Tuning the Porosity of Piezoelectric Zinc Oxide Thin Films Obtained from Molecular Layer-Deposited “Zincones”

Porous zinc oxide (ZnO) thin films were synthesized via the calcination of molecular layer-deposited (MLD) "zincone" layers. The effect of the MLD process temperature (110 degrees C, 125 degrees C) and of the calcination temperature (340 degrees C, 400 degrees C, 500 degrees C) on the chemical, morphological, and crystallographic properties of the resulting ZnO was thoroughly investigated. Spectroscopic ellipsometry reveals that the thickness of the calcinated layers depends on the MLD temperature, resulting in 38-43% and 52-56% of remaining thickness for the 110 degrees C and 125 degrees C samples, respectively. Ellipsometric porosimetry shows that the open porosity of the ZnO thin films depends on the calcination temperature as well as on the MLD process temperature. The maximum open porosity of ZnO derived from zincone deposited at 110 degrees C ranges from 14.5% to 24%, rising with increasing calcination temperature. Compared with the 110 degrees C samples, the ZnO obtained from 125 degrees C zincone yields a higher porosity for low calcination temperatures, namely 18% for calcination at 340 degrees C; and up to 24% for calcination at 500 degrees C. Additionally, the porous ZnO thin films were subjected to piezoelectric measurements. The piezoelectric coefficient, d(33), was determined to be 2.8 pC/N, demonstrating the potential of the porous ZnO as an, e.g., piezoelectric sensor or energy harvester.