  Quantum well (QW) heterostructures have been extensively used for the
realization of a wide range of optical and electronic devices. Exploiting their
potential for further improvement and development requires a fundamental
understanding of their electronic structure. So far, the most commonly used
experimental techniques for this purpose have been all-optical spectroscopy
methods that, however, are generally averaged in momentum space. Additional
information can be gained by angle-resolved photoelectron spectroscopy (ARPES),
which measures the electronic structure with momentum resolution. Here we
report on the use of extremely low energy ARPES (photon energy $\sim$ 7 eV) to
increase its depth sensitivity and access buried QW states, located at 3 nm and
6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures,
respectively. We find that the QW states in cubic-GaN/AlN can indeed be
observed, but not their energy dispersion because of the high surface
roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to
be detected by extremely low energy ARPES. Since the sample surface is much
flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in
momentum space, which can be reconducted to the band structure of the topmost
surface layer of the QW structure. Our results provide important information
about the samples' properties required to perform extremely low energy ARPES
experiments on electronic states buried in semiconductor heterostructures.
