Analysis on reflection spectra in strained ZnO thin films

TL;DR

This study investigates how biaxial in-plane strain affects the optical reflection spectra of ZnO thin films, revealing strain-dependent shifts in excitonic features and calculating related optical parameters.

Contribution

It provides a detailed analysis of strain effects on ZnO excitons using reflectivity spectra and theoretical modeling, which was not previously comprehensively studied.

Findings

E_LT for A-excitons shows a minimum around zero strain.

ELT for B-excitons increases with strain.

Theoretical spectra match experimental data well.

Abstract

Thin films of laser molecular-beam epitaxy grown ZnO films were studied with respect to their optical properties. 4-K reflectivity was used to analyze various samples grown at different biaxial in-plane strain. The spectra show two structures at 3.37 eV corresponding to the A-free exciton transition and at 3.38 eV corresponding to the B-free exciton transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model. Thus, the transverse energies, the longitudinal transversal splitting (ELT,), the oscillator strengths, and the damping parameters were determined for both the A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the energy E_LT for the A-excitons is characterized by a negatively-peaking behavior with a minimum around the zero strain, while ELT for the B-excitons is an increasing function of the strain field values.