(223g) 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).