Sliding Ferroelectric Physics And Device Characteristic Of Two-dimensional Semiconductors

Ferroelectric materials have switchable spontaneous polarization that can be flipped by the external electric field,and are potentially attractive for non-volatile memory devices,such as,ferroelectric random-access memory,ferroelectric effect-field transistors,ferroelectric tunnel junctions and ferroelectric diodes.However,the traditional ferroelectricity is insufficient for meeting the future trend of high-integration artificial intelligence applications because of the critical thickness.Recently,the brand-new sliding ferroelectricity with atomic thickness was discovered in the van der Waals compound.Its out-of-plane polarization stems from the interlayer asymmetric distributed electron charges caused by the special bilayer stacking configurations of two-dimensional materials.The electrical field can easily drive the relative in-plane motion of adjacent layers to flip the out-of-plane polarization direction because of the ultralow energy barrier for changing stacking configurations.To our best knowledge,there are few reports about the sliding ferroelectric semiconductor.In this dissertation,the ferroelectric physical and electronic device of sliding ferroelectric semiconductor was systemically studied,as following:1.The bulk 3R-MoS₂ was synthesized with chemical vapor transport.And then a field-effect transistor was fabricated based on bilayer 3R-stacked MoS₂,where the device shows a typical n-type channel with a mobility of 8.6 cm² V^-1 s^-1,a current on/off ratio over 10⁸ and an ultra-low SS value.Compared with that of the single layer MoS₂ with a center of symmetry and bilayer 2H-MoS₂ with inversion symmetry,the dynamic electric transport of bilayer or trilayer 3R-MoS₂ shows a typical ferroelectric bistable state.In addition,the high Curie phase transition temperature of 3R-MoS₂ has experimentally determined with the help of variable temperature electric transport,second harmonic generation measurement and X-ray power diffraction characteristic.Thanks to the effective symmetric broken of 3R-MoS₂,the ferroelectricity can still be switched when the carrier density of channel is 5.42×10¹² cm^-2.2.The interfacial polarization is cumulative in the 3R-MoS₂,which can be deduced from the result of static electric transport or Kelvin probe force microscopy of bilayer and trilayer.It brings multiple polarization states in multi-layer 3R-MoS₂.Combined with the theory calculations analysis,a generalized model was proposed to describe the ferroelectric switching process in multilayer 3R-MoS₂ and to explain the formation of these intermediate polarization states.The domain walls motion of 3R-MoS₂ will not be pinned by the mobile charge defects due to the absence of out-of-plane ion displacements even under the large electric field in such ferroelectricity.The sliding ferroelectric3R-MoS₂ does not show the loss of polarization after 10⁶ bipolar cycles and the anti-fatigue feature is immune to the pulse width,indicating that the sliding ferroelectricity could provide a solution to the long-standing fatigue issue in ferroelectric material.3.The single-layer graphene with tunable Fermi level was introduced in the drain electrode to fabricated the high-performance sliding ferroelectric semiconductor field-effect transistor.The operation mechanism of device roots in the tunable Schottky barrier through electrically modulating the ferroelectric polarization and the device exhibits a large memory window of 4.5 V,a high on/off ratio of 10⁶and the retention time up to 1000 s.In addition,it was found that the memory windows could be adjusted by electric field of top-gate or light illumination,providing a novel way to develop ferroelectric multifunctional devices.Our work demonstrated here offers a promising avenue towards designing new ferroelectric devices integrated with memory and computing functions.