Observation of anisotropic distribution of microstructure in GaP/GaAs epitaxial layers

Abstract

Anisotropic distribution of microstructure in GaP/GaAs epitaxial layers is investigated by performing high resolution x-ray diffraction (HRXRD) experiments for symmetric reflections at different azimuths. The observed anisotropy leads to the different values of lattice relaxation along the [011¯] and [01¯1¯] directions as revealed by the reciprocal space maps recorded at 0° and 90° azimuths for (42¯2¯) and (422¯) reflections. The anisotropic relaxation process causes large differences in the full width at half maximum of (400) diffraction peaks of omega scans along the [011¯] and [01¯1¯] directions. It therefore provides large differences in the values of microstructural parameters of the GaP epitaxial layer along the two in-plane orthogonal directions. A systematic Williamson-Hall analysis of HRXRD data confirms the presence of large anisotropy of lattice relaxation process along the [011¯] and [01¯1¯] directions. Due to the large lattice mismatch, the GaP/GaAs epilayer presented here possesses grain boundaries and the broadening of the HRXRD pattern is strongly affected by the anisotropy of microstructure and dislocations. The kinetics of dislocations is discussed where it is observed that the misfit strain is primarily relieved by 90° partial dislocations up to a certain thickness of epilayer and thereafter the nucleation of 60° perfect dislocations dominates the strain relaxation process. The effect of the anisotropic distribution of relaxation process is also seen in the surface topography of GaP epilayer as revealed by atomic force microscopy images. The origin of the anisotropic distribution of dislocations is explained by considering the Burgers vector network, and its consequences are also observed in the cross sectional transmission electron microscopy images.