在最新出版的《半导体学报》2019年第6期上，北京大学物理学院戴伦教授介绍了一种室温稳定的二维铁电材料α-In2Se3及其器件应用。二维室温铁电薄膜具有稳定的层状结构、原子级的厚度、弱层间耦合、电极化翻转电压低等优点，有望在探索二维铁电性、发展低功耗器件应用中发挥重要作用。原子级薄的α-In2Se3材料是一种新兴的铁电半导体，其具有耦合的面内和面外铁电性。目前已观察到α-In2Se3的电极化在高达700K的温度下依然稳定。几种基于少层α-In2Se3的原型器件也陆续报道，包括可切换的铁电二极管和铁电半导体晶体管。此外，人们通过将不同的二维材料集成到范德华（vdW）异质结构中还发展出多种新型器件。最近，研究表明铁电体可以具有负电容。将室温稳定的铁电α-In2Se3与其他二维半导体（例如MoS2）集成制备的负电容场效应晶体管可以超越金属氧化物半导体场效应晶体管（MOSFET）的理论极限，是一类新型的高效电子器件。
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专家视点 | 戴伦：室温稳定的二维铁电材料
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Ferroelectric materials, with spontaneous electric polarization that can be reversed by an external electric field, have many technological applications, such as non-volatile memories, field-effect transistors, and sensors. Ferroelectric polarization originates from an asymmetric distribution of atoms in a material’s crystal structure, requiring the material to have two energetically degenerate ground state structures (two stable spontaneous polarization states) with inversion symmetry breaking. Based on Landau-Ginzburg-Devonshire theory, a ferroelectric material below its transition temperature is described by a double well free energy (F) landscape as a function of the electric polarization (P) as shown in Fig. 1. Thin-film ferroelectrics have the advantage to significantly scale down the dimensions of the devices. Moreover, the voltage used to flip the electric polarization can be much lower, satisfying the requirement for low-power consumption devices. However, in conventional thin-film ferroelectrics, the arrangement of atoms produces charges on the material’s surface, generating a strong opposite depolarization field to suppress the polarization.
Figure1. (Color online) Double-well landscape of the free energy F in a ferroelectric as a function of the electric polarization P. Insets: the two energetically degenerate state with opposite electric polarizations of α-In2Se3. The size-view ball-and-stick schematic illustrations are cited from Ref. [3].
Recently, theoretical study predicted that the ferroelectric polarization in 2D semiconducting α-In2Se3 is driven by local covalent bonds (not by long-range interactions), which are strong enough to prevent the depolarization field from suppressing the polarization[3]. In addition, the two opposite electric polarization states only differ at the energetically degenerate positions of the central Se-layer atoms, enabling the locked in-plane and out-of-plane electric polarizations in the 2D α-In2Se3 (Fig. 1). Stimulated by this unprecedented inter-locking of electric dipoles in α-In2Se3, intense experimental efforts were devoted with various techniques. Clear ferroelectric domains, ferroelectric hysteresis loop, and piezoelectricity have been observed in ultrathin samples with the thickness down to 2D limit by using Piezo force microscopy (PFM) and second-harmonic generation (SHG)[4–6]. The electric polarization of α-In2Se3 is observed to be stable at temperature up to 700 K. Besides, several prototype devices based on the few layer α-In2Se3, including switchable ferroelectric diode and ferroelectric semiconductor transistor were demonstrated.
The isolation of 2D layered materials allows to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Recently, it was shown that ferroelectrics can have negative capacitance[7]. Negative-capacitance field-effect transistors (NC-FET) integrated of room-temperature stable ferroelectric α-In2Se3 with other 2D semiconducting channels (e.g. MoS2) could bypass the Boltzmann tyranny of a metal–oxide–semiconductor FET (MOSFET)[8], which is highly needed for energy-efficient electronics beyond fundamental limits.
References：
[1] Birol T. Stable and switchable polarization in 2D. Nature, 2018, 560, 174
[2] Hoffmann M, Fengler F P G, Herzig M, et al. Unveiling the double-well energy landscape in a ferroelectric layer. Nature, 2019, 565, 464
[3] Ding W, Zhu J, Wang Z, et al. Prediction of intrinsic two-dimensional ferroelectrics in In2Se3 and other III2–VI3 van der Waals materials. Nat Commun, 2017, 8, 14596
[4] Xiao J, Zhu H, Wang Y, et al. Intrinsic two-dimensional ferroelectricity with dipole locking. Phys Rev Lett, 2018, 120, 227601
[5] Zhou Y, Wu D, Zhu Y, et al. Out-of-plane piezoelectricity and ferroelectricity in layered α-In2Se3 nanoflakes. Nano Lett, 2017, 17, 5508
[6] Wan S, Li Y, Li W, et al. Room-temperature ferroelectricity and a switchable diode effect in two-dimensional α-In2Se3 thin layers. Nanoscale, 2018, 10, 14885
[7] Khan A I, Chatterjee K, Wang N, et al. Negative capacitance in a ferroelectric capacitor. Nat Mater, 2015, 14, 182
[8] Si M, Su C, Jiang C, et al. Steep-slope hysteresis-free negative capacitance MoS2 transistors. Nat Nano, 2018, 13, 24
点击阅读戴伦研究员文章：
Room-temperature stable two-dimensional ferroelectric materials
Lun Dai
J. Semicond. 2019, 40(6), 060402
doi: 10.1088/1674-4926/40/6/060402
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