Abstract
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.
Highlights
The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions
Similar results of transition metal enrichment in doped ZnO films grown by pulsed-laser deposition (PLD) have been reported [30, 43, 44]
For films doped with nominally 30% Co, the cobalt concentration is slightly higher with increased substrate temperature and slightly lower with a small addition of oxygen background gas
Summary
Cobalt-doped ZnO epitaxial films were deposited via pulsed laser deposition (PLD) [42] onto c-plane oriented sapphire substrates. The ablation targets were prepared through the solidstate reaction of mixed oxide powders. Appropriate amounts of ZnO (Alfa Aesar, PuratronicR , 99.9995%) and Co3O4 (Alfa Aesar, PuratronicR , 99.9985%) powders were ground and mixed in methanol, dried in air, pressed into pellets and sintered at 1000 ◦C for 12 h in air. The targets were mixed to give proportions of Zn1−x Cox O with x = 0.00, 0.02, 0.05, 0.10, 0.15 and 0.30. A KrF excimer laser (248 nm wavelength) was used for target ablation using a repetition rate of 1 Hz and a laser energy density of 1–3 J cm−2. The PLD system is evacuated using a turbomolecular pump to a base pressure of ∼7.0 × 10−6 Torr. Film thicknesses were 150–250 nm as measured by mechanical profilometry
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
