Effect of thickness on physical and waveguiding properties of nanostructured AZO thin films by a sol-gel dip-coating method

In the present work, 3.0 at. % aluminum doped zinc oxide (AZO) thin films were prepared at different number of coatings (6, 12, 18 and 24) on glass substrates by a sol-gel dip-coating process. The effect of film thickness on... [ view full abstract ]

In the present work, 3.0 at. % aluminum doped zinc oxide (AZO) thin films were prepared at different number of coatings (6, 12, 18 and 24) on glass substrates by a sol-gel dip-coating process. The effect of film thickness on structural, morphological, optical and waveguiding properties of AZO thin films was studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-Visible-NIR spectrophotometer and m-lines spectroscopy (MLS). XRD analysis showed that all the AZO films are polycrystalline and exhibit the hexagonal wurtzite crystal structure with a preferential growth orientation along the c-axis. The relative  intensity  of  the  (002)  diffraction  peak  and crystallite size tend  to  increase  with  an increase in  the  number of layers indicating an  improvement  in  the crystallinity  of  the  films. SEM micrographs and AFM images revealed that the 6 layers AZO film exhibits very smooth surface consisted of small grain size particles, along with some pores over the film. In comparison, the surface of the films becomes denser and rougher with the grain size enlarged as the number of layers increases. Optical transmittance spectra showed that all of the multilayer films exhibited a high average transmittance (84-88%) in the visible region with the sharp absorption edge in the UV light region. It was also found that the band gap energy, calculated from the spectra of optical transmittance, show a significant dependence on the number of coatings. MLS measurements at 632.8 nm wavelength put into evidence that multilayer AZO planar waveguides support single and multi-well confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. The film refractive index increase with thickness and reach a saturation value when the number of layers is higher than 12. Moreover, the 24 layers AZO planar waveguide shows propagation loss lower than 1 dB/cm for a wavelength of 632.8 nm.

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