Excitonic absorption and optical properties of ZnO films

The analysis of excitonic effects on the optical properties of thin films is crucial for technological applications. Wide band gap materials like ZnO exhibit strong excitonic features in the dielectric function (ε). The influence of this excitonic absorption on ε was described by Tanguy. Here we demonstrate the behavior of excitons at interfaces in c-axis oriented ZnO thin films on Si and SiO₂ substrates using visible/UV and FTIR ellipsometry. We also performed XRD, XRR, and AFM to characterize the structural properties of our ZnO films. The real and imaginary parts of ε in thin ZnO films on Si are much smaller than in bulk ZnO. We find that the excitonic enhancement decreases monotonically with decreasing film thickness and disappears in thin films. A similar behavior can be seen for ZnO films on SiO₂ as a function of thickness. We will fit our ellipsometric spectra by describing the dielectric function of ZnO using the Tanguy model. We will analyze the dependence of the excitonic Tanguy parameters on film thickness and substrate material.