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The introduction of an undoped (AlGa)As spacer enhances significantly the low-temperature mobility in modulation-doped GaAs-(AlGa)As superlattices. Mobilities increase monotonically with spacer thickness. This indicates that ionized impurity scattering can be further suppressed by increasing the separation between carriers and their parent donors. Hall mobilities of 93 000 cm2/V sec were observed for average Hall densities of 4.9×1016 cm−3 at 4.2 K.
Störmer et al. (Fri,) studied this question.