Thickness-dependent stress relaxation and its unreported effect on optical band gap of Al-doped ZnO thin films have been investigated. The thinnest film (∼84 nm) had a stress of −8.39×109 Nm−2, carrier concentration of 1.73×1019 cm−3 and optical band gap of 3.69 eV, a value significantly higher than the reported ones. With increase in thickness, magnitude of the stress decreased, and correspondingly a redshift of fundamental absorption band edge was observed. A linear dependence of optical band gap on stress in the films with a coefficient of 54.6 meV/GPa has been observed.
The effects of thickness on flexibility and crack initiation in ZnO : Al thin films sputter-deposited on polyethersulfone substrates have been investigated. With an increase in thickness, root-mean-square roughness and average crystallite size increase linearly. It is found that the higher the thickness, the lower is the strain required to initiate cracks in the film. The thinnest film (∼240 nm) exhibits a crack-initiating critical strain of 0.96% and a saturated crack density of 0.10 µm−1. A critical energy release rate of 68.5 J m−2 and a mode I fracture toughness of 3.2 MPa m0.5 are estimated for the films. These parameters are found to exhibit a linear dependence on film thickness.
AlN passivation layer-mediated improvement in tensile failure of ZnO:Al thin films on polyethersulfone substrates is investigated. ZnO:Al films without any passivation layer were brittle with a crack-initiating bending strain εc of only about 1.13% with a saturated crack density ρs of 0.10 μm(-1) and a fracture energy Γ of 49.6 J m(-2). On passivation by an AlN overlayer, the fracture energy of the system increased considerably and a corresponding improvement in εc was observed. AlN layers deposited at higher discharge powers yielded higher fracture energy and exhibited better performance in terms of εc and ρs.
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.
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