Multibit Optoelectronic Memory

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NUS scientists have developed multibit optoelectronic memory using a heterostructure made of two-dimensional (2D) materials for next generation devices.

Optoelectronic memories are devices which can store photon-generated charge carriers when exposed to light. The stored charges can be accessed later for information retrieval. These devices can be used in image capture and spectrum analysis systems. 2D atomically layered materials are promising candidates for the development of next generation optoelectronic memories to meet emerging requirements for device miniaturisation and structural flexibility. However, optoelectronic memories fabricated using 2D materials have been reported to suffer from poor data storage capability with the highest reported figure at about eight distinct storage states.

A team lead by Prof CHEN Wei from both the Department of Chemistry and the Department of Physics, NUS has developed a multibit, non-volatile optoelectronic memory device that is able to store up to 130 distinct states by using a tungsten diselenide/ boron nitride (WSe₂/ BN) heterostructure. The heterostructure, made of 2D materials, comprises a monolayer of WSe₂ on a 20-layer BN. The programming (store data) and erasing (delete data) functions are controlled by adjusting the applied polarity to the device. A negative polarity is applied during the programming function and it causes the photon-generated electrons from the midgap donor-like states of the BN material to transfer into the WSe₂ material. This leaves behind localised (non-mobile) positive charges in the BN material. For the erasing function, a positive polarity is applied. This causes the photon-generated electrons from the valence band in the BN material to recombine with the localised positive charges, returning it to a neutral state.

The amount of electrons transferred into the WSe₂ material is dependent on the duration of light exposure time for the device. A longer exposure time would mean that more electrons are transferred. The researchers found that the continual accumulation of electrons in the WSe₂ material corresponding up to 130 light pulses can be detected before saturation conditions set in. Each of these pulses can be treated as a distinct storage state. During performance testing, they found that the device exhibits a data retention of over 4.5×10⁴ seconds and a cyclic programme/ erase endurance exceeding 200 cycles.

Explaining the significance of the findings, Prof Chen said, “Although there is still a performance gap when compared to commercial silicon-based memory, these devices are advantageous in electronic applications which require structural flexibility. The use of this WSe₂/ BN 2D layered heterostructure provides a method to achieve multibit memory device and may pave the way for the development of next generation optoelectronic memories.”