Enhanced Electron Field Emission from Low-Temperature-Grown GaN Nanocrystals

GaN nanocrystals were synthesized on graphene-coated copper substrates using a two-step low-temperature droplet-epitaxy method. The deposition temperature (100–200 °C) and the nitrogen-ion current density during nitridation (1000–1900 nA/cm2) were varied to tune nanocrystal density and morphology. SEM revealed that growth at 100 °C led to dense ensembles of compact nanocrystals (40 ± 5 nm, 445 nanocrystals/μm2), whereas deposition at 200 °C yielded larger but more sparsely distributed nanocrystals (80 ± 16 nm, 46 nanocrystals/μm2). Increasing the nitrogen-ion current density at 200 °C promoted edge-rich hollow structures. Field-emission measurements showed that compact high-density ensembles delivered higher emission currents due to their large cumulative emitting area, while edge-rich hollow nanocrystals exhibited the best performance with the lowest turn-on field (3.4 V/μm) and the highest field-enhancement factor (β = 1470). These results demonstrate that the substrate temperature and ion current density provide effective control over the morphology and, consequently, the emission behavior of low-temperature GaN nanostructures.