THE CONDUCTION-BAND AND SELECTION-RULES FOR INTERBAND OPTICAL-TRANSITIONS IN STRAINED GE1-XSIX GE AND GE1-XSIX/SI HETEROSTRUCTURES/

The conduction-band alignments of Ge1-xSix/Ge and Ge1-xSix/Si heterost ructures grown on (111) and (001) crystallographic planes, respectivel y, are analysed. We have obtained the selection rules for interband di pole optical transitions in the heterostructures, and discussed the po ssibilities for specifying the types of the lowest conduction-band min ima. We show that this can be done by, for example, exploring the pola rization of different phonon-assisted band-edge optical transitions. T he conduction-band minima may be at different L or Delta points of the Brillouin zone, depending on the structure parameters. Although bulk Ge, Si, and their alloy are indirect-gap semiconductors, the heterostr uctures Ge1-xSix/Ge and Ge1-xSix/Si can have a direct band gap. We fou nd the parameter regions where type-I and type-II band alignment were realized, and those where the band gap was direct in quantum well (QW) structures. It is shown that in direct-gap QW structures grown on (00 1) planes, direct interband optical transitions between the nearest el ectron and hole subbands are allowed, but the same transitions are for bidden for direct-gap structures grown on (111) planes.