The presence of threading and misfit dislocations in modulation-doped heterostructures, and their effect upon the low-temperature electron mobility, were investigated. The density of threading dislocations was varied from 105 to 1011/cm2 by changing the gradient of Ge in the buffer layer. It was found that misfit dislocations always existed in the gradient-Ge buffer, and could also exist at the Si/channel-bottom interface. The former could be separated from the electron channel by including a thick relaxed SiGe buffer layer, and the latter could be avoided by making the Si channel thinner than the critical thickness. It was found that the electron mobility was sensitive to threading dislocations when their density exceeded 3 x 108/cm2, and decreased by 2 orders of magnitude when the threading dislocation density was 1011/cm2. Misfit segments in the graded-Ge buffer limited the mobility when the Si channel was 0.4 or less away from that buffer. Misfit dislocations at the Si/channel-bottom interface strongly scattered the electrons in the channel when a continuous network of misfit segments was created.

K.Ismail: Solid State Phenomena, 1996, 47-48, 409-18