A new photonic, functional memory based on tin oxide slanted nanorod arrays in which both the optical and electrical stimuli can be used to modulate switching characteristics shows potential for developing high-density and high-efficiency computing systems.

Currently, various research groups worldwide are designing and realizing non-volatile, ultrafast, reliable, functional memory systems that outperform traditional silicon-based flash memories. In this big data era, a new class of data storage devices that can overcome the physical limitations of the existing memory technologies is being pursued vigorously. One such class of memories is commonly known as memristor (an acronym for memory resistor), which can store and process data through electrical signals.

Figure 1. (a) Cross-sectional FESEM image depicting the SnO_x slanted nanorod array. (b) Schematic of Al/SnO_x/FTO device upon light illumination. (c) Optically controlled resistive switching in Al/SnO_x/FTO devices under 365 nm light illumination for several switching cycles demonstrating the electrical SET and optical RESET processes.

Recently, researchers from the Centre for Nano and Soft Matter Sciences (CeNS), Bangalore, an autonomous institution of the Department of Science and Technology (DST), Govt. of India, have designed such a functional memory based on tin oxide slanted nanorod arrays that shows great potential for the development of high-density and high-efficient computing systems. In this restive memory (non-linear passive two-terminal electrical component which changes its internal resistance between high and low resistance states), both the optical and electrical stimuli can be used to modulate the switching characteristics, including multilevel cell operation.

The CeNS team consisting of Swathi S. P., Athira M., and S. Angappane developed the photonic memory in which the tin oxide slanted nanorod arrays are used as an active layer. The tin oxide nanostructures are prepared by electron-beam evaporation through a technique called the glancing angle deposition (GLAD) technique.

The electron-beam evaporation is a physical vapor deposition method wherein a focussed electron beam is made to bombard the desired target material, which results in its vaporization, and, eventually, deposition of the target material onto the substrate. GLAD facilitates the preparation of complex nanostructures by manipulating the coordinates (tilt and rotation) of the substrate.

The researchers observed good switching characteristics of the memory devices, including low operating voltages, moderate ON/OFF ratio (refers to the ratio of current in the ON state (low resistance state--LRS) to the OFF state (high resistance state- HRS) of the memory device), longer endurance, and better retention with a self-compliance effect in the dark. Interestingly, an unusual negative photo response with an enlarged ON/OFF ratio of greater than 10⁷ and a faster response time is observed under illumination ranging from ultraviolet (254 and 365 nm) to visible light (405 and 533 nm).

The negative photo response is characterized by the decrease of the current in the active layer of the device upon light illumination. They found that these devices can be electrically SET (switching the device from a high to low resistance state by applying voltage bias) to LRS and optically RESET (switching of the device from low to high resistance state upon exposure to the light) to HRS.

Remarkably, multiple low and high resistance states have been achieved by modulating the programming current and optical stimulus. Moreover, they have presented ample experimental evidence which suggests that the electric field-induced formation and light-induced dissolution of oxygen vacancies are responsible for the optically-stimulated resistance switching. In other words, multiple nanoscale conductive filaments composed of oxygen vacancies (primary defects in oxide-based memory devices) are formed on applying the electrical bias, and the photo-stimulated recombination of the surrounding oxygen ions with the vacancies results in the rupturing of the formed conductive filaments. In this manner, the local conductivity of the tin oxide nanorod array could be modified by the synergistic interplay between the electrical and optical means.

The research recently published in ACS Applied Materials and Interfaces can enable the design and development of photonic memories based on metal oxide nanostructures and help explore their potential applications in artificial visual memory and optoelectronics.

Publication link: https://doi.org/10.1021/acsami.2c22362

For more details, Dr. S. Angappane (angappane[at]cens[dot]res[dot]in) can be contacted.