Observation of luminescence from a highly concentrated Nd center in GaP direct optical excitation and comparison with Nd centers excited under host excitation

We report the studies of direct excitation on Nd-doped GaP samples grown by metalorganic chemical vapor deposition. It is found that extremely sharp photoluminescence excitation lines, with full width at half maximum of less than 0.02 nm at 4 K, can be observed when the excitation photon energy is in resonance with the Nd internal 4f-shell transition energy. We used direct excitation spectra to estimate the concentration of each individual Nd luminescence center, and found that the majority of the Nd ions form one particular kind of noncubic center which cannot be effectively excited through energy transfer from the host crystal. Such a phenomenon is likely to be the reason why the efficiencies of the rare-earth-doped III-V compounds are low. It also suggests that if we can fully utilize all the available rare-earth ions, a much higher efficiency can be achieved.