Fig. 1. (Color online) Integrating ultra-thin high-κ dielectric on 2D materials. (a) The 3D AFM image of HfO₂ deposited directly on MoS₂ by ALD technique forms an island structure^[26]. (b) Schematic of top gate FET based on monolayer 2D TMDC^[7]. (c) Requirement of high-κ dielectric on 2D materials for low power device; (d) Device schematic of the top-gated MoS₂ FET with HfO₂ dielectric deposited by ozone pretreatment^[31]. (e) Structural schematic of few layer MoS₂ covered with the metal oxide buffer layer and HfO₂ film^[33]. (f) Illustration of the atomic structure of the bilayer MoS₂ FETs with a CaF₂ gate dielectric^[35]. A quasi-van der Waals interface is formed between the F-terminated CaF₂ (111) and the MoS₂ channel. (g) PTCA coated graphene. PTCA selectively adheres to graphene on SiO₂ surfaces, providing binding sites for ALD deposition. Inset is a top view of PTCA structure^[38].

Fig. 2. (Color online) Basic concept of NC and ultra-low power NCFETs based on 2D TMDC. (a) Schematic of two capacitors in series. V_int is the voltage at the interface, equaling to the radio of gate voltage V and body factor m. (b) Gibb’s free energy-polarization (U–P) diagram^[58] of FE, DE, and a series of them for capacitance matching and stabilization of negative capacitance. (c) Polarization–Voltage (P–V) curves deducted from U–P curves in (b). An S-shaped curve of FE is shown. (d) Polarization–electric field (P–E) loop of ferroelectric HZO and the schematic of back gate MoS₂ NCFETs. (e) I_D–V_GS curves of a MoS₂ NCFET^[16] on 4 nm AL₂O₃/20 nm HZO (black: forward sweep, red: reverse sweep, grey: gate leakage) and control device with the same channel length of 1.7 μm on 25 nm Al₂O₃ (blue) at V_DS = 0.1 V. (f) A brief summary of representative SS-hysteresis data of reported 2D NCFETs, including MoS₂ FETs with HZO^{[16, 18, 54, 55]}, PVDF^[59], CIPs^{[60, 61]}, and WSe₂ FETs with HZO dielectric^{[56, 57]}, while three typical works based on Si are selected for comparation. According to ITRS 2.0^[6], the requirements for scaled SS, 40 mV/dec in 2024, 25 mV/dec in 2030, are plotted respectively.