Measurements of the energy band offsets of Si1−xGexSi and Ge1−yCyGe heterojunctions

Discontinuities in the energies of the conduction and valence bands at semiconductor heterojunctions are important parameters for device design. We describe experiments using X-ray photoelectron spectroscopy with measurements of valence-band energies with respect to core-levels of metastable, coherently strained Si1−xGex alloy layers and of thick Ge1−yCy alloy layers. For strained Si1−xGex alloys on Si, we have found that the valence band offset increased with the Ge fraction x with most of the offset in the valence band. We obtained a valence band offset of 0.22 eV for x=0.23, in good agreement with theoretical calculations. For Ge1−yCy alloys, we found very little shift in the valence band energies with the C fraction y. Since the optical bandgap of GeC increased with the C fraction y, most of the offset for Ge1-yCyGe heterojunction was in the conduction band. Based on the measurements of the energy band offsets of Si1−xGexSi, we infer that the major portion of bandgrap discontinuity of Ge1−yCy on Si is in the valence band. Ge1−yCy alloys are new matastable materials that open up a new region for group IV heterostructures.