  We report an investigation on the observation of ultrahigh photoresponse
(photogain, G_Pc>106) in single nanowire photodetectors of diameter < 100 nm.
The investigation which is a combination of experimental observations and a
theoretical analysis of the ultrahigh optical response of semiconductor
nanowires, has been carried out with emphasis on Ge nanowires. Semiconductor
nanowire photodetectors show a signature of photogating where G_Pc rolls-off
with increasing illumination intensity. We show that surface band bending due
to depleted surface layers in nanowires induces a strong radial field (~ 108
V/m at the nanowire surface) which causes physical separation of photogenerated
electron-hole pairs. This was established quantitatively through a
self-consistent theoretical model based on coupled Schrodinger and Poisson
Equations. It shows that carrier separation slows down the surface
recombination velocity to a low value (< 1 cm/s) thus reducing the carrier
recombination rate and extending the recombination lifetime by few orders of
magnitude. An important outcome of the model is the prediction of G_Pc ~ 106 in
a single Ge nanowire (with diameter 60 nm), which matches well with our
experimental observation. The model also shows an inverse dependence of G_Pc on
the diameter that has been observed experimentally. Though carried out in
context of Ge nanowires, the physical model developed has general applicability
in other semiconductor nanowires as well.
