Infrared reflectance studies of hillock-like porous zinc oxide thin films

• Multilayer porous assumption qualitatively increased the overall spectra fitting.
• IR reflectance is a sensitive method to probe the multilayer porous structure.
• Hillock-like porous ZnO thin films fabricated using electrochemical etching method.
• The thickness and porosity of the samples were determined.
• Formation of extra resonance hump was due to splitting of reststrahlen band.

We investigated the infrared (IR) reflectance characteristics of hillock-like porous zinc oxide (ZnO) thin films on silicon substrates. The IR reflectance spectra of the porous samples exhibited an extra resonance hump in the reststrahlen region of ZnO compared with the as-grown sample. Oscillation fringes with different behaviors were also observed in the non-reststrahlen region of ZnO. Standard multilayer optic technique was used with the effective medium theory to analyze the observations. Results showed that the porous ZnO layer consisted of several sublayers with different porosities and thicknesses. These findings were confirmed by scanning electron microscopy measurements.