Operating regimes and contact resistance of side-bonded contacts to thin heavily doped semiconductor nanowires

Abstract

In many nano-wire (NW) devices and during NW characterization, the contact is bonded on the surface or the side of the NW. The prior model of such side-bonded contacts assumes partial NW depletion and purely radial tunneling, both restricted to the contacted region. However, the real space-charge extends to the non-contacted NW, aided by the fringing field, and depletes the contacted NW fully for small NW radius, R. In addition, there are non-radial tunneling and generation-recombination near the contact edge. Supported by numerical calculations, the present work shows that when all the effects are included, different regimes of operation manifest in a side-bonded contact, and the space-charge and contact resistance can differ widely from prior predictions. Our calculations span contacts with barrier height, ϕb0 = 0.4–0.8 V on n-type silicon NWs of R = 7.5–20 nm and doping Nd = 1018–1020 cm−3 and include the effects of dielectric confinement, NW length, surface defects, image force barrier lowering, and heavy doping. We find that a side-bonded contact gets fully depleted at the contact edge for Nd≤αFD[4εsψ0/qR2], where ψ0 = contact potential and αFD = 0.73 (0.88) for air (SiO2) ambient. Furthermore, the behavior of a side-bonded contact approaches that of an end-bonded contact for Nd≤αEB[4εsψ0/qR2], where αEB = 0.16 (0.30), while surface space-charge widths in the two contacts match over a much wider Nd range for SiO2 ambient. We express the radial depletion width in the NW as an explicit function of the contact potential based on an available implicit relation.