Study of the relationship between crystal structure and luminescence in rare-earth-implanted Ga_2O_3 nanowires during annealing treatments

A systematical analysis of the correlation between the crystalline quality and the luminescence of rare-earth-implanted s-Ga_2O_3 nanostructures with potential applications in visible and ultraviolet photonics is presented. Europium ions led to red emission while gadolinium ions are efficient ultraviolet emitters. Different degrees of lattice recoveries of the nanostructures have been achieved after implantation by rapid thermal annealing treatments carried out at different temperatures. The recovery process has been analyzed by transmission electron microscopy (TEM), high-resolution TEM, and Raman techniques. High-fluence implantation with either of the two rare earth ions induces partial amorphization of the structures. Partial recrystallization of the nanostructures above 500 oC is revealed by Raman analysis. Nearly complete recovery of the crystal structure is obtained in the annealing temperature range 900-1100 oC, coincident with the expected value for bulk Ga_2O_3. Cathodoluminescence and photoluminescence allowed comparison of the Eu^3+ and Gd^3+ intraionic luminescence lines after annealing at different temperatures and their correlation with the crystallinity. It has been found that the width of the Eu^3+ luminescence lines clearly correlates with the width of the Raman peaks, both decreasing with annealing temperature, which shows the possibility of using the luminescence of this rare earth as a probe for lattice disorder. On the other hand, our results suggest that Gd^3+ lines are much less sensitive to disorder.