Optical response and electronic structure of Zn-doped MgAl ₂O₄

We have studied the optical response and the electronic structure of Zn-doped Mg Al2 O4 using optical transmission, emission, and excitation spectroscopies, x-ray photoemission spectroscopy, and unrestricted Hartree-Fock calculation. Emission lines at 710, 650, and 470 nm observed in pure Mg Al2 O4 are related to the Mg vacancies and Mg-Al antisite defects. Interestingly, the intensities of these emission lines are enhanced by Zn doping. Unrestricted Hartree-Fock calculation for Zn-doped Mg Al2 O4 shows that in-gap impurity states are formed just above the valence-band maximum of Mg Al2 O4 when the Zn ion is substituted for the B-site Al ion. On the other hand, no in-gap state is formed when the Zn ion is substituted for the A -site Mg ion. The position of the Zn (3d) impurity level is identified by the photoemission measurement. The broad spectral features of the defect-induced states in pure Mg Al2 O4 is dramatically reduced by the Zn doping, indicating that holes supplied from the Zn ions at B site are trapped by the defect-induced states.