Investigation of the thickness-dependent optical properties of ZnO films on Si and SiO₂ substrates from the Mid-infrared to the Vacuum-ultraviolet using UV and FTIR spectroscopic ellipsometry

The conventional approach to describe the dielectric function ε as a sum of oscillators sometimes fails because each term only has a single broadening parameter. Instead, we find it more convenient to describe ε as a product of Drude, TO/LO phonon, and electronic interband transition factors. Specifically, we explore the behavior of phonon and excitonic absorption in bulk zinc oxide (ZnO) and ZnO thin films grown on Si and SiO₂ using UV/VIS and FTIR spectroscopic ellipsometry. We characterized the structural properties of our ZnO films with x-ray diffraction, x-ray reflectivity, and atomic force microscopy.

We find that the real and imaginary parts of ε in ZnO films on Si and SiO₂ are much smaller than in bulk. Excitonic enhancement, absorption coefficient, and refractive index decrease monotonically with decreasing film thickness. The impact of the excitonic contribution to ε was described by Tanguy [1]. We also fit our ellipsometric spectra by describing the dielectric function of ZnO using the Tanguy model. Also, we investigate the thickness dependence of the Born effective charge, high-frequency and static dielectric constant, and exciton parameters.

References
[1] C. Tanguy, Phys. Rev. Lett. 75, 4090 (1995).