  Electron Holographic Tomography was used to obtain 3-dimensional
reconstructions of the morphology and electrostatic potential gradient of axial
GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two
opposite directions: GaInP:Zn/InP:S and InP:Sn/GaInP:Zn, using Zn as the p-type
dopant in the GaInP, but with changes to the n-type dopant (S or Sn) in the
InP. Secondary electron and electron beam induced current images obtained using
scanning electron microscopy indicated the presence of p-n junctions in both
cases and current-voltage characteristics measured via lithographic contacts
showed the negative differential resistance, characteristic of band-to-band
tunneling, for both diodes. EHT measurements confirmed a short depletion width
in both cases ($21 \pm 3$ nm), but different built-in potentials, $V_{bi}$, of
1.0 V for the p-type (Zn) to n-type (S) transition, and 0.4 V for both were
lower than the expected 1.5 V for these junctions, if degenerately-doped.
Charging induced by the electron beam was evident in phase images which showed
non-linearity in the surrounding vacuum, most severe in the case of the
nanowire grounded at the \emph{p}-type Au contact. We attribute their lower
$V_{bi}$ to asymmetric secondary electron emission, beam-induced current
biasing and poor grounding contacts.
