The influence of defect states on non-equilibrium carrier dynamics in GaN nanowires

Semiconductor nanowires have recently attracted much attention for their unique properties and potential applications in a number of areas, most notably in nanophotonics. However, the presence of defect states in these quasi-one-dimensional nanostructures can significantly detract from nanophotonic device performance. Here, we use ultrafast optical pump-probe spectroscopy to study the influence of defect states on carrier dynamics in GaN nanowires by probing carrier relaxation through the states responsible for yellow luminescence, an undesirable effect that plagues many GaN-based photonic devices. Faster relaxation is seen in nanowires grown at lower temperatures, which also exhibits higher lasing thresholds. We attribute this to rapid trapping of photoexcited carriers into additional impurity sites that are present at lower growth temperatures. In addition, excitation density-dependent measurements reveal a decrease in carrier lifetimes with increasing pump fluence. These results demonstrate the influence of both radiative and non-radiative defect states on carrier dynamics in GaN nanowires and indicate that relaxation rates can be controlled by varying the growth temperature, which should enable researchers to optimize nanowire properties for a given application.