Photonic Properties of Thin Films Comprised of Plasma-Synthesized Gallium Nitride Quantum Dots

Gallium nitride (GaN) is a promising material for power electronics and optoelectronics owing to its high electron mobility, high breakdown voltage, and wide, direct band gap. The synthesis of thin film and bulk crystal GaN has received much attention, which has resulted in the commercialization of GaN. However, few synthesis methods exist for freestanding GaN quantum dots (QDs), which can possess a tunable band gap above 3.4 eV. These ultra-wide-gap GaN QDs are promising for deep ultraviolet light sources and single photon emitters in quantum information technologies. In this work, Nonequilibrium Plasma Aerotaxy was utilized to synthesize GaN QDs and deposit them in flight as thin films on foreign substrates which are relevant for practical solid-state devices. The QD diameter was controlled between 3.6 and 7.4 nm. The particles were monocrystalline. The photoluminescence of the QD films increased steadily with time in air, after oxidation of the QD surfaces. There were two PL peaks: one in the ultraviolet, hypothesized to originate from band-to-band recombination, and another in the visible, hypothesized to originate from deep-trap-mediated recombination. Surprisingly, both PL peaks possessed a size dependence, in agreement with the Brus equation. Coating the GaN QDs with Al₂O₃ suppressed the PL peak in the visible, confirming that air exposure caused the defect-related PL.