Role ofp-dands-dinteractions in the electronic structure and band gap of Zn1−xMxO (M=Cr, Mn, Fe, Co, Ni, and Cu): Photoelectron and optical spectroscopy and first-principles band structure calculations

Abstract

We report an investigation on the effect of $p$-$d$ and $s$-$d$ interactions in the electronic structure, and especially in the band-gap value, of wurtzite wide-gap diluted magnetic semiconductors Zn${}_{1\ensuremath{-}x}$${M}_{x}$O ($M=\mathrm{Cr}$, Mn, Fe, Co, Ni, Cu). Thin films prepared by pulsed laser deposition are investigated by means of optical absorption at low-temperature and photoelectron spectroscopy. Pure wurzite phase is shown to be maintained for Co and Mn concentrations up to 25$%$ and for Cr up to 10$%$, while in the case of Fe, Ni, and Cu, other phases are present for concentrations higher than 5, 2, and 1$%$, respectively. The band gap of the Zn${}_{1\ensuremath{-}x}$${M}_{x}$O alloy increases at a rate of 9, 22, 4, and 23 meV/$%$$M$ for $M=\mathrm{Cr}$, Mn, Fe, and Co, respectively, and decreases at a rate of about $\ensuremath{-}$14 and $\ensuremath{-}$10 meV/$%$$M$ for $M=\mathrm{Ni}$ and Cu. Photoelectron spectroscopy of the Zn${}_{1\ensuremath{-}x}$${M}_{x}$O valence band for $M=\mathrm{Mn}$ and Co shows that the emergence of the transition metal-related photoemission peak is clearly correlated to a larger binding energy of the O 2$p$ valence-band peaks. A simple model of $p$-$d$ and $s$-$d$ interaction is proposed in which the decrease of Zn 3$d$ electron density below the valence band and the increase of $M$ 3$d$ electron density for $M=\mathrm{Cr}$ to Co lead to higher binding energies of the valence-band maximum and, thus, to a larger band gap. In contrast, for Ni and Cu the 3$d$ electrons lie below the valence-band maximum and push it to lower binding energies, thus decreasing the band gap. This simple model is basically confirmed by first-principles density functional theory band structure calculations. Detailed analyses of the band structures and densities of states show that the $p$-$d$ interaction leads to an increase of the band gap for $M=\mathrm{Mn}$ to Co but a decrease for $M=\mathrm{Ni}$ and Cu. They also suggest that the $s$-$d$ interaction plays the major role or contributes as much as the $p$-$d$ interaction in leading to the increase of the band gap for $M=\mathrm{Cr}$ and Mn, respectively.