO host lattice, we observe disorder-induced lattice vibration modes and structures, which we assign to vibration modes of Co-related precipitates. We present their dependence on the composition, the applied preparation technique, and the annealing process. Furthermore, the possible impact of such precipitate formation on the magnetic properties is discussed.