Anomalous photoluminescence and UV light sensing characteristics of ZnO:Ga nanowires—role of Ga content

Ga doped ZnO nanostructures were synthesized by VLS mechanism in which Ga was simultaneously used as a catalyst as well as dopant with varied Ga layer thickness (1 nm, 3 nm, 5 nm and 10 nm). The synthesized ZnO NWs were studied for optical and UV light sensing characteristics. The morphology of grown nanostructures studied by SEM showed that the diameter of nanowires increased with the thickness of the Ga film increase. XRD patterns revealed that ZnO was present in Wurtzite (hexagonal) phase. In addition to ZnO an impurity phase of Ga₂O₃ was also observed in all synthesized ZnO NWs. The amount of Ga doping affected the position and intensity of ZnO (002) reflection peak in the XRD patterns. This was ascribed to two factors, one is substitution of Ga atoms at the Zn site in the ZnO crystal structure and secondly tendency of Ga to become interstitial. Room temperature and temperature dependent photoluminescence spectroscopy was performed to study the band edge and defected assisted luminescence. In the low temperature PL spectra, DAP transition at (3.30–3.32 eV) and neutral Ga donor bound exciton D^oX lines were observed, which disappeared when the temperature was raised above 100 K, which confirmed the successful Ga doping in ZnO crystal structure. The PL intensity showed anomalous behavior as a consequence of tail edge states, which showed strong dependence on Ga content. It was observed that the activation energy for radiative transitions decreased rapidly while the activation energy for non-radiative did not decrease appreciably with the Ga content. This led to enhanced UV sensing characteristics of ZnO NWs with slow recovery time. It is demonstrated that the recovery time depended on the Ga content as it became slow for large Ga content.