Impact of HfO₂ Dielectric Layer Placement in Hf_0.5Zr_0.5O₂‐Based Ferroelectric Tunnel Junctions for Neuromorphic Applications

Abstract

The use of Hf0.5Zr0.5O2 (HZO) films within hafnia‐based ferroelectric tunnel junctions (FTJ) presents a promising avenue for next‐generation non‐volatile memory devices. HZO exhibits excellent ferroelectric properties, ultra‐thinness, low power consumption, nondestructive readout, and compatibility with silicon devices. In this study, Mo/HZO/n+ Si devices are investigated, incorporating a 1 nm HfO2 dielectric layer at the top and bottom of the HZO ferroelectric layer. Comparing the FTJ device configurations, it is observed that the metal‐ferroelectric‐dielectric‐semiconductor (MFIS) outperforms the metal‐dielectric‐ferroelectric‐semiconductor (MIFS) in terms of ferroelectricity, displaying a high 2Pr value of ≈69 µC cm−2. Additionally, MFIS exhibits lower leakage current, higher tunneling electro‐resistance ratio, and a thin dead layer during short pulse switching, as confirmed through DC double sweeping of I−V characteristics. The modified half‐bias scheme demonstrates a maximum array size of 191 for MFIS, showcasing its superior performance over MIFS. Synaptic characteristics, including potentiation, depression, paired‐pulse facilitation, spike‐rate‐dependent plasticity, and excitatory postsynaptic current, are measured using MFIS, highlighting its outstanding ferroelectric properties. As a physical reservoir, the FTJ device implements 16 states of 4 bits in reservoir computing. Finally, pattern recognition using a deep learning neural network achieves high accuracy with using the Modified National Institute of Standards and Technology dataset.

High-Performance Artificial Synapse Developed by HZO on (110) NSTO

“…Additionally, the fabrication of these FEs is incongruent with current complementary metal oxide semiconductor (CMOS) processes, constraining their advancement in semiconductor applications. These limitations are overcome by fluorite-structured HZO, which features noncentrosymmetric and polar-orthogonal Pca2 1 grains, providing ferroelectric properties even at smaller thicknesses. , Furthermore, HZO compatibility with CMOS processes, high dielectric constant, and wide bandgap make it promising for ferroelectric synapses, particularly in ferroelectric tunnel junctions (FTJs) where ultrathin films (<5 nm) are essential. − However, recent studies have revealed that certain external influences, such as dead layers at the FE/electrode interface and limited screening lengths within the electrodes, result in incomplete screening of polarized charges and formation of depolarization fields (DFs), which can be detrimental to ferroelectric stability . Due to the limitations imposed by DFs, it is not possible to further decrease the thickness of HZO films to enhance their ferroelectric properties.…”