Oxide electronics: Upward mobility rocks!

The technological advancements have made it possible to fabricate silicon and other mainstream semiconductors to purities of 99.999999% and grow single crystals up to 40 cm in diameter that are free from dislocations. The electrical transport becomes a more sensitive test of cleanliness for materials with such a high perfection than any structural or chemical characterization. Electrical cleanliness is measured by the mobility of the charge carriers in the material. Defects such as impurities and dislocations can be minimized in high-quality crystals, but dopants are needed to provide the mobile electrons and those dopants themselves severely limit the mobility. Three different approaches have been used with semiconductors to make two-dimensional electron gas (2DEG) sufficiently clean for fractional quantum Hall effect (FQHE) observations. The first approach is, modulation-doping, the route to GaAs, AlAs and SiGe 2DEG and another approach is 'field-effect' doping, applied to silicon and graphene, and last approach is 'polarization doping', which produces the 2DEG in GaN.