Neutral (Cu-Li) complexes in GaP: The (Cu-Li)Vbound exciton at 2.172 eV

A detailed study of a complex defect in GaP created by low-temperature Li diffusion into previously Cu-diffused GaP is presented. This (Cu-Li${)}_{\mathrm{V}}$ defect is electrically neutral and binds an exciton with the lowest electronic state at 2.172 eV. The electronic structure of this bound exciton (BE) has a triplet-singlet pair as a lowest-energy configuration, with an electron-hole exchange splitting of ≊2 meV, while higher excited BE states are observed around 2.26 eV. Such an electronic structure is common for complex defects in GaP. It can be understood in terms of a hole-attractive centered-cell potential combined with a local strain field of compressive sign and low symmetry. Thermal quenching data are consistent with a conventional Hopfield-Thomas-Lynch model where the hole is bound in a localized potential and the electron is bound by Coulomb forces from the hole by a typical donor binding energy of ≊80 meV. The simplest model for the identity of the defect, consistent with the body of experimental data presented, has the structure of three foreign atoms in a configuration ${\mathrm{Cu}}_{\mathrm{Ga}}$-${\mathrm{Cu}}_{\mathrm{i}}$-${\mathrm{Li}}_{\mathrm{i}}$. Optically detected magnetic resonance data suggest that the ${\mathrm{Li}}_{\mathrm{i}}$ is located off the trigonal ${\mathrm{Cu}}_{\mathrm{Ga}}$-${\mathrm{Cu}}_{\mathrm{i}}$ axis, so that the total configuration of the defect is bent in a (110) plane.