Measurement of the valence-band offset in strained Si/Ge (100) heterojunctions by x-ray photoelectron spectroscopy

We have used x-ray photoelectron spectroscopy to measure the valence-band offset in situ for strained Si/Ge (100) heterojunctions grown by molecular beam epitaxy. Si 2p and Ge 3d core level to valence-band-edge binding energies and Si 2p to Ge 3d core level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x-ray diffraction. Our measurements yield valence-band offset values of 0.83±0.11 eV and 0.22±0.13 eV for Ge on Si (100) and Si on Ge (100), respectively. If we assume that the offset between the weighted averages of the light hole, heavy hole, and spin-orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence-band edge of 0.49±0.13 eV.