Role of p-d and s-d interactions in the electronic structure and band gap of Zn <inf>1-x</inf>M <inf>x</inf>O (M=Cr, Mn, Fe, Co, Ni, and Cu): Photoelectron and optical spectroscopy and first-principles band structure calculations

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-xM xO (M=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-xM xO alloy increases at a rate of 9, 22, 4, and 23 meV/%M for M=Cr, Mn, Fe, and Co, respectively, and decreases at a rate of about -14 and -10 meV/%M for M=Ni and Cu. Photoelectron spectroscopy of the Zn 1-xM xO valence band for M=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 2p valence-band peaks. A simple model of p-d and s-d interaction is proposed in which the decrease of Zn 3d electron density below the valence band and the increase of M 3d electron density for M=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 3d 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=Mn to Co but a decrease for M=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=Cr and Mn, respectively. © 2012 American Physical Society.