Single-crystalline two-dimensional van der Waals dielectrics for electronic devices

Abstract

Two-dimensional (2D) materials provide ideal platforms to conduct basic scientific research and explore many kinds of rich physical and chemical phenomena such as catalysis, ferroelectricity, semiconductor, magnetism, superconductivity, multiferrocity, and topological properties. On the other hand, a unique feature of layered structure, bound together by weak van der Waals (vdW) interactions makes the 2D nanoflakes can be easily transferred or fabricated the heterostructures using transferring process. These two features of 2D vdW layered materials offer a powerful and versatile platform for designing next-generation electronic devices. Among of these 2D vdW materials, 2D dielectrics behave many applications such as capacitor for storing energy, an insulator and a cooling agent in a transformer, and an electret (similarity to magnets). Especially for developing the next-generation electronics, it plays a key role in gating a semiconductor in electronic device. 2D vdWs dielectrics contains normal dielectrics, ferroelectrics and multiferroics. For normal 2D dielectrics, the junction between two layered two-dimensional materials is theoretically free of defects, making it ideal for carrier transport. However, the dielectric constants (k) of existing 2D vdW dielectrics are still low. The dielectric constant of the widely used vdW dielectric material h-BN is only about 3 ~ 5. For the vdW layered 2D ferroelectrics, most of them are semimetals or semiconductors. Actually, only CuInP2S6 has been widely used as a ferroelectric dielectric material. However, the low remanent polarization intensity (4 μC/cm²) is still low. All these challenges promote the urgent demand for discovering new 2D vdW dielectric/ferroelectric materials with good dielectric properties, a wide band gap, high polarization intensity, good air stability and insensitivity under ambient conditions. In this thesis, firstly, a family of high-k 2D vdW single-crystalline dielectrics Cu1-­xM’ xInP2S6 (CM’IPS, M’=Sn, Ag, Mn; 0.05 ≤ x ≤0.4) were grown using CVT method. Owing to their inherent layered structure, the bulk crystal can be easily exfoliated to flat nanoflakes as thin as sub-5 nm in thickness and as large as 450×220 μm². By inducing and optimizing non-ferroelectric phase within the van der Waals ferroelectric CIPS system, we obtained dielectric single crystals of Cu0.95Mn0.05InP2S6 (CMIPS), Cu0.9Sn0.1InP2S6 (CSIPS), and Cu0.8Ag0.2InP2S6 (CAIPS), exhibiting large dielectric constants ranging from 86 to 108. The surfaces are atomically smooth and free of dangling bonds. Due to the high dielectric constant and well-defined vdW heterointerface, few layers MoS2 FETs with CM’IPS gate dielectrics operate with a high on/off ratio (Ion/Ioff >107) and low SS (63 mV/dec). Secondly, we investigated the CVT growth of high-entropy 2D vdW single-crystalline dielectrics Cu1-5x(SnAgMnZnHf)xInP2S6 with high dielectric constant. By inducing entropy engineering in the CM’IPS high-k dielectrics, the thermal stability can be further enhanced. The grown high-entropy Cu0.8(SnAgMnZnHf)0.04InP2S6 (HE-MIPS) single crystals have centimeter dimensions and low cleavage energy. Fabricating nanoflakes with different layers (200 × 100 μm² in size and sub-3.5 nm in thickness) through mechanical exfoliation method is not difficult. HE-MIPS nanoflakes exhibit large dielectric constants about 20, resulting in good gate tunability. Few layers MoS2 FETs with HE-MIPS gate dielectrics with different thickness operate with a steep SS (66 mV/dec), high on/off ratio (Ion/Ioff >107), and low gate leakage current (~pA). Finally, we developed a new 2D van der Waals layered ferroelectric single crystals Bi5Te₂IO₁₁ (BTIO) that have never been report before. The polarization-electric field loop (P-E loop), local PFM results and ferroelectric field-effect transistor (FeFET) performance all demonstrate its ferroelectric properties. BTIO nanoflakes exhibit wide band gap (~3.49 eV), excellent dielectric properties (k=40), and remarkably high polarization intensity (Pr~20 μC/cm²). The FeFET devices show a high on/off ratio (>108) and low leakage current (~0.1 pA) indicating its promising applications as a good vdW ferroelectric dielectric. MFMIS FeFET devices shows long time memory property. Furthermore, Bi5Te₂XO₁₁ (X=Cl, Br, I) and Bi₁+4nTe₂nXO₁+10n (X=Br, I; n ≥ 1) superstructure materials are discussed. This thesis develops promising two-dimensional vdW layered dielectric/ferroelectric single crystals. These materials exhibit good potential in electronic devices, which provide more candidates in 2D dielectrics/ferroelectrics.