In 2021, two papers by Liu et al. (Nat. Nanotechnol. 16, 874–881; 2021) and Wu et al. (Nat. Nanotechnol. 16, 882–887; 2021) independently reported an ultrafast programming speed non-volatile 2D flash memory device. With the Fowler–Nordheim tunnelling mechanism and the Wentzel–Kramers–Brillouin approximation method in mind, Liu et. al. hypothesized that a 2D memory device with a high gate coupling ratio and low-tunnelling-barrier would have a greatly improved programming efficiency. In their flash based on a MoS2/h-BN/multilayer graphene van der Waals heterostructure these materials serve as the channel material, tunnelling layer and floating gate layer, respectively (see Figure). According to Peng Zhou from Fudan University, China, one of the corresponding authors of the paper: “The idea was that in the 2D-based flash memory, the sharp energy band bending of ultrathin materials channel allow for the formation of a triangle barrier before the main tunnelling barrier. This double barrier significantly enhanced the tunnelling efficiency.” In practical terms, this optimization strategy translated into a 20 ns programming speed, channel current ratios of memory state-1/state-0 of ~106 and ten years of data retention.
One year and a half after publication, reflecting on the success of the ultrafast flash memory demonstration, Zhou feels that the broad appeal of their work is down to the fact that they were able to achieve “the programming speed of nonvolatile flash memory on par with that of the volatile memory such as DRAM.” This achievement has high technological relevance and has stimulated a large number of follow-up studies on device optimization, particularly in the design of the effective tunnelling barrier and increasing the gate coupling ratio for even faster operation. At present, engineering efforts have also started to look into integrated architectures for wafer scale operations of 2D flash memory devices; here the main challenge is to preserve the quality of the atomically sharp interface.
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