Atomically thin two-dimensional (2D) materials are promising candidates to develop flash memories with premium performances as compared to conventional bulk materials, because of their ultra-thin thickness and highly tunable electrical properties. So far, most of the reported 2D material based flash memories work in the uni-polar mode, which usually further integrate additional local gate to achieve bi-polar function. However, such approach is volatile, meaning that the gate bias has to be applied persistently to maintain the polarity change and thus increases the power consumption. Here, we report a bi-polar memory based on MoTe₂/h-BN/graphene semi-floating gate (SFG) heterostructure, which has non-volatile and dynamically tunable polarity. The SFG configuration has the channel layer of MoTe₂ and dielectric layer of h-BN half-stacked on the floating gate layer of graphene. The off-graphene half of the MoTe₂ channel can be tuned between n-type and p-type by simultaneously applying ultraviolet (UV) illumination and electrical field through the back gate, which maintains this polarity after the removal of both stimuli. As a result, the SFG memory can work in the non-volatile bi-polar mode, with a on/off ratio of ~ 100 and switching speed of 1 ms. On the other hand, the on-graphene half of the MoTe₂ channel remains n-type under UV illumination and electrical bias, so that the MoTe₂ full floating gate memory maintains n-type, which implements the integration of both n- and p-type memories in a single 2D heterostructure. This capability provides great flexibility for memory devices adapting in various emerging applications.

Flash memories and semiconductor p-n junctions are two elementary but incompatible building blocks of most electronic and optoelectronic devices. The pressing demand to efficiently transfer massive data between memories and logic circuits, as well as for high data storage capability and device integration density, has fueled the rapid growth of technique and material innovations. Two-dimensional (2D) materials are considered as one of the most promising candidates to solve this challenge. However, a key aspect for 2D materials to build functional devices requires effective and accurate control of the carrier polarity, concentration and spatial distribution in the atomically thin structures. Here, a non-volatile opto-electrical doping approach is demonstrated, which enables reversibly writing spatially resolved doping patterns in the MoTe₂ conductance channel through a MoTe₂/hexagonal boron nitride (h-BN) heterostructure. Based on the doping effect induced by the combination of electrostatic modulation and ultraviolet light illumination, a 3-bit flash memory and various homojunctions on the same MoTe₂/BN heterostructure are successfully developed. The flash memory achieved 8 well distinguished memory states with a maximum on/off ratio over 10⁴. Each state showed negligible decay during the retention time of 2,400 s. The heterostructure also allowed the formation of p-p, n-n, p-n, and n-p homojunctions and the free transition among these states. The MoTe₂ p-n homojunction with a rectification ratio of 10³ exhibited excellent photodetection and photovoltaic performance. Having the memory device and p-n junction built on the same structure makes it possible to bring memory and computational circuit on the same chip, one step further to realize near-memory computing.