Metal/P-type GeSn Contacts with Specific Contact Resistivity down to 4.4×10⁻¹⁰ Ω-cm²

Ga and Sn surface-segregated p ⁺ -GeSn (Seg. p ⁺ -GeSn) was grown by molecular beam epitaxy (MBE) to achieve an average active Ga doping concentration of 3.4×10 ²⁰ cm ^-3 and surface Sn composition of more than 8%. This enables the realization of record-low specific contact resistivity ρ _c down to 4.4×10 ^-10 Ω-cm ² . The average ρ _c extracted from 14 sets of Ti/Seg. p ⁺ -GeSn Nano-TLM test structures, a collection of more than 90 devices is 6.5×10 ^-10 Ω-cm ² . This is also the lowest ρ _c for non-laser-annealed contacts. Ti contacts to p ⁺ -GeSn films with and without Ga and Sn surface segregation were fabricated. It is shown that the segregation of Ga and Sn at the Ti/p ⁺ -GeSn interface leads to 50% reduction in ρ _c as compared with a sample without segregation.