Cobalt-doped ZnO Thin Films Research Articles

Swift heavy ion induced modifications in cobalt doped ZnO thin films: Structural and optical studies

Modifications in the structural and optical properties of 100 MeV Ni 7+ ions irradiated cobalt doped ZnO thin films (Zn 1− x Co x O, x = 0.05) prepared by sol–gel route were studied. The films irradiated with a fluence of 1 × 10 13 ions/cm 2 were single phase and show improved crystalline structure with preferred C-axis orientation as revealed from XRD analysis. Effects of irradiation on bond structure of thin films were studied by FTIR spectroscopy. The spectrum shows no change in bonding structure of Zn–O after irradiation. Improved quality of films is further supported by FTIR studies. Optical properties of the pristine and irradiated samples have been determined by using UV–vis spectroscopic technique. Optical absorption spectra show an appreciable red shift in the band gap of irradiated Zn 1− x Co x O thin film due to sp–d interaction between Co 2+ ions and ZnO band electrons. Transmission spectra show absorption band edges at 1.8 eV, 2.05 eV and 2.18 eV corresponding to d–d transition of Co 2+ ions in tetrahedral field of ZnO. The AFM study shows a slight increase in grain size and surface roughness of the thin films after irradiation.

Structure and magnetism of cobalt-doped ZnO thin films

The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions. Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30 at.%. The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice. Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry. Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD. These particles are well oriented with the ZnO crystal structure. By increasing the base pressure of the vacuum chamber to pressures above 1×10−5 Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ∼0.08 μB/Co. Depositions in the presence of oxygen background gas at 0.02 mTorr decreases the magnetization. The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO. Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation. Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out.

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO 3 (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42 μ B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21 μ B/Co and 0.65 μ B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.

Raman studies of ZnO:Co thin films

AbstractIn this paper we investigate cobalt doped ZnO thin films prepared via different sol–gel methods. XRD shows only the diffraction peaks from wurtzite ZnO without secondary phase, as widely reported in literature, for the Co contents of our samples of 3 to 12%. Raman spectroscopy reveals the existence of antiferromagnetic cobalt oxides like CoO and Co3O4 already in intermediately doped samples. Furthermore, the Raman shifts of the doped samples exhibit a shift to higher energy with increasing Co doping content. Unfortunately, Raman spectroscopy is not sensitive to metallic Co and, therefore, cannot be employed to explore the presence of Co clusters. However, we demonstrate that XRD is not a suitable method to rule out the existence of magnetic secondary phases in ZnO doped with transition metals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)