  We demonstrate a new fabrication process for hybrid
semiconductor-superconductor heterostructures based on anodic oxidation (AO),
allowing controlled thinning of epitaxial Al films. Structural and transport
studies of oxidized epitaxial Al films grown on insulating GaAs substrates
reveal spatial non-uniformity and enhanced critical temperature and magnetic
fields. Oxidation of epitaxial Al on hybrid InAs heterostructures with a
conducting quantum well show similarly enhanced superconducting properties
transferred to the two-dimensional electron gas (2DEG) by proximity effect,
with critical perpendicular magnetic fields up to 3.5 T. An insulating AlOx
film, that passivates the heterostructure from exposure to air, is obtained by
complete oxidation of the Al. It simultaneously removes the need to strip Al
which damages the underlying semiconductor. AO passivation yielded 2DEG
mobilities two times higher than similar devices with Al removed by wet
etching. An AO-passivated Hall bar showed quantum Hall features emerging at a
transverse field of 2.5 T, below the critical transverse field of thinned
films, eventually allowing transparent coupling of quantum Hall effect and
superconductivity. AO thinning and passivation are compatible with standard
lithographic techniques, giving lateral resolution below <50 nm. We demonstrate
local patterning of AO by realizing a semiconductor-based Josephson junction
operating up to 0.3 T perpendicular.
