  Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is
presented. The structures fabrication procedure is based on delta-doped layers
grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap
layer to protect the active semiconductor layers during ex situ transfer. The
superconductor is then sputter deposited in situ after thermal desorption of
the protective layer. Two types of structures in particular will be discussed,
i.e., a reference junction and the engineered one that contains an additional
insulating AlGaAs barrier inserted during the growth in the semiconductor. This
latter configuration may give rise to controlled interference effects and
realizes the model introduced by de Gennes and Saint-James in 1963. While both
structures show reflectionless tunneling-dominated transport, only the
engineered junction shows additionally a low-temperature single marked
resonance peaks superimposed to the characteristic Andreev-dominated subgap
conductance. The analysis of coherent magnetotransport in both microstructures
is successfully performed within the random matrix theory of Andreev transport
and ballistic effects are included by directly solving the Bogoliubov-de Gennes
equations. The impact of junction morphology on reflectionless tunneling and
the application of the employed fabrication technique to the realization of
complex semiconductor-superconductor systems are furthermore discussed.
