Surface Morphology and Roughness of Sulfur-Doped ZnO Thin Films: Analysis Based on Atomic Force Microscopy

The surface morphology of undoped ZnO as well as 3 at. % sulfur-doped ZnO (ZnO:S) thin films were examined utilizing atomic force microscopy (AFM). Surface characteristics evaluations and comparisons were made based on 2D and 3D AFM images, line profile analyses, and roughness parameters; Ra, Rq, Rz, Rt Rsk, and Rku. The undoped ZnO medium showed a smooth surface, with moderate height fluctuations and a comparatively narrow Gaussian-like height. On the other hand, ZnO:S film showed much higher surface roughness and topographical alternation with larger and more symmetrical height histograms. Both the Rq/Ra ratios for both started at around the theoretical Gaussian value (~1.25) with the skewness and kurtosis parameters showing distinctly different degrees of surface symmetry and texture. Sulfur incorporation was shown to change the grain morphology, to introduce peak-to-valley contrast and to increase the overall surface area. The morphological improvements further show that ZnO:S thin films could be more adequate for applications where high surface activity is essential, provided by gas sensing and catalysis. This study presents a quantitative and qualitative evaluation of the influence of sulfur doping on the surface morphology of ZnO at the nanoscale level.