Photoluminescence studies of the 1.911-eV Cu-related complex in GaP

The optical properties of the 1.911-eV bound exciton (BE) in Cu-doped GaP have been investigated with several photoluminescence techniques. Photoluminescence (PL) and photoluminescence excitation (PLE) data at different temperatures are combined with magneto-optical data from Zeeman and high-resolution optically detected magnetic resonance (ODMR) measurements. The doping conditions required for this BE spectrum to appear, as well as isotope experiments, prove that the defect binding the exciton involves Cu. A neutral and isoelectronic complex is suggested, most probably a linear Cu-Ga associate with a $〈100〉$-oriented symmetry axis as determined from ODMR data. The axial compressional strain field consistent with this defect geometry completely decouples the spin-orbit coupling of the bound-hole states, leaving pure-spin hole states with lowest energy. The combination of two pure-spin particles accounts for the $J=1\mathrm{}$ spin-triplet and $J=0\mathrm{}$ singlet states for the bound exciton. The spin-forbidden transition from the $J=1\mathrm{}$ triplet to the spin-free ($J=0\mathrm{}$) ground state is the 1.911-eV transition clearly resolved in the emission spectrum at low temperatures, while the $J=0\mathrm{}$ BE state is observable at 2.002 eV in PLE. The extraordinary large exchange splitting of 91 meV is related to the unusual degree of BE localization. Both particles of the exciton are tightly bound, as supported by measurements of the activation energy for the thermal quenching of the 1.911-eV emission. We interpret this tight binding of both particles as a recently recognized but probably general property of an exciton bound to an associate with an attractive localized potential for holes as well as for electrons. In the present case, the hole is bound in the strong central-cell potential of ${\mathrm{Cu}}_{\mathrm{Ga}}$, while the electron also experiences a strong short-range component of binding potential arising from the effect of the local compressive strain field on the lowest conduction band in GaP. A splitting of hole states in the axial field of the defect is estimated to about 280 meV, exceeding the spin-orbit splitting by at least 200 meV.