  Over the last two decades, prototype devices for future classical and quantum
computing technologies have been fabricated, by using scanning tunneling
microscopy and hydrogen resist lithography to position phosphorus atoms in
silicon with atomic-scale precision. Despite these successes, phosphine remains
the only donor precursor molecule to have been demonstrated as compatible with
the hydrogen resist lithography technique. The potential benefits of
atomic-scale placement of alternative dopant species have, until now, remained
unexplored. In this work, we demonstrate successful fabrication of atomic-scale
structures of arsenic-in-silicon. Using a scanning tunneling microscope tip, we
pattern a monolayer hydrogen mask to selectively place arsenic atoms on the
Si(001) surface using arsine as the precursor molecule. We fully elucidate the
surface chemistry and reaction pathways of arsine on Si(001), revealing
significant differences to phosphine. We explain how these differences result
in enhanced surface immobilization and in-plane confinement of arsenic compared
to phosphorus, and a dose-rate independent arsenic saturation density of
$0.24{\pm}0.04$ ML. We demonstrate the successful encapsulation of arsenic
delta-layers using silicon molecular beam epitaxy, and find electrical
characteristics that are competitive with equivalent structures fabricated with
phosphorus. Arsenic delta-layers are also found to offer improvement in
out-of-plane confinement compared to similarly prepared phosphorus layers,
while still retaining >80% carrier activation and sheet resistances of $<2
k{\Omega}/{\square}$. These excellent characteristics of arsenic represent
opportunities to enhance existing capabilities of atomic-scale fabrication of
dopant structures in silicon, and are particularly important for
three-dimensional devices, where vertical control of the position of device
components is critical.
