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

The use of Hf_0.5Zr_0.5O₂ (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 2P_r 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.