Interface-Protected Optical Synapse Device Operating Stably in Air
Objective 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.Methods 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).Results and Discussions 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.Conclusions 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.
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