  Atomically controlled crystal growth of thin films has established
foundations of nanotechnology aimed at the development of advanced functional
devices. Crystallization under non-equilibrium conditions allows engineering of
new materials with their atomically-flat interfaces in the heterostructures
exhibiting novel physical properties. From a fundamental point of view,
knowledge of the electronic structures of thin films and their interfaces is
indispensable to understand the origins of their functionality which further
evolves into realistic device application. In view of extreme surface
sensitivity of the conventional vacuum-ultraviolet (VUV) angle-resolved
photoemission spectroscopy (ARPES), with a probing depth of several angstroms,
experiments on thin films have to use sophisticated in-situ sample transfer
systems to avoid surface contamination. In this Letter, we put forward a method
to circumvent these difficulties using soft X-ray (SX) ARPES. A GaAs:Be thin
film in our samples was protected by an amorphous As layer with an thickness of
$\sim 1$ nm exceeding the probing depth of the VUV photoemission with photon
energy $h\nu$ around 100 eV. The increase of the probing depth with increasing
$h\nu$ towards the SX region has clearly exposed the bulk band dispersion
without any surface treatment. Any contributions from potential interface
states between the thin film and the amorphous capping layer has been below the
detection limit. Our results demonstrate that SX-ARPES enables the observation
of coherent three-dimensional band dispersion of buried heterostructure layers
through an amorphous capping layer, breaking through the necessity of surface
cleaning of thin film samples. Thereby, this opens new frontiers in diagnostics
of authentic momentum-resolved electronic structure of protected thin-film
heterostructures.
