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Two dimensional electron gases (2DEGs) realized at GaAs/AlGaAs singleinterfaces by molecular-beam epitaxy (MBE) reach mobilities of about 15 millioncm^2/Vs if the AlGaAs alloy is grown after the GaAs. Surprisingly, themobilities may drop to a few millions for the identical but invertedAlGaAs/GaAs interface, i.e. reversed layering. Here we report on a series ofinverted heterostructures with varying growth parameters including temperature,doping, and composition. Minimizing the segregation of both dopants andbackground impurities leads to mobilities of 13 million cm^2/Vs for invertedstructures. The dependence of the mobility on electron density tunes by a gateor by illumination is found to be the identical if no doping layers existbetween the 2DEG and the respective gate. Otherwise, it differs significantlycompared to normal interface structures. Reducing the distance of the 2DEG tothe surface down to 50nm requires an additional doping layer between 2DEG andsurface in order to compensate for the surface-Schottky barrier. Thesuitability of such shallow inverted structures for futuresemiconductor-superconductor hybrid systems is discussed. Lastly, ourunderstanding of the improved inverted interface enables us to produceoptimized double-sided doped quantum wells exhibiting an electron mobility of40 million cm^2/Vs at 1K.

The improved inverted AlGaAs/GaAs interface: its relevance for high-mobility quantum wells and hybrid systems