Determination of the local Al concentration in Al_xGa _1-xAs-GaAs quantum well structures using the (200) diffraction intensity obtained with a 10 Å electron beam

A novel method for determining the local concentration of Al in the Al _xGa_1-xAs layer of Al_xGa_1-xAs-GaAs multiple quantum well structures is reported. By scanning a 10 Å electron beam across the interface, the (200) dark-field scanning transmission electron microscopy (STEM) image shows the contrast of the Al_xGa _1-xAs-GaAs multilayer since the intensity of the (200) diffraction is sensitive to the Al concentration. The line scan intensity profile of the (200) diffraction, along a uniform specimen region of known thickness, shows the intensity variation of the (200) diffraction and reflects the local content of Al in each region. The simulation of the nanodiffraction patterns produces a chart of the (200) diffraction intensity versus the Al concentration for the determination of the local change of the Al concentration. A molecular beam epitaxy grown Al_xGa_1-xAs-GaAs specimen (x=0.57 as determined from Raman spectroscopy) is tested and the dark-field STEM studies show two thin layers of x=0.46 at the 1/3 and 2/3 height level within every Al_xGa_1-xAs layer.