Abrupt degenerately-doped silicon nanowire tunnel junctions.

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.