Fine structure and Zeeman effect of the excited state of the green emitting copper centre in zinc oxide

In the excitation spectrum of the green emission band of ZnO:Cu crystals, zero phonon lines have been detected for the first time. In the liquid-helium temperature range, a line with Δ = 2 cm⁻¹ at λ = 433.5 nm coincides with the well-known zero-phonon line of the emission. Additional lines are located at 432.2 and 431.4 nm. In a distance of 525 cm⁻¹ from these zero-phonon lines, phonon satellites superpose on the low-energy part of the broad excitation band. On its high-energy slope pronounced structures appear in the excitonic region. With selected highly-doped crystals, the new zero-phonon lines could also be detected in absorption.Zeeman experiments produce anisotropic splittings of the common ground state of all three lines with g = 0.7 ± 0.1 and g = 1.5 ± 0.1 which are approximately the values known from emission. Different anisotropic g-factors around g = 2 are derived for the terminal states of the absorption lines.The experimental data are compatible with a model of the investigated centre in which the ground state is Cu²⁺ and the excited state is a Cu⁺ with a loosely bound hole: (Cu⁺)⁺ 3]. As to the position of the impurity levels within the forbidden gap, still two possibilities exist: the excited hole could either be localized near the oxygen neighbours so that a depth of some tenth of an eV results, or effective-mass-like states could be involved in a distance of only a few meV above the top of the valence band.