Investigation of the effects of thickness on physical properties of AZO sol-gel films for photonic device applications

Abstract

Aluminum doped zinc oxide (AZO) films are synthesized on glass substrates using different number of coatings (6, 12, 18 and 24) by the sol-gel dip-coating technique. The effects of thickness on structural, morphological, electrical optical and waveguiding properties of the prepared films are studied through X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall effect measurements, UV–Visible–NIR spectrophotometer and m-lines spectroscopy (MLS). XRD analysis shows that all AZO films exhibit the hexagonal wurtzite crystal structure with a preferential growth orientation along the c-axis. The intensity of the (002) diffraction peak and crystallite size tend to increase with increase in the thickness indicating an improvement in the crystallinity of the films. Furthermore, the observation of the high-frequency E2 mode in the Raman spectra supports the hexagonal wurtzite structure of ZnO in the films. SEM and AFM images indicate that microstructure parameters, such as grain size and surface roughness depend sensitively on the thickness of the film. The electrical resistivity and carrier concentration are found to increase and decrease, respectively, as the film thickness increased. Optical transmittance spectra demonstrate that all the films exhibit high average transmittance (86.8–89.1%) in the visible spectral range. Moreover, the band gap energy shows a red-shift with the film thickness. MLS measurements at a 632.8 nm wavelength put into evidence that AZO planar waveguides support single and multi-well confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. Both TE and TM refractive indexes are found to increase with thickness and reach saturation values when the number of coatings is greater than 18. Moreover, the thickest AZO planar waveguide demonstrates a propagation loss lower than 1 dB/cm.