Electronic structure and p-type conduction mechanism of spinel cobaltite oxide thin films

This work reports a fundamental study on the electronic structure, optical properties, and defect chemistry of a series of Co-based spinel oxide (Co3O4, ZnCo2O4, and CoAl2O4) epitaxial thin films using x-ray photoemission and absorption spectroscopies, optical spectroscopy, transport measurements, and density functional theory. We demonstrate that ZnCo2O4 has a fundamental bandgap of 1.3 eV, much smaller than the generally accepted values, which range from 2.26 to 2.8 eV. The valence band edge mainly consists of occupied Co 3d t(2g)(6) with some hybridization with O 2p/Zn 3d, and the conduction band edge of unoccupied e(g)* state. However, optical transition between the two band edges is dipole forbidden. Strong absorption occurs at photon energies above 2.6 eV, explaining the reasonable transparency of ZnCo2O4. A detailed defect chemistry study indicates that Zn vacancies formed at high oxygen pressure are the origin of a high p-type conductivity of ZnCo2O4, and the hole conduction mechanism is described by small-polaron hoping model. The high p-type conductivity, reasonable transparency, and large work function make ZnCo2O4 a desirable p-type transparent semiconductor for various optoelectronic applications. Using the same method, the bandgap of Co3O4 is further proved to be similar to 0.8 eV arising from the tetrahedrally coordinated Co2+ cations. Our work advances the fundamental understanding of these materials and provides significant guidance for their use in catalysis, electronic, and solar applications.