AZO thin films grown by confocal RF sputtering: role of deposition time on microstructural, optical, luminescence and electronic properties

In this work, 2% aluminum-doped zinc oxide (AZO) thin films are deposited on quartz substrates by radiofrequency magnetron sputtering in confocal configuration at room temperature (RT). The effects of various deposition times (20, 40 and 90 min) on microstructure, surface morphology, optical, luminescence and electrical properties are investigated using different characterization techniques. X-ray diffraction measurements reveal that all AZO thin films have a wurtzite hexagonal structure with a more intense preferential growth direction (002). Better crystallinity and larger grains are observed by increasing deposition time (thickness). Atomic force microscopy images show that the morphology and surface roughness depend on deposition time. The results from the measurements by UV–Visible spectrophotometry put into evidence that all the films are transparent with an average transmittance reaching 78% after 90 min of deposition. Moreover, the bandgap energy decreases from 3.57 to 3.38 eV whereas the refractive index increases from 2.26 to 2.30 with increasing deposition time from 20 to 90 min. Room temperature photoluminescence spectra show a decrease in the entire emission by increasing deposition time while a violet emission is observed only for the AZO film grown for 90 min. Hall Effect measurements demonstrate that electrical properties of the AZO films are improved with increasing sputtering time. Indeed, with the increase of deposition time, the Hall mobility increased from 0.92 to 3.30 cm²/V s and the resistivity decreased from 2.36 × 10^–3 to 1.48 × 10^–3 Ω cm.