  Understanding the relaxation and recombination processes of excited states in
two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is
essential for developing efficient optical and (opto)electronic devices which
integrate new 2D materials with more conventional 3D ones. In this work, we
unveil the carrier dynamics and charge transfer in a monolayer of WSe$_2$ on a
GaAs substrate. We use time-resolved differential reflectivity to study the
charge relaxation processes involved in the junction and how they change when
compared to an electrically decoupled heterostructure, WSe$_2$/hBN/GaAs. We
observe that the monolayer in direct contact with the GaAs substrate presents
longer optically-excited carrier lifetimes (3.5 ns) when compared with the
hBN-isolated region (1 ns), consistent with a strong reduction of radiative
decay and a fast charge transfer of a single polarity. Through low-temperature
measurements, we find evidence of a type-II band alignment for this
heterostructure with an exciton dissociation that accumulates electrons in the
GaAs and holes in the WSe$_2$. The type-II band alignment and fast
photo-excited carrier dissociation shown here indicate that WSe$_2$/GaAs is a
promising junction for new photovoltaic and other optoelectronic devices,
making use of the best properties of new (2D) and conventional (3D)
semiconductors.
