Change in photoluminescence spectra of Eu-doped GaN powders due to the aggregation of nanosized grains into micrometer-sized conglomerations

In this letter, we report a phenomenon of the aggregation of nanocrystalline grains of GaN:Eu into micrometer-sized conglomerates having optical properties different from those of the initial GaN:Eu grains. The nanocrystalline GaN:Eu powders exhibit no emission related to the GaN band gap and a very broad yellow/red PL band associated with recombination in the GaN host matrix via surface states, in addition to the strong Eu3+-related emission, i.e., the D05→FJ7 transitions with J=1, 2, 3, and 4. For the microcrystalline powders, the yellow/red photoluminescence (PL) band disappears and a strong PL band related to the GaN band gap arises. In addition, the Eu3+-related transitions start to split into individual lines typical of Eu-doped bulk-like GaN crystals. These differences in PL spectra have been correlated with the surface/volume ratio of the GaN grains. It has been concluded that, for very small grains (<50nm), surface states determine the optical properties of GaN:Eu powders. Postgrowth processing, which is able to change the size of the GaN grains, seems to be very useful in order to improve and to control the optical properties of GaN:Eu powders.