Optoelectronic Synapses for Neuromorphic Computing Using ITO/Nb-doped SrTiO₃ Memristor

From the viewpoint of processing speed and safety, edge computing, which operates physically close to the user or data source, has been attracting attention [1]. In order to enable data processing at any edge, a method that reduces computational cost while maintaining high learning performance is required. One of the candidates is reservoir computing (RC), which is a computational framework suitable for processing time series-data. RC transforms time-series data into spatio-temporal patterns with a high-dimensional space of mutually conjugate nonlinear elements called reservoirs, which can be easily analyzed for patterns using a simple learning algorithm at readout time [2]. Physical reservoir computing (PRC) is expected to achieve low power consumption and simple device structures because it uses the physical dynamics of real physical systems as a reservoir. Among PRCs, a tin-doped indium oxide (ITO)/Nb-doped SrTiO₃ (Nb:STO) junction, which works as an artificial optoelectronic synapse that respond to not only electric stimuli but also photonic ones, is interesting because human beings receive much of the information from the environment through visual perception [３].<br/>In this paper, we discuss the mechanism of synaptic behavior of ITO/Nb:STO junctions, focusing on the correlation between photo- and electric- responses. For this purpose, we prepared ITO/Nb:STO junction that shows a large hysteresis in<i> I</i>-<i>V</i> characteristics, meaning that the ITO/Nb:STO junction can work as a resistive random access memory with a large memory window. We also demonstrated the influence of the correlation between optical and electrical on PRC.<br/>We have evaluated the photoresponsive characteristics of ITO/Nb:STO junctions irradiated with ultraviolet (UV) light from the ITO transparent electrode side. The measurement was performed by applying a constant read out voltage (<i>V</i>_read) to the device using a source measure unit (Keysight B2901A), and the current value of the device was read out. As a result, we found that the photoresponsive behavior of the ITO/Nb:STO junction is significantly different depending on whether <i>V</i>_read is applied or not.<br/>First, we irradiated the ITO/Nb:STO junction with UV light and measured the photoinduced current at <i>V</i>_read of 0 V, i.e., with no external voltage application. It was found that the current value quickly increased with UV irradiation, and quickly decayed to the initial current level when UV irradiation was stopped. This characteristic is considered to be suitable for applications such as photo sensors [４].<br/>Secondly, we performed the same experiment, except that a finite <i>V</i>_read was applied. When <i>V</i>_read was set to lower than -0.3 V, it was found that the current value nonlinearly and drastically increased with UV irradiation. Interestingly, when the UV irradiation is stopped, the current kept retaining large value although the current slightly decayed. In this case, the current never relaxed to the initial current value. This behavior is similar to Persistent Photoconductivity (PPC), which has already been observed in STO [５]. We have found for the first time that both photosensor-like mode with quick response and PPC-like mode with very slow but strong response can be controllably switched by an externally applied voltage to the device. This behavior is also interesting from a physical point of view.<br/>In addition, by using the nonlinear photoresponse behavior and the slow relaxation caused by the PPC, we have confirmed that the 4-bit data written by light irradiation ON as “1” and no light irradiation as “0” can be distinguished.<br/>[1] W. Shi <i>et al</i>., IEEE Internet of Things Journal 3, 637 (2016).<br/>[2] G. Tanaka <i>et al</i>., Neural Networks 115, 100 (2019).<br/>[３] S. Gao <i>et al</i>., ACS Nano 13, 2634 (2019).<br/>[４] L. Wu <i>et al</i>., Adv. Funct. Mater 31, 2010439 (2021).<br/>[５] MC. Tarun <i>et al</i>., PRL 111, 187403 (2013).