  The memory window of floating gate (FG) type non-volatile memory (NVM)
devices is a fundamental figure of merit used not only to evaluate the
performance, such as retention and endurance, but also to discuss the
feasibility of advanced functional memory devices. However, the memory window
of two dimensional (2D) materials based NVM devices is historically determined
from round sweep transfer curves, while that of conventional Si NVM devices is
determined from high and low threshold voltages (Vths), which are measured by
single sweep transfer curves. Here, it is elucidated that the memory window of
2D NVM devices determined from round sweep transfer curves is overestimated
compared with that determined from single sweep transfer curves. The floating
gate voltage measurement proposed in this study clarifies that the Vths in
round sweep are controlled not only by the number of charges stored in floating
gate but also by capacitive coupling between floating gate and back gate. The
present finding on the overestimation of memory window enables to appropriately
evaluate the potential of 2D NVM devices.
