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.