Graded electronic structure in a 3 nm strainedGe40Si60quantum well

Spatially resolved electron energy loss spectra of the Si 2p core excitation are obtained in transmission through a single cross section of a 3 nm thick ${\mathrm{Ge}}_{40}$ ${\mathrm{Si}}_{60}$ quantum well. The spectra yield the positions of the ${\mathit{L}}_1$ and ${\mathrm{\Delta }}_1$ conduction band minima, and the biaxial strain splitting of ${\mathrm{\Delta }}_1$. From annular dark field images, the well composition appears to be graded over three to four layers on the cap side. The conduction band offset varies from 0 eV at the well edge to +25 meV in the well center, relative to the Si substrate and cap. The biaxial strain splitting is 0.28 eV in the well center and shows an asymmetry with position that is consistent with the annular dark field data.