  In this paper, we study band-to-band and intersubband characteristics of
GaN/AlN heterostructures (planar and nanowires) structurally designed to absorb
in the short-wavelength infrared region, particularly at 1.55 microns. We
compare the effect of doping the GaN sections with Si and Ge, and we discuss
the variation of free-carrier screening with the doping density and
well/nanodisk size. We observe that nanowire heterostructures consistently
present longer photoluminescence decay times than their planar counterparts,
which supports the existence of an in-plane piezoelectric field associated to
the shear component of the strain tensor, leading to lateral electron-hole
separation. We report intersubband absorption covering 1.45 microns to 1.75
microns using Ge-doped quantum wells, with comparable performance to
well-studied Si-doped planar heterostructures. We also report comparable
intersubband absorption in Si- and Ge-doped nanowire heterostructures
indicating that the choice of dopant is not an intrinsic barrier for observing
intersubband phenomena. In addition, we calculate the spectral shift of the
intersubband absorption due to many body effects as a function of the doping
concentration.
