Neutral (Cu-Li) complexes in GaP: The (Cu-Li)IIIbound exciton at 2.242 eV

A photoluminescence study is reported in order to identify a new radiative center in GaP co-doped with Cu and Li. This center, denoted (Cu-Li${)}_{\mathrm{III}}$, binds an exciton with binding energy 110 meV. The bound-exciton spectrum shows sharp electronic lines and a structured phonon sideband with well-resolved replicas. The group of electronic transitions consists of spin-only-like singlet-triplet pairs, which are characteristic of tightly bound holes in a strong compressive axial crystal field. Two such singlet-triplet pairs are observed for the (Cu-Li${)}_{\mathrm{III}}$ center, instead of the usual one pair. A model taking into account the splitting of electron states as well as hole states is proposed to explain this additional splitting. The experimental work includes photoluminescence spectroscopy and time-resolved and Zeeman measurements as well as optically detected magnetic resonance measurements. The careful doping procedure includes isotope doping with both ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ ${,}^{65}$Cu and ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ ${,}^7$Li. The Li-isotope doping was clearly reflected in the phonon sideband. A neutral molecular associate of isoelectronic nature is suggested, with the (Cu-Li${)}_{\mathrm{III}}$ center involving ${\mathrm{Cu}}_{\mathrm{Ga}}$ and a pair of interstitial atoms, ${\mathrm{Cu}}_{\mathrm{i}}$ and ${\mathrm{Li}}_{\mathrm{i}}$.