464360 Electrostatic Double Layer Flash Memory Based on Two-Dimensional Crystals
A new type of flash memory will be presented based on the electrostatic doping of two-dimensional crystals using ions. The proposed device consists of two, 2D crystals separated by a 2D electrolyte through which the ions can pass. The top 2D crystal comprises the channel of a field-effect transistor (FET), while bottom 2D crystal is a backgate. When the ions are near the surface of the channel, they induce image charge in the channel and the device is the low resistance, or ON state. When the ions are pulled back to the backgate by an applied field, the device is in the high resistance or OFF state. The 2D electrolyte is cobalt crown ether phthalocyanine (CoCrPc) plus a salt, which can be deposited on the surface of 2D crystals simply by drop casting and annealing. The crown ethers solvate metal ions, and the ions can pass through the cavity of the crowns. Density functional theory (DFT) calculations show that the crown ethers present a small barrier to ion transport required for fast (nanosecond) switching, but the height of the barrier will be increased for long retention both by the image charge induced in the channel and by modulating the gate bias. The current-voltage characteristics of a simplified device architecture (monolayer CoCrPc:LiClO4 on graphene) will be presented. The graphene FET can be reconfigureably programmed by the 2D electrolyte, achieving sheet carrier densities of 4 x 1012 cm-2 at a low lithium concentration of 20 crown ethers to 1 Li+. Based on the geometric packing of the molecules, as determined by scanning tunneling microscopy, the doping density is predicted to increase to 5 x 1013 cm-2 at a crown ether to lithium ratio of 1:1. State retention measurements show that the two states can be retained for at least 30 minutes (maximum time measured to date) with a memory window of 10 uA. In future work, the graphene will be replaced with MoS2 because both the doping density and memory window are predicted to increase.
This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of six SRC STARnet Centers, sponsored by MARCO and DARPA, and NSF grant #ECCS-GOALI-1408425.
Figure: Electrostatic double layer 2D crystal memory. (a) Cross section of proposed deice; ions induce image charge in the channel in the ON state, and are pulled back to the backgate in the OFF state. (b) Schematic of simplified device; 2D electrolyte and LiClO4 on graphene FET (c) Transfer characteristics of simplified device at varying programming voltages; the shift in the Dirac point indicates reconfigureable doping by the 2D electrolyte corresponding to the two positions shown in (d).