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 < 11 (2) over bar0 > 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(lambda) and b(lambda), of the E-2(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 degrees X-A emissions and the A-exciton binding energy all show linear dependence on the in-plane strain with a negative slope.