X-ray spectroscopic studies of the electronic structure of chromium based p-type transparent conducting oxides

The valence band structure of $p$-type transparent oxides---crystalline ${\text{Mg}}_{x}{\text{Cr}}_{2\ensuremath{-}x}{\text{O}}_{3}$ and nanocrystalline ${\text{Cu}}_{x}{\text{CrO}}_{y}$---is analyzed as a function of incoming photon energy. The valence band of both $p$-type transparent conducting oxides shows striking similarities to measurements on crystalline ${\text{CuCrO}}_{2}$:Mg with all films showing that chromium states compose the top of the valence band, suggesting that the valence-band structure is dominated by the presence of the Cr-${\mathrm{O}}_{6}$ octahedra. A comparison of the valence band between the best performing $p$-type, crystalline ${\mathrm{CuCrO}}_{2}$:Mg, with crystalline ${\text{Mg}}_{x}{\text{Cr}}_{2\ensuremath{-}x}{\text{O}}_{3}$ and nanocrystalline ${\text{Cu}}_{x}{\text{CrO}}_{y}$ shows that the chromium $3d$ states are fixed irrespective of changes in long-range crystallographic order. This indicates little spatial overlap between adjacent Cr $3d$ states. This further confirms the conduction mechanism via hopping for chromium based $p$-type TCOs as the Cr $3d$ states are localized within the Cr-${\mathrm{O}}_{6}$ octahedra.