Electronic structure of oxysulfide (LaO)CuS and(La1−xCaxO)Cu1−xNixS(x<~0.10)studied by photoemission and inverse-photoemission spectroscopies

Abstract

Valence-band and conduction-band electronic structure of the transparent p-type semiconductor (LaO)CuS and the Ca and Ni co-doped system $({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{O}){\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{S}$ $(x<~0.10)$ has been investigated by means of photoemission and inverse-photoemission spectroscopies. It is confirmed that the valence bands of (LaO)CuS are composed of the bonding and antibonding states of the Cu $3d$ ${(t}_{2g})$ and S $3p$ orbitals as well as the nearly localized Cu $3d$ ${(e}_{g}),$ and the O $2p$ states. With increasing x up to $x=0.03,$ the structure due to the Cu $3d$ ${(e}_{g})$ states shifts away from the Fermi level ${(E}_{F})$ and the energy separation between the valence-band maximum and conduction-band minimum decreases rapidly, followed by an appearance of new density of states (DOS) derived from the Ni $3d$ and S $3p$ states just above ${E}_{F}$ of (LaO)CuS. For $x>~0.03,$ the new DOS shows up with x, while the energy position of the Cu $3d$ ${(e}_{g})$-derived structure no longer shifts. The finite DOS at ${E}_{F}$ has been observed as a clear structure for $x=0.10.$ These results are not understood in terms of a rigid-band model. We propose a different band model for $({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{O}){\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{S}$ taking into account the Ni $3d$ and S $3p$-derived new DOS.