Contrast in triple bound excitons in copper-doped gallium phosphide

Abstract

The bound exciton (BE) recombination no-phonon lines at 1.911 eV and 2.177 eV in copper doped gallium phosphide have been studied under uniaxial stress. The 1.911 eV centre exhibits splitting characteristic of a BE at an axial centre having rhombohedral I symmetry. The electronic degeneracy within each centre is not reduced, in agreement with ODMR and Zeeman spectroscopy. By contrast, the stress splitting of the 2.177 eV (COL) no-phonon line indicates a pure trigonal centre with additional electronic degeneracy in the initial BE state characteristic of an E to A transition with no splitting under (111) stress. The difference is attributed to the character of the electron which is deduced to be in a valley-orbit E state for the 2.177 eV BE, but has normal A character for the 1.911 eV BE. This difference may account for the failure of ODMR studies on the 2.177 eV system while the 1.911 eV BE shows very strong ODMR signal strength, previously incomprehensible since both BE behave as near isotropic spin triplets.