Low-leakage WSe2 FET gate-stack using titanyl phthalocyanine seeding layer for atomic layer deposition of Al2O3

Summary form only given. The fabrication of top-gated transition metal dichalchogenide (TMD) field-effect transistors (FETs), requires a uniform and pinhole-free gate dielectric. For realization of TMD tunnel FETs (TFETs) a high-k gate dielectric with subnanometer equivalent oxide thickness (EOT) is required. However, deposition of a thin, uniform high-k dielectric on a TMD surface without functionalization is challenging [1]. This is due to the lack of dangling bonds on the TMD surface. In this report, titanyl (TiO) phthalocyanine (C₃₂H₁₈N₈), TiOPc, is used successfully as a seeding layer on WSe₂ to enable the atomic layer deposition of Al₂O₃. TiOPc is an organic semiconductor with a band gap of approximately 2 eV in air [2]. We demonstrate the first top-gated TMD FET with an EOT as low as 2.2 nm (ε_Al2O3 = 9) and a gate leakage current density of 0.2 pA/μm² at 1 V gate bias. The best prior report we have found, by Liu [3] on MoS₂, achieved an EOT of 6.9 nm with a gate leakage current density of 2 pA/μm². As a point of reference the ITRS [4] targets an ultimate EOT for CMOS approaching 0.4 nm with a current density below 0.25 pA/μm². Here, we compare the electrical characteristics of top-gated TiOPc-seeded Al₂O₃ WSe₂ FETs against the characteristics of the same transistor with an ALD Al₂O₃ back gate. We also report the improvements obtained by post-deposition high vacuum annealing.