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Hafnia-based ferroelectric memory devices with excellent ferroelectricity, low power consumption, and fast operation speed have attracted considerable interest with the ever-growing desire for nonvolatile memory in flexible electronics. However, hafnia films are required to perform a high temperature (>500 °C) annealing process for crystallization into the ferroelectric orthorhombic phase. It can hinder the integration of hafnia ferroelectric films on flexible substrates including plastic and polymer, which are not endurable at high temperatures above 300 °C. Here, we propose the extremely low-temperature (∼250 °C) process for crystallization of Hf 0.5 Zr 0.5 O 2 (HZO) thin films by applying a focused-microwave induced annealing method. HZO thin films on a flexible mica substrate exhibits robust remnant polarization (2P r ∼ 50 μC/cm 2 ), which is negligibly changed under bending tests. In addition, the electrical characteristics of a HZO capacitor on the mica substrate were evaluated, and ferroelectric thin film transistors (Fe-TFTs), using a HZO gate insulator, were fabricated on mica substrates for flexible synapse applications. Symmetric potentiation and depression characteristics are successfully demonstrated in the Fe-TFT memory devices, and the synaptic devices result in high recognition accuracy of 91.44%. The lowtemperature annealing method used in this work are promising for forming hafnia-based Fe-TFT memory devices as a building block on a flexible platform.

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The passivation layers that should be formed on flexible electronics devices require high mechanical stability. Therefore, organic passivation has been used to enhance the electrical characteristics of various devices such as thin-film transistors (TFTs), organic light emitting diodes, and capacitors under mechanical stress. However, the conventional deposition of an organic film based on spin coating results in excessive thickness and the potential for chemical damage due to by-products such as organic solvents throughout the process. Here, we present the effects of a poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) organic passivation layer deposited by initiative chemical vapor deposition (iCVD) on the electrical and mechanical stabilities of oxide TFTs subjected to 30 000 repetitive tensile bending cycles. The highly ultrathin (50 nm) and excellent roughness (Rq = 0.33 nm) of the pV3D3 film assisted in preserving the mechanical stability of the device under external mechanical stress, and degradation of the electrical properties was suppressed compared with a device using SiO2 passivation. The mechanical properties of the type of passivation layer, including its Young’s modulus, affected the degradation of the electrical properties and reliability characteristics under repetitive bending. Finite-element structural simulations indicated a 15% reduction in equivalent stress applied to each layer of the device when pV3D3 (versus SiO2 passivation) was used. The iCVD-deposited pV3D3 film used in this study is a powerful candidate to act as the passivation layer of flexible electronics by strengthening the electrical stability of a device under external mechanical stress.

Careful application of Auger depth profiling (ADP) in connection with quantitative consideration of electron escape depth and surface roughness allows to obtain a large depth resolution (0.5 nm at 10 nm film thickness) and a reduction of chemical changes due to the measurement. The Al‐SiO2 interlayer is very thin (about five atom layers) and may be described well with a simple penetration‐reaction model. The reduction of SiO2 and formation of Al2O3 can be caused by X‐rays, electron beam, or ion beam.