Optical properties of the Cu-related characteristic-orange-luminescence center in GaP

A detailed investigation on optical properties of the characteristic-orange-luminescence (COL) center in GaP is reported, combining photoluminescence data with dye-laser-excited excitation spectra. Evidence from the doping conditions required to produce this defect suggests an identification of the COL center with a defect containing Cu only. The novel optical data support this view, since the COL spectrum is identified as originating from an exciton bound to a nonlinear isoelectronic ${\mathrm{Cu}}_I-{\mathrm{Cu}}_{\mathrm{Ga}}-{\mathrm{Cu}}_I$ associate. The complicated local mode coupling and the rather strong coupling to the lattice continuum modes is expected for such a defect structure, where the ${\mathrm{Cu}}_{\mathrm{Ga}}$ can be considerably relaxed. The strong compressive axial strain field created by this defect causes a splitting of the hole states at the defect and decouples the spin and orbital angular momentum of these states. For a complete decoupling the bound exciton is formed by combining a pure spin hole state and an electron. This results in the observed $J=1\mathrm{}$ spin triplet as the lowest bound exciton state and a higher $J=0\mathrm{}$ singlet state. From the rich structure observed in excitation spectra a large exchange splitting of 23.2 meV is obtained between the $J=1\mathrm{}$ ground state and the $J=0\mathrm{}$ state. No orbitally excited states of one particle in the Coulomb field of the other are observed for this bound exciton, probably a consequence of the fact that both electronic particles are relatively deeply bound. A typical feature for this class of defects seems to be that transitions involving the singlet $J=0\mathrm{}$ state have a much stronger total oscillator strength than those involving the $J=1\mathrm{}$ ground state. This is a consequence of a spin selection rule $\Delta S=0\mathrm{}$, also manifested by the long decay time of the $J=1\mathrm{}$ bound exciton emission ($\tau \approx 100$ μs).