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 <110> 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, a_λ and b_λ, 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 FX_A, FX_B, and D°X_A emissions and the A-exciton binding energy all show linear dependence on the in-plane strain with a negative slope.