Compositional Mapping of the AlGaN Alloy Composition in Graded Buffer Structures Using Cathodoluminescence

Herein, the use of cathodoluminescence (CL) hyperspectral mapping in the quantification of the AlGaN alloy composition in graded buffer structures is explored. The quantification takes advantage of the known parabolic dependence of the AlGaN bandgap on the alloy composition allowing the AlGaN near-band-edge (NBE) emission energy recorded from CL to be converted to a composition. The proposed quantification method is first applied to cleaved cross-sections of two nominally step-graded AlGaN buffer structures each containing five AlGaN layers with different compositions. By comparing the compositions obtained from CL to those calculated using X-ray diffraction, a close agreement between values from both techniques is observed. However, due to a change in the bowing parameter, some deviation is observed for layers with compositions near 75%. The method is then applied to cleaved cross-sections of an AlGaN buffer whose group III precursor flow molar ratio is varied linearly throughout the growth. Herein, the hyperspectral nature of the CL datasets is exploited such as to produce composition maps by converting the relevant AlGaN-NBE emission energy at each pixel of the CL data to a composition.