A 400Å thick strained layer, grown epitaxially on GaAs, was chosen for this study. XAFS experiments could not be used to study the InGaAs local structure in this sample because of strong interfering signals from the GaAs substrate and cap. The structure of the GaAs/InGaAs/GaAs sample is shown in Fig. 7a.
The lattice spacing of the 400Å InGaAs layer was larger than the lattice spacing of the GaAs substrate and cap, and the InGaAs diffraction peak was well separated from the GaAs peak. Figure 7b shows the measured x-ray diffraction pattern as a function of the Bragg angle near the GaAs (004) Bragg peak for a photon energy of eV. The intense peak at was produced by the GaAs substrate, while the broad peak at was produced by the InGaAs layer. The InGaAs peak is broad and has ``subsidiary'' side lobes, due to the finite thickness of the InGaAs layer. Because the InGaAs and GaAs peaks were well separated in , the Ga and As DAFS signals from the InGaAs peak could be measured without interference from the Ga and As in the substrate.