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

DOI: 10.1103/PhysRevB.100.115301 DOI Help

Authors: X. C. Huang (Xiamen University; Southern University of Science and Technology) , J. Y. Zhang (Xiamen University; Southern University of Science and Technology) , M. Wu (Xiamen University) , S. Zhang (University of Electronic Science and Technology of China) , H. Y. Xiao (University of Electronic Science and Technology of China) , W. Q. Han (Southern University of Science and Technology) , T.-l. Lee (Diamond Light Source) , A. Tadich (Australian Synchrotron; La Trobe University) , D.-c. Qi (La Trobe University; Queensland University of Technology) , L. Qiao (University of Electronic Science and Technology of China) , L. Chen (Southern University of Science and Technology) , K. H. L. Zhang (Xiamen University)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 100

State: Published (Approved)
Published: September 2019
Diamond Proposal Number(s): 21432

Abstract: This work reports a fundamental study on the electronic structure, optical properties, and defect chemistry of a series of Co-based spinel oxide ( Co 3 O 4 , ZnCo 2 O 4 , and CoAl 2 O 4 ) epitaxial thin films using x-ray photoemission and absorption spectroscopies, optical spectroscopy, transport measurements, and density functional theory. We demonstrate that ZnCo 2 O 4 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 3 d t 6 2 g with some hybridization with O 2 p /Zn 3 d , 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 ZnCo 2 O 4 . A detailed defect chemistry study indicates that Zn vacancies formed at high oxygen pressure are the origin of a high p -type conductivity of ZnCo 2 O 4 , and the hole conduction mechanism is described by small-polaron hoping model. The high p -type conductivity, reasonable transparency, and large work function make ZnCo 2 O 4 a desirable p -type transparent semiconductor for various optoelectronic applications. Using the same method, the bandgap of Co 3 O 4 is further proved to be ∼0.8 eV arising from the tetrahedrally coordinated Co 2 + cations. Our work advances the fundamental understanding of these materials and provides significant guidance for their use in catalysis, electronic, and solar applications.

Journal Keywords: Density of states; Electrical conductivity; Electronic structure; Cobaltates; Spinel; Transparent conducting oxides; Ultrathin films; Wide band gap systems; Density functional theory; Epitaxy; Laser ablation; Resistivity measurements

Subject Areas: Materials, Physics

Instruments: I09-Surface and Interface Structural Analysis

Other Facilities: Australian Synchrotron