Neuromorphic computing, with its high parallelism, is considered a promising method for further improving the efficiency of integrated computing systems in the post-Moore era. As the fundamental components of hardware-based neuromorphic systems, analog synaptic devices have undergone considerable research progress in recent years. Among them, SiO₂ trapping-based synaptic devices have unique advantages in terms of system integration, such as easy fabrication and high CMOS process compatibility. However, the electrochemical activity of oxygen makes the electron-trapping states unstable in air, which leads to an unstable operation of the device in air. Here, we used an interfacial layer of Si₃N₄ to block oxygen molecules and protect the trapped electrons at the SiO₂ interface. The experimental results demonstrate the feasibility of this method. Based on the device with 7 nm Si₃N₄, we mimicked some common synaptic plasticities, including EPSC, PPF, pulse duration-dependent plasticity, pulse number-dependent plasticity, and pulse frequency-dependent plasticity. In addition, by studying the device behaviors with different Si₃N₄ thicknesses, we discuss the interface protection mechanism of Si₃N₄.

Considering that the device relies on SiO₂ interface trapping to maintain a nonvolatile state, the physical protection of the interface is a reasonable approach to minimize damage to the trapped state. In addition, another requirement for the blocking layer is to allow only the electron to tunnel through itself; hence, it is possible to prevent the penetration of oxygen molecules while simultaneously maintaining electron trapping at the SiO₂ interface. Based on this concept, we used ultrathin and dense Si₃N₄ as the blocking layer to ensure electron tunneling, whereas large-sized O₂ was isolated. Subsequently, we grew an ultrathin and dense Si₃N₄ layer on the SiO₂ interface and constructed a device as shown in Fig. 3(a).

A B-B SJ device is constructed using a symmetrical Au pair as the electrode connecting the signal input and C₈-BTBT as the device channel layer, as shown in Fig. 1(a). As the pulse increases, as shown in Fig. 2, there is an obvious state of deterioration in the air, indicating failure of the synaptic function. Subsequently, an ultrathin and dense Si₃N₄ layer is grown on the SiO₂ interface and a device is constructed as shown in Fig. 3(a). These results indicate that the device with Si₃N₄ can operate stably in air, exhibiting several pulse-pattern-dependent plasticities. The pulse intensity-dependent plasticity of the device is shown in Fig. 3(f). When the pulse light intensity increases from 10.8 to 546 μW/cm², the EPSC of the device can simultaneously increase from 1.72 to 5 nA. We quantify this relationship into a functional relationship and present it in Fig. 3(g). The PPF fitting of the device is shown in Fig. 4(c), where C₁=1.043, C₂=0.213,τ₁=50 ms, and τ₂=4363 ms, which is consistent with biological features. The single-, double-, and ten-pulse tests on the device are shown in Fig. 4(d), which shows the corresponding decay times of 19, 26, and 49 s. The pulse duration-dependent plasticity, pulse number-dependent plasticity, and pulse frequency-dependent plasticity of the device are shown in Fig. 4(e), Fig. 5(a), and Fig. 5(c), respectively. These results adequately demonstrate that the Si₃N₄ interface protection can enable synaptic devices to stably operate in air. In addition, to study the interface protection mechanism of Si₃N₄, the device with 5 nm Si₃N₄ is also tested. By laterally comparing the device behaviors in 0, 5, and 7 nm Si₃N₄ devices, we found that the protecting effect of Si₃N₄ improved with its thickness and that 7 nm Si₃N₄ could ensure stable operation of the device in air.

We found that oxygen-induced electrochemical reactions could destroy electron-trapping states, inevitably making SiO₂ trapping-based memristives unstable in air. We experimentally demonstrate that using a Si₃N₄ protective layer on SiO₂ can markedly improve the operating stability of the device in air because Si₃N₄ with a suitable thickness can effectively block oxygen molecules from contacting the SiO₂ interface but allow electrons to pass through. Subsequently, common synaptic plasticity behaviors, such as EPSC, PPF, pulse duration-dependent plasticity, pulse number-dependent plasticity, and pulse frequency-dependent plasticity, are mimicked in air with the 7 nm Si₃N₄ device. In addition, by showing the Si₃N₄ thickness-dependent state-updating behavior, we demonstrate the modulating effects of Si₃N₄ on interface protection.