Electronic contribution to secondary electron compositional contrast in the scanning electron microscope

Scanning electron microscopy of cleavage surfaces through a variable thickness SiGe0.25Si0.75 heterostructure is shown to reveal the high sensitivity of the secondary electron signal to small changes in band structure. Ge0.25Si0.75 layers that are coherently strained appear brighter in secondary electron micrographs than equal thickness layers of the unstrained Ge0.25Si0.75 alloy. This effect has been studied quantitatively and is explained in terms of the 0.1 eV strain-induced raising of the Ge0.25Si0.75 valence band edge resulting in an increased secondary electron escape probability.