  Hyperdoping consists of the intentional introduction of deep-level dopants
into a semiconductor in excess of equilibrium concentrations. This causes a
broadening of dopant energy levels into an intermediate band between the
valence and conduction bands.[1,2] Recently, bulk Si hyperdoped with chalcogens
or transition metals has been demonstrated to be an appropriate
intermediate-band material for Si-based short-wavelength infrared
photodetectors.[3-5] Intermediate-band nanowires could potentially be used
instead of bulk materials to overcome the Shockley-Queisser limit and to
improve efficiency in solar cells,[6-9] but fundamental scientific questions in
hyperdoping Si nanowires require experimental verification. The development of
a method for obtaining controlled hyperdoping levels at the nanoscale
concomitant with the electrical activation of dopants is, therefore, vital to
understanding these issues. Here, we show a CMOS-compatible technique based on
non-equilibrium processing for the controlled doping of Si at the nanoscale
with dopant concentrations several orders of magnitude greater than the
equilibrium solid solubility. Through the nanoscale spatially controlled
implantation of dopants, and a bottom-up template-assisted solid phase
recrystallization of the nanowires with the use of millisecond-flash lamp
annealing, we form Se-hyperdoped Si/SiO2 core/shell nanowires that have a
room-temperature sub-band gap optoelectronic photoresponse when configured as a
photoconductor device.
