Long Duration Persistent Photocurrent in 3 nm Thin Doped Indium Oxide for Integrated Light Sensing and In‐Sensor Neuromorphic Computation

Abstract

Miniaturization and energy consumption by computational systems remain major challenges to address. Optoelectronics based synaptic and light sensing provide an exciting platform for neuromorphic processing and vision applications offering several advantages. It is highly desirable to achieve single‐element image sensors that allow reception of information and execution of in‐memory computing processes while maintaining memory for much longer durations without the need for frequent electrical or optical rehearsals. In this work, ultra‐thin (<3 nm) doped indium oxide (In2O3) layers are engineered to demonstrate a monolithic two‐terminal ultraviolet (UV) sensing and processing system with long optical state retention operating at 50 mV. This endows features of several conductance states within the persistent photocurrent window that are harnessed to show learning capabilities and significantly reduce the number of rehearsals. The atomically thin sheets are implemented as a focal plane array (FPA) for UV spectrum based proof‐of‐concept vision system capable of pattern recognition and memorization required for imaging and detection applications. This integrated light sensing and memory system is deployed to illustrate capabilities for real‐time, in‐sensor memorization, and recognition tasks. This study provides an important template to engineer miniaturized and low operating voltage neuromorphic platforms across the light spectrum based on application demand.

Visible-Active Artificial Synapses Based on Ultrathin Indium Oxide

“…X-ray photoelectron spectroscopy (XPS) is used to determine the chemical composition of the printed In 2 O 3 nanosheets as depicted in Figure e–g. The XPS analysis shows the In 3+ oxidation peaks of In 2 O 3 at 444.2 eV and 452 eV and the presence of oxygen peaks at 529.9 eV, which is in accordance with that of c- In 2 O 3 reported in the literature . The remaining oxygen peaks observed in the XPS analysis can be attributed to organic contaminants or the oxygen from the Si/SiO 2 substrate …”

“…As shown in Figure b, the device shows approximately 2 orders of magnitude change in current upon illumination with a slow decaying photocurrent upon stimulus removal. The persistent photocurrent (PPC) characteristics observed in the device arises from the presence of defects from oxygen vacancies (V o ) and atomic-scale thickness. , The photoresponse of the material in the visible spectrum can be attributed to the postsynthesis annealing process, which improves the material carrier mobility and possibly allows for new V o -based energy levels for easy electronic transitions with lesser energy between the valence band (VB) and conduction band (CB). , As reported previously, the improved UV photodetection and carrier mobility in LM-printed In 2 O 3 have been associated with reduced grain boundaries and lower recombination rate arising from the annealing process . With increasing grain sizes due to annealing, there is a reduction in the intergranular distorted region in the polycrystalline sheet, resulting in improved mobility due to fewer barriers .…”

“…Figure c shows the replication of SM characteristics using a 500 ms optical pulse of 455 nm under 4 mW/cm 2 incident power density, which results in 72.2 nA photocurrent generation. Such a phenomenon is akin to the successful firing of neurotransmitters, resulting in a change in synaptic weight/PSC in biological synapses known as excitatory postsynaptic current (EPSC), indicating the establishment of communication in the brain. , It is demonstrated in the next section that our device can even detect a 200 ms optical pulse of 455 nm, indicating the capability of the ultrathin wide-band gap system to distinguish visible light-based optical pulses in the millisecond level. Furthermore, as demonstrated in previous studies, the device would be able to detect shorter pulses than those demonstrated in this study if the readout voltage is increased, as it can enable a higher number of photogenerated carriers due to efficient charge separation. , SM information when acquired consciously without hindrance is converted into STM, which is stored for a short period in the brain .…”

“…In recent years, atomically thin semiconductors are being widely investigated for varied applications including artificial synaptic devices. − Such optoelectronic devices with PPC characteristics have been used to devise synapses functioning across the electromagnetic spectrum. − With time, there has been widespread application of such materials and their heterostructures to devise visible-active synaptic applications. ,, However, these devices often require high driving bias or additional stimuli to engineer/prolong their PPC property. , Metal oxide-based semiconductors form a coveted category of materials for optoelectronic synapses due to their superlative optoelectronic properties and well-established synthesis processes . Despite their high photocurrent generation and high mobility, their poor to absent response to visible wavelengths deems them unsuitable for visible-active synaptic applications. , In recent years, liquid metal printing has allowed the realization of many nonstratified metal oxides in ultrathin morphology with properties different from their bulk counterparts, thereby allowing the facile realization of large-scale 2D materials applicable for various semiconducting and conducting applications , and enabling the engineering and application of many such atomically thin LM-printed oxides for UV-active optoelectronic synapses. ,, However, there has not been much development in engineering these oxides for optoelectronic synaptic applications in the visible region.…”

Toward Smart, Flexible, and Omnidirectional Self-Powered Photodetection by an All-Solution-Processed In2O3/Pbl2 Heterojunction

“…Although amorphous In 2 O 3 films have been widely studied in electrical devices, their photodetection performance is rarely reported due to the famous persistent photocurrent (PPC) phenomenon. ,, The PPC phenomenon originates from the ionized oxygen vacancies and is prevalent in oxide semiconductors, which prevents the recombination of photogenerated carriers after removing the light signal, resulting in a slow photoresponse speed and unstable dynamic photoresponse. ,− On the other hand, it is well-known that the In cation is a creator of oxygen vacancy (Vo) due to the weak In–O bond strength, and Vo acts as an electron source site . Hence, the electron concentration in amorphous films is high, resulting in a large dark current and a small I light / I dark ratio in photodetectors.…”