Vapor-transport synthesis and annealing study of Zn_xMg_1-xO nanowire arrays for selective, solar-blind UV-C detection

This work uniquely reports the synthesis of Zn_xMg_1-xO nanowires and submicron columns by utilizing a traditional carbothermal reduction process toward forming ZnO nanowire ultraviolet detectors, while simultaneously utilizing Mg₃N₂ as the source of Mg. To investigate the relationship between Mg content in the ZnO lattice and the cutoff wavelength for high spectral responsivity, the nanowires were annealed in a series of designed conditions, whereas chemical, nanostructural, and optoelectronic characteristics were compared before and after treatment. Postanneal scanning electron micrographs revealed a reduction of the average ensemble nanowire dimensions, which was correlated to the modification of ZnO lattice parameters stemming from Zn²⁺ dissociation and Mg²⁺ substitution (confirmed via Raman spectroscopy). The analysis of cathodoluminescence spectra revealed a blueshift of the peak alloy band-edge emission along with a redshift of the ZnO band-edge emission; and both were found to be strong functions of the annealing temperature. The conversion of Zn₂SiO₄ to Mg₂SiO₄ (in O₂) and MgSiO₃ (in Ar) was found to correspond to transformations (shifting and scaling) of high-energy luminescence peaks and was confirmed with X-ray diffraction analysis. The tunability of the cutoff photodetection wavelength was evaluated as the nanowire arrays exhibited selective absorption by retaining elevated conduction under high-energy UV-C irradiation after thermal treatment but exhibiting suppressed conductivity and a single order of magnitude reduction in both spectral responsivity (R_λ) and photoconductive gain (G) under UV-A illumination. Noise analysis revealed that the variation of detectivity (D^∗) depended on the regime of ultraviolet irradiation, and that these variations are related to thermal noise resulting from oxygen-related defects on both nanowire and substrate surfaces. These results suggest a minor design tradeoff between the noise characteristics of thermally treated ZnMgO nanowire array UV detectors and the tunability of their spectral sensitivity.