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1. Implementation of high-performance and high-yield nanoscale hafnium zirconium oxide based ferroelectric tunnel junction devices on 300 mm wafer platform

   https://doi.org/10.1116/6.0002097  

   Liehr, Maximilian; Hazra, Jubin; Beckmann, Karsten; Mukundan, Vineetha; Alexandrou, Ioannis; Yeow, Timothy; Race, Joseph; Tapily, Kandabara; Consiglio, Steven; Kurinec, Santosh K; et al (January 2023, Journal of Vacuum Science & Technology B)

   In this work, hafnium zirconium oxide (HZO)-based 100 × 100 nm2 ferroelectric tunnel junction (FTJ) devices were implemented on a 300 mm wafer platform, using a baseline 65 nm CMOS process technology. FTJs consisting of TiN/HZO/TiN were integrated in between metal 1 (M1) and via 1 (V1) layers. Cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy analysis confirmed the targeted thickness and composition of the FTJ film stack, while grazing incidence, in-plane x-ray diffraction analysis demonstrated the presence of orthorhombic phase Pca21 responsible for ferroelectric polarization observed in HZO films. Current measurement, as a function of voltage for both up- and down-polarization states, yielded a tunneling electroresistance (TER) ratio of 2.28. The device TER ratio and endurance behavior were further optimized by insertion of thin Al2O3 tunnel barrier layer between the bottom electrode (TiN) and ferroelectric switching layer (HZO) by tuning the band offset between HZO and TiN, facilitating on-state tunneling conduction and creating an additional barrier layer in off-state current conduction path. Investigation of current transport mechanism showed that the current in these FTJ devices is dominated by direct tunneling at low electric field (E < 0.4 MV/cm) and by Fowler–Nordheim (F–N) tunneling at high electric field (E > 0.4 MV/cm). The modified FTJ device stack (TiN/Al2O3/HZO/TiN) demonstrated an enhanced TER ratio of ∼5 (2.2× improvement) and endurance up to 106 switching cycles. Write voltage and pulse width dependent trade-off characteristics between TER ratio and maximum endurance cycles (Nc) were established that enabled optimal balance of FTJ switching metrics. The FTJ memory cells also showed multi-level-cell characteristics, i.e., 2 bits/cell storage capability. Based on full 300 mm wafer statistics, a switching yield of >80% was achieved for fabricated FTJ devices demonstrating robustness of fabrication and programming approach used for FTJ performance optimization. The realization of CMOS-compatible nanoscale FTJ devices on 300 mm wafer platform demonstrates the promising potential of high-volume large-scale industrial implementation of FTJ devices for various nonvolatile memory applications. 

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2. 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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3. Ferroelectric Schottky diodes of CuInP2S6 nanosheet

   https://doi.org/10.1063/5.0160928  

   Yao, Jinyuan; Liu, Yongtao; Ding, Shaoqing; Zhu, Yanglin; Mao, Zhiqiang; Kalinin, Sergei V.; Liu, Ying (October 2023, Applied Physics Letters)

   Ferroelectricity in van der Waals (vdW) layered material has attracted a great deal of interest recently. CuInP2S6 (CIPS), the only vdW layered material whose ferroelectricity in the bulk was demonstrated by direct polarization measurements, was shown to remain ferroelectric down to a thickness of a few nanometers. However, its ferroelectric properties have just started to be explored in the context of potential device applications. We report here the preparation and measurements of metal-ferroelectric semiconductor-metal heterostructures using nanosheets of CIPS obtained by mechanical exfoliation. Four bias voltage and polarization dependent resistive states were observed in the current–voltage characteristics, which we attribute to the formation of ferroelectric Schottky diode, along with switching behavior. 

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4. Ferroelectric Tunneling Junctions Based on Aluminum Oxide/ Zirconium-Doped Hafnium Oxide for Neuromorphic Computing

   https://doi.org/10.1038/s41598-019-56816-x  

   Ryu, Hojoon; Wu, Haonan; Rao, Fubo; Zhu, Wenjuan (December 2019, Scientific Reports)

   Abstract Ferroelectric tunneling junctions (FTJs) with tunable tunneling electroresistance (TER) are promising for many emerging applications, including non-volatile memories and neurosynaptic computing. One of the key challenges in FTJs is the balance between the polarization value and the tunneling current. In order to achieve a sizable on-current, the thickness of the ferroelectric layer needs to be scaled down below 5 nm. However, the polarization in these ultra-thin ferroelectric layers is very small, which leads to a low tunneling electroresistance (TER) ratio. In this paper, we propose and demonstrate a new type of FTJ based on metal/Al₂O₃/Zr-doped HfO₂/Si structure. The interfacial Al₂O₃layer and silicon substrate enable sizable TERs even when the thickness of Zr-doped HfO₂(HZO) is above 10 nm. We found that F-N tunneling dominates at read voltages and that the polarization switching in HZO can alter the effective tunneling barrier height and tune the tunneling resistance. The FTJ synapses based on Al₂O₃/HZO stacks show symmetric potentiation/depression characteristics and widely tunable conductance. We also show that spike-timing-dependent plasticity (STDP) can be harnessed from HZO based FTJs. These novel FTJs will have high potential in non-volatile memories and neural network applications. 

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5. Ferroelectric behavior of sputter deposited Al0.72Sc0.28N approaching 5 nm thickness

   https://doi.org/10.1063/5.0147224  

   Zheng, Jeffrey X.; Fiagbenu, Merrilyn Mercy; Esteves, Giovanni; Musavigharavi, Pariasadat; Gunda, Akhil; Jariwala, Deep; Stach, Eric A.; Olsson, Roy H. (May 2023, Applied Physics Letters)

   Ferroelectric Al1−xScxN has raised much interest in recent years due to its unique ferroelectric properties and complementary metal oxide semiconductor back-end-of-line compatible processing temperatures. Potential applications in embedded nonvolatile memory, however, require ferroelectric materials to switch at relatively low voltages. One approach to achieving a lower switching voltage is to significantly reduce the Al1−xScxN thickness. In this work, ferroelectric behavior in 5–27 nm films of sputter deposited Al0.72Sc0.28N has been studied. We find that the 10 kHz normalized coercive field increases from 4.4 to 7.3 MV/cm when reducing the film thickness from 27.1 to 5.4 nm, while over the same thickness range, the characteristic breakdown field of a 12.5 μm radius capacitor increases from 8.3 to 12.1 MV/cm. The 5.4 nm film demonstrates ferroelectric switching at 5.5 V when excited with a 500 ns pulse and a switching speed of 60 ns. 

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