  We have confirmed the presence of narrow, degenerately-doped axial silicon
nanowire (SiNW) $p$-$n$ junctions via off-axis electron holography (EH). SiNWs
were grown via the vapor-solid-liquid (VLS) mechanism using gold (Au) as the
catalyst, silane (SiH$_{4}$), diborane (B$_{2}$H$_{6}$) and phosphine
(PH$_{3}$) as the precursors, and hydrochloric acid (HCl) to stabilize the
growth. Two types of growth were carried out, and in each case we explored
growth with both $n$/$p$ and $p$/$n$ sequences. In the first type, we abruptly
switched the dopant precursors at the desired junction location, and in the
second type we slowed the growth rate at the junction to allow the dopants to
readily leave the Au catalyst. We demonstrate degenerately-doped $p$/$n$ and
$n$/$p$ nanowire segments with abrupt potential profiles of $1.02\pm0.02$ and
$0.86\pm0.3$ V, and depletion region widths as narrow as $10\pm1$ nm via EH.
Low temperature current-voltage measurements show an asymmetric curvature in
the forward direction that resemble planar gold-doped tunnel junctions, where
the tunneling current is hidden by a large excess current. The results
presented herein show that the direct VLS growth of degenerately-doped axial
SiNW $p$-$n$ junctions is feasible, an essential step in the fabrication of
more complex SiNW-based devices for electronics and solar energy.
