Near-edge conduction band electronic states in SiGe alloys

Spatially resolved electron energy loss spectroscopy (EELS) measurements in GeSi alloys illustrate the relationship of atomic structure to local electronic structure. Extending earlier measurements, where electronic structure was found to be controlled by composition in relaxed alloys, measurements in anisotropically strained alloys show splitting of normally degenerate band edges into two components. In a strained Si quantum well, this allows the engineered band offset to be followed from the GeSi substrate through the well to the alloy-capping layer. In the high-mobility conduction channel, the band edge is found to be very sharp, in spite of obvious composition roughness. Near a misfit dislocation under the Si well, the band edge can shift by as much as 0.25 eV due to local strain. Within the core of the defect, however, strictly local behavior dominates the observations. Local conduction band splitting and in-gap states are both observed.