Optical Characterization of ZnO Nanoforest for Hardware Security Applications

Disordered systems or materials with intrinsic variability in some measurable quantity are recently being considered for hardware security applications like physical unclonable functions or PUFs. An inexpensive optical PUF system based on a dense nanoforest of ZnO nanorods is presented here. A relatively concentrated seed solution used in the synthesis process results in these high density nanorods grown mostly in the upright direction with some vertical hollow spots in between neighboring nanorods. This nanorod arrangement grown on optically reflective substrates works as a nano-porous thin film and produces Fabry-Pérot fringes under perpendicularly incident focused white light. The local variations in density, height, diameter, orientation angle, and hexagonal perfection of the ZnO nanorods give rise to unique and reproducible scattering light spectra that vary significantly from one spot on the sample to another. Over 1500 optical measurements are performed to show the strongly variable optical characteristics. These distinct spectra show promise as the raw responses for ZnO-based optical PUFs.