  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.

Materials Science