  The downscaling of silicon-based structures and proto-devices has now reached
the single atom scale, representing an important milestone for the development
of a silicon-based quantum computer. One especially notable platform for atomic
scale device fabrication is the so-called SiP delta-layer, consisting of an
ultra dense and sharp layer of dopants within a semiconductor host. Whilst
several alternatives exist, phosphorus dopants in silicon have drawn the most
interest, and it is on this platform that many quantum proto-devices have been
successfully demonstrated. Motivated by this, both calculations and experiments
have been dedicated to understanding the electronic structure of the SiP
delta-layer platform. In this work, we use high resolution angle-resolved
photoemission spectroscopy (ARPES) to reveal the structure of the electronic
states which exist because of the high dopant density of the SiP delta-layer.
In contrast to published theoretical work, we resolve three distinct bands, the
most occupied of which shows a large anisotropy and significant deviation from
simple parabolic behaviour. We investigate the possible origins of this fine
structure, and conclude that it is primarily a consequence of the dielectric
constant being large (ca. double that of bulk Si). Incorporating this factor
into tight binding calculations leads to a major revision of band structure;
specifically, the existence of a third band, the separation of the bands, and
the departure from purely parabolic behaviour. This new understanding of the
bandstructure has important implications for quantum proto-devices which are
built on the SiP delta-layer platform.
