Low-Temperature Processed Ni/GeSn Optimal Contacts for Junctionless GeSn-on-Si FinFETs

Abstract

For junctionless FETs (JLFETs), an optimal ohmic contact is needed to achieve maximum drive current. The scaling of the source/drain (S/D) contact area impacts the contact resistivity ( $\rho _{c}$ ) of FETs, which limits their on current and switching speed. Minimizing the S/D series resistance along with ohmic contacts is the critical factor in JLFET design due to moderate doping levels at S/D. The Ni and Ge contacts optimized at a low temperature of $350~^{\circ }$ C by forming gas annealing (FGA) process and the computed contact resistance ( ${R}_{c}$ ), sheet resistance ( ${R}_{\text {sh}}$ ), and contact resistivity ( $\rho _{c}$ ) for Ni/p-GeSn contacts are $2.04\times 10^{-{3}}~\Omega \cdot \text {cm}$ , $63.96~\Omega $ /□, and $6.18\times 10^{-{8}}~\Omega \cdot \text {cm}^{{2}}$ , respectively. The impact of capping metal resistance ( ${R}_{m}$ ) is analytically examined for Ni/p-GeSn contacts using the modified circular transmission line model (cTLM). Furthermore, to study the metal cap resistance ( ${R}_{m}$ ) effect pragmatically, the optimized GeSn channel FinFET with width/length (W/L) 20/90 nm is analyzed by incorporating an extra metal cap at contacts and its electrical characteristics were compared with the control sample. The result demonstrate that the effect of metal resistance is very significant in low sheet resistance ( ${R}_{\text {sh}}$ ) materials, where ${R}_{\text {sh}}$ is close to ${R}_{m}$ .