The origin of the α, β, γ blue no-phonon transitions in ZnO:Cu-A deep-level problem

Abstract

The first satisfactory assignment is given for the three closely spaced zero-phonon lines which appear near 2.86 eV in the absorption and photoluminescence excitation spectra of Cu-doped ZnO crystals. The zero-phonon lines, designed alpha , beta , gamma , represent charge-transfer type transitions in which a hole is excited from the Cu 3d shell to form a bound state with a neutral deep-acceptor-like configuration, i.e. Cu_zn²⁺(3d⁹) to (Cu_Zn¹⁺(3d¹⁰))h. The three transitions terminate on different excited hole states which derive from the special nature of thMe wurtzite valence band structure. A qualitative description of these excited hole wave-functions, consistent with the magneto-optical data, is obtained using a Wannier-function basis for the upper valence bands of ZnO and including a small admixture of Cu impurity 3d orbital character in the bound state. It is shown that the energies of the bound hole states in the excited configuration (Cu¹⁺)h are determined by the Wannier-function energies and by spin-orbit interaction with the heavy Cu impurity core, whereas the hole g-factors and the polarisations of the alpha , beta , gamma , lines reflect mainly the symmetry properties of the lattice Wannier functions.