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1. Unveiling the Apparent “Negative Capacitance” Effects Resulting from Pulse Measurements of Ferroelectric-Dielectric Bilayer Capacitors

   Liu, Zhan ; Jiang, Hao ; Ordway, Brandon ; and Ma, T.P. ( October 2020 , IEEE electron device letters)

   Naeemi, A.J. (Ed.)

   Apparent ‘Negative Capacitance’ (NC) effects have been observed in some ferroelectric-dielectric (FE-DE) bilayers by pulse measurements, and the associated results have been published that claim to be direct evidence to support the quasi-static ‘negative capacitance’ (QSNC) idea. However, the ‘NC’ effects only occur when sufficiently high voltage is applied, and even exist in stand-alone FE capacitors. These results contradict the QSNC theory, as it predicts that once stabilized (requires a DE layer), the FE remains in the ‘NC’ state regardless of the applied voltage. In this letter, by the use of Nucleation-Limited-Switching (NLS) model, we present our results obtained from simulation of pulse measurements on samples that are similar to the published ones. The simulation results indicate that reverse polarization switching occurs upon the falling edge of the pulses, which leads to the apparent hysteresis-free NC effect. This work provides an alternative interpretation of the experimental results without invoking the QSNC theory. 

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2. Domain wall enabled steep slope switching in MoS2 transistors towards hysteresis-free operation

   https://doi.org/10.1038/s41699-022-00353-1  

   Song, Jingfeng ; Qi, Yubo ; Xiao, Zhiyong ; Wang, Kun ; Li, Dawei ; Kim, Seung-Hyun ; Kingon, Angus I. ; Rappe, Andrew M. ; Hong, Xia ( November 2022 , npj 2D Materials and Applications)

   The device concept of ferroelectric-based negative capacitance (NC) transistors offers a promising route for achieving energy-efficient logic applications that can outperform the conventional semiconductor technology, while viable operation mechanisms remain a central topic of debate. In this work, we report steep slope switching in MoS₂transistors back-gated by single-layer polycrystalline PbZr_0.35Ti_0.65O₃. The devices exhibit current switching ratios up to 8 × 10⁶within an ultra-low gate voltage window of$$V_{{{\mathrm{g}}}} = \pm \! 0.5$$$V_g=\pm 0.5$V and subthreshold swing (SS) as low as 9.7 mV decade⁻¹at room temperature, transcending the 60 mV decade⁻¹Boltzmann limit without involving additional dielectric layers. Theoretical modeling reveals the dominant role of the metastable polar states within domain walls in enabling the NC mode, which is corroborated by the relation between SS and domain wall density. Our findings shed light on a hysteresis-free mechanism for NC operation, providing a simple yet effective material strategy for developing low-power 2D nanoelectronics.

    

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3. Ferroelectric transistor model based on self-consistent solution of 2D Poisson's, non-equilibrium Green's function and multi-domain Landau Khalatnikov equations

   https://doi.org/10.1109/IEDM.2017.8268385  

   Saha, A. K. ; Sharma, P. ; Dabo, I. ; Datta, S. ; Gupta, S. K. ( December 2017 , International Electron Device Meetings)

   We present a physics-based model for ferroelectric/negative capacitance transistors (FEFETs/ NCFETs) without an inter-layer metal between ferroelectric and dielectric in the gate stack. The model self-consistently solves 2D Poisson's equation, non-equilibrium Green's function (NEGF) based charge and transport equations, and multi-domain Landau Khalatnikov (LK) equations with the domain interaction term. The proposed simulation framework captures the variation of ferroelectric (FE) polarization (P) along the gate length due to non-uniform electric field (E) along the channel. To calibrate the LK equations, we fabricate and characterize 10nm HZO films. Based on the calibrated model, we analyze the gate/drain voltage dependence of P distribution in the FE and its effect on the channel potential and current-voltage characteristics. Our results highlight the importance of larger domain interaction to boost the benefits of FEFETs with subthreshold swing (SS) as small as ~50mV/decade achieved at room temperature. As domain interaction increases, the characteristics of FEFETs without inter-layer metal (SS, negative drain induced barrier lowering (DIBL), negative output conductance) approach those of FEFETs with inter-layer metal. 

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4. Demonstration of Wide Bandgap AlGaN/GaN Negative‐Capacitance High‐Electron‐Mobility Transistors (NC‐HEMTs) Using Barium Titanate Ferroelectric Gates

   https://doi.org/10.1002/aelm.202000074  

   Chandrasekar, Hareesh ; Razzak, Towhidur ; Wang, Caiyu ; Reyes, Zeltzin ; Majumdar, Kausik ; Rajan, Siddharth ( July 2020 , Advanced Electronic Materials)

   A negative‐capacitance high electron mobility transistor (NC‐HEMT) with low hysteresis in the subthreshold region is demonstrated in the wide bandgap AlGaN/GaN material system using sputtered BaTiO₃as a “weak” ferroelectric gate in conjunction with a conventional SiN_xdielectric. An enhancement in the capacitance for BaTiO₃/SiN_xgate stacks is observed in comparison to control structures with SiN_xgate dielectrics directly indicating the negative capacitance contribution of the ferroelectric BaTiO₃layer. A significant reduction in the minimum subthreshold slope for the NC‐HEMTs is obtained in contrast to standard metal‐insulator‐semiconductor HEMTs with SiN_xgate dielectrics—97.1 mV dec⁻¹versus 145.6 mV dec⁻¹—with almost no hysteresis in theI_D–V_Gtransfer curves. These results are promising for the integration of ferroelectric perovskite oxides with III‐Nitride devices toward NC‐field‐effect transistor switches with reduced power consumption.

    

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5. Vibrational fingerprints of ferroelectric HfO2

   https://doi.org/10.1038/s41535-022-00436-8  

   Fan, Shiyu ; Singh, Sobhit ; Xu, Xianghan ; Park, Kiman ; Qi, Yubo ; Cheong, S. W. ; Vanderbilt, David ; Rabe, Karin M. ; Musfeldt, J. L. ( December 2022 , npj Quantum Materials)

   Abstract Hafnia (HfO 2 ) is a promising material for emerging chip applications due to its high- κ dielectric behavior, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO 2 crystals stabilized with yttrium (chemical formula HfO 2 :  x Y, where x  = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO 2 and paves the way for an analysis of mode contributions to high- κ dielectric and ferroelectric properties for chip technologies. 

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