Origin of Prestress-Driven Optical Modulations of Flexible ZnO Thin Films Processed in Stretching Mode

Experimental verification of optical modulation with external stress has not been easily available in flexible systems. Here, we intentionally induced extra stress in wide band gap ZnO thin films by a unique prestress-driven deposition processing that utilizes a stretching mode. The stretching mode provides homogeneous but biaxial stresses in the hexagonal wurtzite structure, leading to the extension of the c-axis and the contraction of the a-axis. As a result, the reduction of the optical band gap by ∼150 meV was observed for the strain of ∼4.87%. The band gap narrowing was found to occur from the respective downward and upward shifts of the conduction band minimum and valence band maximum under the applied stress. The experimental evidence of optical modulations was supported by the theoretical calculations using density functional theory. The reduced strong interactions between Zn d and O p orbitals were assumed to be responsible for the band gap narrowing.