image: Figure 1 | Working principle of the critical BTBT memory. a, Schematic diagram of the critical BTBT memory. b, Negative photomemory characteristics of the critical BTBT memory in response to multiple 520 nm laser pulses with a duration of 500 ns. c, band alignments are depicted at Vg < 0 with a constant Vd. Only when the critical BTBT condition is satisfied, does the device exhibits negative and non-volatile photo memory characteristics.
Credit: by Hangyu Xu, Runzhang Xie et al.
In conventional processing system designs, sensory units are typically separated from memory and computation components. Visual data is first transmitted to binary memory through high-power analogue-to-digital conversion (ADC) and subsequently processed by computational units. This separation leads to significant data transfer overhead, causing speed mismatches and power consumption challenges. While optoelectronic memory has emerged as a promising solution to integrate sensing and memory, reducing the need for data transfer, it still faces limitations. Technologies based on photogating and the Fowler-Nordheim tunnelling mechanism show potential but are hindered by unclear trap energy levels and high barriers, which result in slow response times and the necessity for high operating voltages.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Weida Hu from the Shanghai Institute of Technical Physics has made a significant advancement in this area. The researchers designed an optoelectronic memory device based on critical band-to-band tunnelling (BTBT), utilizing black phosphorus (BP) on indium selenium (InSe) materials. With deliberate band alignment, their device (critical BTBT memory) satisfies the critical BTBT condition, exhibiting cumulative photomemory current and a low operating voltage. The infinitesimal barrier enables the device's operating wavelength to extend into the near-infrared range (940 nm). Additionally, considering that motion can be viewed as the streaming of images across successive frames, this motion can be stored in optoelectronic memory devices. When paired with an interframe algorithm, the movement trajectory is uniquely defined. With no motion blur, the critical BTBT memory exhibits superior optoelectronic performance for precise moving target tracking and recognition, thus advancing the development of neuromorphic vision hardware.
To clarify the underlying mechanism, the researchers performed a series of comparative experiments using identical device structures but different materials. They found that only when the band alignment satisfies the critical BTBT condition does the device exhibit negative and non-volatile current under optical stimulation. Additionally, they observed two distinct negative differential resistance (NDR) points, which serve as conclusive evidence for the existence of hole memory and the electron tunnelling process. The findings mark a significant advancement in the field of ultrafast, high-bandwidth intelligent optoelectronic memory, showcasing a unique tunnelling mechanism that promises to push the boundaries of performance and efficiency. Based on these findings, the researchers summarized the operational principle of the critical BTBT memory as follows:
“The anomalous optoelectronic memory characteristics are attributed to two main factors: (1) The photo-generated carriers are rapidly separated via critical BTBT; (2) The spatial overlap between the electron tunnelling region and the hole storage region is minimized, leading to a low recombination rate. We have done a lot of experiments and simulations and found that this photoelectric storage phenomenon does not occur if either of these principles is violated.”
“The discovery of dual NDR points is strong evidence supporting our assumption. Thanks to the critical BTBT, our device demonstrates record photo-memory speed across a wide wavelength range.” they added.
“The presented technique can be used for moving target tracking and recognition, enhancing system efficiency. By reducing exposure time, it becomes particularly effective for fast-moving target scenarios, minimizing motion blur and significantly improving accuracy. This innovation could lay the foundation for the development of advanced neuromorphic vision hardware, with the potential to transform human life by enabling smarter, more responsive systems in various applications.” the scientists forecast.
Journal
Light Science & Applications
Article Title
Critical band-to-band-tunnelling based optoelectronic memory