Lattice, band, and spin engineering in Zn1−xCoxO

This work was conducted to investigate lattice, band, and spin engineering of magnetic Zn1−xCoxO layers towards quantum barriers in ZnO. Lattice distortions by doping with Co ions caused a flat tetrahedron in the host, leading to an increase of spontaneous polarization in Zn1−xCoxO compared to ZnO based on the point-charge model. The band-gap energy increased linearly with the Co concentrations, which was very similar to the band-gap widening in Zn1−xMgxO derived from sp hybridization. The Co (3d) states were located in the mid-gap, which remained unchanged following changes in Co concentrations. Large magneto-optical effects were induced at the band edge due to sp-d exchange interactions. However, magneto-optical activity was reduced in heavily doping concentrations above x = 0.16 because of antiferromagnetic coupling between nearest-neighbor Co ions. The high magnetic activity at x = 0.10 is related to competition between the complex Co-related configurations, such as singles, pairs, open and closed triples, in Co-doped ZnO layers. Magnetic Zn1−xCoxO therefore has an effective layer composition for applications of quantum barriers.