Space-charge and current non-uniformities, and contact resistivity of end-bonded metal contacts to thin heavily doped semiconductor nanowires

Prior work calculated the contact resistivity, ρcN, of end-bonded metal contacts on n-type silicon NanoWires (NWs) of radius, R = 5-10 nm and doping, Nd = 1018-1020 cm-3; it described the internal physics briefly and qualitatively, and considered long NWs and high barrier heights, øb0 = 0.8-1.0 V. The present work extends the ρcN calculations to low øb0 = 0.4-0.6 V contacts with long NWs, and to low to high øb0 = 0.4-0.8 V contacts with short NWs of length, LNW ≤ long NW space-charge width. It calculates the space-charge and current distributions in contacts with LNW = 50 nm-2 μm and low to high øb0 = 0.4-0.8 V to provide the following insights: (i) radial non-uniformity in space-charge width and current are high enough to violate the plane space-charge edge assumption employed to model NW junctions analytically. The non-uniformity peaks at Nd where the bulk depletion width ≈2.14R for large metals and small surface charge. (ii) Low øb0 contacts are tunneling dominated, and their ρcN versus Nd behaviour differs qualitatively from that of high øb0 contacts. (iii) Practical LNW can be ≤long NW space-charge width, for Nd ≤ 1019 cm-3. Shortening the LNW reduces the space-charge non-uniformity and increases (reduces) the tunneling [generation-recombination (GR)] current. However, the current non-uniformity is unaffected, and the change in current or ρcN is disproportionately smaller than that in LNW, since the tunneling or GR causing these occur over a small region near the junction which is less affected by NW shortening. All our calculations include the effects of contact geometry, surface defects, dielectric confinement, image force, and heavy doping. Our work provides an experimentalist clear qualitative understanding over a wide range of conditions.