P-type zinc oxide spinels: application to transparent conductors and spintronics

Abstract

We report on the electronic and optical properties of two theoretically predicted stable spinel compounds of the form ZnB $_{2}$ O $_{4}$ , where B = Ni or Cu; neither compound has been previously synthesized, so we compare them to the previously studied p-type ZnCo $_{2}$ O $_{4}$ spinel. These new materials exhibit spin polarization, which is useful for spintronics applications, and broad conductivity maxima near the valence band edge that indicate good p-type dopability. We show that $3{\rm{d}}$ electrons on the octahedrally coordinated Zn atom fall deep within the valence band and do not contribute significantly to the electronic structure near the band edge of the material, while the O $2{\rm{p}}$ and tetrahedrally coordinated B $3{\rm{d}}$ electrons hybridize broadly in the shallow valence states, resulting in increasing curvature (i.e., decreased electron effective mass) of valence bands near the band edge. In particular, ZnCu $_{2}$ O $_{4}$ exhibits high electrical conductivities in the p-doping region near the valence band edge that, at $\sigma =2\times {{10}^{4}}\ {\rm{S}}\;{\rm{c}}{{{\rm{m}}}^{-1}}$ , are twice the maximum found for ZnCo $_{2}$ O $_{4}$ , a previously synthesized compound in this class of materials. This material also exhibits ferromagnetism in all of its most stable structures, which makes it a good candidate for further study as a dilute magnetic semiconductor.

Keywords

• Boltzmann equation
• density functional theory
• electrical conductivity
• optical absorption
• transparent conducting oxides
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