  Suppressing electron scattering is essential to achieve high-mobility
two-dimensional electron systems (2DESs) that are clean enough to probe exotic
interaction-driven phenomena. In heterostructures it is common practice to
utilize modulation doping, where the ionized dopants are physically separated
from the 2DES channel. The doping-well structure augments modulation doping by
providing additional screening for all types of charged impurities in the
vicinity of the 2DES, which is necessary to achieve record-breaking samples.
Despite its prevalence in the design of ultra-high-mobility 2DESs, the working
principles of the doping-well structure have not been reported. Here we
elaborate on the mechanics of electron transfer from doping wells to the 2DES,
focusing on GaAs/AlGaAs samples grown by molecular beam epitaxy. Based on this
understanding we demonstrate how structural parameters in the doping well can
be varied to tune the properties of the 2DES.
