Investigation of the correlation between dielectric function, thickness and morphology of nano-granular ZnO very thin films

•c-Axis oriented ZnO thin films were grown with different morphological states.•The morphology and structures are controlled by controlling the thickness.•The optical properties are correlated to morphological evolution.•Two growth behaviors and property evolutions are identified around a critical thickness.

Thin nano-granular ZnO layers were prepared using a sol–gel synthesis and spin-coating deposition process with a thickness ranging between 20 and 120 nm. The complex dielectric function (ϵ) of the ZnO film was determined from spectroscopic ellipsometry measurements. Up to a critical thickness close to 60 nm, the magnitude of both the real and the imaginary parts of ϵ rapidly increases and then slowly tends to values closer to the bulk ZnO material. This trend suggests a drastic change in the film porosity at both sides of this critical thickness, due to the pre-heating and post-crystallization processes, as confirmed by additional characterization of the structure and the morphology of the ZnO films.