Thickness-dependent lattice relaxation and the associated optical properties of ZnO epitaxial films grown on Si (111)

Abstract

The evolution of the strain state as a function of layer thickness of (0001) oriented ZnO epitaxial films grown by pulsed-laser deposition on Si (111) substrates with a thin oxide Y2O3 buffer layer was investigated by high resolution X-ray diffraction (XRD). The ZnO layers experience a tensile strain, which gradually diminishes with increasing layer thickness. Regions with a nearly strain-free lattice develop as the layer thickness exceeds a critical value and are correlated with the emergence of the oriented crack channels. The influence of the biaxial strain to the vibrational and optical properties of the ZnO layers were also studied by micro-Raman, optical reflectance, and photoluminescence. The deformation-potential parameters, aλ and bλ, of the E2(high) phonon mode are determined to be −740.8 ± 8.4 and −818.5 ± 14.8 cm−1, respectively. The excitonic transitions associated with the FXA, FXB, and D°XA emissions and the A-exciton binding energy all show linear dependence on the in-plane strain with a negative slope.