Amorphous phase as possible origin of additional absorption bands in polycrystalline ZnO films

作者：Mera J; Cordoba C; Gomez A; Paucar C; Moran O^*

来源：Journal of Non-Crystalline Solids, 2012, 358(23): 3229-3233.

DOI：10.1016/j.jnoncrysol.2012.08.025

• Semiconductors
• Chemical synthesis
• Optical properties

The optical absorption edge characteristics of undoped zinc oxide (ZnO) films with varying thickness (similar to 100 to similar to 1000 nm) prepared by means of polymeric precursor method (Pechini) on glass substrates have been investigated. Besides the normal absorption edge at about 3.3 eV, related to the transition between the valence and the conduction bands, the films feature additional absorption bands in the energy region around 4 eV. The experimental results show that this band is redshifted with increasing film thickness approaching a value of similar to 3.7 eV for the thickest films (similar to 1000 nm). In contrast, increasing film thickness resulted in a significant blue shift of the bulk absorption edge stabilizing at similar to 3.3 eV for the thickest films which corresponds to the single-crystal value. Although the appearance of additional absorption bands in ZnO has been attributed to excitonic resonances as those related to the exciton/longitudinal optical (LO) phonons, this argument is hardly sustainable for ZnO films with a high concentration of defects as those reported in the present investigation. The achieved results suggest that this phenomenon is rather linked to reduced crystallinity of the films. Certainly, the redshift of the additional optical band gap back towards the single-crystal value with increasing film thickness was strong evidence that the additional absorption bands should be related to an amorphous phase. Particularly, it is possible that the amorphortization of the film increases the localized states in the conduction band. The improvement of the crystal quality with increasing film thickness was distinctly verified by X-ray diffraction analysis.

• 出版日期2012-12-1