Defect engineering and carrier dynamics in gallium-doped zinc oxide nanowires for light-emitting applications

Abstract

Gallium (Ga) dopant-induced modulation of defects and recombination dynamics in wurtzite zinc oxide (ZnO) nanowires are investigated by cathodoluminescence (CL) and transient photoluminescence (PL) spectroscopy, complemented by density functional theory (DFT). The results reveal that high doping levels of Ga (>3 atom%) in ZnO nanowires grown in an oxygen-rich environment lead to the formation of Ga_Zn–V_Zn intraband states, which act as optically active luminescence centers. The emission lines of Ga-induced donor-bound excitons and acceptor-bound complexes confirm the formation of Ga_Zn and Ga_Zn–V_Zn defects in Ga-doped ZnO nanowires. The presence of these bound complexes significantly reduces the bandgap and broadens the near-band edge (NBE) emission of ZnO. The formation of Ga_Zn–V_Zn defects significantly suppresses the characteristic V_Zn-related green luminescence (GL) and introduces a new recombination channel of orange luminescence (OL). Temperature-dependent CL and time-resolved PL analyses reveal that this OL band, attributed to the Ga_Zn–V_Zn center laying at 0.62 eV over the valence band, exhibits a slow decay time constant of 5.4 μs. The simulation of the spectral line shape of this OL band using the Franck–Condon model reveals the thermodynamic transition level of 630 meV above the valence band and an electron–phonon coupling strength of 6.4 for this OL center. Ga-doped ZnO nanowire arrays are used to fabricate nanowire-based light-emitting diodes (LEDs), which show a low threshold voltage of 4.1 volts and intense orange electroluminescence. These Ga-doped ZnO nanowires grown in an oxygen-rich environment can be used as efficient orange-coloured light emitters in photonic and optoelectronic devices.