Charge–Ferroelectric Transition in Ultrathin Na_0.5Bi_4.5Ti₄O₁₅ Flakes Probed via a Dual‐Gated Full van der Waals Transistor

Abstract

control, which enhances the immunity to short channel effects. [1-5] Ferroelectric field-effect transistors (FeFETs) with a 2D channel and ferroelectric insulator take advantage of the large polarization charge density and high dielectric constant have recently attracted great attention in nonvolatile memory and low-power logic applications. [6-12] However, direct deposition of conventional ferroelectric insulators, such as hafnium or zirconium oxide (HfO 2 , ZrO 2) onto the 2D channel is challenging due to the absence of dangling bonds at the 2D semiconductor surface. Meanwhile, the manifest demonstration of ferroelectricity is critical to the thickness and doping of HfO 2 or ZrO 2 film. Transistors with polymer ferroelectric (e.g., poly[(vinylidene fluoride)-co-trifluoroethylene)] [P(VDF-TrFE)]) gated 2D channel have been thoroughly explored due to the facile device preparation. [13-16] However, the low polarizing performance and poor reliability of polymer ferroelectrics compared with inorganic ferroelectrics limit their applications in real device applications. Inorganic CuInP 2 S 6 (CIPS) down to ≈4 nm has recently been demonstrated to be compatible with van der Waals (vdW) heterostructure as room-temperature ferroelectric insulator for 2D FeFETs. [17-19] Nevertheless, its relatively low Curie point of 315 K (T c , phase transition from ferroelectric to paraelectric) hinders high temperature ferroelectric applications. Same issue is also found for some of the organic polymer ferroelectrics. Aurivillius bismuth layer-structured ferroelectric (BLSF) Na 0.5 Bi 4.5 Ti 4 O 15 (NBIT) has received significant attention in virtue of their high Curie temperature of ≈650 °C, high resistivity, low leakage current, low dielectric loss, low aging rate, and excellent thermal stability, which are attractive for hightemperature piezoelectric devices and ferroelectric nonvolatile random-access memory storage devices. [20-25] A high dielectric constant of 140-180 at room temperature has been reported in the NBIT synthesized by solid-state reaction process, and it is stable over an ultrawide temperature range from room temperature to ≈400 °C. [26] However, NBIT prepared by traditional solid-state reaction methods inevitably suffers from these issues, such as significant impurity phase, irregular flake shape, rough surface. [27] The nanoscale NBIT flakes synthesized by wet chemical methods have the problem of severe agglomeration Ferroelectric field-effect transistors (FeFETs) have recently attracted enormous attention owing to their applications in nonvolatile memories and low-power logic electronics. However, the current mainstream thin-film-based ferroelectrics lack good compatibility with the emergent 2D van der Waals (vdW) heterostructures. In this work, the synthesis of thin ferroelectric Na 0.5 Bi 4.5 Ti 4 O 15 (NBIT) flakes by a molten-salt method is reported. With a dry-transferred NBIT flake serving as the top-gate dielectric, dual-gate molybdenum disulfide (MoS 2) FeFETs are fabricated in a full vdW stacking structur...

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

“…For instance, metal-insulator transition has been observed in the MoS 2 transistor due to barrier-free graphene vdW contacts. [116] One order of magnitude higher mobility of 50 cm 2 V −1 s −1 has been measured in monolayer WS 2 with graphene vdW contact compared to the one with 3D metal contact. [112] The 2D-vdW-contacted device is easily embedded within h-BN encapsulation to prevent surface contaminations, offering an ideal device platform to investigate intrinsic properties for 2D semiconductors.…”

“…Conductive-AFM [41] MoS ML graphene [116] N-type 0 (Ohmic) NbS 2 [119] 0.08 45 WS 2 ML graphene N-type 300 (FB)…”

Reconfigurable Quasi‐Nonvolatile Memory/Subthermionic FET Functions in Ferroelectric–2D Semiconductor vdW Architectures

“…[16][17][18][19] Previously, we reported programmable device functions by tuning the competition between interfacial vacancy-induced charge dynamics and ferroelectric polarization in the 2D vdW FeFET with dualgate modulation. [20] Furthermore, oxygen vacancy migration in oxides has been commonly utilized to demonstrate resistive switching memory. [21] Nevertheless, the behavior of oxygen vacancies and their impact on FeFET functionalities have not been extensively investigated.…”

Polarization‐Induced Band‐Alignment Transition and Nonvolatile p‐n Junctions in 2D Van der Waals Heterostructures

“…[25] Although P(VDF-TrFE) organic ferroelectrics offer a flexible way to control the interfacial structure and doping polarity by using spincoating them on 2D channel materials, the low polarizing performance and poor stability limit their applications. [26] Third, most of the conventional ferroelectrics are insulators with larger band gaps (>3.5 eV), thus they cannot be directly used for the fabrication of semiconductor heterojunctions with 2D materials as the channel for multi-function device applications.…”