Characterization of defects and strain in the (Al_xGa_(1−x))_0.5In_0.5P/ GaAs system by synchrotron X-ray topography

Synchrotron white beam X-ray topography and monochromatic beam topography have been applied to study GaAs substrates and (Al_xGa_(1−x))_0.5In_0.5P epitaxial layers grown on them that are being developed for LEDs. The GaAs substrates are 15° offcut toward [1 1 0] and mainly consist of dislocations in the {1 1 1} slip planes and low angle grain boundaries. The nominally lattice matched (Al_xGa_(1−x))_0.5In_0.5P layers contain defects such as threading dislocations replicated from the substrates, inclusions and misfit dislocations generated during organometallic vapor phase epitaxy (OMVPE) growth. The existence of misfit dislocations (MDs) indicates that strain relaxation has taken place to accommodate the in-plane lattice mismatch between epitaxial layer and substrate. The driving force for the glide of threading dislocations and relaxation efficiency of dislocations with different Burgers vectors and glide planes has been analyzed in the wafers with 15° offcut to predict the Burgers vectors and line directions of dislocations that dominate strain relaxation. Burgers vectors have been determined using ray tracing simulations. Reciprocal space maps (RSMs) recorded from the wafers to measure the strain and tilt of the epilayers show both in-plane and out-of-plane lattice mismatch as well as tilt between epilayer and substrate. A modified Nagai′s model has been developed to analyze and account for in-plane and out-of-plane tilt.